View source for Biology:Marine life

{{Short description|Organisms that live in salt water}}
<noinclude>[[File:General characteristics of a large marine ecosystem.jpg|thumb|upright=1.8|right|General characteristics of a [[Earth:Large marine ecosystem|large marine ecosystem]] (Gulf of Alaska)]]</noinclude>
[[File:Orca porpoising.jpg|thumb|upright=1.5|right|[[Biology:Killer whale|Killer whale]]s (orcas) are highly visible marine apex predators that hunt many large species. But most biological activity in the ocean takes place with microscopic marine organisms that cannot be seen individually with the naked eye, such as marine bacteria and phytoplankton.<ref name="Cavicchioli_2019" />]]
{{marine life sidebar}}

'''Marine life''', '''sea life''', or '''ocean life''' is the [[Biology:Aquatic plant|plants]], animals, and other organisms that live in the salt water of [[Earth:Sea|sea]]s or [[Earth:Ocean|ocean]]s, or the [[Earth:Brackish water|brackish water]] of coastal [[Earth:Estuary|estuaries]]. At a fundamental level, marine life affects the nature of the planet. Marine organisms, mostly [[Biology:Marine microorganisms|microorganisms]], [[Earth:Oxygen cycle|produce oxygen]] and [[Biology:Biosequestration|sequester carbon]]. Marine life, in part, shape and protect shorelines, and some marine organisms even help create new land (e.g. [[Biology:Coral|coral]] building [[Earth:Coral reef|reefs]]).

Most life forms evolved initially in [[Biology:Marine habitats|marine habitats]]. By volume, oceans provide about 90% of the living space on the planet.<ref name="NOAA2019" /> The earliest [[Biology:Vertebrate|vertebrate]]s appeared in the form of [[Biology:Fish|fish]],<ref>{{cite web |url=https://www.livescience.com/46262-fossil-ancestor-jawed-vertebrates.html |title=Tiny Fish May Be Ancestor of Nearly All Living Vertebrates |work=Live Science |date=11 June 2014 }}</ref> which live exclusively in water. Some of these evolved into [[Biology:Amphibian|amphibian]]s, which spend portions of their lives in water and portions on land. One group of amphibians evolved into reptiles and mammals and a few subsets of each returned to the ocean as sea snakes, [[Biology:Sea turtle|sea turtle]]s, [[Biology:Pinniped|seals]], [[Biology:Sirenia|manatee]]s, and whales. Plant forms such as [[Biology:Kelp|kelp]] and other [[Biology:Algae|algae]] grow in the water and are the basis for some underwater ecosystems. [[Earth:Plankton|Plankton]] forms the general foundation of the ocean food chain, particularly phytoplankton which are key [[Earth:Primary producer|primary producer]]s.

Marine invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters, including breathing tubes as in [[Biology:Siphon (mollusc)|mollusc siphons]]. Fish have [[Biology:Fish gill|gills]] instead of lungs, although some species of fish, such as the [[Biology:Lungfish|lungfish]], have both. [[Biology:Marine mammal|Marine mammal]]s (e.g. dolphins, whales, [[Biology:Otter|otter]]s, and seals) need to surface periodically to breathe air.

{{As of|2023}}, more than 242,000 marine [[Biology:Species|species]] have been documented, and perhaps two million marine species are yet to be documented. An average of 2,332 new species per year are being described.<ref name="Drogin_2009">{{cite news |title=Mapping an ocean of species |last=Drogin |first=B |url=http://articles.latimes.com/2009/aug/02/nation/na-fish2 |work={{wipe|Los Angeles Times}} |date=2 August 2009 |access-date=18 August 2009}}</ref><ref>{{Cite journal |last1=Bouchet |first1=Philippe |last2=Decock |first2=Wim |last3=Lonneville |first3=Britt |last4=Vanhoorne |first4=Bart |last5=Vandepitte |first5=Leen |date=June 2023 |title=Marine biodiversity discovery: the metrics of new species descriptions |url=https://www.researchgate.net/publication/371877966 |journal=Frontiers in Marine Science |volume=10 |issue=3389 |doi=10.3389/fmars.2023.929989 |via=[[Company:ResearchGate|ResearchGate]] |doi-access=free }}</ref>

Marine species range in size from the microscopic like [[Earth:Phytoplankton|phytoplankton]], which can be as small as 0.02 [[Micrometre|micrometre]]s, to huge cetaceans like the [[Biology:Blue whale|blue whale]] &ndash; the largest known animal, reaching {{cvt|33|m|ft}} in length.<ref>{{cite book |last=Paul |first=GS |title=The Princeton Field Guide to Dinosaurs |url=https://archive.org/details/princetonfieldgu00paul_514 |url-access=limited |chapter=The Evolution of Dinosaurs and their World |location=Princeton |publisher=Princeton University Press |year=2010 |page=[https://archive.org/details/princetonfieldgu00paul_514/page/n19 19] |isbn=978-0-691-13720-9}}</ref><ref>{{Cite book |last=Bortolotti |first=Dan |title=Wild blue: a natural history of the world's largest animal |date=2008 |isbn=978-0-312-38387-9 |location=New York |publisher=Thomas Dunn Books |oclc=213451450}}</ref> Marine microorganisms, including protists and bacteria and their associated [[Biology:Marine viruses|viruses]], have been variously estimated as constituting about 70%<ref name="Bar-On_2018">{{cite journal |vauthors=Bar-On YM, Phillips R, Milo R |title=The biomass distribution on Earth |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=115 |issue=25 |pages=6506–6511 |date=June 2018 |pmid=29784790 |pmc=6016768 |doi=10.1073/pnas.1711842115 |bibcode=2018PNAS..115.6506B |doi-access=free }}</ref> or about 90%<ref>{{cite web |url=https://ocean.si.edu/ecosystems/census-marine-life/census-marine-life-overview |title=Census Of Marine Life |work=Smithsonian |date=30 April 2018 |access-date=29 October 2020 }}</ref><ref name="Cavicchioli_2019">{{cite journal |vauthors=Cavicchioli R, Ripple WJ, Timmis KN, Azam F, Bakken LR, Baylis M, Behrenfeld MJ, Boetius A, Boyd PW, Classen AT, Crowther TW, Danovaro R, Foreman CM, Huisman J, Hutchins DA, Jansson JK, Karl DM, Koskella B, Mark Welch DB, Martiny JB, Moran MA, Orphan VJ, Reay DS, Remais JV, Rich VI, Singh BK, Stein LY, Stewart FJ, Sullivan MB, van Oppen MJ, Weaver SC, Webb EA, Webster NS |display-authors=6 |title=Scientists' warning to humanity: microorganisms and climate change |journal=Nature Reviews. Microbiology |volume=17 |issue=9 |pages=569–586 |date=September 2019 |pmid=31213707 |pmc=7136171 |doi=10.1038/s41579-019-0222-5 }} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> of the total marine biomass. Marine life is studied scientifically in both marine biology and in biological oceanography. The term ''marine'' comes from the [[Social:Latin|Latin]] ''mare'', meaning "sea" or "ocean".


==Water==
[[File:Earth elevation histogram 2.svg|thumb|upright=1.3|Elevation histogram showing the percentage of the Earth's surface above and below sea level]]

There is no life without water.<ref>{{cite book |vauthors=Xiao-Feng P |date=2014 |url=https://books.google.com/books?id=O_26CgAAQBAJ&q=%22Water%3A+Molecular+Structure+And+Properties%22 |title=Water: Molecular Structure And Properties |chapter=Chapter 5 |pages=390–461 |publisher=World Scientific |isbn=9789814440448 }}</ref> It has been described as the ''universal solvent'' for its ability to [[Physics:Solvent|dissolve]] many substances,<ref>{{Greenwood&Earnshaw2nd|page=620|name-list-style=vanc}}</ref><ref>{{cite web |title=Water, the Universal Solvent |url=http://water.usgs.gov/edu/solvent.html |website=USGS |access-date=27 June 2017 |archive-url=https://web.archive.org/web/20170709141251/https://water.usgs.gov/edu/solvent.html |archive-date=9 July 2017 |url-status=live |df=dmy-all }}</ref> and as the ''solvent of life''.<ref>{{Cite book |title=Campbell Biology |vauthors=Reece JB |date=31 October 2013 |publisher=Pearson |isbn=9780321775658 |edition=10 |page=48 }}</ref> Water is the only common substance to exist as a [[Earth:Ice|solid]], liquid, and [[Physics:Water vapor|gas]] under conditions normal to life on Earth.<ref>{{Cite book |title=Campbell Biology |vauthors=Reece JB |date=31 October 2013 |publisher=Pearson |isbn=9780321775658 |edition=10 |page=44 }}</ref> The Nobel Prize winner [[Biography:Albert Szent-Györgyi|Albert Szent-Györgyi]] referred to water as the ''mater und matrix'': the mother and womb of life.<ref>{{cite book |vauthors=Collins JC |date=1991 |url=https://books.google.com/books?id=Rm_wAAAAMAAJ&q=%22materund+matrix%22 |title=The Matrix of Life: A View of Natural Molecules from the Perspective of Environmental Water |publisher=Molecular Presentations |isbn=978-0-9629719-0-7 }}</ref>

[[File:Composition of seawater.jpg|thumb|upright=1.3|Composition of seawater. Quantities in relation to 1&nbsp;kg or 1&nbsp;litre of sea water.]]

The abundance of surface water on Earth is a unique feature in the [[Astronomy:Solar System|Solar System]]. Earth's [[Earth:Hydrosphere|hydrosphere]] consists chiefly of the oceans but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of {{convert|2000|m|ft}}. The deepest underwater location is [[Earth:Challenger Deep|Challenger Deep]] of the Mariana Trench in the [[Earth:Pacific Ocean|Pacific Ocean]], having a depth of {{convert|10900|m|mi}}.<ref group=note name="trench_depth"/><ref name=kaiko7000/>

Conventionally, the planet is divided into five separate oceans, but these oceans all connect into a single world ocean.<ref>{{cite web |url=https://oceanservice.noaa.gov/facts/howmanyoceans.html |title=How many oceans are there? |work=NOAA |date=9 April 2020 }}</ref> The mass of this world ocean is 1.35{{e|18}}&nbsp;metric tons or about 1/4400 of Earth's total mass. The world ocean covers an area of {{val|3.618|e=8|u=km2}} with a mean depth of {{val|3682|u=m}}, resulting in an estimated volume of {{val|1.332|e=9|u=km3}}.<ref name="Charette_2010"/> If all of Earth's crustal surface was at the same elevation as a smooth sphere, the depth of the resulting world ocean would be about {{convert|2.7|km|mi}}.<ref>{{cite encyclopedia |vauthors=Duxbury AC, [[Biography:Claudia Cenedese|Cenedese C]] |date=7 May 2021 |url=https://www.britannica.com/EBchecked/topic/559627/sphere-depth-of-the-ocean |title=Sphere depth of the ocean&nbsp;– hydrology |encyclopedia=Encyclopædia Britannica |access-date=12 April 2015}}</ref><ref>{{cite web |url=https://ase.tufts.edu/cosmos/print_chapter.asp?id=4 |title=Third rock from the Sun&nbsp;– restless Earth |work=NASA's Cosmos |access-date=12 April 2015}}</ref>

[[File:Diagram of the Water Cycle.jpg|thumb|left|The Earth's [[Physics:Water cycle|water cycle]]]]

About 97.5% of the water on Earth is [[Chemistry:Saline water|saline]]; the remaining 2.5% is [[Earth:Fresh water|fresh water]]. Most fresh water &ndash; about 69% &ndash; is present as ice in [[Earth:Ice cap|ice cap]]s and [[Earth:Glacier|glacier]]s.<ref>{{cite web |url=https://water.usgs.gov/edu/earthwherewater.html |title=The World's Water |vauthors=Perlman H |date=17 March 2014 |access-date=12 April 2015 |work=USGS Water-Science School }}</ref> The average salinity of Earth's oceans is about {{convert|35|g|oz}} of salt per kilogram of seawater (3.5% salt).<ref name="Kennish_2001"/> Most of the salt in the ocean comes from the weathering and erosion of rocks on land.<ref>{{Cite web |url=https://oceanservice.noaa.gov/facts/whysalty.html |title=Why is the ocean salty? }}</ref> Some salts are released from [[Earth:Volcano|volcanic activity]] or extracted from cool [[Earth:Igneous rock|igneous rock]]s.<ref name="Mullen_2002"/>

The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.<ref name="Morris_2007"/> Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.<ref name="Michon_2006"/> Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.<ref name="Sample_2005"/>

[[File:PIA19048 realistic color Europa mosaic edited.jpg|thumb|upright=1|Jupiter's moon Europa may have an underground ocean which supports life.]]

Altogether the ocean occupies 71 percent of the world surface,<ref name="NOAA2019">{{cite web |title=National Oceanic and Atmospheric Administration – Ocean |url=http://www.noaa.gov/ocean.html |publisher=NOAA |access-date=20 Feb 2019 }}</ref> averaging nearly {{convert|3.7|km|mi}} in depth.<ref name="NOAA2015">{{cite web |title=Volumes of the World's Oceans from ETOPO1 |url=http://ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html |publisher=[[Organization:National Oceanic and Atmospheric Administration|NOAA]] |access-date=20 Feb 2019 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20150311032757/http://ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html |archive-date=2015-03-11 }}</ref> By volume, the ocean provides about 90 percent of the living space on the planet.<ref name="NOAA2019" /> The science fiction writer [[Biography:Arthur C. Clarke|Arthur C. Clarke]] has pointed out it would be more appropriate to refer to planet Earth as planet Ocean.<ref>{{cite web |url=https://quoteinvestigator.com/2017/01/25/water-planet/ |title=Planet "Earth": We Should Have Called It "Sea" |work=Quote Invertigator |date=25 January 2017 }}</ref><ref>{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2002/14mar_grace_oceans |title=Unveiling Planet Ocean |work=pNASA Science |date=14 March 2002 |access-date=19 February 2019 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008093758/https://science.nasa.gov/science-news/science-at-nasa/2002/14mar_grace_oceans |url-status=dead }}</ref>

However, water is found elsewhere in the solar system. [[Astronomy:Europa (moon)|Europa]], one of the moons orbiting [[Astronomy:Jupiter|Jupiter]], is slightly smaller than the [[Astronomy:Moon|Earth's moon]]. There is a strong possibility a large saltwater ocean exists beneath its ice surface.<ref name="NASA-20150512">{{cite web |vauthors=Dyches P, Brown D |title=NASA Research Reveals Europa's Mystery Dark Material Could Be Sea Salt |url=http://www.jpl.nasa.gov/news/news.php?feature=4586 |date=12 May 2015 |work=[[Organization:NASA|NASA]] |access-date=12 May 2015 }}</ref> It has been estimated the outer crust of solid ice is about 10–30&nbsp;km (6–19&nbsp;mi) thick and the liquid ocean underneath is about 100&nbsp;km (60&nbsp;mi) deep.<ref>{{Cite web |vauthors=Adamu Z |date=1 October 2012 |work=CNN Light Years |url=http://lightyears.blogs.cnn.com/2012/10/01/a-moon-of-jupiter-may-have-water-temporarily/?hpt=us_bn4 |title=Water near surface of a Jupiter moon only temporary |access-date=24 April 2019 |archive-date=5 October 2012 |archive-url=https://web.archive.org/web/20121005011205/http://lightyears.blogs.cnn.com/2012/10/01/a-moon-of-jupiter-may-have-water-temporarily/?hpt=us_bn4 |url-status=dead }}</ref> This would make Europa's ocean over twice the volume of the Earth's ocean. There has been speculation Europa's ocean could support life,<ref name="Tritt_2002">{{cite web |url=http://people.msoe.edu/~tritt/sf/europa.life.html |title=Possibility of Life on Europa |vauthors=Tritt CS |access-date=10 August 2007 |publisher=Milwaukee School of Engineering |date=2002 |url-status=dead |archive-url=https://web.archive.org/web/20070609150109/http://people.msoe.edu/~tritt/sf/europa.life.html |archive-date=9 June 2007 }}</ref><ref name="Schulze-Makuch_2001">{{cite web |title=Alternative Energy Sources Could Support Life on Europa |url=http://www.geo.utep.edu/pub/dirksm/geobiowater/pdf/EOS27March2001.pdf |vauthors=Schulze-Makuch D, Irwin LN |work=Departments of Geological and Biological Sciences, [[Organization:University of Texas at El Paso|University of Texas at El Paso]] |date=2001 |access-date=21 December 2007 |archive-url=https://web.archive.org/web/20060703033956/http://www.geo.utep.edu/pub/dirksm/geobiowater/pdf/EOS27March2001.pdf |archive-date=3 July 2006 }}</ref> and could be capable of supporting multicellular [[Biology:Microorganism|microorganism]]s if [[Earth:Hydrothermal vent|hydrothermal vent]]s are active on the ocean floor.<ref name="EuropaPlanetary">{{Cite web |url=http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |title=Projects: Europa Mission Campaign |publisher=The Planetary Society |vauthors=Friedman L |date=14 December 2005 |access-date=8 August 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110811002508/http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |archive-date=11 August 2011 }}</ref> [[Astronomy:Enceladus|Enceladus]], a small icy moon of Saturn, also has what appears to be [[Astronomy:Enceladus#Subsurface water ocean|an underground ocean]] which actively vents warm water from the moon's surface.<ref>{{cite web |url=http://astrobiology.com/2015/03/ocean-within-enceladus-may-harbor-hydrothermal-activity.html |title=Ocean Within Enceladus May Harbor Hydrothermal Activity |work=NASA Press Release |date=11 March 2015 }}</ref>

{{clear}}

==Evolution==
{{Life timeline}}

===Historical development===
The [[Earth:Age of the Earth|Earth]] is about 4.54 billion years old.<ref name="USGS1997">{{cite web |url=http://pubs.usgs.gov/gip/geotime/age.html |title=Age of the Earth |date=9 July 2007 |publisher=United States Geological Survey |access-date=2015-05-31 }}</ref><ref name="Dalrymple 2001 205–221">{{cite journal |vauthors=Dalrymple GB |title=The age of the Earth in the twentieth century: a problem (mostly) solved. |publisher=Geological Society |location=London |journal=Special Publications |date=January 2001 |volume=190 |issue=1 |pages=205–21 |doi=10.1144/GSL.SP.2001.190.01.14 |bibcode=2001GSLSP.190..205D |s2cid=130092094 }}</ref><ref name="Manhes_1980">{{cite journal |vauthors=Manhes G, Allègre CJ, Dupré B, Hamelin B |date=May 1980 |title=Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |volume=47 |issue=3 |pages=370–382 |bibcode=1980E&PSL..47..370M |doi=10.1016/0012-821X(80)90024-2 |issn=0012-821X }}</ref> The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,<ref name="Schopf_2007">{{cite journal |vauthors=Schopf JW, Kudryavtsev AB, Czaja AD, Tripathi AB |author-link1=J. William Schopf |date=5 October 2007 |title=Evidence of Archean life: Stromatolites and microfossils |journal=Precambrian Research |volume=158 |pages=141–155 |issue=3–4 |doi=10.1016/j.precamres.2007.04.009 |issn=0301-9268 |bibcode=2007PreR..158..141S }}</ref><ref name="Raven_Johnson_2002">{{cite book |vauthors=Raven PH, Johnson GB |title=Biology |date=2002 |publisher=McGraw-Hill |location=Boston |isbn=978-0-07-112261-0 |edition=6th |page=68 }}</ref> during the [[Earth:Eoarchean|Eoarchean]] era after a geological [[Earth:Crust (geology)|crust]] started to solidify following the earlier molten [[Earth:Hadean|Hadean]] Eon. [[Biology:Microbial mat|Microbial mat]] fossils have been found in 3.48 billion-year-old [[Earth:Sandstone|sandstone]] in Western Australia.<ref>{{cite journal |vauthors=Baumgartner RJ, Van Kranendonk MJ, Wacey D, Fiorentini ML, Saunders M, Caruso S, Pages A, Homann M, Guagliardo P |display-authors=6 |year=2019 |title=Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life |url=https://discovery.ucl.ac.uk/id/eprint/10087275/1/Baumgartner%20et%20al%202019%20accepted.pdf|journal=Geology |volume=47 |issue=11 |pages=1039–1043 |doi=10.1130/G46365.1 |bibcode=2019Geo....47.1039B |s2cid=204258554 }}</ref><ref>{{cite web |url=https://phys.org/news/2019-09-earliest-life-scientists-microbial-ancient.html |title=Earliest signs of life: Scientists find microbial remains in ancient rocks |work=Phys.org |date=26 September 2019 }}</ref> Other early physical evidence of a [[Earth:Biogenic substance|biogenic substance]] is [[Chemistry:Graphite|graphite]] in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland<ref name="Ohtomo_2014">{{cite journal | vauthors = Ohtomo Y, Kakegawa T, Ishida A, Nagase T, Rosing MT |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025 |issn=1752-0894}}</ref> as well as "remains of [[Chemistry:Biotic material|biotic life]]" found in 4.1 billion-year-old rocks in Western Australia.<ref name="Borenstein_2015">{{cite news | vauthors = Borenstein S |title=Hints of life on what was thought to be desolate early Earth |url=https://apnews.com/e6be2537b4cd46ffb9c0585bae2b2e51 |date=19 October 2015 |work=Associated Press |access-date=2018-10-09}}</ref><ref name="Bell_2015">{{cite journal | vauthors = Bell EA, Boehnke P, Harrison TM, Mao WL | title = Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 47 | pages = 14518–21 | date = November 2015 | pmid = 26483481 | pmc = 4664351 | doi = 10.1073/pnas.1517557112 | bibcode = 2015PNAS..11214518B | doi-access = free }}</ref> According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in the [[Astronomy:Universe|universe]]."<ref name="Borenstein_2015" /> <!---Nevertheless, [[Astronomy:Late Heavy Bombardment#Geological consequences on Earth|several studies]] suggest that life on Earth may have started even earlier,<ref name="Tenenbaum_2002">{{cite web | vauthors = Tenenbaum D |title=When Did Life on Earth Begin? Ask a Rock |url=http://www.astrobio.net/exclusive/293/when-did-life-on-earth-begin-ask-a-rock |date=14 October 2002 |work=Astrobiology Magazine |access-date=2014-04-13 |archive-url=https://web.archive.org/web/20210628022131/https://www.astrobio.net/news-exclusive/when-did-life-on-earth-begin-ask-a-rock/ |archive-date=2021-06-28 |url-status=usurped}}</ref> as early as 4.25 billion years ago according to one study,<ref name="Courtland_2008">{{cite web | vauthors = Courtland R |title=Did newborn Earth harbour life? |url=https://www.newscientist.com/article/dn14245-did-newborn-earth-harbour-life.html |date=2 July 2008 |work=New Scientist |access-date=2014-04-13}}</ref> and 4.4 billion years ago according to another study.<ref name="Steenhuysen_2019">{{cite web | vauthors = Steenhuysen J |title=Study turns back clock on origins of life on Earth |url=https://www.reuters.com/article/2009/05/20/us-asteroids-idUSTRE54J5PX20090520 |date=20 May 2009 |work={{wipe|Reuters}} |access-date=2014-04-13}}</ref>--->

All organisms on Earth are descended from a [[Biology:Common descent|common ancestor]] or ancestral [[Biology:Gene pool|gene pool]].<ref name="Penny_1999">{{cite journal | vauthors = Penny D, Poole A | title = The nature of the last universal common ancestor | journal = Current Opinion in Genetics & Development | volume = 9 | issue = 6 | pages = 672–7 | date = December 1999 | pmid = 10607605 | doi = 10.1016/S0959-437X(99)00020-9 }}</ref><ref>{{cite journal | vauthors = Theobald DL | title = A formal test of the theory of universal common ancestry | journal = Nature | volume = 465 | issue = 7295 | pages = 219–22 | date = May 2010 | pmid = 20463738 | doi = 10.1038/nature09014 | bibcode = 2010Natur.465..219T | s2cid = 4422345 }}</ref> 
Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.<ref name="Doolittle_2000">{{cite journal | vauthors = Doolittle WF | title = Uprooting the tree of life | journal = Scientific American | volume = 282 | issue = 2 | pages = 90–5 | date = February 2000 | pmid = 10710791 | doi = 10.1038/scientificamerican0200-90 | url = http://shiva.msu.montana.edu/courses/mb437_537_2004_fall/docs/uprooting.pdf | access-date = 2015-04-05 | bibcode = 2000SciAm.282b..90D | archive-url = https://web.archive.org/web/20060907081933/http://shiva.msu.montana.edu/courses/mb437_537_2004_fall/docs/uprooting.pdf | archive-date = 2006-09-07 | author-link = Ford Doolittle }}</ref> The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.<ref>{{cite journal | vauthors = Peretó J | title = Controversies on the origin of life | journal = International Microbiology | volume = 8 | issue = 1 | pages = 23–31 | date = March 2005 | pmid = 15906258 | url = http://www.im.microbios.org/0801/0801023.pdf | url-status = dead | archive-url = https://web.archive.org/web/20150824074726/http://www.im.microbios.org/0801/0801023.pdf | archive-date = 24 August 2015 }}</ref> The beginning of life may have included self-replicating molecules such as [[Biology:RNA|RNA]]<ref>{{cite journal | vauthors = Joyce GF | title = The antiquity of RNA-based evolution | journal = Nature | volume = 418 | issue = 6894 | pages = 214–21 | date = July 2002 | pmid = 12110897 | doi = 10.1038/418214a | bibcode = 2002Natur.418..214J | s2cid = 4331004 | author-link = Gerald Joyce }}</ref> and the assembly of simple cells.<ref>{{cite journal | vauthors = Trevors JT, Psenner R | title = From self-assembly of life to present-day bacteria: a possible role for nanocells | journal = FEMS Microbiology Reviews | volume = 25 | issue = 5 | pages = 573–82 | date = December 2001 | pmid = 11742692 | doi = 10.1111/j.1574-6976.2001.tb00592.x | doi-access = free }}</ref> In 2016 scientists reported a set of 355 [[Biology:Gene|gene]]s from the [[Biology:Last universal common ancestor|last universal common ancestor]] (LUCA) of all life, including microorganisms, living on [[Astronomy:Earth|Earth]].<ref name="NYT-20160725">{{cite news | vauthors = Wade N |author-link=Nicholas Wade |title=Meet Luca, the Ancestor of All Living Things |url=https://www.nytimes.com/2016/07/26/science/last-universal-ancestor.html |date=25 July 2016 |work={{wipe|New York Times}} |access-date=25 July 2016 }}</ref>

Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.<ref>{{cite journal | vauthors = Bapteste E, Walsh DA | title = Does the 'Ring of Life' ring true? | journal = Trends in Microbiology | volume = 13 | issue = 6 | pages = 256–61 | date = June 2005 | pmid = 15936656 | doi = 10.1016/j.tim.2005.03.012 }}</ref> The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of unique organisms, but organisms that share [[Biology:Homology|morphological similarities]]. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groups—similar to a family tree.<ref>{{cite web | vauthors = Darwin C | title = On The Origin of the Species | location = London | publisher = John Murray | date = 1859 | url = http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16 }}</ref> However, modern research has suggested that, due to [[Biology:Horizontal gene transfer in evolution|horizontal gene transfer]], this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.<ref>{{cite journal | vauthors = Doolittle WF, Bapteste E | title = Pattern pluralism and the Tree of Life hypothesis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 7 | pages = 2043–9 | date = February 2007 | pmid = 17261804 | pmc = 1892968 | doi = 10.1073/pnas.0610699104 | bibcode = 2007PNAS..104.2043D | doi-access = free }}</ref><ref>{{cite journal | vauthors = Kunin V, Goldovsky L, Darzentas N, Ouzounis CA | title = The net of life: reconstructing the microbial phylogenetic network | journal = Genome Research | volume = 15 | issue = 7 | pages = 954–9 | date = July 2005 | pmid = 15965028 | pmc = 1172039 | doi = 10.1101/gr.3666505 }}</ref>

Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.<ref name=Jablonski>{{cite journal | vauthors = Jablonski D | title = The future of the fossil record | journal = Science | volume = 284 | issue = 5423 | pages = 2114–6 | date = June 1999 | pmid = 10381868 | doi = 10.1126/science.284.5423.2114 | s2cid = 43388925 }}</ref> By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.

{{clear}}

{{PhylomapA|size=320px|align=left|caption=[[Biology:Phylogenetic tree|Evolutionary tree]] showing the divergence of modern species from their common ancestor in the centre.<ref name="Ciccarelli_2006">{{cite journal | vauthors = Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P | title = Toward automatic reconstruction of a highly resolved tree of life | journal = Science | volume = 311 | issue = 5765 | pages = 1283–7 | date = March 2006 | pmid = 16513982 | doi = 10.1126/science.1123061 | bibcode = 2006Sci...311.1283C | s2cid = 1615592 | citeseerx = 10.1.1.381.9514 | author-link6 = Peer Bork }}</ref> The three domains are coloured, with [[Biology:Bacteria|bacteria]] blue, [[Biology:Archaea|archaea]] green and [[Biology:Eukaryote|eukaryote]]s red.}}
More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set of [[Biology:Nucleotide|nucleotide]]s and [[Chemistry:Amino acid|amino acid]]s.<ref>{{cite journal | vauthors = Mason SF | title = Origins of biomolecular handedness | journal = Nature | volume = 311 | issue = 5981 | pages = 19–23 | date = 6 September 1984 | pmid = 6472461 | doi = 10.1038/311019a0 | bibcode = 1984Natur.311...19M | s2cid = 103653 }}</ref> The development of [[Biology:Molecular genetics|molecular genetics]] has revealed the record of evolution left in organisms' genomes: dating when species diverged through the [[Biology:Molecular clock|molecular clock]] produced by mutations.<ref>{{cite journal | vauthors = Wolf YI, Rogozin IB, Grishin NV, Koonin EV | title = Genome trees and the tree of life | journal = Trends in Genetics | volume = 18 | issue = 9 | pages = 472–9 | date = September 2002 | pmid = 12175808 | doi = 10.1016/S0168-9525(02)02744-0 | author-link4 = Eugene Koonin }}</ref> For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analysing the few areas where they differ helps shed light on when the common ancestor of these species existed.<ref>{{cite journal | vauthors = Varki A, Altheide TK | title = Comparing the human and chimpanzee genomes: searching for needles in a haystack | journal = Genome Research | volume = 15 | issue = 12 | pages = 1746–58 | date = December 2005 | pmid = 16339373 | doi = 10.1101/gr.3737405 | doi-access = free | author-link1 = Ajit Varki }}</ref>

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.<ref name="Cavalier-Smith_2006">{{cite journal | vauthors = Cavalier-Smith T | title = Cell evolution and Earth history: stasis and revolution | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 361 | issue = 1470 | pages = 969–1006 | date = June 2006 | pmid = 16754610 | pmc = 1578732 | doi = 10.1098/rstb.2006.1842 | author-link = Thomas Cavalier-Smith }}</ref><ref>{{cite journal | vauthors = Schopf JW | title = Fossil evidence of Archaean life | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 361 | issue = 1470 | pages = 869–85 | date = June 2006 | pmid = 16754604 | pmc = 1578735 | doi = 10.1098/rstb.2006.1834 | author-link = J. William Schopf }}
* {{cite journal | vauthors = Altermann W, Kazmierczak J | title = Archean microfossils: a reappraisal of early life on Earth | journal = Research in Microbiology | volume = 154 | issue = 9 | pages = 611–7 | date = November 2003 | pmid = 14596897 | doi = 10.1016/j.resmic.2003.08.006 | doi-access = free }}</ref> No obvious changes in [[Biology:Morphology|morphology]] or cellular organisation occurred in these organisms over the next few billion years.<ref name="Schopf_1994">{{cite journal | vauthors = Schopf JW | title = Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 91 | issue = 15 | pages = 6735–42 | date = July 1994 | pmid = 8041691 | pmc = 44277 | doi = 10.1073/pnas.91.15.6735 | bibcode = 1994PNAS...91.6735S | doi-access = free }}</ref> The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called [[Biology:Endosymbiont|endosymbiosis]].<ref name="rgruqh">{{cite journal | vauthors = Poole AM, Penny D | title = Evaluating hypotheses for the origin of eukaryotes | journal = BioEssays | volume = 29 | issue = 1 | pages = 74–84 | date = January 2007 | pmid = 17187354 | doi = 10.1002/bies.20516 }}</ref><ref name="Dyall">{{cite journal | vauthors = Dyall SD, Brown MT, Johnson PJ | title = Ancient invasions: from endosymbionts to organelles | journal = Science | volume = 304 | issue = 5668 | pages = 253–7 | date = April 2004 | pmid = 15073369 | doi = 10.1126/science.1094884 | bibcode = 2004Sci...304..253D | s2cid = 19424594 | author-link3 = Patricia J. Johnson }}</ref> The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or [[Biology:Hydrogenosome|hydrogenosome]]s.<ref>{{cite journal | vauthors = Martin W | title = The missing link between hydrogenosomes and mitochondria | journal = Trends in Microbiology | volume = 13 | issue = 10 | pages = 457–9 | date = October 2005 | pmid = 16109488 | doi = 10.1016/j.tim.2005.08.005 }}</ref> Another engulfment of [[Biology:Cyanobacteria|cyanobacteria]]l-like organisms led to the formation of chloroplasts in algae and plants.<ref>{{cite journal | vauthors = Lang BF, Gray MW, Burger G | title = Mitochondrial genome evolution and the origin of eukaryotes | journal = Annual Review of Genetics | volume = 33 | pages = 351–97 | date = December 1999 | pmid = 10690412 | doi = 10.1146/annurev.genet.33.1.351 }}
* {{cite journal | vauthors = McFadden GI | title = Endosymbiosis and evolution of the plant cell | journal = Current Opinion in Plant Biology | volume = 2 | issue = 6 | pages = 513–9 | date = December 1999 | pmid = 10607659 | doi = 10.1016/S1369-5266(99)00025-4 }}</ref>

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[[File:Tree of Living Organisms 2.png|thumb|upright=1.3 |right|Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes]]

The history of life was that of the [[Biology:Unicellular organism|unicellular]] eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.<ref name="Cavalier-Smith_2006" /><ref>{{cite journal | vauthors = DeLong EF, Pace NR | title = Environmental diversity of bacteria and archaea | journal = Systematic Biology | volume = 50 | issue = 4 | pages = 470–8 | date = August 2001 | pmid = 12116647 | doi = 10.1080/106351501750435040 | citeseerx = 10.1.1.321.8828 |  author-link1 = Edward DeLong }}</ref> The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as [[Biology:Sponge|sponge]]s, [[Biology:Brown algae|brown algae]], [[Biology:Cyanobacteria|cyanobacteria]], slime moulds and [[Biology:Myxobacteria|myxobacteria]].<ref>{{cite journal | vauthors = Kaiser D | title = Building a multicellular organism | journal = Annual Review of Genetics | volume = 35 | pages = 103–23 | date = December 2001 | pmid = 11700279 | doi = 10.1146/annurev.genet.35.102401.090145 | s2cid = 18276422 | author-link = A. Dale Kaiser }}</ref> In 2016 scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.<ref name="NYT-20160107">{{cite news | vauthors = Zimmer C |author-link=Carl Zimmer |title=Genetic Flip Helped Organisms Go From One Cell to Many |url=https://www.nytimes.com/2016/01/12/science/genetic-flip-helped-organisms-go-from-one-cell-to-many.html |date=7 January 2016 |work=The New York Times |access-date=7 January 2016 }}</ref>

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over a span of about 10 million years, in an event called the [[Earth:Cambrian explosion|Cambrian explosion]]. Here, the majority of [[Biology:Phylum|types]] of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.<ref name="Valentine_1999">{{cite journal | vauthors = Valentine JW, Jablonski D, Erwin DH | title = Fossils, molecules and embryos: new perspectives on the Cambrian explosion | journal = Development | volume = 126 | issue = 5 | pages = 851–9 | date = February 1999 | pmid = 9927587 | doi = 10.1242/dev.126.5.851 |  author-link1 = James W. Valentine }}</ref> Various triggers for the Cambrian explosion have been proposed, including the accumulation of [[Chemistry:Oxygen|oxygen]] in the [[Astronomy:Atmosphere|atmosphere]] from photosynthesis.<ref>{{cite journal | vauthors = Ohno S | title = The reason for as well as the consequence of the Cambrian explosion in animal evolution | journal = Journal of Molecular Evolution | volume = 44 | issue = Suppl. 1 | pages = S23-7 | date = January 1997 | pmid = 9071008 | doi = 10.1007/PL00000055 | s2cid = 21879320 | bibcode = 1997JMolE..44S..23O }}
* {{cite journal | vauthors = Valentine JW, Jablonski D | title = Morphological and developmental macroevolution: a paleontological perspective | journal = The International Journal of Developmental Biology | volume = 47 | issue = 7–8 | pages = 517–22 | year = 2003 | pmid = 14756327 | url = http://www.ijdb.ehu.es/web/paper.php?doi=14756327 | access-date = 2014-12-30 }}</ref>

About 500 million years ago, plants and fungi started colonising the land. Evidence for the appearance of the first land plants occurs in the [[Earth:Ordovician|Ordovician]], around {{Ma|450}}, in the form of fossil spores.<ref name="Wellman2003">{{cite journal | vauthors = Wellman CH, Osterloff PL, Mohiuddin U | title = Fragments of the earliest land plants | journal = Nature | volume = 425 | issue = 6955 | pages = 282–5 | date = September 2003 | pmid = 13679913 | doi = 10.1038/nature01884 | s2cid = 4383813 | bibcode = 2003Natur.425..282W | url = http://eprints.whiterose.ac.uk/106/1/wellmanch1.pdf }}</ref> Land plants began to diversify in the Late Silurian, from around {{Ma|430}}.<ref>{{cite book | vauthors = Barton N |year=2007 |title=Evolution |pages=273–274|publisher=CSHL Press |url=https://books.google.com/books?id=mMDFQ32oMI8C |access-date=30 September 2012|isbn=9780199226320}}</ref> The colonisation of the land by plants was soon followed by [[Biology:Arthropod|arthropod]]s and other animals.<ref>{{cite journal | vauthors = Waters ER | title = Molecular adaptation and the origin of land plants | journal = Molecular Phylogenetics and Evolution | volume = 29 | issue = 3 | pages = 456–63 | date = December 2003 | pmid = 14615186 | doi = 10.1016/j.ympev.2003.07.018 }}</ref> [[Biology:Insect|Insect]]s were particularly successful and even today make up the majority of animal species.<ref>{{cite journal | vauthors = Mayhew PJ | title = Why are there so many insect species? Perspectives from fossils and phylogenies | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 82 | issue = 3 | pages = 425–54 | date = August 2007 | pmid = 17624962 | doi = 10.1111/j.1469-185X.2007.00018.x | s2cid = 9356614 | author-link = Peter Mayhew (biologist) }}</ref> [[Biology:Amphibian|Amphibian]]s first appeared around 364 million years ago, followed by early [[Biology:Amniote|amniote]]s and [[Biology:Bird|bird]]s around 155 million years ago (both from "[[Biology:Reptile|reptile]]"-like lineages), [[Biology:Mammal|mammal]]s around 129 million years ago, [[Biology:Homininae|homininae]] around 10 million years ago and modern humans around 250,000 years ago.<ref>{{cite journal | vauthors = Carroll RL |author-link=Robert L. Carroll |date=May 2007 |title=The Palaeozoic Ancestry of Salamanders, Frogs and Caecilians |journal=Zoological Journal of the Linnean Society |volume=150 |issue=Supplement s1 |pages=1–140 |doi=10.1111/j.1096-3642.2007.00246.x |issn=1096-3642 |doi-access=free }}</ref><ref>{{cite journal | vauthors = Wible JR, Rougier GW, Novacek MJ, Asher RJ | title = Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary | journal = Nature | volume = 447 | issue = 7147 | pages = 1003–6 | date = June 2007 | pmid = 17581585 | doi = 10.1038/nature05854 | bibcode = 2007Natur.447.1003W | s2cid = 4334424 }}</ref><ref>{{cite journal | vauthors = Witmer LM | title = Palaeontology: An icon knocked from its perch | journal = Nature | volume = 475 | issue = 7357 | pages = 458–9 | date = July 2011 | pmid = 21796198 | doi = 10.1038/475458a | s2cid = 205066360 | author-link = Lawrence Witmer }}</ref> However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.<ref name=Schloss>{{cite journal | vauthors = Schloss PD, Handelsman J | title = Status of the microbial census | journal = Microbiology and Molecular Biology Reviews | volume = 68 | issue = 4 | pages = 686–91 | date = December 2004 | pmid = 15590780 | pmc = 539005 | doi = 10.1128/MMBR.68.4.686-691.2004 | author-link2 = Jo Handelsman }}</ref>

Estimates on the number of Earth's current [[Biology:Species|species]] range from 10 million to 14 million,<ref name="MillerSpoolman2012">{{cite book | vauthors = Miller GT, Spoolman S| title = Environmental Science | publisher = Cengage Learning | date = January 2012 | isbn = 978-1-133-70787-5 | chapter = Chapter 4.1: What is Biodiversity and Why is it Important? | page = 62 | chapter-url = https://books.google.com/books?id=NYEJAAAAQBAJ&pg=PA62 }}</ref> of which about 1.2 million have been documented and over 86 percent have not yet been described.<ref name="PLoS-20110823">{{cite journal | vauthors = Mora C, Tittensor DP, Adl S, Simpson AG, Worm B | title = How many species are there on Earth and in the ocean? | journal = PLOS Biology | volume = 9 | issue = 8 | pages = e1001127 | date = August 2011 | pmid = 21886479 | pmc = 3160336 | doi = 10.1371/journal.pbio.1001127 |  doi-access = free }}</ref>

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== Microorganisms ==
{{multiple image
 | align = right
 | direction = horizontal
 | width = 220
 | header = microbial mats
 | header_align = center
 | image1 = Cyanobacterial-algal mat.jpg
 | caption1 = [[Biology:Microbial mat|Microbial mat]]s are the earliest form of life on Earth for which there is good [[Biology:Fossil|fossil]] evidence. The image shows a cyanobacterial-algal mat.
 | image2 = Stromatolites in Sharkbay.jpg
 | caption2 = [[Earth:Stromatolite|Stromatolite]]s are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment.
}}
{{main|Biology:Marine microorganism}}

Microorganisms make up about 70% of the marine biomass.<ref name="Bar-On_2018"/> A [[Biology:Microorganism|microorganism]], or microbe, is a [[Engineering:Microscope|microscopic]] organism too small to be recognised with the naked eye. It can be [[Biology:Unicellular organism|single-celled]]<ref name=Brock>{{cite book |veditors=Madigan M, Martinko J |title=Brock Biology of Microorganisms |edition=13th |publisher=Pearson Education |year=2006 |isbn=978-0-321-73551-5 |page=1096 }}</ref> or [[Biology:Multicellular organism|multicellular]]. Microorganisms are diverse and include all [[Biology:Bacteria|bacteria]] and [[Biology:Archaea|archaea]], most [[Biology:Protozoa|protozoa]] such as [[Biology:Algae|algae]], [[Biology:Fungi|fungi]], and certain microscopic animals such as [[Biology:Rotifer|rotifer]]s.

Many [[Physics:Macroscopic scale|macroscopic]] animals and [[Biology:Plant|plant]]s have microscopic [[Biology:Juvenile (organism)|juvenile stages]]. Some microbiologists also classify [[Biology:Virus|virus]]es (and viroids) as microorganisms, but others consider these as nonliving.<ref>{{Cite journal |vauthors=Rybicki EP |title=The classification of organisms at the edge of life, or problems with virus systematics |journal=South African Journal of Science |volume=86 |pages=182–6 |year=1990 |issn=0038-2353}}</ref><ref name=pmid13481308>{{cite journal |vauthors=Lwoff A |title=The concept of virus |journal=Journal of General Microbiology |volume=17 |issue=2 |pages=239–53 |date=October 1957 |pmid=13481308 |doi=10.1099/00221287-17-2-239 |doi-access=free }}</ref>

Microorganisms are crucial to nutrient recycling in [[Earth:Ecosystem|ecosystem]]s as they act as [[Biology:Decomposer|decomposer]]s. Some microorganisms are [[Biology:Pathogen|pathogen]]ic, causing disease and even death in plants and animals.<ref>{{cite web |url=https://www.who.int/healthinfo/bodgbd2002revised/en/index.html |archive-url=https://web.archive.org/web/20060819224428/http://www.who.int/healthinfo/bodgbd2002revised/en/index.html |url-status=dead |archive-date=19 August 2006 |title=2002 WHO mortality data |access-date=20 January 2007 }}</ref> As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for virtually all the [[Earth:Photosynthesis|photosynthesis]] that occurs in the ocean, as well as the cycling of [[Chemistry:Carbon|carbon]], [[Chemistry:Nitrogen|nitrogen]], [[Chemistry:Phosphorus|phosphorus]], other nutrients and trace elements.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2015/12/151210181647.htm |title=Functions of global ocean microbiome key to understanding environmental changes |date=10 December 2015 |website=www.sciencedaily.com |publisher=University of Georgia |access-date=11 December 2015}}</ref>
[[File:Relative scale.svg|thumb|upright 1.2|The range of sizes shown by prokaryotes (bacteria and archaea) and viruses relative to those of other organisms and biomolecules]]
{{clade
|label1='''[[Biology:Marine microorganisms|Marine microorganisms]]&nbsp;&nbsp;&nbsp;'''
|1={{clade
   |1 = Viruses [[File:Bacteriophage2.png|45px]]
   |label2=&nbsp; Cellular life &nbsp;
   |2={{clade
   |label1=&nbsp;[[Biology:Marine prokaryotes|Prokaryotes]]&nbsp;
   |1={{clade
      |1 = Bacteria [[File:Prokaryote cell.svg|45px]]
      |2 = Archaea &nbsp; &nbsp; [[File:Haloquadratum walsbyi00 (cropped).png|25px]]
      }}
   |label2=Eukaryotes
   |2={{clade
      |1 = Protists [[File:Diatomee - Diatom (fossile) - Stephanopyxis sp. - 630x (14117161428).jpg|55px]]
      |2 = [[Biology:Microfungi|Microfungi]] &nbsp; [[File:S cerevisiae under DIC microscopy.jpg|25px]]
      |4 = Microanimals &nbsp; [[File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png|40px]]
      }}
   }}
}}
}}

{{clear}}

[[File:Microbial Loop.jpg|thumb|left| Marine [[Biology:Microbial loop|microbial loop]]]]

Microscopic life undersea is diverse and still poorly understood, such as for the role of [[Biology:Virus|virus]]es in marine ecosystems.<ref>{{cite journal |vauthors=Suttle CA |title=Viruses in the sea |journal=Nature |volume=437 |issue=7057 |pages=356–61 |date=September 2005 |pmid=16163346 |doi=10.1038/nature04160 |s2cid=4370363 |bibcode=2005Natur.437..356S }}</ref> Most marine viruses are [[Biology:Bacteriophage|bacteriophage]]s, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.<ref name="Shors_2017">{{cite book |vauthors=Shors T |date=2017 |title=Understanding Viruses |edition=3rd |publisher=Jones and Bartlett Publishers |isbn=978-1-284-02592-7}}</ref>{{rp|5}} They infect and destroy bacteria in aquatic microbial communities, and are the most important mechanism of [[Earth:Carbon cycle|recycling carbon]] in the marine environment. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth.<ref name="Shors_2017" />{{rp|593}} Viral activity may also contribute to the [[Biology:Biological pump|biological pump]], the process whereby [[Chemistry:Carbon|carbon]] is [[Earth:Carbon sequestration|sequestered]] in the deep ocean.<ref name="pmid17853907">{{cite journal |vauthors=Suttle CA |title=Marine viruses--major players in the global ecosystem |journal=Nature Reviews. Microbiology |volume=5 |issue=10 |pages=801–12 |date=October 2007 |pmid=17853907 |doi=10.1038/nrmicro1750 |s2cid=4658457 }}</ref>

[[File:Ocean mist and spray 2.jpg|thumb|upright=1.3 |right| Sea spray containing marine microorganisms can be swept high into the atmosphere where they become [[Earth:Aeroplankton|aeroplankton]], and can travel the globe before falling back to earth.]]
[[File:Marine critters.jpg|thumb|left|Under a magnifier, a splash of seawater teems with life.]]

A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.<ref>{{cite web | vauthors = Morrison J | date = 11 January 2016 | url = https://www.smithsonianmag.com/science-nature/living-bacteria-are-riding-earths-air-currents-180957734/ | title = Living Bacteria Are Riding Earth's Air Currents | work = Smithsonian Magazine }}</ref> Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in [[Earth:Sea spray|sea spray]]. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.<ref>{{cite news | vauthors = Robbins J |title=Trillions Upon Trillions of Viruses Fall From the Sky Each Day |url=https://www.nytimes.com/2018/04/13/science/virosphere-evolution.html |date=13 April 2018 |work={{wipe|The New York Times}} |access-date=14 April 2018 }}</ref><ref name="ISME-2018">{{cite journal | vauthors = Reche I, D'Orta G, Mladenov N, Winget DM, Suttle CA | title = Deposition rates of viruses and bacteria above the atmospheric boundary layer | journal = The ISME Journal | volume = 12 | issue = 4 | pages = 1154–1162 | date = April 2018 | pmid = 29379178 | pmc = 5864199 | doi = 10.1038/s41396-017-0042-4 }}</ref>

Microscopic organisms live throughout the [[Biology:Biosphere|biosphere]]. The mass of [[Biology:Prokaryote|prokaryote]] microorganisms &mdash; which includes bacteria and archaea, but not the nucleated [[Biology:Microorganism#Eukaryotes|eukaryote microorganisms]] &mdash; may be as much as 0.8 trillion tons of carbon (of the total biosphere [[Earth:Biomass (ecology)|mass]], estimated at between 1 and 4 trillion tons).<ref name="AGCI-2014">{{cite web |author=Staff |title=The Biosphere |url=http://www.agci.org/classroom/biosphere/index.php |date=2014 |work=Aspen Global Change Institute |access-date=10 November 2014 |archive-date=2 September 2010 |archive-url=https://web.archive.org/web/20100902111048/http://www.agci.org/classroom/biosphere/index.php |url-status=dead }}</ref> Single-celled [[Biology:Piezophile|barophilic]] marine microbes have been found at a depth of {{convert|10900|m|ft|abbr=on}} in the Mariana Trench, the deepest spot in the Earth's oceans.<ref name="LS-20130317">{{cite web | vauthors = Choi CQ | title=Microbes Thrive in Deepest Spot on Earth |url=http://www.livescience.com/27954-microbes-mariana-trench.html |date=17 March 2013 |publisher=LiveScience |access-date=17 March 2013 }}</ref><ref name="NG-20130317">{{cite journal | vauthors = Glud RN, Wenzhöfer F, Middelboe M, Oguri K, Turnewitsch R, Canfield DE, Kitazato H |title=High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth |doi=10.1038/ngeo1773 |date=17 March 2013 |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |volume=6 |issue=4 |pages=284–288 |bibcode = 2013NatGe...6..284G }}</ref> Microorganisms live inside rocks {{convert|580|m|ft|abbr=on}} below the sea floor under {{convert|2590|m|ft|abbr=on}} of ocean off the coast of the northwestern United States,<ref name="LS-20130317" /><ref name="LS-20130314">{{cite web | vauthors = Oskin B |title=Intraterrestrials: Life Thrives in Ocean Floor |url= http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |date=14 March 2013 |publisher=LiveScience |access-date=17 March 2013 }}</ref> as well as {{convert|2400|m|ft mi|abbr=on}} beneath the seabed off Japan.<ref name="BBC-20141215-RM">{{cite news | last = Morelle | first=Rebecca |  name-list-style=vanc |title=Microbes discovered by deepest marine drill analysed |url=https://www.bbc.com/news/science-environment-30489814 |date=15 December 2014 |work=BBC News |access-date=15 December 2014 }}</ref> The greatest known temperature at which microbial life can exist is {{convert|122|°C|°F|abbr=on}} (''Methanopyrus kandleri'').<ref>{{cite journal | vauthors = Takai K, Nakamura K, Toki T, Tsunogai U, Miyazaki M, Miyazaki J, Hirayama H, Nakagawa S, Nunoura T, Horikoshi K | display-authors = 6 | title = Cell proliferation at 122 degrees C and isotopically heavy CH4 production by a hyperthermophilic methanogen under high-pressure cultivation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 31 | pages = 10949–54 | date = August 2008 | pmid = 18664583 | pmc = 2490668 | doi = 10.1073/pnas.0712334105 | bibcode = 2008PNAS..10510949T | doi-access = free }}</ref> In 2014, scientists confirmed the existence of microorganisms living {{convert|800|m|ft|abbr=on}} below the ice of [[Earth:Antarctica|Antarctica]].<ref name="NAT-20140820">{{cite journal | vauthors = Fox D | title = Lakes under the ice: Antarctica's secret garden | journal = Nature | volume = 512 | issue = 7514 | pages = 244–6 | date = August 2014 | pmid = 25143097 | doi = 10.1038/512244a | doi-access = free | bibcode = 2014Natur.512..244F }}</ref><ref name="FRB-20140820">{{cite web | vauthors = Mack E |title=Life Confirmed Under Antarctic Ice; Is Space Next? |url=https://www.forbes.com/sites/ericmack/2014/08/20/life-confirmed-under-antarctic-ice-is-space-next/ |date=20 August 2014 |work={{wipe|Forbes}} |access-date=21 August 2014 }}</ref> According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."<ref name="LS-20130317" />

{{clear}}

===Marine viruses===
{{main|Biology:Marine viruses}}

Viruses are small [[Biology:Pathogen|infectious agent]]s that do not have their own [[Biology:Metabolism|metabolism]] and can [[Biology:Replicate|replicate]] only inside the living [[Biology:Cell|cells]] of other organisms.<ref name="pmid20010599">{{cite journal | vauthors = Wimmer E, Mueller S, Tumpey TM, Taubenberger JK | title = Synthetic viruses: a new opportunity to understand and prevent viral disease | journal = Nature Biotechnology | volume = 27 | issue = 12 | pages = 1163–72 | date = December 2009 | pmid = 20010599 | pmc = 2819212 | doi = 10.1038/nbt.1593 }}</ref> Viruses can infect all types of life forms, from [[Biology:Animal|animal]]s and [[Biology:Plant|plant]]s to [[Biology:Microorganism|microorganism]]s, including [[Biology:Bacteria|bacteria]] and [[Biology:Archaea|archaea]].<ref name="pmid16984643">{{cite journal | vauthors = Koonin EV, Senkevich TG, Dolja VV | title = The ancient Virus World and evolution of cells | journal = Biology Direct | volume = 1 | pages = 29 | date = September 2006 | pmid = 16984643 | pmc = 1594570 | doi = 10.1186/1745-6150-1-29 | doi-access = free }}</ref> The linear size of the average virus is about one one-hundredth that of the average [[Biology:Bacterium|bacterium]]. Most viruses cannot be seen with an [[Engineering:Optical microscope|optical microscope]] so [[Engineering:Electron microscope|electron microscope]]s are used instead.<ref>{{cite book | vauthors = Topley WW, Wilson GS, Collier LH, Balows A, Sussman M |title=Topley and Wilson's Microbiology and Microbial Infections. |date=1998 |publisher=Arnold |location=London |isbn=978-0-340-66316-5 |edition=9th | volume = 1 | pages = 33–37 | veditors =  Mahy BW, Collier L }}</ref>

Viruses are found wherever there is life and have probably existed since living cells first evolved.<ref name="pmid16494962">{{cite journal | vauthors = Iyer LM, Balaji S, Koonin EV, Aravind L | title = Evolutionary genomics of nucleo-cytoplasmic large DNA viruses | journal = Virus Research | volume = 117 | issue = 1 | pages = 156–84 | date = April 2006 | pmid = 16494962 | doi = 10.1016/j.virusres.2006.01.009 | url = https://zenodo.org/record/1259447 }}</ref> The origin of viruses is unclear because they do not form fossils, so [[Biology:Molecular biology|molecular techniques]] have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arise.<ref name="pmid20660197">{{cite journal | vauthors = Sanjuán R, Nebot MR, Chirico N, Mansky LM, Belshaw R | title = Viral mutation rates | journal = Journal of Virology | volume = 84 | issue = 19 | pages = 9733–48 | date = October 2010 | pmid = 20660197 | pmc = 2937809 | doi = 10.1128/JVI.00694-10 }}</ref>

Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the [[Biology:Three-domain system|three domains]].<ref name="Mahy Gen 28">{{cite book | veditors = Mahy WJ, Van Regenmortel MH |title=Desk Encyclopedia of General Virology |publisher=Academic Press |location=Oxford |year=2009 |pages=28 |isbn=978-0-12-375146-1}}</ref> But the origins of viruses in the [[Biology:Evolutionary history of life|evolutionary history of life]] are unclear: some may have [[Biology:Evolution|evolved]] from [[Biology:Plasmid|plasmid]]s—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of [[Biology:Horizontal gene transfer in evolution|horizontal gene transfer]], which increases [[Biology:Genetic diversity|genetic diversity]].<ref name="Canchaya" />

{{multiple image
 | align     = left
 | direction = horizontal
 | header    = Bacteriophages (phages)
 | image1    = Phage.jpg
 | width1    = 160
 | caption1  = Multiple phages attached to a bacterial cell wall at 200,000× magnification
 | image2    = Tailed phage.png
 | width2    = 166
 | caption2  = Diagram of a typical [[Biology:Caudovirales|tailed phage]]
}}
[[File:Cyanophages.png|thumb|left| These are cyanophages, viruses that infect cyanobacteria (scale bars indicate 100&nbsp;nm)]]

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.<ref name="pmid26965225">{{cite journal | vauthors = Koonin EV, Starokadomskyy P | title = Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question | journal = Studies in History and Philosophy of Biological and Biomedical Sciences | volume = 59 | pages = 125–34 | date = October 2016 | pmid = 26965225 | pmc = 5406846 | doi = 10.1016/j.shpsc.2016.02.016 }}</ref> They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through [[Biology:Natural selection|natural selection]]. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators<ref name="pmid26965225"/> and as "organisms at the edge of life".<ref name="ReferenceA">{{cite journal| vauthors = Rybicki EP |year = 1990|title = The classification of organisms at the edge of life, or problems with virus systematics|journal = South African Journal of Science |volume = 86|pages = 182–186}}</ref>

[[File:Caudovirales.svg|thumb|160px|In terms of individual counts, tailed phage are the most abundant biological entities in the sea.]]
[[Biology:Bacteriophage|Bacteriophage]]s, often just called ''phages'', are viruses that [[Biology:Parasite|parasite]] bacteria and archaea. [[Biology:Marine bacteriophage|Marine phages]] parasite marine bacteria and archaea, such as [[Biology:Cyanobacteria|cyanobacteria]].<ref name=Mann>{{cite journal | vauthors = Mann NH | title = The third age of phage | journal = PLOS Biology | volume = 3 | issue = 5 | pages = e182 | date = May 2005 | pmid = 15884981 | pmc = 1110918 | doi = 10.1371/journal.pbio.0030182 | doi-access = free }}</ref> They are a common and diverse group of viruses and are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. Generally there are about 1 million to 10 million viruses in each mL of seawater, or about ten times more double-stranded DNA viruses than there are cellular organisms,<ref name="Wommack_2000">{{cite journal | vauthors = Wommack KE, Colwell RR | title = Virioplankton: viruses in aquatic ecosystems | journal = Microbiology and Molecular Biology Reviews | volume = 64 | issue = 1 | pages = 69–114 | date = March 2000 | pmid = 10704475 | pmc = 98987 | doi = 10.1128/MMBR.64.1.69-114.2000 }}</ref><ref name="Suttle_2005">{{cite journal | vauthors = Suttle CA | title = Viruses in the sea | journal = Nature | volume = 437 | issue = 7057 | pages = 356–61 | date = September 2005 | pmid = 16163346 | doi = 10.1038/nature04160 | bibcode = 2005Natur.437..356S | s2cid = 4370363 }}</ref> although estimates of viral abundance in seawater can vary over a wide range.<ref>{{cite journal | vauthors = Bergh O, Børsheim KY, Bratbak G, Heldal M | title = High abundance of viruses found in aquatic environments | journal = Nature | volume = 340 | issue = 6233 | pages = 467–8 | date = August 1989 | pmid = 2755508 | doi = 10.1038/340467a0 | bibcode = 1989Natur.340..467B | s2cid = 4271861 }}</ref><ref>{{cite journal | vauthors = Wigington CH, Sonderegger D, Brussaard CP, Buchan A, Finke JF, Fuhrman JA, Lennon JT, Middelboe M, Suttle CA, Stock C, Wilson WH, Wommack KE, Wilhelm SW, Weitz JS | display-authors = 6 | title = Re-examination of the relationship between marine virus and microbial cell abundances | journal = Nature Microbiology | volume = 1 | pages = 15024 | date = January 2016 | issue = 3 | pmid = 27572161 | doi = 10.1038/nmicrobiol.2015.24 | s2cid = 52829633 | url = http://www.vliz.be/imisdocs/publications/23/301523.pdf }}</ref> [[Biology:Caudovirales|Tailed bacteriophages]] appear to dominate marine ecosystems in number and diversity of organisms.<ref name=Mann /> Bacteriophages belonging to the families [[Biology:Corticovirus|Corticoviridae]],<ref>{{cite journal | vauthors = Krupovic M, Bamford DH | title = Putative prophages related to lytic tailless marine dsDNA phage PM2 are widespread in the genomes of aquatic bacteria | journal = BMC Genomics | volume = 8 | pages = 236 | date = July 2007 | pmid = 17634101 | pmc = 1950889 | doi = 10.1186/1471-2164-8-236 | doi-access = free }}</ref> [[Biology:Inoviridae|Inoviridae]]<ref>{{cite journal | vauthors = Xue H, Xu Y, Boucher Y, Polz MF | title = High frequency of a novel filamentous phage, VCY φ, within an environmental Vibrio cholerae population | journal = Applied and Environmental Microbiology | volume = 78 | issue = 1 | pages = 28–33 | date = January 2012 | pmid = 22020507 | pmc = 3255608 | doi = 10.1128/AEM.06297-11 | bibcode = 2012ApEnM..78...28X }}</ref> and [[Biology:Microviridae|Microviridae]]<ref name=Roux>{{cite journal | vauthors = Roux S, Krupovic M, Poulet A, Debroas D, Enault F | title = Evolution and diversity of the Microviridae viral family through a collection of 81 new complete genomes assembled from virome reads | journal = PLOS ONE | volume = 7 | issue = 7 | pages = e40418 | year = 2012 | pmid = 22808158 | pmc = 3394797 | doi = 10.1371/journal.pone.0040418 | bibcode = 2012PLoSO...740418R | doi-access = free }}</ref> are also known to infect diverse marine bacteria.

Microorganisms make up about 70% of the marine biomass.<ref name="Bar-On_2018"/> It is estimated viruses kill 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmful [[Biology:Algal bloom|algal bloom]]s,<ref name="Suttle_2005"/> which often kill other marine life.<ref name="cdc.gov">{{cite web
|url=https://www.cdc.gov/hab/redtide/|title=Harmful Algal Blooms: Red Tide: Home {{!}}CDC HSB|publisher=www.cdc.gov|access-date=2014-12-19}}</ref>
The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.<ref name="pmid17853907" />

There are also archaeal viruses which replicate within [[Biology:Archaea|archaea]]: these are double-stranded DNA viruses with unusual and sometimes unique shapes.<ref name="Lawrence">{{cite journal | vauthors = Lawrence CM, Menon S, Eilers BJ, Bothner B, Khayat R, Douglas T, Young MJ | title = Structural and functional studies of archaeal viruses | journal = The Journal of Biological Chemistry | volume = 284 | issue = 19 | pages = 12599–603 | date = May 2009 | pmid = 19158076 | pmc = 2675988 | doi = 10.1074/jbc.R800078200 | doi-access = free }}</ref><ref name=Prangishvili>{{cite journal | vauthors = Prangishvili D, Forterre P, Garrett RA | title = Viruses of the Archaea: a unifying view | journal = Nature Reviews. Microbiology | volume = 4 | issue = 11 | pages = 837–48 | date = November 2006 | pmid = 17041631 | doi = 10.1038/nrmicro1527 | s2cid = 9915859 }}</ref> These viruses have been studied in most detail in the [[Earth:Thermophile|thermophilic]] archaea, particularly the orders [[Biology:Sulfolobales|Sulfolobales]] and [[Biology:Thermoproteales|Thermoproteales]].<ref>{{cite journal | vauthors = Prangishvili D, Garrett RA | title = Exceptionally diverse morphotypes and genomes of crenarchaeal hyperthermophilic viruses | journal = Biochemical Society Transactions | volume = 32 | issue = Pt 2 | pages = 204–8 | date = April 2004 | pmid = 15046572 | doi = 10.1042/BST0320204 | s2cid = 20018642 | url = https://curis.ku.dk/ws/files/51497971/0320204.pdf }}</ref>

Viruses are an important natural means of [[Biology:Horizontal gene transfer|transferring genes]] between different species, which increases [[Biology:Genetic diversity|genetic diversity]] and drives evolution.<ref name="Canchaya">{{cite journal | vauthors = Canchaya C, Fournous G, Chibani-Chennoufi S, Dillmann ML, Brüssow H | title = Phage as agents of lateral gene transfer | journal = Current Opinion in Microbiology | volume = 6 | issue = 4 | pages = 417–24 | date = August 2003 | pmid = 12941415 | doi = 10.1016/S1369-5274(03)00086-9 }}</ref> It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth.<ref name="pmid11536914">{{cite journal | vauthors = Forterre P, Philippe H | title = The last universal common ancestor (LUCA), simple or complex? | journal = The Biological Bulletin | volume = 196 | issue = 3 | pages = 373–5; discussion 375–7 | date = June 1999 | pmid = 11536914 | doi = 10.2307/1542973 | jstor = 1542973 }}</ref> Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.<ref name="pmid17853907" />

{{clear}}

===Marine bacteria===
[[File:Vibrio vulnificus 01.png|thumb|right|''[[Biology:Vibrio vulnificus|Vibrio vulnificus]]'', a virulent bacterium found in estuaries and along coastal areas]]
[[File:Pelagibacter.jpg|thumb| ''[[Biology:Pelagibacter ubique|Pelagibacter ubique]]'', the most abundant bacteria in the ocean, plays a major role in the global carbon cycle.]]
{{Further|Biology:Marine prokaryotes|Biology:Bacterioplankton}}

[[Biology:Bacteria|Bacteria]] constitute a large domain of prokaryotic [[Biology:Microorganism|microorganism]]s. Typically a few [[Micrometre|micrometre]]s in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on [[Astronomy:Earth|Earth]], and are present in most of its [[Earth:Habitat|habitat]]s. Bacteria inhabit soil, water, [[Earth:Hot spring|acidic hot springs]], [[Physics:Radioactive waste|radioactive waste]],<ref>{{cite journal | vauthors = Fredrickson JK, Zachara JM, Balkwill DL, Kennedy D, Li SM, Kostandarithes HM, Daly MJ, Romine MF, Brockman FJ | display-authors = 6 | title = Geomicrobiology of high-level nuclear waste-contaminated vadose sediments at the hanford site, washington state | journal = Applied and Environmental Microbiology | volume = 70 | issue = 7 | pages = 4230–41 | date = July 2004 | pmid = 15240306 | pmc = 444790 | doi = 10.1128/AEM.70.7.4230-4241.2004 | bibcode = 2004ApEnM..70.4230F }}</ref> and the deep portions of [[Earth:Crust (geology)|Earth's crust]]. Bacteria also live in symbiotic and parasitic relationships with plants and animals.

Once regarded as [[Biology:Plant|plant]]s constituting the class ''Schizomycetes'', bacteria are now classified as prokaryotes. Unlike cells of animals and other [[Biology:Eukaryote|eukaryote]]s, bacterial cells do not contain a [[Biology:Cell nucleus|nucleus]] and rarely harbour [[Biology:Cell membrane|membrane-bound]] [[Biology:Organelle|organelle]]s. Although the term ''bacteria'' traditionally included all prokaryotes, the [[Scientific classification|scientific classification]] changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that [[Biology:Evolution|evolved]] from an ancient common ancestor. These evolutionary domains are called ''Bacteria'' and ''[[Biology:Archaea|Archaea]]''.<ref name="Woese">{{cite journal | vauthors = Woese CR, Kandler O, Wheelis ML | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 87 | issue = 12 | pages = 4576–9 | date = June 1990 | pmid = 2112744 | pmc = 54159 | doi = 10.1073/pnas.87.12.4576 | bibcode = 1990PNAS...87.4576W | doi-access = free }}</ref>

The ancestors of modern bacteria were unicellular microorganisms that were the [[Biology:Abiogenesis|first forms of life]] to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.<ref name="Schopf_1994"/><ref>{{cite journal | vauthors = DeLong EF, Pace NR | title = Environmental diversity of bacteria and archaea | journal = Systematic Biology | volume = 50 | issue = 4 | pages = 470–8 | date = August 2001 | pmid = 12116647 | doi = 10.1080/106351501750435040 | citeseerx = 10.1.1.321.8828 }}</ref> Although bacterial [[Biology:Fossil|fossil]]s exist, such as [[Earth:Stromatolite|stromatolite]]s, their lack of distinctive [[Biology:Morphology|morphology]] prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial [[Biology:Phylogenetics|phylogeny]], and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.<ref>{{cite journal | vauthors = Brown JR, Doolittle WF | title = Archaea and the prokaryote-to-eukaryote transition | journal = Microbiology and Molecular Biology Reviews | volume = 61 | issue = 4 | pages = 456–502 | date = December 1997 | doi = 10.1128/mmbr.61.4.456-502.1997 | pmid = 9409149 | pmc = 232621 }}</ref> <!-- The [[Biology:Most recent common ancestor|most recent common ancestor]] of bacteria and archaea was probably a [[Earth:Thermophile|hyperthermophile]] that lived about 2.5 billion–3.2 billion years ago.<ref>{{cite journal |vauthors=Di Giulio M |title=The universal ancestor and the ancestor of bacteria were hyperthermophiles |journal=Journal of Molecular Evolution |volume=57 |issue=6 |pages=721–30 |year=2003 |pmid=14745541 |doi=10.1007/s00239-003-2522-6 |bibcode=2003JMolE..57..721D}}</ref><ref>{{cite journal |vauthors=Battistuzzi FU, Feijao A, Hedges SB |title=A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land |journal=BMC Evolutionary Biology |volume=4 |pages=44 |year=2004 |pmid=15535883 |pmc=533871 |doi=10.1186/1471-2148-4-44 |doi-access=free }}</ref> -->
Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into [[Biology:Endosymbiont|endosymbiotic]] associations with the ancestors of eukaryotic cells, which were themselves possibly related to the [[Biology:Archaea|Archaea]].<ref name=Dyall /><ref name="rgruqh"/> This involved the engulfment by proto-eukaryotic cells of [[Biology:Alphaproteobacteria|alphaproteobacteria]]l symbionts to form either [[Biology:Mitochondrion|mitochondria]] or [[Biology:Hydrogenosome|hydrogenosome]]s, which are still found in all known Eukarya. Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation of [[Biology:Chloroplast|chloroplast]]s in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.<ref>{{cite journal | vauthors = Lang BF, Gray MW, Burger G | title = Mitochondrial genome evolution and the origin of eukaryotes | journal = Annual Review of Genetics | volume = 33 | pages = 351–97 | year = 1999 | pmid = 10690412 | doi = 10.1146/annurev.genet.33.1.351 }}</ref><ref>{{cite journal | vauthors = McFadden GI | title = Endosymbiosis and evolution of the plant cell | journal = Current Opinion in Plant Biology | volume = 2 | issue = 6 | pages = 513–9 | date = December 1999 | pmid = 10607659 | doi = 10.1016/S1369-5266(99)00025-4 }}</ref> This is known as secondary endosymbiosis.

<gallery mode="packed" heights="140px" style="float:left;">
File:Sulphide bacteria crop2.jpg|The marine ''[[Biology:Thiomargarita namibiensis|Thiomargarita namibiensis]]'', the largest known bacterium
File:Potomac river eutro.jpg|[[Biology:Cyanobacteria|Cyanobacteria]] [[Biology:Algal bloom|blooms]] can contain lethal [[Chemistry:Cyanotoxin|cyanotoxin]]s.
File:Glaucocystis sp.jpg|The [[Biology:Chloroplast|chloroplast]]s of [[Biology:Glaucophyte|glaucophyte]]s have a [[Biology:Peptidoglycan|peptidoglycan]] layer, evidence suggesting their endosymbiotic origin from [[Biology:Cyanobacteria|cyanobacteria]].<ref name="keeling">{{cite journal|journal = American Journal of Botany|year = 2004|volume = 91|pages = 1481–1493|title = Diversity and evolutionary history of plastids and their hosts|vauthors = Keeling PJ|doi = 10.3732/ajb.91.10.1481|issue = 10|pmid = 21652304|s2cid = 17522125}}</ref>
File:Alvinella pompejana01.jpg|Bacteria can be beneficial. This Pompeii worm, an [[Earth:Extremophile|extremophile]] found only at [[Earth:Hydrothermal vent|hydrothermal vent]]s, has a protective cover of bacteria.
</gallery>

{{clear}}

The largest known bacterium, the marine ''[[Biology:Thiomargarita namibiensis|Thiomargarita namibiensis]]'', can be visible to the naked eye and sometimes attains {{convert|0.75|mm|µm|abbr=on}}.<ref>{{Citation|work=Max Planck Institute for Marine Microbiology |date=8 April 1999 |title=The largest Bacterium: Scientist discovers new bacterial life form off the African coast |url=http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1999/news17_99.htm |url-status=dead |archive-url=https://web.archive.org/web/20100120043846/http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1999/news17_99.htm |archive-date=20 January 2010 }}</ref><ref>{{Citation |title=List of Prokaryotic names with Standing in Nomenclature - Genus Thiomargarita |url=https://lpsn.dsmz.de/genus/thiomargarita |df=dmy-all }}</ref>

===Marine archaea===
[[File:Morning-Glory Hotspring.jpg|thumb|right|Archaea were initially viewed as extremophiles living in harsh environments, such as the yellow archaea pictured here in a hot spring, but they have since been found in a much broader range of habitats.<ref name=Bang2015>{{cite journal | vauthors = Bang C, Schmitz RA | title = Archaea associated with human surfaces: not to be underestimated | journal = FEMS Microbiology Reviews | volume = 39 | issue = 5 | pages = 631–48 | date = September 2015 | pmid = 25907112 | doi = 10.1093/femsre/fuv010 | doi-access = free }}</ref>]]
{{Further|Biology:Marine prokaryotes}}

The [[Biology:Archaea|archaea]] (Greek for ''ancient''<ref>{{cite web | url = http://www.etymonline.com/index.php?l=a&p=41 | title = Archaea | work = Online Etymology Dictionary | access-date = 17 August 2016 }}</ref>) constitute a domain and [[Biology:Kingdom|kingdom]] of [[Biology:Unicellular organism|single-celled]] [[Biology:Microorganism|microorganism]]s. These microbes are prokaryotes, meaning they have no [[Biology:Cell nucleus|cell nucleus]] or any other membrane-bound [[Biology:Organelle|organelle]]s in their cells.

Archaea were initially classified as [[Biology:Bacteria|bacteria]], but this classification is outdated.<ref>{{cite journal | vauthors = Pace NR | title = Time for a change | journal = Nature | volume = 441 | issue = 7091 | pages = 289 | date = May 2006 | pmid = 16710401 | doi = 10.1038/441289a | s2cid = 4431143 | bibcode = 2006Natur.441..289P | doi-access = free }}</ref> Archaeal cells have unique properties separating them from the other two domains of life, [[Biology:Bacteria|Bacteria]] and Eukaryota. The Archaea are further divided into multiple recognized [[Biology:Phylum|phyla]]. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of their [[Biology:Nucleic acid|nucleic acid]]s in samples from their environment.

Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells of ''[[Biology:Haloquadratum|Haloquadratum walsbyi]]''.<ref>{{cite journal | vauthors = Stoeckenius W | title = Walsby's square bacterium: fine structure of an orthogonal procaryote | journal = Journal of Bacteriology | volume = 148 | issue = 1 | pages = 352–60 | date = October 1981 | pmid = 7287626 | pmc = 216199 | doi = 10.1128/JB.148.1.352-360.1981 }}</ref> Despite this morphological similarity to bacteria, archaea possess [[Biology:Gene|gene]]s and several [[Biology:Metabolic pathway|metabolic pathway]]s that are more closely related to those of eukaryotes, notably the [[Biology:Enzyme|enzyme]]s involved in transcription and [[Biology:Translation|translation]]. Other aspects of archaeal biochemistry are unique, such as their reliance on [[Chemistry:Ether lipid|ether lipid]]s in their [[Biology:Cell membrane|cell membrane]]s, such as [[Chemistry:Archaeol|archaeol]]s. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to [[Chemistry:Ammonia|ammonia]], [[Physics:Ion|metal ions]] or even [[Chemistry:Hydrogen|hydrogen gas]]. Salt-tolerant archaea (the [[Biology:Haloarchaea|Haloarchaea]]) use sunlight as an energy source, and other species of archaea [[Earth:Carbon fixation|fix carbon]]; however, unlike plants and [[Biology:Cyanobacteria|cyanobacteria]], no known species of archaea does both. Archaea reproduce asexually by [[Binary fission|binary fission]], [[Medicine:Fragmentation (reproduction)|fragmentation]], or [[Biology:Budding|budding]]; unlike bacteria and eukaryotes, no known species forms [[Biology:Spore|spore]]s.

Archaea are particularly numerous in the oceans, and the archaea in [[Earth:Plankton|plankton]] may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life and may play roles in both the [[Earth:Carbon cycle|carbon cycle]] and the [[Biology:Nitrogen cycle|nitrogen cycle]].

<gallery mode="packed" heights="150px" style="float:left;">
File:Halobacteria with scale.jpg|Halobacteria, found in water near saturated with salt, are now recognised as archaea.
File:Haloquadratum walsbyi00.jpg|Flat, square-shaped cells of the archaea ''[[Biology:Haloquadratum|Haloquadratum walsbyi]]''
File:Methanosarcina barkeri fusaro.gif|''[[Biology:Methanosarcina|Methanosarcina barkeri]]'', a marine archaea that produces [[Chemistry:Methane|methane]]
File:Thermophile bacteria2.jpg| [[Earth:Thermophile|Thermophile]]s, such as ''[[Biology:Pyrolobus fumarii|Pyrolobus fumarii]]'', survive well over 100&nbsp;°C.
File:Pyrococcus furiosus.png| Drawing of another marine thermophile, ''[[Biology:Pyrococcus furiosus|Pyrococcus furiosus]]''
</gallery>

{{clear}}

===Marine protists===
<!-- [[File:Ceratium hirundinella.jpg|thumb|100px| [[Biology:Dinoflagellate|Dinoflagellate]]]]
[[File:Aplanonet3.jpg|thumb| Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel]] -->
{{main|Biology:Marine protists}}

[[Biology:Protist|Protist]]s are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Life originated as [[Biology:Prokaryote|single-celled prokaryotes]] ([[Biology:Bacteria|bacteria]] and [[Biology:Archaea|archaea]]) and later evolved into [[Biology:Eukaryote|more complex eukaryotes]]. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. The term [[Biology:Protist|protist]] came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they are paraphyletic (lacking a common ancestor). Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungus-like,<ref>{{cite journal | vauthors = Whittaker RH, [[Biography:Lynn Margulis|Margulis L]] | title = Protist classification and the kingdoms of organisms | journal = Bio Systems | volume = 10 | issue = 1–2 | pages = 3–18 | date = April 1978 | pmid = 418827 | doi = 10.1016/0303-2647(78)90023-0 }}</ref> or a mixture of these.<ref>{{cite journal | vauthors = Faure E, Not F, Benoiston AS, Labadie K, Bittner L, Ayata SD | title = Mixotrophic protists display contrasted biogeographies in the global ocean | journal = The ISME Journal | volume = 13 | issue = 4 | pages = 1072–1083 | date = April 2019 | pmid = 30643201 | pmc = 6461780 | doi = 10.1038/s41396-018-0340-5 }}</ref>

{|class="wikitable"
! colspan=8 |{{centre|Protists according to how they get food}}
|-
! colspan=2 |Type of protist
! Description
! colspan=2 | Example
! Other examples
|-
! width=90px | Plant-like
| width=90px | {{center|[[Biology:Algae|Algae]]<br />([[Biology:Marine life#Algae|see below]])}}
| [[Biology:Autotroph|Autotroph]]ic protists that make their own food without needing to consume other organisms, usually by using photosynthesis
| [[File:Cyanidium O5A.jpg|100px]]
| Red algae, ''[[Biology:Cyanidiophyceae|Cyanidium sp.]]''
| [[Biology:Green algae|Green algae]], [[Biology:Brown algae|brown algae]], [[Biology:Diatom|diatom]]s and some [[Biology:Dinoflagellate|dinoflagellate]]s. Plant-like protists are important components of phytoplankton discussed below.
|-
! Animal-like
| {{center|Protozoans}}
| [[Biology:Heterotroph|Heterotroph]]ic protists that get their food consuming other organisms
| [[File:Haeckel Spumellaria detail.png|100px]]
| Radiolarian protist as drawn by [[Biography:Ernst Haeckel|Haeckel]]
| Foraminiferans, and some marine amoebae, ciliates and [[Biology:Flagellate|flagellate]]s.
|-
! Fungus-like
| {{center|Slime moulds<br />and<br />slime nets}}
| Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed 
| [[File:Aplanonet3.jpg|100px]]
| Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel
| Marine lichen
|-
! Mixotropes 
| {{center|Various}}
| Mixotrophic and osmotrophic protists that get their food from a combination of the above 
| [[File:Euglena mutabilis - 400x - 1 (10388739803) (cropped).jpg|100px]]
| ''[[Biology:Euglena|Euglena mutabilis]]'', a photosynthetic [[Biology:Flagellate|flagellate]]
| Many marine mixotropes are found among protists, including among ciliates, [[Biology:Rhizaria|Rhizaria]] and dinoflagellates <ref name=Leles2017/>
|-
|}

{{multiple image
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 | footer     = [[Biology:Choanoflagellate|Choanoflagellate]]s, unicellular "collared" [[Biology:Flagellate|flagellate]] protists, are thought to be the closest living relatives of the [[Biology:Animal|animal]]s.<ref name="Budd2015">{{cite journal | vauthors = Budd GE, Jensen S | title = The origin of the animals and a 'Savannah' hypothesis for early bilaterian evolution | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 92 | issue = 1 | pages = 446–473 | date = February 2017 | pmid = 26588818 | doi = 10.1111/brv.12239 | doi-access = free }}</ref>
----
[[File:Nuvola apps kaboodle.svg|16px]] [https://www.youtube.com/watch?v=9rtUEPuzhbE&ab_channel=JourneytotheMicrocosmos Getting to know our single-celled ancestors] - ''MicroCosmos''
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}}

Protists are highly diverse organisms currently organised into 18 phyla, but are not easy to classify.<ref>{{cite journal | vauthors = Cavalier-Smith T | title = Kingdom protozoa and its 18 phyla | journal = Microbiological Reviews | volume = 57 | issue = 4 | pages = 953–94 | date = December 1993 | pmid = 8302218 | pmc = 372943 | doi = 10.1128/MMBR.57.4.953-994.1993 }}</ref><ref>{{cite journal | vauthors = Corliss JO | title = Should there be a separate code of nomenclature for the protists? | journal = Bio Systems | volume = 28 | issue = 1–3 | pages = 1–14 | year = 1992 | pmid = 1292654 | doi = 10.1016/0303-2647(92)90003-H }}</ref> Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting a large number of eukaryotic microbial communities have yet to be discovered.<ref>{{cite journal | vauthors = Slapeta J, Moreira D, López-García P | title = The extent of protist diversity: insights from molecular ecology of freshwater eukaryotes | journal = Proceedings. Biological Sciences | volume = 272 | issue = 1576 | pages = 2073–81 | date = October 2005 | pmid = 16191619 | pmc = 1559898 | doi = 10.1098/rspb.2005.3195 }}</ref><ref>{{cite journal | vauthors = Moreira D, López-García P | title = The molecular ecology of microbial eukaryotes unveils a hidden world | journal = Trends in Microbiology | volume = 10 | issue = 1 | pages = 31–8 | date = January 2002 | pmid = 11755083 | doi = 10.1016/S0966-842X(01)02257-0 }}</ref> There has been little research on mixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protist [[Earth:Biomass (ecology)|biomass]].<ref name=Leles2017>{{cite journal | vauthors = Leles SG, Mitra A, Flynn KJ, Stoecker DK, Hansen PJ, Calbet A, McManus GB, Sanders RW, Caron DA, Not F, Hallegraeff GM, Pitta P, Raven JA, Johnson MD, Glibert PM, Våge S | display-authors = 6 | title = Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance | journal = Proceedings. Biological Sciences | volume = 284 | issue = 1860 | page = 20170664 | date = August 2017 | pmid = 28768886 | pmc = 5563798 | doi = 10.1098/rspb.2017.0664 }}</ref>

<gallery caption="Single-celled and microscopic protists" mode="packed" heights="130px" style="float:left;">
File:Diatoms through the microscope.jpg| [[Biology:Diatom|Diatom]]s are a major algae group generating about 20% of world oxygen production.<ref name="livescience.com">{{cite web | vauthors = Alverson A | date = 11 June 2014 | work = Live Science | url = https://www.livescience.com/46250-teasing-apart-the-diatom-genome.html | title = The Air You're Breathing? A Diatom Made That }}</ref>
File:Diatom algae Amphora sp.jpg| Diatoms have glass like cell walls made of [[Chemistry:Silica|silica]] and called [[Biology:Frustule|frustule]]s.<ref>{{cite web | title=More on Diatoms | website=University of California Museum of Paleontology | url=http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html | access-date=27 June 2019 | archive-url=https://web.archive.org/web/20121004130024/http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html | archive-date=4 October 2012 | url-status=dead }}</ref>
File:Triceratium morlandii var. morlandii.jpg|Fossil diatom frustule from 32 to 40 [[Physics:Mya (unit)|mya]]
File:Podocyrtis papalis Ehrenberg - Radiolarian (30448963206).jpg| Radiolarian
File:Gephyrocapsa oceanica color (lightened).jpg| Single-celled alga, ''Gephyrocapsa oceanica''
File:CSIRO ScienceImage 7609 SEM dinoflagellate.jpg| Two [[Biology:Dinoflagellate|dinoflagellate]]s
File:Zooxanthellae.jpg| [[Biology:Zooxanthellae|Zooxanthellae]] is a photosynthetic algae that lives inside hosts like [[Biology:Coral|coral]].
File:Paramecium bursaria.jpg| A single-celled ciliate with green zoochlorellae living inside endosymbiotically.
File:Euglenoid movement.jpg| Euglenoid
File:The ciliate Frontonia sp.jpg| This ciliate is digesting [[Biology:Cyanobacteria|cyanobacteria]]. The [[Biology:Cytostome|cytostome]] or mouth is at the bottom right.
</gallery>

{{clear}}

[[File:Frontonia ingesting a diatom.ogg|thumb|Video of a ciliate ingesting a diatom]]

In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually [[Biology:Multicellular organism|multi-celled]] and are typically [[Physics:Macroscopic|macroscopic]]. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, [[Biology:Colony|colonial]], and multicellular forms.<ref>{{cite journal | vauthors = Devreotes P | title = Dictyostelium discoideum: a model system for cell-cell interactions in development | journal = Science | volume = 245 | issue = 4922 | pages = 1054–8 | date = September 1989 | pmid = 2672337 | doi = 10.1126/science.2672337 | bibcode = 1989Sci...245.1054D }}</ref> Other marine protist are neither single-celled nor microscopic, such as [[Biology:Seaweed|seaweed]].

{{anchor|Macroscopic protists}}
<gallery caption="Macroscopic protists (see also unicellular macroalgae →)" mode="packed" heights="130px" style="float:left;">
File:Chaos carolinensis Wilson 1900.jpg| The single-celled giant amoeba has up to 1000 [[Biology:Cell nucleus|nuclei]] and reaches lengths of {{cvt|5|mm|in}}.
File:Gromia in situ closeup.png| ''[[Biology:Gromia sphaerica|Gromia sphaerica]]'' is a large spherical testate amoeba which makes mud trails. Its diameter is up to {{cvt|3.8|cm|in}}.<ref name="Matz2008">{{cite journal | vauthors = Matz MV, Frank TM, Marshall NJ, Widder EA, Johnsen S | title = Giant deep-sea protist produces bilaterian-like traces | journal = Current Biology | volume = 18 | issue = 23 | pages = 1849–54 | date = December 2008 | pmid = 19026540 | doi = 10.1016/j.cub.2008.10.028 | publisher = Elsevier Ltd | s2cid = 8819675 }}</ref>
File:Spiculosiphon oceana AB.png| ''[[Biology:Spiculosiphon oceana|Spiculosiphon oceana]]'', a unicellular foraminiferan with an appearance and lifestyle that mimics a [[Biology:Sponge|sponge]], grows to 5&nbsp;cm long.
File:Xenophyophore.jpg| The [[Biology:Xenophyophore|xenophyophore]], another single-celled foraminiferan, lives in [[Earth:Abyssal zone|abyssal zone]]s. It has a giant shell up to {{cvt|20|cm|in}} across.<ref>{{Cite journal| vauthors = Gooday AJ, Da Silva AA, Pawlowski J |date=2011-12-01|title=Xenophyophores (Rhizaria, Foraminifera) from the Nazaré Canyon (Portuguese margin, NE Atlantic)|journal=Deep-Sea Research Part II: Topical Studies in Oceanography|series=The Geology, Geochemistry, and Biology of Submarine Canyons West of Portugal|volume=58|issue=23–24|pages=2401–2419|doi=10.1016/j.dsr2.2011.04.005|bibcode=2011DSRII..58.2401G}}</ref> 
File:Giant Kelp.jpg| Giant kelp, a [[Biology:Brown algae|brown algae]], is not a true plant, yet it is multicellular and can grow to 50m.
</gallery>

{{clear}}

Protists have been described as a taxonomic grab bag where anything that doesn't fit into one of the main [[Biology:Kingdom|biological kingdoms]] can be placed.<ref>{{cite book | vauthors = Neil AC, Reece JB, Simon EJ | date = 2004 | url = https://books.google.com/books?id=lRhFAQAAIAAJ&q=protists+%22taxonomic+grab+bag%22 | title = Essential biology with physiology | publisher = Pearson/Benjamin Cummings | page = 291 | isbn = 978-0-8053-7503-9 }}</ref> Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.<ref name="O'Malley">{{Cite journal | doi = 10.1007/s10539-012-9354-y| title = The other eukaryotes in light of evolutionary protistology| journal = Biology & Philosophy| volume = 28| issue = 2| pages = 299–330| year = 2012| vauthors = O'Malley MA, Simpson AG, Roger AJ | s2cid = 85406712}}</ref><ref name="Adl 05">{{cite journal | vauthors = Adl SM, Simpson AG, Farmer MA, Andersen RA, Anderson OR, Barta JR, Bowser SS, Brugerolle G, Fensome RA, Fredericq S, James TY, Karpov S, Kugrens P, Krug J, Lane CE, Lewis LA, Lodge J, Lynn DH, Mann DG, McCourt RM, Mendoza L, Moestrup O, Mozley-Standridge SE, Nerad TA, Shearer CA, Smirnov AV, Spiegel FW, Taylor MF | display-authors = 6 | title = The new higher level classification of eukaryotes with emphasis on the taxonomy of protists | journal = The Journal of Eukaryotic Microbiology | volume = 52 | issue = 5 | pages = 399–451 | year = 2005 | pmid = 16248873 | doi = 10.1111/j.1550-7408.2005.00053.x | s2cid = 8060916 | doi-access = free }}</ref> This more constrained definition excludes [[Biology:Seaweed|seaweed]]s and [[Biology:Slime mold|slime mold]]s.<ref>{{Cite book | url = https://books.google.com/books?id=9IWaqAOGyt4C | title = Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth | vauthors = Margulis L, Chapman MJ | date = 2009-03-19 | publisher = Academic Press | isbn = 9780080920146 }}</ref>

===Marine microanimals===
{{ external media
| headerimage=
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| video1 = [https://www.youtube.com/watch?v=UZZ_RmpzSFQ&ab_channel=JourneytotheMicrocosmos Copepods: The Diatom-Devouring King of Plankton] - ''Journey to the Microcosmos''
}}

As juveniles, animals develop from microscopic stages, which can include [[Biology:Spore|spore]]s, [[Biology:Egg|egg]]s and [[Biology:Larva|larva]]e. At least one microscopic animal group, the [[Biology:Parasite|parasitic]] [[Biology:Cnidaria|cnidaria]]n [[Biology:Myxozoa|Myxozoa]], is unicellular in its adult form, and includes marine species. Other adult marine microanimals are multicellular. Microscopic adult [[Biology:Arthropod|arthropod]]s are more commonly found inland in freshwater, but there are marine species as well. Microscopic adult marine [[Biology:Crustacean|crustacean]]s include some [[Biology:Copepod|copepod]]s, [[Biology:Cladocera|cladocera]] and [[Biology:Tardigrade|tardigrade]]s (water bears). Some marine [[Biology:Nematode|nematode]]s and [[Biology:Rotifer|rotifer]]s are also too small to be recognised with the naked eye, as are many [[Biology:Loricifera|loricifera]], including the recently discovered [[Biology:Anaerobic organism|anaerobic]] species that spend their lives in an [[Earth:Anoxic waters|anoxic]] environment.<ref>{{cite journal | vauthors = Fang J | title = Animals thrive without oxygen at sea bottom | journal = Nature | volume = 464 | issue = 7290 | pages = 825 | date = April 2010 | pmid = 20376121 | doi = 10.1038/464825b | bibcode = 2010Natur.464..825F | doi-access = free }}</ref><ref>{{cite web |url=http://www.sciencenews.org/view/generic/id/58154/description/Multicelled_animals_may_live_oxygen-free |title=Briny deep basin may be home to animals thriving without oxygen |work=Science News |date=2013-09-23}}</ref> Copepods contribute more to the secondary productivity and [[Physics:Carbon sink|carbon sink]] of the world oceans than any other group of organisms.<ref>{{Cite journal |last1=Jónasdóttir |first1=Sigrún Huld |last2=Visser |first2=André W. |last3=Richardson |first3=Katherine |last4=Heath |first4=Michael R. |date=2015-09-29 |title=Seasonal copepod lipid pump promotes carbon sequestration in the deep North Atlantic |journal=Proceedings of the National Academy of Sciences |language=en |volume=112 |issue=39 |pages=12122–12126 |doi=10.1073/pnas.1512110112 |issn=0027-8424 |pmc=4593097 |pmid=26338976 |doi-access=free }}</ref><ref>{{Cite journal |last1=Pinti |first1=Jérôme |last2=Jónasdóttir |first2=Sigrún H. |last3=Record |first3=Nicholas R. |last4=Visser |first4=André W. |date=2023-03-07 |title=The global contribution of seasonally migrating copepods to the biological carbon pump |url=https://aslopubs.onlinelibrary.wiley.com/doi/10.1002/lno.12335 |journal=Limnology and Oceanography |language=en |volume=68 |issue=5 |pages=1147–1160 |doi=10.1002/lno.12335 |bibcode=2023LimOc..68.1147P |s2cid=257422956 |issn=0024-3590|doi-access=free }}</ref> While [[Biology:Mite|mite]]s are not normally thought of as marine organisms, most species of the family [[Biology:Halacaridae|Halacaridae]] live in the sea.<ref>{{Cite journal |last1=Pepato |first1=Almir R. |last2=Vidigal |first2=Teofânia H.D.A. |last3=Klimov |first3=Pavel B. |date=2018 |title=Molecular phylogeny of marine mites (Acariformes: Halacaridae), the oldest radiation of extant secondarily marine animals |journal=Molecular Phylogenetics and Evolution |language=en |volume=129 |pages=182–188 |doi=10.1016/j.ympev.2018.08.012 |pmid=30172010 |s2cid=52145427 |doi-access=free }}</ref>

<gallery mode="packed" heights="150px" style="float:left;" caption="Marine microanimals">
File:Copepod 2.jpg| Over 10,000 marine species are [[Biology:Copepod|copepod]]s, small, often microscopic [[Biology:Crustacean|crustacean]]s
File:Gastrotrich.jpg| Darkfield photo of a [[Biology:Gastrotrich|gastrotrich]], a worm-like animal living between sediment particles
File:Pliciloricus enigmatus.jpg|Armoured ''[[Biology:Pliciloricus enigmaticus|Pliciloricus enigmaticus]]'', about 0.2&nbsp;mm long, live in spaces between marine gravel.
File:Echiniscus testudo Doyere 1840 Pl 12 Fig 1.png| Drawing of a [[Biology:Tardigrade|tardigrade]] (water bear) on a grain of sand
File:Squatinella sp. (Rädertierchen - Rotifera) - 160x (13402418244).jpg| [[Biology:Rotifer|Rotifer]]s, usually 0.1–0.5&nbsp;mm long, may look like protists but have many cells and belongs to the Animalia.
</gallery>

{{clear}}

==Fungi==
[[File:Lichen rock.jpeg|thumb|right|Lichen on a rock in a marine splash zone. Lichens are mutualistic associations between a fungus and an alga or cyanobacterium.]]
[[File:Littoraria irrorata.jpg|thumb| A sea snail, ''Littoraria irrorata'', covered in lichen. This snail farms intertidal ascomycetous fungi.]]

Over 1500 species of [[Biology:Fungi|fungi]] are known from marine environments.<ref>{{cite journal |vauthors=Hyde KD, Jones EG, Leaño E, Pointing SB, Poonyth AD, Vrijmoed LL |title=Role of fungi in marine ecosystems |journal=Biodiversity and Conservation |year=1998 |volume=7 |issue=9 |pages=1147–1161 |doi=10.1023/A:1008823515157 |s2cid=22264931 }}</ref> These are parasitic on marine algae or animals, or are saprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, wood and other substrata.<ref>{{cite book | vauthors = Kirk PM, Cannon PF, Minter DW, Stalpers J | title = Dictionary of the Fungi | edition = 10 | publisher = CABI | date = 2008 }}</ref> Spores of many species have special appendages which facilitate attachment to the substratum.<ref>{{cite journal |vauthors=Hyde KD, Greenwood R, Jones EG |title=Spore attachment in marine fungi |journal=Botanica Marina |year=1989 |volume=32 |issue=3 |pages=205–218 |doi=10.1515/botm.1989.32.3.205 |s2cid=84879817}}</ref> Marine fungi can also be found in [[Earth:Sea foam|sea foam]] and around hydrothermal areas of the ocean.<ref name="LeCalvez_2009"/> A diverse range of unusual secondary [[Biology:Metabolite|metabolite]]s is produced by marine fungi.<ref>{{cite journal |vauthors=San-Martin A, Orejarena S, Gallardo C, Silva M, Becerra J, Reinoso RO, Chamy MC, Vergara K, Rovirosa J |title=Steroids from the marine fungus Geotrichum sp |journal=Journal of the Chilean Chemical Society |year=2008 |volume=53 |issue=1 |pages=1377–1378 |doi=10.4067/S0717-97072008000100011 |doi-access=free }}</ref>

[[Earth:Mycoplankton|Mycoplankton]] are [[Biology:Saprotrophic nutrition|saprotropic]] members of the [[Earth:Plankton|plankton]] communities of [[Biology:Marine biology|marine]] and [[Earth:Limnology|freshwater]] [[Earth:Aquatic ecosystem|ecosystems]].<ref>{{cite book | veditors = Jones EB, Hyde KD, Pang KL | date = 2014 | url = https://books.google.com/books?id=mXfnBQAAQBAJ | title = Freshwater fungi: and fungal-like organisms | location = Berlin/Boston | publisher = De Gruyter | isbn = 9783110333480 }}</ref><ref>{{cite book |year=2012 | veditors = Jones EB, Pang KL | title=Marine Fungi, and Fungal-like Organisms  |url=http://www.degruyter.com/view/product/177990 |series=Marine and Freshwater Botany |location=Berlin, Boston |publisher=De Gruyter |publication-date=August 2012 |isbn=978-3-11-026406-7 |access-date=3 September 2015 |ref=jones2012 |doi=10.1515/9783110264067 }}</ref> They are composed of filamentous free-living [[Biology:Fungus|fungi]] and yeasts associated with planktonic particles or [[Earth:Phytoplankton|phytoplankton]].<ref>{{cite journal | vauthors = Wang X, Singh P, Gao Z, Zhang X, Johnson ZI, Wang G | title = Distribution and diversity of planktonic fungi in the West Pacific Warm Pool | journal = PLOS ONE | volume = 9 | issue = 7 | pages = e101523 | year = 2014 | pmid = 24992154 | pmc = 4081592 | doi = 10.1371/journal.pone.0101523.s001 | bibcode = 2014PLoSO...9j1523W | doi-access = free }}</ref> Similar to [[Biology:Bacterioplankton|bacterioplankton]], these aquatic fungi play a significant role in [[Biology:Heterotroph|heterotroph]]ic [[Chemistry:Mineralization (soil science)|mineralization]] and nutrient cycling.<ref>{{cite book | vauthors = Wang G, Wang X, Liu X, Li Q  | chapter = Diversity and biogeochemical function of planktonic fungi in the ocean| veditors = Raghukumar C |title=Biology of marine fungi | series = Progress in Molecular and Subcellular Biology|location=Berlin, Heidelberg |publisher=Springer-Verlag |year=2012 | volume = 53|pages=71–88 |isbn=978-3-642-23341-8 |chapter-url=https://www.springer.com/us/book/9783642233418 |access-date=3 September 2015 |doi=10.1007/978-3-642-23342-5 |s2cid=39378040 }}</ref> Mycoplankton can be up to 20&nbsp;mm in diameter and over 50&nbsp;mm in length.<ref>{{cite journal | vauthors = Damare S, Raghukumar C | title = Fungi and macroaggregation in deep-sea sediments | journal = Microbial Ecology | volume = 56 | issue = 1 | pages = 168–77 | date = July 2008 | pmid = 17994287 | doi = 10.1007/s00248-007-9334-y | s2cid = 21288251 }}</ref>

A typical milliliter of seawater contains about 10<sup>3</sup> to 10<sup>4</sup> fungal cells.<ref>{{cite journal | vauthors = Kubanek J, Jensen PR, Keifer PA, Sullards MC, Collins DO, Fenical W | title = Seaweed resistance to microbial attack: a targeted chemical defense against marine fungi | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 12 | pages = 6916–21 | date = June 2003 | pmid = 12756301 | pmc = 165804 | doi = 10.1073/pnas.1131855100 | bibcode = 2003PNAS..100.6916K | doi-access = free }}</ref> This number is greater in coastal ecosystems and [[Earth:Estuary|estuaries]] due to nutritional runoff from terrestrial communities. A higher diversity of mycoplankton is found around coasts and in surface waters down to 1000 metres, with a [[Earth:Water column|vertical profile]] that depends on how abundant [[Earth:Phytoplankton|phytoplankton]] is.<ref name=":1">{{cite journal | vauthors = Gao Z, Johnson ZI, Wang G | title = Molecular characterization of the spatial diversity and novel lineages of mycoplankton in Hawaiian coastal waters | journal = The ISME Journal | volume = 4 | issue = 1 | pages = 111–20 | date = January 2010 | pmid = 19641535 | doi = 10.1038/ismej.2009.87 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Panzer K, Yilmaz P, Weiß M, Reich L, Richter M, Wiese J, Schmaljohann R, Labes A, Imhoff JF, Glöckner FO, Reich M | display-authors = 6 | title = Identification of Habitat-Specific Biomes of Aquatic Fungal Communities Using a Comprehensive Nearly Full-Length 18S rRNA Dataset Enriched with Contextual Data | journal = PLOS ONE | volume = 10 | issue = 7 | pages = e0134377 | date = 2015-07-30 | pmid = 26226014 | pmc = 4520555 | doi = 10.1371/journal.pone.0134377 | bibcode = 2015PLoSO..1034377P | doi-access = free }}</ref> This profile changes between seasons due to changes in nutrient availability.<ref name=":3">{{cite journal | vauthors = Gutierrez MH, Pantoja S, Quinones RA, Gonzalez RR | trans-title = First record of filamentous fungi in the coastal upwelling ecosystem off central Chile | title = Primer registro de hongos filamentosos en el ecosistema de surgencia costero frente a Chile central | language = Spanish | journal = Gayana |year=2010 | volume = 74 | issue = 1 | pages =  66–73 }}</ref> Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion by [[Physics:Turbulence|turbulence]] and oxygen generated by [[Biology:Phototroph|photosynthetic organisms]].<ref name="Sridhar2009">{{Cite book |title = Plankton Dynamics of Indian Waters | vauthors = Sridhar KR |publisher = Pratiksha Publications |year = 2009 |location = Jaipur, India |pages = 133–148 |chapter = 10. Aquatic fungi – Are they planktonic?}}</ref>

Marine fungi can be classified as:<ref name="Sridhar2009" />
* Lower fungi - adapted to marine habitats ([[Biology:Zoospore|zoosporic]] fungi, including mastigomycetes: oomycetes and [[Biology:Chytridiomycetes|chytridiomycetes]])
* Higher fungi - filamentous, modified to planktonic lifestyle ([[Biology:Hyphomycetes|hyphomycetes]], [[Biology:Ascomycota|ascomycetes]], [[Biology:Basidiomycota|basidiomycetes]]). Most mycoplankton species are higher fungi.<ref name=":1" />

[[Biology:Lichen|Lichen]]s are [[Biology:Mutualism|mutualistic]] associations between a fungus, usually an ascomycete, and an alga or a [[Biology:Cyanobacteria|cyanobacterium]]. Several lichens are found in marine environments.<ref>{{cite web | url = http://ocean.otr.usm.edu/~w529014/index_files/Page2025.htm | title = Species of Higher Marine Fungi | archive-url = https://web.archive.org/web/20130422084649/http://ocean.otr.usm.edu/~w529014/index_files/Page2025.htm | archive-date=22 April 2013 | publisher = University of Mississippi | access-date = 5 February 2012 }}</ref> Many more occur in the splash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.<ref>{{cite journal | vauthors = Hawksworth DL | title = Freshwater and marine lichen-forming fungi. | journal = Fungal Diversity | date = 2000 | volume = 5 | pages = 1–7 | url = http://www.fungaldiversity.org/fdp/sfdp/FD_5_1-7.pdf }}</ref> Some lichens live a long time; one species has been dated at 8,600 years.<ref>{{cite web |url=https://www.nps.gov/glac/learn/nature/lichens.htm |title=Lichens |publisher=National Park Service, US Department of the Interior, Government of the United States |date=22 May 2016 |access-date=4 April 2018 }}</ref> However their lifespan is difficult to measure because what defines the same lichen is not precise.<ref name=ELNGDL>{{cite web |url=http://www.earthlife.net/lichens/growth.html |title=The Earth Life Web, Growth and Development in Lichens |date=14 February 2020 |publisher=earthlife.net }}</ref> Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the same lichen, and two lichens of different ages can merge, raising the issue of whether it is the same lichen.<ref name=ELNGDL/>
The [[Biology:Sea snail|sea snail]] ''[[Biology:Littoraria irrorata|Littoraria irrorata]]'' damages plants of ''[[Biology:Spartina|Spartina]]'' in the sea marshes where it lives, which enables spores of intertidal ascomycetous fungi to colonise the plant. The snail then eats the fungal growth in preference to the grass itself.<ref>{{cite journal | vauthors = Silliman BR, Newell SY | title = Fungal farming in a snail | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 26 | pages = 15643–8 | date = December 2003 | pmid = 14657360 | pmc = 307621 | doi = 10.1073/pnas.2535227100 | bibcode = 2003PNAS..10015643S | doi-access = free }}</ref>

According to fossil records, fungi date back to the late [[Earth:Proterozoic|Proterozoic]] era 900-570 million years ago. Fossil marine lichens 600 million years old have been discovered in China.<ref>{{cite journal | vauthors = Yuan X, Xiao S, Taylor TN | title = Lichen-like symbiosis 600 million years ago | journal = Science | volume = 308 | issue = 5724 | pages = 1017–20 | date = May 2005 | pmid = 15890881 | doi = 10.1126/science.1111347 | s2cid = 27083645 | bibcode = 2005Sci...308.1017Y }}</ref> It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in the [[Earth:Paleozoic|Paleozoic]] era (390 million years ago).<ref>{{Cite book|title = Marine Fungi: and Fungal-like Organisms|url = https://books.google.com/books?id=RcF97cHppPsC|publisher = Walter de Gruyter|date = 2012-08-31|isbn = 9783110264067| vauthors = Jones EB, Pang KL }}</ref>

==Origin of animals==
[[File:DickinsoniaCostata.jpg|thumb| ''Dickinsonia'' may be the earliest animal. They appear in the fossil record 571 million to 541 million years ago.]]

The earliest [[Biology:Animal|animal]]s were marine [[Biology:Invertebrate|invertebrate]]s, that is, [[Biology:Vertebrate|vertebrate]]s came later. Animals are multicellular [[Biology:Eukaryote|eukaryote]]s,<ref group=note name=Myxozoa/> and are distinguished from plants, algae, and fungi by lacking [[Biology:Cell wall|cell wall]]s.<ref>{{cite web |url=http://micro.magnet.fsu.edu/cells/animalcell.html |title=Animal Cell Structure | vauthors = Davidson MW |date=26 May 2005 |website=Molecular Expressions |publisher=[[Organization:Florida State University|Florida State University]] |location=Tallahassee, Fla. |access-date=2008-09-03}}</ref> [[Biology:Marine invertebrates|Marine invertebrates]] are animals that inhabit a marine environment apart from the vertebrate members of the [[Biology:Chordate|chordate]] phylum; invertebrates lack a [[Biology:Vertebral column|vertebral column]]. Some have evolved a shell or a hard [[Biology:Exoskeleton|exoskeleton]].

The earliest animal fossils may belong to the genus ''[[Biology:Dickinsonia|Dickinsonia]]'',<ref name="Vogel">{{cite web | vauthors = Vogel G |title=This fossil is one of the world's earliest animals, according to fat molecules preserved for a half-billion years |url=https://www.science.org/content/article/fossil-one-world-s-earliest-animals-according-fat-molecules-preserved-half-billion |website=Science |publisher=AAAS |access-date=21 September 2018|date=2018-09-20 }}</ref> 571 million to 541 million years ago.<ref>{{cite journal | vauthors = Bobrovskiy I, Hope JM, Ivantsov A, Nettersheim BJ, Hallmann C, Brocks JJ | title = Ancient steroids establish the Ediacaran fossil ''Dickinsonia'' as one of the earliest animals | journal = Science | volume = 361 | issue = 6408 | pages = 1246–1249 | date = September 2018 | pmid = 30237355 | doi = 10.1126/science.aat7228 | doi-access = free | bibcode = 2018Sci...361.1246B | hdl = 1885/230014 | hdl-access = free }}</ref> Individual ''Dickinsonia'' typically resemble a bilaterally symmetrical ribbed oval. They kept growing until they were covered with sediment or otherwise killed,<ref name=Retallack2007>{{cite journal | vauthors = Retallack GJ | year = 2007 | title = Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil | journal = Alcheringa: An Australasian Journal of Palaeontology | volume = 31 | issue = 3 | pages = 215–240 | url = http://www.informaworld.com/index/781217204.pdf | doi = 10.1080/03115510701484705| bibcode = 2007Alch...31..215R | s2cid = 17181699 }}</ref> and spent most of their lives with their bodies firmly anchored to the sediment.<ref name=PalAss>{{cite journal | vauthors = Sperling EA, Vinther J | title = A placozoan affinity for Dickinsonia and the evolution of late Proterozoic metazoan feeding modes | journal = Evolution & Development | volume = 12 | issue = 2 | pages = 201–9 | date = 2010 | pmid = 20433459 | doi = 10.1111/j.1525-142X.2010.00404.x | s2cid = 38559058 }}</ref> Their [[Affinity (taxonomy)|taxonomic affinities]] are presently unknown, but their mode of growth is consistent with a [[Biology:Bilateria|bilateria]]n affinity.<ref>{{cite journal | vauthors = Gold DA, Runnegar B, Gehling JG, Jacobs DK | title = Ancestral state reconstruction of ontogeny supports a bilaterian affinity for Dickinsonia | journal = Evolution & Development | volume = 17 | issue = 6 | pages = 315–24 | year = 2015 | pmid = 26492825 | doi = 10.1111/ede.12168 | s2cid = 26099557 }}</ref>

Apart from ''Dickinsonia'', the earliest widely accepted animal fossils are the rather modern-looking [[Biology:Cnidaria|cnidaria]]ns (the group that includes [[Biology:Coral|coral]], [[Biology:Jellyfish|jellyfish]], [[Biology:Sea anemone|sea anemone]]s and ''[[Biology:Hydra (genus)|Hydra]]''), possibly from around {{ma|580|Ma}}<ref>{{cite journal | vauthors = Chen JY, Oliveri P, Gao F, Dornbos SQ, Li CW, Bottjer DJ, Davidson EH | title = Precambrian animal life: probable developmental and adult cnidarian forms from Southwest China | journal = Developmental Biology | volume = 248 | issue = 1 | pages = 182–96 | date = August 2002 | pmid = 12142030 | doi = 10.1006/dbio.2002.0714 | url = https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf | archive-url = https://web.archive.org/web/20130526203745/https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf | url-status = dead | df = dmy-all |  archive-date = 26 May 2013 | first6 = David J. | first7 = Eric H. | access-date = 2015-02-04 }}</ref> The Ediacara biota, which flourished for the last 40 million years before the start of the [[Earth:Cambrian|Cambrian]],<ref name="Grazhdankin2004">{{cite journal | vauthors = Grazhdankin D |date=June 2004 |title=Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution |journal=Paleobiology |volume=30 |issue=2 |pages=203–221 |doi=10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 |s2cid=129376371 |issn=0094-8373 }}</ref> were the first animals more than a very few centimetres long. Like ''Dickinsonia'', many were flat with a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate [[Biology:Kingdom|kingdom]], Vendozoa.<ref name="Seilacher1992">{{cite journal | vauthors = Seilacher A |author-link=Adolf Seilacher |date=August 1992 |title=Vendobionta and Psammocorallia: lost constructions of Precambrian evolution |url=http://jgs.lyellcollection.org/content/149/4/607.abstract |journal=Journal of the Geological Society |volume=149 |issue=4 |pages=607–613 |doi=10.1144/gsjgs.149.4.0607 |issn=0016-7649 |access-date=2015-02-04 |bibcode=1992JGSoc.149..607S |s2cid=128681462 }}</ref> Others, however, have been interpreted as early [[Biology:Mollusca|mollusc]]s (''[[Biology:Kimberella|Kimberella]]''<ref name="Martin2000">{{cite journal | vauthors = Martin MW, Grazhdankin DV, Bowring SA, Evans DA, Fedonkin MA, Kirschvink JL | title = Age of Neoproterozoic bilatarian body and trace fossils, White Sea, Russia: implications for metazoan evolution | journal = Science | volume = 288 | issue = 5467 | pages = 841–5 | date = May 2000 | pmid = 10797002 | doi = 10.1126/science.288.5467.841 | s2cid = 1019572 | bibcode = 2000Sci...288..841M }}</ref><ref name="FedonkinWaggoner1997KimberellaMollusc">{{cite journal | vauthors = Fedonkin MA, Waggoner BM |date=28 August 1997 |title=The late Precambrian fossil ''Kimberella'' is a mollusc-like bilaterian organism |journal=Nature |volume=388 |issue=6645 |pages=868–871 |bibcode=1997Natur.388..868F |doi=10.1038/42242 |s2cid=4395089 |issn=0028-0836 |doi-access=free }}</ref>), [[Biology:Echinoderm|echinoderm]]s (''[[Biology:Arkarua|Arkarua]]''<ref>{{cite journal | vauthors = Mooi R, David B | date=December 1998 |title=Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms |journal=American Zoologist |volume=38 |issue=6 |pages=965–974 |doi=10.1093/icb/38.6.965 |issn=1540-7063 |doi-access=free }}</ref>), and [[Biology:Arthropod|arthropod]]s (''[[Biology:Spriggina|Spriggina]]'',<ref>{{cite conference |url= https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |title=''Spriggina'' is a trilobitoid ecdysozoan | vauthors = McMenamin MA |author-link=Mark McMenamin |date=September 2003 |conference=Geoscience Horizons Seattle 2003 |conference-url=https://www.geosociety.org/meetings/2003/ |volume=35 |issue=6 |series=Abstracts with Programs |publisher=Geological Society of America |location=Boulder, Colo. |page=105 |oclc=249088612 |access-date=2007-11-24 |archive-url=https://web.archive.org/web/20160412064305/https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |archive-date=12 April 2016 |url-status=dead }} Paper No. 40-2 presented at the Geological Society of America's 2003 Seattle Annual Meeting (2–5 November 2003) on 2 November 2003, at the Washington State Convention Center.</ref> ''[[Biology:Parvancorina|Parvancorina]]''<ref>{{cite journal | vauthors = Lin JP, Gon III SM, Gehling JG, Babcock LE, Zhao YL, Zhang XL, Hu SX, Yuan JL, Yu MY, Peng J |display-authors=6 |year=2006 |title=A ''Parvancorina''-like arthropod from the Cambrian of South China |journal=Historical Biology: An International Journal of Paleobiology |volume=18 |issue=1 |pages=33–45 |doi=10.1080/08912960500508689 |s2cid=85821717 |issn=1029-2381}}</ref>). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that ''Kimberella'' was at least a [[Biology:Triploblasty|triploblastic]] bilaterian animal, in other words, an animal significantly more complex than the cnidarians.<ref name="Butterfield2006">{{cite journal | vauthors = Butterfield NJ | title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale | journal = BioEssays | volume = 28 | issue = 12 | pages = 1161–6 | date = December 2006 | pmid = 17120226 | doi = 10.1002/bies.20507 | s2cid = 29130876 }}</ref>

[[Biology:Small shelly fauna|Small shelly fauna]] are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, ''Cloudina'', shows signs of successful defense against predation and may indicate the start of an [[Biology:Evolutionary arms race|evolutionary arms race]]. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates," ''[[Biology:Halkieriid|Halkieria]]'' and ''[[Biology:Microdictyon|Microdictyon]]'', were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals.<ref name="Bengtson2004">{{cite journal | vauthors = Bengtson S | title = Early skeletal fossils | journal = Paleontological Society Papers | date = November 2004 | volume = 10 | pages = 67–78 | doi = 10.1017/S1089332600002345 }}</ref>

===Body plans and phyla===
[[File:Kimberella NT.jpg|thumb|right|''Kimberella'', an early mollusc important for understanding the Cambrian explosion. Invertebrates are grouped into different phyla (body plans).]]

Invertebrates are grouped into different [[Biology:Phylum|phyla]]. Informally phyla can be thought of as a way of grouping organisms according to their [[Biology:Body plan|body plan]].<ref>{{cite book | vauthors = Valentine JW | year = 2004 | title = On the Origin of Phyla | publisher = University Of Chicago Press | location = Chicago | isbn = 978-0-226-84548-7 | page = 7 | quote = Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy. }}</ref><ref name=Valentine2004>{{Cite book|url=https://books.google.com/books?id=DMBkmHm5fe4C |title=On the Origin of Phyla |isbn=9780226845487 | vauthors = Valentine JW |date=2004-06-18|publisher=University of Chicago Press }}</ref>{{rp|33}} A body plan refers to a blueprint which describes the shape or [[Biology:Morphology|morphology]] of an organism, such as its symmetry, [[Biology:Segmentation|segmentation]] and the disposition of its [[Biology:Appendage|appendage]]s. The idea of body plans originated with [[Biology:Vertebrate|vertebrate]]s, which were grouped into one phylum. But the vertebrate body plan is only one of many, and invertebrates consist of many phyla or body plans. The history of the discovery of body plans can be seen as a movement from a worldview centred on vertebrates, to seeing the vertebrates as one body plan among many. Among the pioneering zoologists, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has [[Biology:Phylum#Animal phyla|risen to 35]].<ref name=Valentine2004/>

[[File:Marine animal biodiversity.png|thumb|left| Taxonomic biodiversity of accepted marine species, according to ''WoRMS'', 18 October 2019.<ref>{{Cite web|url=http://www.marinespecies.org/|title=WoRMS - World Register of Marine Species|website=www.marinespecies.org}}</ref><ref>{{cite journal | vauthors = Novak BJ, Fraser D, Maloney TH | title = Transforming Ocean Conservation: Applying the Genetic Rescue Toolkit | journal = Genes | volume = 11 | issue = 2 | page = 209 | date = February 2020 | pmid = 32085502 | pmc = 7074136 | doi = 10.3390/genes11020209 | doi-access = free }}</ref>]]
[[File:20191108 Opabinia regalis.png|thumb|right|''Opabinia'', an extinct stem group arthropod appeared in the Middle Cambrian.<ref name=Gould1989>{{cite book | vauthors = Gould SJ |  date = 1990 | url = https://archive.org/details/wonderfullifebur00goul <!-- quote=Gould 1989. --> | title = Wonderful Life: The Burgess Shale and the Nature of History | publisher = W. W. Norton | isbn = 978-0-393-30700-9}}</ref>{{rp|124–136}}]]

Historically body plans were thought of as having evolved rapidly during the [[Earth:Cambrian explosion|Cambrian explosion]],<ref name=AmSci>{{cite journal| vauthors = Erwin D, Valentine J, Jablonski D |title=Recent fossil finds and new insights into animal development are providing fresh perspectives on the riddle of the explosion of animals during the Early Cambrian |journal=American Scientist |date=1997 |issue=March–April |url=http://www.americanscientist.org/issues/num2/the-origin-of-animal-body-plans/3}}</ref> but a more nuanced understanding of animal evolution suggests a gradual development of body plans throughout the early Palaeozoic and beyond.<ref name="Budd2000"/> More generally a phylum can be defined in two ways: as described above, as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).<ref name="Budd2000">{{cite journal | vauthors = Budd GE, Jensen S | title = A critical reappraisal of the fossil record of the bilaterian phyla | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 75 | issue = 2 | pages = 253–95 | date = May 2000 | pmid = 10881389 | doi = 10.1111/j.1469-185X.1999.tb00046.x | s2cid = 39772232 }}</ref>

In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of [[Earth:Precambrian|Precambrian]] animal fossils.<ref name="Bengtson2004" /> A re-analysis of fossils from the [[Earth:Burgess Shale|Burgess Shale]] lagerstätte increased interest in the issue when it revealed animals, such as ''[[Biology:Opabinia|Opabinia]]'', which did not fit into any known [[Biology:Phylum|phylum]]. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.<ref>{{cite book | vauthors = Gould SJ |title=Wonderful life: the Burgess Shale and the nature of history |date=1989 |location=New York |isbn=978-0-393-02705-1 |edition=First}}</ref> Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups<ref name="Budd2003">{{cite journal | vauthors = Budd GE | title = The Cambrian fossil record and the origin of the phyla | journal = Integrative and Comparative Biology | volume = 43 | issue = 1 | pages = 157–65 | date = February 2003 | pmid = 21680420 | doi = 10.1093/icb/43.1.157 | doi-access = free | author-link = Graham Budd }}</ref>—for example that ''Opabinia'' was a member of the [[Biology:Lobopodia|lobopod]]s, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern [[Biology:Tardigrade|tardigrade]]s.<ref name="Budd1996">{{cite journal | vauthors = Budd GE | date=March 1996 |title=The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group |journal=Lethaia |volume=29 |issue=1 |pages=1–14 |doi=10.1111/j.1502-3931.1996.tb01831.x |issn=0024-1164 }}</ref> Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.<ref name="Marshall2006Explaining">{{cite journal | vauthors = Marshall CR | date=May 2006 |title=Explaining the Cambrian 'Explosion' of Animals |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=355–384 |bibcode=2006AREPS..34..355M |doi=10.1146/annurev.earth.33.031504.103001 |s2cid=85623607 |issn=1545-4495 }}</ref>

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=== Earliest animals {{anchor|basal_animal_anchor}}===

The [[Biology:Basal (phylogenetics)|deepest-branching]] animals — the earliest animals that appeared during evolution — are marine non-vertebrate organisms. The earliest animal phyla are the [[Biology:Porifera|Porifera]], [[Biology:Ctenophora|Ctenophora]], [[Biology:Placozoa|Placozoa]] and [[Biology:Cnidaria|Cnidaria]]. No member of these [[Biology:Clade|clade]]s exhibit body plans with [[Biology:Symmetry in biology|bilateral symmetry]].

{{Clade |style=<!-- font-size:80%; line-height:80%; --> float:left;
|{{Clade
  |1=[[Biology:Choanoflagellate|Choanoflagellata]][[File:Cronoflagelado2.svg|50 px]] &nbsp; <small>unicellular protists thought to be the closest living relatives of animals</small>
   |sublabel1=<small>950 mya</small> &nbsp;&nbsp;&nbsp;
   |label2 = &nbsp; Animals &nbsp; &nbsp; &nbsp; 
   |sublabel2=<small>760 mya</small> &nbsp;&nbsp;&nbsp;
  |2={{Clade
      |1= &nbsp; [[Biology:Porifera|Porifera]] &nbsp; &nbsp; [[File:Aplysina archeri (Stove-pipe Sponge-pink variation).jpg|30 px]] &nbsp; <small>sponges – asymmetric</small>
      |2={{Clade
         |1= &nbsp; [[Biology:Ctenophora|Ctenophora]] &nbsp; &nbsp; [[File:Comb jelly 2.jpg|45 px]] &nbsp; <small>comb jellies – biradial symmetry</small>
         |2={{Clade
            |1= &nbsp; [[Biology:Placozoa|Placozoa]] &nbsp; &nbsp; [[File:Trichoplax 2.jpg|40 px]] &nbsp; <small>simplest animals – asymmetric</small>
            |2={{Clade
               |1= &nbsp; [[Biology:Cnidaria|Cnidaria]] &nbsp; &nbsp; [[File:Cubozoa - Tripedalia drawing.jpg|30 px]] &nbsp; <small>have tentacles with stingers – radial symmetry</small>
               |2={{Clade
                  |1= &nbsp; ''bilaterians &nbsp; [[File:Goldfish varieties and tropical aquarium fishes; a complete guide to aquaria and related subjects (1917) (14597751849).jpg|45 px]] &nbsp; <small>all remaining animals – bilateral symmetry</small>'' &nbsp;→&nbsp;
               }}
            }}
         }}
      }}
   }}
}}
}}

{{multiple image
 | align = right
 | direction = horizontal
 | footer    = There has been much controversy over which invertebrate phyla, [[Biology:Sponge|sponge]]s or [[Biology:Ctenophora|comb jellies]], is the most [[Biology:Basal (phylogenetics)|basal]].<ref>{{cite journal | vauthors = King N, Rokas A | title = Embracing Uncertainty in Reconstructing Early Animal Evolution | journal = Current Biology | volume = 27 | issue = 19 | pages = R1081–R1088 | date = October 2017 | pmid = 29017048 | pmc = 5679448 | doi = 10.1016/j.cub.2017.08.054 }}</ref> Currently, sponges are more widely considered to be the most basal.<ref>{{cite journal | vauthors = Feuda R, Dohrmann M, Pett W, Philippe H, Rota-Stabelli O, Lartillot N, Wörheide G, Pisani D | display-authors = 6 | title = Improved Modeling of Compositional Heterogeneity Supports Sponges as Sister to All Other Animals | journal = Current Biology | volume = 27 | issue = 24 | pages = 3864–3870.e4 | date = December 2017 | pmid = 29199080 | doi = 10.1016/j.cub.2017.11.008 | doi-access = free | hdl = 11572/302898 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = Nielsen C | title = Early animal evolution: a morphologist's view | journal = Royal Society Open Science | volume = 6 | issue = 7 | pages = 190638 | date = July 2019 | pmid = 31417759 | pmc = 6689584 | doi = 10.1098/rsos.190638 | bibcode = 2019RSOS....690638N }}</ref>
 | image1    = Aplysina archeri (Stove-pipe Sponge-pink variation).jpg|
 | width1    = 130
 | image2    = Comb jelly 2.jpg
 | width2    = 222
}}

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====Marine sponges====
[[File:Callyspongia sp. (Tube sponge).jpg|thumb|right|Sponges are perhaps the most basal animals. They have no nervous, digestive or circulatory system.]]

[[Biology:Sponge|Sponge]]s are animals of the [[Biology:Phylum|phylum]] [[Biology:Porifera|Porifera]] (from Modern Latin for ''bearing pores''<ref>{{cite web | url = http://www.etymonline.com/index.php?term=Porifera | title = Porifera (n.) | work = Online Etymology Dictionary | access-date = 18 August 2016 }}</ref>). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of [[Biology:Cell|cell]]s. They have unspecialized cells that can [[Biology:Cellular differentiation|transform into other types]] and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have [[Biology:Nervous system|nervous]], digestive or [[Biology:Circulatory system|circulatory system]]s. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.

Sponges are similar to other animals in that they are multicellular, heterotrophic, lack [[Biology:Cell wall|cell wall]]s and produce sperm cells. Unlike other animals, they lack true [[Biology:Tissue|tissues]] and [[Biology:Organ (anatomy)|organs]], and have no body symmetry. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum. Many sponges have internal skeletons of [[Biology:Spongin|spongin]] and/or spicules of [[Chemistry:Calcium carbonate|calcium carbonate]] or [[Chemistry:Silicon dioxide|silicon dioxide]]. All sponges are sessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding {{convert|8800|m|mi|abbr=on}}. Some sponges live to great ages; there is evidence of the deep-sea glass sponge ''[[Biology:Monorhaphis chuni|Monorhaphis chuni]]'' living about 11,000 years.<ref name=Petralia2014>{{cite journal | vauthors = Petralia RS, Mattson MP, Yao PJ | title = Aging and longevity in the simplest animals and the quest for immortality | journal = Ageing Research Reviews | volume = 16 | pages = 66–82 | date = July 2014 | pmid = 24910306 | pmc = 4133289 | doi = 10.1016/j.arr.2014.05.003 }}</ref><ref>{{cite journal | vauthors = Jochum KP, Wang X, Vennemann TW, Sinha B, Müller WE | year = 2012 | title = Siliceous deep-sea sponge ''Monorhaphis chuni'': A potential paleoclimate archive in ancient animals | journal = Chemical Geology | volume = 300 | pages = 143–151 | doi = 10.1016/j.chemgeo.2012.01.009 | bibcode = 2012ChGeo.300..143J }}</ref>

While most of the approximately 5,000–10,000&nbsp;known species feed on [[Biology:Bacteria|bacteria]] and other food particles in the water, some host [[Earth:Photosynthesis|photosynthesizing]] micro-organisms as [[Biology:Endosymbiont|endosymbiont]]s and these alliances often produce more food and oxygen than they consume. A few species of sponge that live in food-poor environments have become [[Biology:Carnivore|carnivore]]s that prey mainly on small [[Biology:Crustacean|crustacean]]s.<ref>{{cite journal | vauthors = Vacelet J, Duport E | title = Prey capture and digestion in the carnivorous sponge Asbestopluma hypogea (Porifera: Demospongiae). | journal = Zoomorphology | date = November 2004 | volume = 123 | issue = 4 | pages = 179–90 | doi = 10.1007/s00435-004-0100-0 | s2cid = 24484610 }}</ref>

<gallery mode="packed" heights="130px" style="float:left;">
File:Sponges in Caribbean Sea, Cayman Islands.jpg|Sponge biodiversity. There are four sponge species in this photo.
File:Callyspongia vaginalis (Branching Vase Sponge - pink variation).jpg|Branching vase sponge
File:Euplectella aspergillum (cropped).jpg|[[Biology:Venus' flower basket|Venus' flower basket]] at a depth of 2572 meters
File:Barrel sponge (Xestospongia testudinaria).jpg|Barrel sponge
File:Szi - Hyalonema.jpg|The long-living ''[[Biology:Monorhaphis chuni|Monorhaphis chuni]]''
</gallery>

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[[Biography:Carl Linnaeus|Linnaeus]] mistakenly identified sponges as plants in the order [[Biology:Algae|Algae]].<ref name=worms>{{cite web |url=http://www.marinespecies.org/aphia.php?p=sourceget&id=44036 |title=''Spongia Linnaeus'', 1759 |publisher=[[Organization:World Register of Marine Species|World Register of Marine Species]] |access-date=18 July 2012 }}</ref> For a long time thereafter sponges were assigned to a separate subkingdom, [[Biology:Parazoa|Parazoa]] (meaning ''beside the animals'').<ref>{{cite journal | vauthors = Rowland SM, Stephens T |year=2001 |title=Archaeocyatha: A history of phylogenetic interpretation |journal=Journal of Paleontology |volume=75 |pages=1065–1078 |issue=6 |jstor=1307076 |doi=10.1666/0022-3360(2001)075<1065:AAHOPI>2.0.CO;2 |s2cid=86211946 }}</ref> They are now classified as a [[Biology:Paraphyly|paraphyletic]] [[Biology:Phylum|phylum]] from which the higher animals have evolved.<ref>{{cite journal |vauthors=Sperling EA, Pisani D, Peterson KJ |title=Poriferan paraphyly and its implications for Precambrian palaeobiology |journal=Geological Society, London, Special Publications |date=1 January 2007 |volume=286 |issue=1 |pages=355–368 |doi=10.1144/SP286.25 |url=http://www.dartmouth.edu/~peterson/Sperling,%20Pisani%20and%20Peterson.pdf |access-date=22 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20090509061759/http://www.dartmouth.edu/~peterson/Sperling,%20Pisani%20and%20Peterson.pdf |archive-date=9 May 2009 |bibcode=2007GSLSP.286..355S |s2cid=34175521 }}</ref>

====Ctenophores====

Ctenophores (from Greek for ''carrying a comb''), commonly known as comb jellies, are a phylum that live worldwide in marine waters. They are the largest non-colonial animals to swim with the help of cilia (hairs or combs).<ref name="RuppertBarnes2004Ctenophora">{{cite book | vauthors = Ruppert EE, Fox RS, Barnes RD | title=Invertebrate Zoology | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | pages=[https://archive.org/details/isbn_9780030259821/page/182 182–195] | url=https://archive.org/details/isbn_9780030259821/page/182 }}</ref> Coastal species need to be tough enough to withstand waves and swirling sediment, but some oceanic species are so fragile and transparent that it is very difficult to capture them intact for study.<ref name="MillsNotesFromExpert">{{cite web| title=Ctenophores – some notes from an expert | vauthors = Mills CE | url= http://faculty.washington.edu/cemills/Ctenophores.html | access-date=2009-02-05}}</ref> In the past ctenophores were thought to have only a modest presence in the ocean, but it is now known they are often significant and even dominant parts of the planktonic biomass.<ref name=Brusca2003 />{{rp|269}}

The phylum has about 150 known species with a wide range of body forms. Sizes range from a few millimeters to {{convert|1.5|m|abbr=on}}. [[Biology:Cydippida|Cydippids]] are egg-shaped with their cilia arranged in eight radial comb rows, and deploy retractable tentacles for capturing prey. The benthic platyctenids are generally combless and flat. The coastal beroids have gaping mouths and lack tentacles. Most adult ctenophores prey on microscopic larvae and [[Biology:Rotifer|rotifer]]s and small [[Biology:Crustacea|crustacea]]ns but beroids prey on other ctenophores.

{{clear right}}

<gallery mode="packed">
File:LightRefractsOf comb-rows of ctenophore Mertensia ovum.jpg| Light [[Physics:Diffraction|diffracting]] along the comb rows of a cydippid, left tentacle deployed, right retracted
File:Ctenophore.jpg| Deep-sea ctenophore trailing tentacles studded with tentilla (sub-tentacles)
File:Aulacoctena cydippid ctenophore.jpg|Egg-shaped cydippid ctenophore
File:Coeloplana astericola (Benthic ctenophores) on Echniaster luzonicus (Seastar).jpg|Group of small benthic [[Biology:Coeloplana astericola|creeping comb jellies]] streaming tentacles and living symbiotically on a starfish.
File:Lobate ctenophore.jpg|''[[Biology:Lobata|Lobata]]'' sp. with paired thick lobes
File:Mnemiopsis leidyi 2.jpg|The sea walnut has a transient anus which forms only when it needs to defecate.<ref>{{cite magazine| title=Animal with an anus that comes and goes could reveal how ours evolved| vauthors = Le Page M | magazine=New Scientist| url=https://www.newscientist.com/article/2195656-animal-with-an-anus-that-comes-and-goes-could-reveal-how-ours-evolved/|  date=March 2019}}</ref>
</gallery>

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[[File:Ctenophore2.jpg|thumb|upright=1.4| The beroid ctenophore, mouth gaping, preys on other ctenophores.]]

Early writers combined ctenophores with cnidarians. Ctenophores resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralized nerve net rather than a brain. Also like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of [[Biology:Cell|cells]] on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. While cnidarians exhibit radial symmetry, ctenophores have two anal canals which exhibit biradial symmetry (half-turn rotational symmetry).<ref>{{cite journal | vauthors = Martindale MQ, Finnerty JR, Henry JQ | title = The Radiata and the evolutionary origins of the bilaterian body plan | journal = Molecular Phylogenetics and Evolution | volume = 24 | issue = 3 | pages = 358–65 | date = September 2002 | pmid = 12220977 | doi = 10.1016/s1055-7903(02)00208-7 }}</ref> The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on [[Biology:Molecular phylogenetics|molecular phylogenetics]], is that cnidarians and [[Biology:Bilateria|bilateria]]ns are more closely related to each other than either is to ctenophores.<ref name=Brusca2003>{{cite book | vauthors = Brusca RC, Brusca GJ | date = 2003 | title = Invertebrates | edition = Second | publisher = Sinauer Associates | isbn = 978-0-87893-097-5}}</ref>{{rp|222}}

{{ external media
| headerimage=
| float  = left
| width  = 280px
| video1 = [https://www.youtube.com/watch?v=5Ny0F6jbX2U&ab_channel=EVNautilus Iridescent red ctenophore] — ''EVNautilus''
}}

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====Placozoa====
[[Biology:Placozoa|Placozoa]] (from Greek for ''flat animals'') have the simplest structure of all animals. They are a [[Biology:Basal (phylogenetics)|basal]] form of free-living (non-parasitic) [[Biology:Multicellular organism|multicellular organism]]<ref>{{MeshName|Placozoa}}</ref> that do not yet have a common name.<ref>{{cite book|language=de| vauthors = Wehner R, Gehring W | title=Zoologie|edition=24th|publisher=Thieme|location= Stuttgart|date=June 2007|page=696}}</ref> They live in marine environments and form a phylum containing sofar only three described species, of which the first, the classical ''[[Biology:Trichoplax adhaerens|Trichoplax adhaerens]]'', was discovered in 1883.<ref>{{cite book | vauthors = Schulze FE | chapter = ''Trichoplax adhaerens'' n. g., n. s. | title = Zoologischer Anzeiger | publisher = Elsevier | location = Amsterdam and Jena | volume = 6 | date = 1883 | page = 92 }}</ref> Two more species have been discovered since 2017,<ref>{{Cite journal |vauthors=Eitel M, Francis WR, Osigus HJ, Krebs S, Vargas S, Blum H, Williams GA, Schierwater B, Wörheide G |display-authors=6 |date=2017-10-13 |title=A taxogenomics approach uncovers a new genus in the phylum Placozoa |url=https://www.biorxiv.org/content/early/2017/10/13/202119 |journal=bioRxiv |language=en |pages=202119 |doi=10.1101/202119 |s2cid=89829846 }}</ref><ref>{{cite journal | vauthors = Osigus HJ, Rolfes S, Herzog R, Kamm K, Schierwater B | title = Polyplacotoma mediterranea is a new ramified placozoan species | journal = Current Biology | volume = 29 | issue = 5 | pages = R148–R149 | date = March 2019 | pmid = 30836080 | doi = 10.1016/j.cub.2019.01.068 | doi-access = free }}</ref> and genetic methods indicate this phylum has a further 100 to 200 undescribed species.<ref>{{cite web | url = http://www.marinespecies.org/photogallery.php?album=5163&pic=24069 | title = Trichoplax adhaerens | work = WoRMS | date = 2009 }}</ref>

{{multiple image
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 | width1    = 182
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[[File:Exodigestion in Trichoplax adhaerens.jpg|thumb|upright=1.5|right| Crawling motility and food uptake by ''[[Biology:Trichoplax adhaerens|T.&nbsp;adhaerens]]'']]

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''Trichoplax'' is a small, flattened, animal about one mm across and usually about 25&nbsp;µm thick. Like the amoebae they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement. ''Trichoplax'' lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers.<ref name="SmithVaroqueaux2014">{{cite journal | vauthors = Smith CL, Varoqueaux F, Kittelmann M, Azzam RN, Cooper B, Winters CA, Eitel M, Fasshauer D, Reese TS | display-authors = 6 | title = Novel cell types, neurosecretory cells, and body plan of the early-diverging metazoan Trichoplax adhaerens | journal = Current Biology | volume = 24 | issue = 14 | pages = 1565–1572 | date = July 2014 | pmid = 24954051 | pmc = 4128346 | doi = 10.1016/j.cub.2014.05.046 }}</ref> The outer layer of simple [[Biology:Epithelium|epithelial cells]] bear cilia which the animal uses to help it creep along the seafloor.<ref name="IZ">{{cite book | vauthors = Barnes RD |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia|pages= 84–85|isbn= 978-0-03-056747-6}}</ref> ''Trichoplax'' feed by engulfing and absorbing food particles – mainly microbes and organic detritus – with their underside.

====Marine cnidarians====
[[File:Nematostella vectensis (I1419) 999 (30695685804).jpg|thumb|right|Cnidarians, like this starlet sea anemone, are the simplest animals to organise cells into tissue. Yet they have the same genes that form the vertebrate (including human) head.]]

Cnidarians (from Greek for ''nettle'') are distinguished by the presence of stinging cells, specialized cells that they use mainly for capturing prey. Cnidarians include [[Biology:Coral|coral]]s, [[Biology:Sea anemone|sea anemone]]s, [[Biology:Jellyfish|jellyfish]] and hydrozoans. They form a phylum containing over 10,000<ref name=Zhang2011>{{cite journal| vauthors = Zhang ZQ | title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness | journal=Zootaxa| volume=3148| year=2011| pages=7–12| url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf| doi=10.11646/zootaxa.3148.1.3 }}</ref> [[Biology:Species|species]] of [[Biology:Animal|animal]]s found exclusively in aquatic (mainly marine) environments. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of [[Biology:Epithelium|epithelium]] that are mostly one [[Biology:Cell|cell thick]]. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single [[Biology:Body orifice|orifice]] and body cavity that are used for digestion and [[Biology:Respiration (physiology)|respiration]].

Fossil cnidarians have been found in rocks formed about {{ma|580}}. Fossils of cnidarians that do not build [[Earth:Biomineralization|mineralized]] structures are rare. Scientists currently think cnidarians, ctenophores and bilaterians are more closely related to [[Biology:Calcarea|calcareous sponges]] than these are to other [[Biology:Sponge|sponge]]s, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.

Cnidarians are the simplest animals in which the cells are organised into tissues.<ref>{{cite web |url=http://genome.jgi-psf.org/Nemve1/Nemve1.home.html |title=''Nematostella vectensis'' v1.0 |work=Genome Portal |publisher=University of California |access-date=2014-01-19}}</ref> The [[Biology:Starlet sea anemone|starlet sea anemone]] is used as a model organism in research.<ref name=Nematostella>{{cite web |url=http://www.nematostella.org/ |title=''Nematostella'' |publisher=Nematostella.org |access-date=2014-01-18 |archive-url=https://web.archive.org/web/20060508222254/http://www.nematostella.org/ |archive-date=8 May 2006 |url-status=dead }}</ref> It is easy to care for in the laboratory and a protocol has been developed which can yield large numbers of embryos on a daily basis.<ref name=Genikhovich>{{cite journal | vauthors = Genikhovich G, Technau U | title = The starlet sea anemone Nematostella vectensis: an anthozoan model organism for studies in comparative genomics and functional evolutionary developmental biology | journal = Cold Spring Harbor Protocols | volume = 2009 | issue = 9 | pages = pdb.emo129 | date = September 2009 | pmid = 20147257 | doi = 10.1101/pdb.emo129 }}</ref> There is a remarkable degree of similarity in the gene sequence conservation and complexity between the sea anemone and vertebrates.<ref name=Genikhovich/> In particular, genes concerned in the formation of the head in vertebrates are also present in the anemone.<ref>{{cite news |title=Where Does Our Head Come From? Brainless Sea Anemone Sheds New Light on the Evolutionary Origin of the Head |url=https://www.sciencedaily.com/releases/2013/02/130220084436.htm#.USs4Y9Dvwpg.twitter |newspaper=Science Daily |date=2013-02-12 |access-date=2014-01-18}}</ref><ref>{{cite journal | vauthors = Sinigaglia C, Busengdal H, Leclère L, Technau U, Rentzsch F | title = The bilaterian head patterning gene six3/6 controls aboral domain development in a cnidarian | journal = PLOS Biology | volume = 11 | issue = 2 | pages = e1001488 | year = 2013 | pmid = 23483856 | pmc = 3586664 | doi = 10.1371/journal.pbio.1001488 | doi-access = free }}</ref>

<gallery mode="packed" heights="130" style="float:left;">
File:Sea anemone in tidepools.jpg|Sea anemones are common in tidepools.
File:Striped colonial anemone.jpg|Their [[Biology:Tentacle|tentacle]]s sting and paralyse small fish.
File:Coral detail.jpg|Close up of polyps on the surface of a [[Biology:Coral|coral]], waving their tentacles.
File:Maldives small island.jpg|If an island sinks below the sea, coral growth can keep up with rising water and form an [[Earth:Atoll|atoll]].
File:Red-paper-lantern-jellyfish-Karen-Osborn-Smithsonian-Institution.png|The mantle of the red paper lantern jellyfish crumples and expands like a paper lantern.<ref>{{cite web |title=Red Paper Lantern Jellyfish |url=http://www.realmonstrosities.com/2012/04/red-paper-lantern-jellyfish.html |website=Real Monstrosities |access-date=25 October 2015}}</ref>
</gallery>

{{clear}}

<gallery mode="packed" heights="170px" style="float:left;">
File:Portuguese Man-O-War (Physalia physalis).jpg| The [[Biology:Portuguese man o' war|Portuguese man o' war]] is a [[Biology:Colony|colonial]] siphonophore
File:Marrus orthocanna.jpg| ''[[Biology:Marrus orthocanna|Marrus orthocanna]]'' another colonial siphonophore, assembled from two types of zooids.
File:Porpita porpita.jpg|''[[Biology:Porpita porpita|Porpita porpita]]'' consists of a colony of hydroids<ref name="Blue Button">{{cite web | url = http://www.beachhunter.net/thingstoknow/jellyfish/blue-button-jellyfish.htm | title = Blue Buttons in Florida | work = BeachHunter.net }}</ref>
File:Largelionsmanejellyfish.jpg|[[Biology:Lion's mane jellyfish|Lion's mane jellyfish]], largest known jellyfish<ref>{{cite book | vauthors = Karleskint G, Turner R, Small J | date = 2012 | url = https://books.google.com/books?id=uBXTCQAAQBAJ&q=%22lion%27s+mane+jellyfish%22+%22Introduction+to+Marine+Biology%22&pg=PA445 | title = Introduction to Marine Biology | publisher = Cengage Learning | edition = 4th | page = 445 | isbn = 978-1-133-36446-7 }}</ref>
File:Turritopsis dohrnii (cropped).jpg| ''[[Biology:Turritopsis dohrnii|Turritopsis dohrnii]]'' achieves [[Biology:Biological immortality|biological immortality]] by transferring its cells back to childhood.<ref>{{cite journal| vauthors = Bavestrello G, Sommer C, Sarà M |year=1992|title=Bi-directional conversion in Turritopsis nutricula (Hydrozoa)|journal=Scientia Marina|volume=56|issue=2–3|pages=137–140}}</ref><ref name="piraino-96">{{cite journal| vauthors = Piraino S, Boero F, Aeschbach B, Schmid V |year=1996|title=Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa)|journal=Biological Bulletin|volume=190|issue=3|pages=302–312|doi=10.2307/1543022|pmid=29227703|jstor=1543022|s2cid=3956265}}</ref>
File:Chironex fleckeri (sea wasp).jpg| The sea wasp is the most lethal jellyfish in the world.<ref name="Fenner1996">{{Cite journal|vauthors=Fenner PJ, Williamson JA |title=Worldwide deaths and severe envenomation from jellyfish stings |journal=The Medical Journal of Australia |volume=165 |issue=11–12 |pages=658–61 |year=1996 |pmid=8985452 |doi=10.5694/j.1326-5377.1996.tb138679.x |s2cid=45032896 }}</ref>
</gallery>

{{clear}}

==Bilateral invertebrate animals==
[[File:Bilaterian body plan.svg|thumb|upright=1.7|right|Idealised wormlike bilaterian body plan. With a cylindrical body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable [[Biology:Peristalsis|peristaltic motion]].]]

Some of the earliest bilaterians were wormlike, and the original bilaterian may have been a bottom dwelling worm with a single body opening.<ref name=Cannon2016 /> A bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a [[Biology:Coelom|coelom]] or pseudocoelom.{{efn|The earliest Bilateria may have had only a single opening, and no coelom.<ref name="Cannon2016">{{cite journal | vauthors = Cannon JT, Vellutini BC, Smith J, Ronquist F, Jondelius U, Hejnol A | title = Xenacoelomorpha is the sister group to Nephrozoa | journal = Nature | volume = 530 | issue = 7588 | pages = 89–93 | date = February 2016 | pmid = 26842059 | doi = 10.1038/nature16520 | s2cid = 205247296 | bibcode = 2016Natur.530...89C | url = http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 }}</ref>}} Animals with this bilaterally symmetric [[Biology:Body plan|body plan]] have a head (anterior) end<!--necessary! not implying there's an actual head--> and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.<ref name="Minelli2009"/><ref name=Brusca2016/>

Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth.<ref name=Finnerty>{{cite journal | vauthors = Finnerty JR | title = Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals? | journal = BioEssays | volume = 27 | issue = 11 | pages = 1174–80 | date = November 2005 | pmid = 16237677 | doi = 10.1002/bies.20299 | url = http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf | access-date = 27 August 2019 | url-status = dead | archive-url = https://web.archive.org/web/20140810012815/http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf | archive-date = 10 August 2014 }}</ref> The body stretches back from the head, and many bilaterians have a combination of circular [[Biology:Muscle|muscle]]s that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;<ref name=Brusca2016/> these enable soft-bodied animals with a [[Biology:Hydrostatic skeleton|hydrostatic skeleton]] to move by [[Biology:Peristalsis|peristalsis]].<ref name=Quillin>{{cite journal | vauthors = Quillin KJ | title = Ontogenetic scaling of hydrostatic skeletons: geometric, static stress and dynamic stress scaling of the earthworm lumbricus terrestris | journal = The Journal of Experimental Biology | volume = 201 | issue = 12 | pages = 1871–83 | date = May 1998 | pmid = 9600869 | doi = 10.1242/jeb.201.12.1871 | doi-access = free }}</ref> They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary [[Biology:Larva|larva]]e which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely simplified body structures.<ref name="Minelli2009">{{cite book| vauthors = Minelli A |title=Perspectives in Animal Phylogeny and Evolution |url=https://books.google.com/books?id=jIASDAAAQBAJ&pg=PA53 |year=2009 |publisher=Oxford University Press |isbn=978-0-19-856620-5 |page=53}}</ref><ref name=Brusca2016>{{Cite book |url=http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |title=Introduction to the Bilateria and the Phylum Xenacoelomorpha {{!}} Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |work=Invertebrates | vauthors = Brusca RC |date=2016 |publisher=Sinauer Associates |pages=345–372 |isbn=978-1605353753}}</ref>

{{clear}}

[[File:Ikaria wariootia (cropped).jpg|thumb|upright=0.6| ''[[Biology:Ikaria wariootia|Ikaria wariootia]]'', an early bilaterian<ref>{{cite web | vauthors =  Specktor B | url = https://www.livescience.com/bilaterian-worm-animal-ancestor.html | title = This primeval worm may be the ancestor of all animals] | work = Live Science' | date = 26 March 2020 }}</ref>]]
{{clade
|label1= ←&nbsp; bilaterians &nbsp; &nbsp;   
|1={{clade |style = float:right;
   |label1= &nbsp; &nbsp; [[Biology:Xenacoelomorpha|Xenacoelomorpha]] &nbsp; 
   |1={{clade
      |1= &nbsp; [[File:Proporus sp.png|60 px]] &nbsp; <small>basal bilaterians (lack a true gut)<ref name="Cannon2016"/></small>
      }}
   |label3=[[Biology:Nephrozoa|Nephrozoa]] 
   |3={{clade
      |label1=[[Biology:Protostome|protostome]]s 
      |sublabel1=<small>610 mya</small>
      |1={{clade
         |1= &nbsp; [[File:J J Wild Pseudocarcinus cropped 2.jpg|55 px]] &nbsp; <small>develops mouth first</small> &nbsp;→&nbsp;
         }}
      |label2= &nbsp;  &nbsp; [[Biology:Deuterostome|deuterostome]]s &nbsp;
      |sublabel2=<small>650 mya</small>
      |2={{clade
         |1= [[File:Carassius wild golden fish 2013 G1 (2).jpg|65 px]] &nbsp; <small>develops anus first</small> &nbsp;→&nbsp;
         }}
      }}
   }}
}}

===Protostomes===

Protostomes (from [[Social:Ancient Greek|Greek]] for ''first mouth'') are a superphylum of [[Biology:Animal|animal]]s. It is a sister clade of the deuterostomes (from Greek for ''second mouth''), with which it forms the [[Biology:Nephrozoa|Nephrozoa]] clade. Protostomes are distinguished from deuterostomes by the way their [[Biology:Embryonic development|embryos develop]]. In protostomes the first opening that develops becomes the [[Biology:Mouth|mouth]], while in deuterostomes it becomes the anus.<ref name="NYT-20170130">{{cite news | vauthors = Wade N |title=This Prehistoric Human Ancestor Was All Mouth |url=https://www.nytimes.com/2017/01/30/science/this-prehistoric-human-ancestor-was-all-mouth.html |date=30 January 2017 |work={{wipe|The New York Times}} |access-date=31 January 2017}}</ref><ref name="NAT-20170130">{{cite journal | vauthors = Han J, Morris SC, Ou Q, Shu D, Huang H | title = Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China) | journal = Nature | volume = 542 | issue = 7640 | pages = 228–231 | date = February 2017 | pmid = 28135722 | doi = 10.1038/nature21072 | s2cid = 353780 | bibcode = 2017Natur.542..228H }}</ref>
{{Clade
  |label1 = ←&nbsp; Protostomes
  |sublabel1= <small>(extant)</small>
  |1={{clade
     |label1= &nbsp; [[Biology:Ecdysozoa|Ecdysozoa]] &nbsp; |sublabel1= &nbsp; <small>>529 mya</small>
     |1={{clade
        |1= &nbsp; [[Biology:Scalidophora|Scalidophora]] [[File:Adult priapulid 2.jpg|40 px]] <small>penis worms and mud dragons</small>
        |2={{clade
           |1= &nbsp; [[Biology:Arthropod|arthropod]]s &nbsp; [[File:J J Wild Pseudocarcinus cropped 2.jpg|60 px]] <small>mainly [[Biology:Crustacean|crustacean]]s</small>
           |2= &nbsp; [[Biology:Nematode|nematode]]s &nbsp; [[File:CelegansGoldsteinLabUNC 2.jpg|50 px]] <small>roundworms</small>
           }}
        }}
     |label2=Spiralia
     |2={{clade
        |label1=[[Biology:Gnathifera (clade)|Gnathifera]]
        |1={{clade
           |1= &nbsp; [[Biology:Rotifer|rotifer]]s [[File:Bdelloid Rotifer (cropped).jpg|40 px]]
           |2= &nbsp; arrow worms [[File:Chaetoblack 3.png|70 px]]
           }}
        |label2= &nbsp; Platytrochozoa &nbsp; |sublabel2= &nbsp; <small>580 mya</small>
        |2={{clade
           |1= &nbsp; flatworms [[File:Pseudoceros liparus - Blue Pseudoceros Flatworm 2.jpg|80 px]]
           |label2= &nbsp; [[Biology:Lophotrochozoa|Lophotrochozoa]] &nbsp; |sublabel2= &nbsp; <small>550 mya</small>
           |2={{Clade
              |1= &nbsp; [[Biology:Mollusc|mollusc]]s [[File:Nautilus - Ekaterina Tutynina 2.jpg|60 px]] <small>gastropods, bivalves and [[Biology:Cephalopod|cephalopod]]s</small>
              |2= &nbsp; ringed worms [[File:Polychaeta (no) 2.jpg|60 px]]
              }}
           }}
       }}
   }}
}}

====Marine worms====
{{Further|Biology:Marine worm|Biology:Sea worm}}
[[File:Chaetoblack.png|thumb|upright=1.4|Many marine worms are related only distantly, so they form a number of different phyla. The worm shown is an arrow worm, found worldwide as a predatory component of plankton.]]

Worms (Old English for ''serpents'') form a number of phyla. Different groups of marine worms are related only distantly, so they are found in several different phyla such as the [[Biology:Annelida|Annelida]] (segmented worms), [[Biology:Chaetognatha|Chaetognatha]] (arrow worms), [[Biology:Phoronida|Phoronida]] (horseshoe worms), and [[Biology:Hemichordata|Hemichordata]]. All worms, apart from the Hemichordata, are protostomes. The Hemichordata are 
deuterostomes and are discussed in their own section below.

The typical body plan of a worm involves long cylindrical tube-like bodies and no limbs. [[Biology:Marine worm|Marine worm]]s vary in size from microscopic to over {{convert|1|m|ft}} in length for some marine polychaete worms (bristle worms)<ref>{{cite web |url=https://www.bbc.co.uk/cornwall/content/articles/2009/04/07/nature_worm_feature.shtml |title=Cornwall – Nature – Superstar Worm |work=BBC }}</ref> and up to {{convert|58|m|ft|}} for the marine nemertean worm (bootlace worm).<ref>Mark Carwardine (1995) ''The Guinness Book of Animal Records''. Guinness Publishing. p. 232.</ref> Some marine worms occupy a small variety of [[Earth:Parasitism|parasitic]] niches, living inside the bodies of other animals, while others live more freely in the marine environment or by burrowing underground. Many of these worms have specialized tentacles used for exchanging oxygen and carbon dioxide and also may be used for reproduction. Some marine worms are tube worms, such as the giant tube worm which lives in waters near underwater volcanoes and can withstand temperatures up to 90 degrees [[Celsius]]. [[Biology:Platyhelminthes|Platyhelminthes]] (flatworms) form another worm phylum which includes a class of parasitic tapeworms. The marine tapeworm ''Polygonoporus giganticus'', found in the gut of [[Biology:Sperm whale|sperm whale]]s, can grow to over 30&nbsp;m (100&nbsp;ft).<ref name="time-1957">{{cite news |date=1957-04-08 |url=http://www.time.com/time/magazine/article/0,9171,809356-1,00.html |archive-url=https://web.archive.org/web/20080627143218/http://www.time.com/time/magazine/article/0,9171,809356-1,00.html |url-status=dead |archive-date=27 June 2008 |title=The Persistent Parasites |magazine=Time}}</ref><ref name="hargis">{{cite tech report |year=1985 |veditors=Hargis W |title=Parasitology and pathology of marine organisms of the world ocean |url=https://spo.nmfs.noaa.gov/content/tr-25-parasitology-and-pathology-marine-organisms-world-ocean |publisher=[[Organization:National Oceanic and Atmospheric Administration|National Oceanic and Atmospheric Administration]]}}</ref>

[[Biology:Nematode|Nematode]]s (roundworms) constitute a further worm phylum with tubular [[Biology:Digestion|digestive system]]s and an opening at both ends.<ref>{{cite web |url=http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm |url-status=dead |archive-url=https://web.archive.org/web/20060914101343/http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm |archive-date=14 September 2006 |title=Classification of Animal Parasites}}</ref><ref>{{cite journal | vauthors = Garcia LS | title = Classification of human parasites, vectors, and similar organisms | journal = Clinical Infectious Diseases | volume = 29 | issue = 4 | pages = 734–6 | date = October 1999 | pmid = 10589879 | doi = 10.1086/520425 | doi-access = free }}</ref> Over 25,000 nematode species have been described,<ref>{{cite journal | vauthors = Hodda M | year = 2011 | title = Phylum Nematoda Cobb, 1932. In: Zhang, Z.-Q. Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness | journal = Zootaxa | volume = 3148 | pages = 63–95 | doi = 10.11646/zootaxa.3148.1.11 }}</ref><ref>{{cite journal | vauthors = Zhang Z | year = 2013 | title = Animal biodiversity: An update of classification and diversity in 2013. In: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013) | journal = Zootaxa | volume = 3703 | issue = 1| pages = 5–11 | doi = 10.11646/zootaxa.3703.1.3 | s2cid = 85252974 | doi-access = free }}</ref> of which more than half are parasitic. It has been estimated another million remain undescribed.<ref name="Lambshead 1993">
{{cite journal | vauthors = Lambshead PJ | title = Recent developments in marine benthic biodiversity research | journal = Oceanis | year = 1993 | volume = 19 | issue = 6 | pages = 5–24 }}</ref> They are ubiquitous in marine, freshwater and terrestrial environments, where they often outnumber other animals in both individual and species counts. They are found in every part of the earth's [[Earth:Lithosphere|lithosphere]], from the top of mountains to the bottom of [[Earth:Oceanic trench|oceanic trench]]es.<ref name="Borgonie 2011">{{cite journal | vauthors = Borgonie G, García-Moyano A, Litthauer D, Bert W, Bester A, van Heerden E, Möller C, Erasmus M, Onstott TC | display-authors = 6 | title = Nematoda from the terrestrial deep subsurface of South Africa | journal = Nature | volume = 474 | issue = 7349 | pages = 79–82 | date = June 2011 | pmid = 21637257 | doi = 10.1038/nature09974 | hdl-access = free | s2cid = 4399763 | bibcode = 2011Natur.474...79B | hdl = 1854/LU-1269676 | url = https://biblio.ugent.be/publication/1269676 }}</ref> By count they represent 90% of all animals on the [[Earth:Seabed|ocean floor]].<ref name="pmid18164201">{{cite journal | vauthors = Danovaro R, Gambi C, Dell'Anno A, Corinaldesi C, Fraschetti S, Vanreusel A, Vincx M, Gooday AJ | display-authors = 6 | title = Exponential decline of deep-sea ecosystem functioning linked to benthic biodiversity loss | journal = Current Biology | volume = 18 | issue = 1 | pages = 1–8 | date = January 2008 | pmid = 18164201 | doi = 10.1016/j.cub.2007.11.056 | s2cid = 15272791 | doi-access = free }}
*{{cite press release |date=27 December 2007 |title=Deep-sea species' loss could lead to oceans' collapse, study suggests |website=EurekAlert! |url=http://www.eurekalert.org/pub_releases/2007-12/cp-dsl122007.php}}</ref> Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.<ref name="isbn0-903874-22-9">{{cite book | vauthors = Platt HM | chapter = foreword |veditors=Lorenzen S, Lorenzen SA | title = The phylogenetic systematics of freeliving nematodes | publisher = The Ray Society | location = London | year = 1994 | isbn = 978-0-903874-22-9 }}</ref>

<gallery mode="packed" heights="150px" style="float:left;">
The bootlace worm can grow to {{convert|58|m|ft|}}.
File:Riftia tube worm colony Galapagos 2011.jpg|Giant tube worms cluster around [[Earth:Hydrothermal vent|hydrothermal vent]]s.
File:CSIRO ScienceImage 2818 Group of Nematodes.jpg|[[Biology:Nematode|Nematode]]s are ubiquitous [[Biology:Pseudocoelomate|pseudocoelomate]]s which can parasite marine plants and animals.
File:Nerr0328.jpg|Bloodworms are typically found on the bottom of shallow marine waters.
</gallery>

{{clear}}

====Marine molluscs====
{{multiple image
 | align = right
 | direction = vertical
 | width     = 220
 | image1    = Squid komodo.jpg
 | caption1  = [[Biology:Bigfin reef squid|Bigfin reef squid]] displaying vivid [[Physics:Iridescence|iridescence]] at night. [[Biology:Cephalopod|Cephalopod]]s are the most neurologically advanced invertebrates.<ref name=Barnes2001>{{Cite book| vauthors = Barnes RS, Calow P, Olive PJ | year=2001| title=The Invertebrates, A Synthesis | edition=3rd| publisher=Blackwell Science | location =UK}}</ref>
 | image2    = Glaucus atlanticus 1 cropped.jpg
 | caption2  = [[Biology:Glaucus atlanticus|Blue dragon]], a pelagic [[Biology:Sea slug|sea slug]]
 | image3    = Bolinus brandaris 2.jpg
 | caption3  = ''[[Biology:Bolinus brandaris|Bolinus brandaris]]'', a [[Biology:Sea snail|sea snail]] from which the [[History:Phoenicia|Phoenicia]]ns extracted royal [[Tyrian purple]] dye colour code: #66023C <span style="background-color:#66023C; color:#66023C;">_____</span><ref>{{cite web | url = http://www.green-lion.net/colour_purple.html | archive-url = https://web.archive.org/web/20140228184525/http://www.green-lion.net/colour_purple.html | url-status = dead | archive-date = 28 February 2014 | title = Tyrian Purple | work = Green Lion | date = 28 February 2014 }}</ref>
 | image4    = Archimollusc-en.svg
 | caption4  = Hypothetical ancestral mollusc
}}


[[Biology:Mollusc|Mollusc]]s (Latin for ''soft'') form a [[Biology:Phylum|phylum]] with about 85,000 extant recognized [[Biology:Species|species]].<ref name=Chapman2009>{{cite book | vauthors = Chapman AD | date = 2009 | url = http://www.environment.gov.au/biodiversity/abrs/publications/other/species-numbers/2009/04-02-groups-invertebrates.html#mollusca | title = Numbers of Living Species in Australia and the World | edition = 2nd | publisher = Australian Biological Resources Study | location = Canberra | isbn = 978-0-642-56860-1}}</ref> They are the largest [[Biology:Marine biology|marine]] phylum in terms of species count, containing about 23% of all the named marine organisms.<ref name=Hancock2008>{{cite web|url=http://www.austmus.gov.au/display.cfm?id=2897|title=Recognising research on molluscs|access-date=2009-03-09|publisher=Australian Museum|year=2008| vauthors = Hancock R |url-status=dead|archive-url=https://web.archive.org/web/20090530042720/http://www.austmus.gov.au/display.cfm?id=2897|archive-date=30 May 2009|df=dmy-all}}</ref> Molluscs have more varied forms than other invertebrate phyla. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat.

[[File:Giant Clam (Tridacna).jpg|thumb|upright=1|left| Drawing of a [[Biology:Giant clam|giant clam]] (''NOAA'')]]

The mollusc phylum is divided into 9 or 10 [[Biology:Taxonomy|taxonomic]] [[Biology:Class|classes]]. These classes include gastropods, bivalves and [[Biology:Cephalopod|cephalopod]]s, as well as other lesser-known but distinctive classes. Gastropods with protective shells are referred to as [[Biology:Snail|snail]]s, whereas gastropods without protective shells are referred to as [[Biology:Slug|slug]]s. Gastropods are by far the most numerous molluscs in terms of species.<ref name="PonderWinstonLindberg">{{Cite book | veditors = Ponder WF, Lindberg DR | year = 2008 | title = Phylogeny and Evolution of the Mollusca | publisher = Berkeley: University of California Press | page = 481 | isbn = 978-0-520-25092-5 }}</ref> Bivalves include [[Biology:Clam|clam]]s, [[Biology:Oyster|oyster]]s, [[Biology:Cockle (bivalve)|cockles]], [[Biology:Mussel|mussel]]s, [[Biology:Scallop|scallop]]s, and numerous other families. There are about 8,000 marine bivalves species (including [[Earth:Brackish water|brackish water]] and estuarine species). A deep sea ocean quahog clam has been reported as [[Biology:Ming (clam)|having lived 507 years]]<ref name="Munro2012">{{cite journal | vauthors = Munro D, Blier PU | title = The extreme longevity of Arctica islandica is associated with increased peroxidation resistance in mitochondrial membranes | journal = Aging Cell | volume = 11 | issue = 5 | pages = 845–55 | date = October 2012 | pmid = 22708840 | doi = 10.1111/j.1474-9726.2012.00847.x | s2cid = 205634828 | doi-access = free }}</ref> making it the longest recorded life of all animals apart from [[Biology:Colony|colonial]] animals, or near-colonial animals like sponges.<ref name=Petralia2014/>

<gallery caption="Gastropods and bivalves" mode="packed" heights="140px" style="float:left;">
File:Nembrotha aurea B.jpg|Marine gastropods are [[Biology:Sea snail|sea snail]]s or [[Biology:Sea slug|sea slug]]s. This [[Biology:Nudibranch|nudibranch]] is a sea slug.
File:Catalonia VilassarDeDalt CaudelCargol Conquilla.JPG|The sea snail ''[[Biology:Syrinx aruanus|Syrinx aruanus]]'' has a shell up to 91&nbsp;cm long, the largest of any living gastropod.
File:Placopecten magellanicus.jpg|Molluscs usually have eyes. Bordering the edge of the mantle of a [[Biology:Scallop|scallop]], a bivalve mollusc, can be over 100 [[Biology:Mollusc eye|simple eyes]].
File:Mytilus edulis illustration.png|Common mussel, another bivalve
</gallery>
{{clear left}}

[[Biology:Cephalopod|Cephalopod]]s include [[Biology:Octopus|octopus]], [[Biology:Squid|squid]] and [[Biology:Cuttlefish|cuttlefish]]. About 800 living species of marine cephalopods have been identified,<ref>{{cite web |url=http://cephbase.eol.org/ |title=Welcome to CephBase |work=CephBase |access-date=29 January 2016 |archive-date=12 January 2016 |archive-url=https://web.archive.org/web/20160112225313/http://cephbase.eol.org/ |url-status=dead }}</ref> and an estimated 11,000 extinct [[Biology:Taxon|taxa]] have been described.<ref name=mollusca12>{{The Mollusca|volume=12|name-list-style=vanc}}</ref> They are found in all oceans, but there are no fully freshwater cephalopods.<ref>{{Cite web | url=http://www.abc.net.au/science/articles/2013/01/16/3670198.htm | title=Are there any freshwater cephalopods?| website=Australian Broadcasting Corporation| date=2013-01-16}}</ref>

<gallery caption="[[Biology:Cephalopod|Cephalopod]]s" mode="packed" heights="150px" style="float:left;">
File:Nautilus Palau.JPG|The nautilus is a [[Biology:Living fossil|living fossil]] little changed since it evolved 500 million years ago as one of the first [[Biology:Cephalopod|cephalopod]]s.<ref name=Callaway2008>{{cite magazine|url=https://www.newscientist.com/article/dn14033-simpleminded-nautilus-reveals-flash-of-memory.html|title=Simple-Minded Nautilus Shows Flash of Memory| vauthors = Callaway E |date=2 June 2008|magazine=New Scientist|access-date=7 March 2012}}</ref><ref name=Phillips2008>{{cite journal|title=Living Fossil Memories| vauthors = Phillips K |date=15 June 2008|page=iii|volume=211|doi=10.1242/jeb.020370|journal=Journal of Experimental Biology|issue=12|s2cid=84279320}}</ref><ref name=Crook2008>{{cite journal| vauthors = Crook R, Basil J |year=2008|title=A biphasic memory curve in the chambered nautilus, ''Nautilus pompilius'' L. (Cephalopoda: Nautiloidea)|journal=Journal of Experimental Biology|volume=211|pages=1992–1998|doi=10.1242/jeb.018531|issue=12|pmid=18515730|s2cid=6305526}}</ref>
File:Dactylioceras NT.jpg| Reconstruction of an [[Biology:Ammonite|ammonite]], a highly successful early cephalopod that appeared 400 [[Physics:Mya (unit)|mya]].
File:HPIM1795.JPG| Cephalopods, like this [[Biology:Cuttlefish|cuttlefish]], use their mantle cavity for [[Biology:Aquatic locomotion#Jet propulsion|jet propulsion]].
File:Calmarcolossal.jpg|[[Biology:Colossal squid|Colossal squid]], the largest of all invertebrates<ref name=BBC2008>{{cite news| vauthors = Black R |title=Colossal squid out of the freezer|work=BBC News|date=26 April 2008|url=http://news.bbc.co.uk/1/hi/sci/tech/7367774.stm }}</ref>
</gallery>
{{clear left}}

Molluscs have such diverse shapes that many textbooks base their descriptions of molluscan anatomy on a generalized or ''hypothetical ancestral mollusc''. This generalized mollusc is unsegmented and bilaterally symmetrical with an underside consisting of a single muscular foot. Beyond that it has three further key features. Firstly, it has a muscular cloak called a [[Biology:Mantle (mollusc)|mantle]] covering its viscera and containing a significant cavity used for breathing and [[Biology:Excretion|excretion]]. A [[Biology:Gastropod shell|shell]] secreted by the mantle covers the upper surface. Secondly (apart from bivalves) it has a rasping tongue called a [[Biology:Radula|radula]] used for feeding. Thirdly, it has a [[Biology:Nervous system|nervous system]] including a complex digestive system using microscopic, muscle-powered hairs called cilia to exude [[Biology:Mucus|mucus]]. The generalized mollusc has two paired [[Biology:Ventral nerve cord|nerve cords]] (three in bivalves). The [[Biology:Brain|brain]], in species that have one, encircles the [[Biology:Esophagus|esophagus]]. Most molluscs have [[Biology:Mollusc eye|eyes]] and all have sensors detecting chemicals, vibrations, and touch.<ref name="Ruppert_2004">{{cite book |title=Invertebrate Zoology |edition=7th | vauthors = Ruppert RE, Fox RS, Barnes RD |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 }}</ref><ref name=Hayward1996>{{Cite book| vauthors = Hayward PJ |title=Handbook of the Marine Fauna of North-West Europe|year=1996|publisher=Oxford University Press|isbn=978-0-19-854055-7}}</ref>

Good evidence exists for the appearance of marine gastropods, cephalopods and bivalves in the [[Earth:Cambrian|Cambrian]] period {{ma|Cambrian|{{period end|Cambrian}}}}.

{{clear}}

====Marine arthropods====
{{Annotated image |float=right| image=Lobster line drawing.jpg |caption=Segments and [[Biology:Tagma|tagmata]] of an arthropod<ref name="Ruppert_2004" />{{rp|518–52}} The thorax bears the main locomotory appendages. The head and thorax are fused in some arthropods, such as [[Biology:Crab|crab]]s and [[Biology:Lobster|lobster]]s.|width=220 |image-width=120 |height=195
| annotations=
{{Annotation|164|45|Head}}
{{Annotation|120|65|___________|color=green}}
{{Annotation|157|93|[[Biology:Thorax|Thorax]]}}
{{Annotation|120|110|___________|color=green}}
{{Annotation|145|145|[[Biology:Abdomen|Abdomen]]}}
{{Annotation|120|170|___________|color=green}}
}}

[[File:Pneumodesmus newmani - MUSE.JPG|thumb|First known air-breathing animal to colonise land, the millipede ''Pneumodesmus newmani'',<ref name=Wilson2004>{{cite journal | vauthors = Wilson HM, Anderson LI |date=January 2004 |title=Morphology and taxonomy of Paleozoic millipedes (Diplopoda: Chilognatha: Archipolypoda) from Scotland |url= http://jpaleontol.geoscienceworld.org/content/78/1/169.abstract |journal=Journal of Paleontology |volume=78 |issue=1 |pages=169–184 |doi=10.1666/0022-3360(2004)078<0169:MATOPM>2.0.CO;2 |s2cid=131201588 }}</ref> lived in the Early Devonian.<ref>{{cite journal | vauthors = Suarez SE, Brookfield ME, Catlos EJ, Stöckli DF | title = A U-Pb zircon age constraint on the oldest-recorded air-breathing land animal | journal = PLOS ONE | volume = 12 | issue = 6 | pages = e0179262 | year = 2017 | pmid = 28658320 | pmc = 5489152 | doi = 10.1371/journal.pone.0179262 | bibcode = 2017PLoSO..1279262S | doi-access = free }}</ref>]]

[[Biology:Arthropod|Arthropod]]s (Greek for ''jointed feet'') have an [[Biology:Exoskeleton|exoskeleton]] (external [[Biology:Skeleton|skeleton]]), a [[Biology:Segmentation|segmented]] body, and jointed [[Biology:Appendage|appendage]]s (paired appendages). They form a [[Biology:Phylum|phylum]] which includes [[Biology:Insect|insect]]s, [[Biology:Arachnid|arachnid]]s, myriapods, and [[Biology:Crustacean|crustacean]]s. Arthropods are characterized by their jointed limbs and [[Biology:Arthropod cuticle|cuticle]] made of [[Chemistry:Chitin|chitin]], often mineralised with [[Chemistry:Calcium carbonate|calcium carbonate]]. The arthropod [[Biology:Body plan|body plan]] consists of segments, each with a pair of [[Biology:Appendage|appendage]]s. The rigid cuticle inhibits growth, so arthropods replace it periodically by [[Biology:Moulting|moulting]]. Their versatility has enabled them to become the most species-rich members of all [[Earth:Guild (ecology)|ecological guilds]] in most environments.

The evolutionary ancestry of arthropods dates back to the [[Earth:Cambrian|Cambrian]] period and is generally regarded as [[Biology:Monophyletic|monophyletic]]. However, basal relationships of arthropods with extinct phyla such as [[Biology:Lobopodia|lobopodia]]ns have recently been debated.<ref>{{cite journal | vauthors = Campbell LI, Rota-Stabelli O, Edgecombe GD, Marchioro T, Longhorn SJ, Telford MJ, Philippe H, Rebecchi L, Peterson KJ, Pisani D | display-authors = 6 | title = MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of Arthropoda | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 38 | pages = 15920–4 | date = September 2011 | pmid = 21896763 | pmc = 3179045 | doi = 10.1073/pnas.1105499108 | bibcode = 2011PNAS..10815920C | doi-access = free }}</ref><ref>{{cite journal | vauthors = Smith FW, Goldstein B | title = Segmentation in Tardigrada and diversification of segmental patterns in Panarthropoda | journal = Arthropod Structure & Development | volume = 46 | issue = 3 | pages = 328–340 | date = May 2017 | pmid = 27725256 | doi = 10.1016/j.asd.2016.10.005 }}</ref>
{{clade
|label1=&nbsp;[[Biology:Panarthropoda|Panarthropoda]] &nbsp; &nbsp;
  |1={{clade
    |1= tardigrades &nbsp; [[File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png|50px]] &nbsp; <small>water bears</small>
    |label2= &nbsp; [[Biology:Lobopodia|Lobopodia]] &nbsp;
  |2={{clade
     |1= [[Biology:Onychophora|velvet worms]] &nbsp; [[File:Velvet worm.png|80px]] &nbsp; <small>(terrestrial)</small>
     |2= arthropods &nbsp; [[File:Scylla serrata.jpg|60 px]] <small>mainly crustaceans</small>
    }}
  }}
}}
{{multiple image
 | align = left
 | direction = horizontal
 | image1    = Aysheaia2.jpg
 | caption1  = Some palaeontologists think Lobopodia represents a basal grade which lead to an arthropod body plan.<ref name="Budd2001">{{cite journal | vauthors = Budd GE | title = Why are arthropods segmented? | journal = Evolution & Development | volume = 3 | issue = 5 | pages = 332–42 | year = 2001 | pmid = 11710765 | doi = 10.1046/j.1525-142X.2001.01041.x | s2cid = 37935884 }}</ref>
 | total_width    = 360
 | image2    = SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2.png
 | alt2      = 
 | caption2  = Tardigrades (water bears) are a phylum of eight-legged, segmented microanimals able to survive in extreme conditions.
}}

<gallery caption="Arthropod fossils and living fossils" mode="packed" heights="150px" style="float:left;">
File:Cheirurus ingricus.png| Fossil [[Biology:Trilobite|trilobite]]. [[Biology:Trilobite|Trilobite]]s first appeared about 521 [[Megaannum|Ma]]. They were highly successful and were found everywhere in the ocean for 270 Ma.<ref name=firstlife>{{cite web |url=http://firstlifeseries.com/learn-more/ |title= David Attenborough's First Life |access-date=2011-03-10 |url-status=dead |archive-url=https://web.archive.org/web/20110126162514/http://firstlifeseries.com/learn-more/ |archive-date=26 January 2011 |df=dmy-all }}</ref>
File:20191203 Anomalocaris canadensis.png| The ''[[Biology:Anomalocaris|Anomalocaris]]'' ("abnormal shrimp") was one of the first apex predators and first appeared about 515 Ma.
File:Jaekelopterus rhenaniae reconstruction.jpg|The largest known arthropod, the [[Biology:Eurypterid|sea scorpion]] ''Jaekelopterus rhenaniae'', has been found in estuarine strata from about 390 Ma. It was up to {{convert|2.5|m|ft|abbr=on}} long.<ref name=Braddy2007>{{cite journal | vauthors = Braddy SJ, Poschmann M, Tetlie OE | title = Giant claw reveals the largest ever arthropod | journal = Biology Letters | volume = 4 | issue = 1 | pages = 106–9 | date = February 2008 | pmid = 18029297 | pmc = 2412931 | doi = 10.1098/rsbl.2007.0491 }}</ref><ref name=Cressey2007>{{cite journal| vauthors = Daniel C |title=Giant sea scorpion discovered|journal=Nature|date=21 November 2007|access-date=10 June 2013|url=http://www.nature.com/news/2007/071120/full/news.2007.272.html|doi=10.1038/news.2007.272}}</ref>
File:Limulus (cropped).jpg| Xiphosurans, the group including modern [[Biology:Horseshoe crab|Horseshoe crab]]s appeared around 480 Ma.<ref>{{Cite journal |last1=Bicknell |first1=Russell D. C. |last2=Pates |first2=Stephen |date=2020 |title=Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida |journal=Frontiers in Earth Science |volume=8 |page=98 |doi=10.3389/feart.2020.00098 |bibcode=2020FrEaS...8...98B |issn=2296-6463 |doi-access=free }}</ref>
</gallery>

{{clear}}

Extant marine arthropods range in size from the microscopic [[Biology:Crustacean|crustacean]] ''[[Biology:Stygotantulus|Stygotantulus]]'' to the [[Biology:Japanese spider crab|Japanese spider crab]]. Arthropods' primary internal cavity is a hemocoel, which accommodates their internal [[Biology:Organ (anatomy)|organs]], and through which their haemolymph - analogue of [[Biology:Blood|blood]] - circulates; they have [[Biology:Circulatory system#Open circulatory system|open circulatory systems]]. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their [[Biology:Nervous system|nervous system]] is "ladder-like", with paired [[Biology:Anatomical terms of location#Dorsal and ventral|ventral]] [[Biology:Ventral nerve cord|nerve cords]] running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their [[Biology:Brain|brain]]s are formed by fusion of the ganglia of these segments and encircle the [[Biology:Esophagus|esophagus]]. The respiratory and [[Biology:Excretion|excretory]] systems of arthropods vary, depending as much on their environment as on the [[Biology:Subphylum|subphylum]] to which they belong.

<gallery caption="Modern crustaceans" mode="packed" heights="150px" style="float:left;">
File:Hyperia.jpg|Many crustaceans are very small, like this tiny amphipod, and make up a significant part of the ocean's [[Earth:Zooplankton|zooplankton]].
File:Macrocheira kaempferi.jpg|The [[Biology:Japanese spider crab|Japanese spider crab]] has the longest leg span of any arthropod, reaching {{convert|5.5|m|ft}} from claw to claw.<ref>{{cite journal|year = 1920|title = An ugly giant crab of Japan|url = https://books.google.com/books?id=xCkDAAAAMBAJ&pg=PA42|journal = Popular Science|volume = 96|issue = 6|page = 42 }}</ref>
File:J J Wild Pseudocarcinus cropped.jpg|The [[Biology:Tasmanian giant crab|Tasmanian giant crab]] is long-lived and slow-growing, making it vulnerable to overfishing.<ref name=Currie2009>{{cite book| vauthors = Currie DR, Ward TM |year=2009 |url=http://www.sardi.sa.gov.au/__data/assets/pdf_file/0019/109522/No_345_SA_Giant_Crab_Fishery_Assessment_Report_2007_08Final_Report_published_alias.pdf|title=South Australian Giant Crab (''Pseudocarcinus gigas'') Fishery|id=Fishery Assessment Report for PIRSA|publisher=South Australian Research and Development Institute|access-date=9 December 2013|archive-url=https://web.archive.org/web/20120328121906/http://www.sardi.sa.gov.au/__data/assets/pdf_file/0019/109522/No_345_SA_Giant_Crab_Fishery_Assessment_Report_2007_08Final_Report_published_alias.pdf|archive-date=28 March 2012|url-status=dead}}</ref>
File:Odontodactylus scyllarus 2.png|[[Biology:Mantis shrimp|Mantis shrimp]] have the most advanced eyes in the animal kingdom,<ref name=Kilday2005>{{cite news|url=http://archive.dailycal.org/article/19671/mantis_shrimp_boasts_most_advanced_eyes|title=Mantis shrimp boasts most advanced eyes|vauthors=Kilday P|newspaper=The Daily Californian|date=28 September 2005|access-date=23 September 2016|archive-date=29 September 2012|archive-url=https://web.archive.org/web/20120929000217/http://archive.dailycal.org/article/19671/mantis_shrimp_boasts_most_advanced_eyes|url-status=dead}}</ref> and smash prey by swinging their club-like [[Biology:Raptorial|raptorial]] claws.<ref name="Patek and Caldwell">{{cite journal| vauthors = Patek SN, Caldwell RL |year=2005|journal=Journal of Experimental Biology|volume=208|pages=3655–3664|title=Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp|doi=10.1242/jeb.01831|pmid=16169943|issue=19|s2cid=312009|url=http://www.escholarship.org/uc/item/9938507n }}</ref>
</gallery>

{{clear}}

Arthropod vision relies on various combinations of [[Biology:Compound eye|compound eye]]s and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many [[Biology:Seta|seta]]e (bristles) that project through their cuticles. Arthropod methods of reproduction are diverse: terrestrial species use some form of internal fertilization while marine species lay eggs using either internal or [[Biology:External fertilization|external fertilization]]. Arthropod hatchlings vary from miniature adults to grubs that lack jointed limbs and eventually undergo a total [[Biology:Metamorphosis|metamorphosis]] to produce the adult form.

===Deuterostomes===

In [[Biology:Deuterostome|deuterostome]]s the first opening that develops in the growing embryo becomes the [[Biology:Anus|anus]], while in protostomes it becomes the mouth. Deuterostomes form a superphylum of [[Biology:Animal|animal]]s and are the sister clade of the protostomes.<ref name="NYT-20170130" /><ref name="NAT-20170130" /> It is once considered that the earliest known deuterostomes are ''[[Biology:Saccorhytus|Saccorhytus]]'' fossils from about 540 million years ago.<ref>{{Cite journal |last1=Han |first1=Jian |last2=Morris |first2=Simon Conway |last3=Ou |first3=Qiang |last4=Shu |first4=Degan |last5=Huang |first5=Hai |date=2017 |title=Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China) |url=https://www.nature.com/articles/nature21072 |journal=Nature |language=en |volume=542 |issue=7640 |pages=228–231 |doi=10.1038/nature21072 |pmid=28135722 |bibcode=2017Natur.542..228H |s2cid=353780 |issn=1476-4687}}</ref> However, another study considered that ''Saccorhytus'' is more likely to be an [[Biology:Ecdysozoa|ecdysozoa]]n.<ref>{{Cite journal |last1=Liu |first1=Yunhuan |last2=Carlisle |first2=Emily |last3=Zhang |first3=Huaqiao |last4=Yang |first4=Ben |last5=Steiner |first5=Michael |last6=Shao |first6=Tiequan |last7=Duan |first7=Baichuan |last8=Marone |first8=Federica |last9=Xiao |first9=Shuhai |last10=Donoghue |first10=Philip C. J. |date=2022-08-17 |title=Saccorhytus is an early ecdysozoan and not the earliest deuterostome |url=https://www.nature.com/articles/s41586-022-05107-z |journal=Nature |volume=609 |issue=7927 |language=en |pages=541–546 |doi=10.1038/s41586-022-05107-z |pmid=35978194 |bibcode=2022Natur.609..541L |s2cid=251646316 |issn=1476-4687|hdl=1983/454e7bec-4cd4-4121-933e-abeab69e96c1 |hdl-access=free }}</ref>
{{clade |label1 = ←&nbsp; deuterostomes &nbsp; &nbsp; 
  |sublabel1= <small>(extant)</small>
  |1={{clade
     |label1=&nbsp; &nbsp; ambulacrarians &nbsp; &nbsp;
     |1={{clade
        |1= &nbsp; [[Biology:Echinoderm|echinoderm]]s &nbsp; [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
        |2= &nbsp; [[Biology:Hemichordate|hemichordate]]s &nbsp; [[File:Balanoglossus by Spengel 1893.png|50 px]]
         }}
     |label2= chordates
     |2={{clade
        |1= &nbsp; [[Biology:Cephalochordate|cephalochordate]]s &nbsp; [[File:Branchiostoma lanceolatum (Pallas, 1774).jpg|80 px]]
        |2={{clade
           |1= &nbsp; [[Biology:Tunicate|tunicate]]s &nbsp; [[File:Tunicate komodo.jpg|60 px]]
           |2= &nbsp; ''[[Biology:Vertebrate|vertebrate]]s'' &nbsp;→&nbsp;
        }}
      }}
   }}
}}

====Echinoderms====
[[File:Haeckel Asteridea Larvae.jpg|thumb|right|upright=1.2| Adult echinoderms have fivefold symmetry but as larvae have bilateral symmetry. This is why they are in the Bilateria.]]

[[Biology:Echinoderm|Echinoderm]]s (Greek for ''spiny skin'') is a phylum which contains only marine invertebrates. The phylum contains about 7000 living [[Biology:Species|species]],<ref name="adw">{{cite web|url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Echinodermata.html|title=Animal Diversity Web - Echinodermata|publisher=University of Michigan Museum of Zoology|access-date=26 August 2012}}</ref> making it the second-largest grouping of [[Biology:Deuterostome|deuterostome]]s, after the [[Biology:Chordata|chordates]].

Adult echinoderms are recognizable by their radial symmetry (usually five-point) and include [[Biology:Starfish|starfish]], [[Biology:Sea urchin|sea urchin]]s, [[Biology:Sand dollar|sand dollar]]s, and [[Biology:Sea cucumber|sea cucumber]]s, as well as the [[Biology:Crinoid|sea lilies]].<ref>{{Cite web |date=2023-06-08 |title=Echinoderm {{!}} Definition, Characteristics, Species, & Facts {{!}} Britannica |url=https://www.britannica.com/animal/echinoderm |access-date=2023-06-24 |website=www.britannica.com |language=en}}</ref> Echinoderms are found at every ocean depth, from the [[Earth:Intertidal zone|intertidal zone]] to the [[Earth:Abyssal zone|abyssal zone]]. They are unique among animals in having bilateral symmetry at the larval stage, but fivefold symmetry (pentamerism, a special type of radial symmetry) as adults.<ref name="Lander">{{cite web | url=http://lanwebs.lander.edu/faculty/rsfox/invertebrates/asterias.html | title=''Asterias forbesi'' | vauthors = Fox R | work=Invertebrate Anatomy OnLine | publisher=Lander University | access-date=14 June 2014}}</ref>

Echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the [[Earth:Continental shelf|biotic desert]] of the [[Earth:Deep sea|deep sea]], as well as shallower oceans. Most echinoderms are able to [[Biology:Regeneration|regenerate]] tissue, organs, limbs, and reproduce asexually; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their [[Biology:Ossification|ossified]] [[Biology:Skeleton|skeleton]]s, which are major contributors to many [[Earth:Limestone|limestone]] formations, and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries.

<gallery mode="packed" heights="180px" style="float:left;">
File:Riccio Melone a Capo Caccia adventurediving.it.jpg|[[Biology:Echinoderm|Echinoderm]] literally means "spiny skin", as this [[Biology:Echinus (sea urchin)|water melon sea urchin]] illustrates.
File:Ochre sea star.jpg|The ochre sea star was the first keystone predator to be studied. They limit [[Biology:Mussel|mussel]]s which can overwhelm intertidal communities.<ref name=Holsinger>Holsinger, K. (2005). Keystone species. Retrieved 10 May 2010, from {{cite web |url=http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html | vauthors = Holsinger K | date = 11 October 2005 | work = University of Connecticut |title=Keystone species |access-date=2010-05-12 |url-status=dead |archive-url=https://web.archive.org/web/20100630134633/http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html |archive-date=30 June 2010 |df=dmy-all }}</ref>
File:Colorful crinoids at shallow waters of Gili Lawa Laut.JPG|Colorful [[Biology:Crinoid|sea lilies]] in shallow waters
</gallery>

{{clear}}

<gallery mode="packed" heights="190px" style="float:left;">
File:Espardenya (animal).jpg|[[Biology:Sea cucumber|Sea cucumber]]s filter feed on plankton and suspended solids.
File:Scotoplanes globosa and crab (cropped).jpg|The [[Biology:Scotoplanes|sea pig]], a deep water sea cucumber, is the only echinoderm that uses legged locomotion.
File:Enypniastes sp Indonesia.jpg| A benthopelagic and bioluminescent [[Biology:Enypniastes|swimming sea cucumber]], 3200 metres deep
</gallery>

{{clear}}

It is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. Aside from the hard-to-classify ''[[Biology:Arkarua|Arkarua]]'' (a [[Earth:Precambrian|Precambrian]] animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the [[Earth:Cambrian|Cambrian]].

====Hemichordates====
{{multiple image
 | align = right
 | direction = horizontal
 | header    = Gill (pharyngeal) slits
 | width1    = 200
 | image1    = Saccoglossus.jpg
 | alt1      = 
 | caption1  = The [[Biology:Acorn worm|acorn worm]] is associated with the development of gill slits.
 | width2    = 233
 | image2    = Gill slits.png
 | alt2      = 
 | caption2  = Gill slits in an acorn worm (left) and tunicate (right)
 | footer    = Gill slits have been described as "the foremost morphological innovation of early deuterostomes".<ref>{{cite journal | vauthors = Simakov O, Kawashima T, Marlétaz F, Jenkins J, Koyanagi R, Mitros T, Hisata K, Bredeson J, Shoguchi E, Gyoja F, Yue JX, Chen YC, Freeman RM, Sasaki A, Hikosaka-Katayama T, Sato A, Fujie M, Baughman KW, Levine J, Gonzalez P, Cameron C, Fritzenwanker JH, Pani AM, Goto H, Kanda M, Arakaki N, Yamasaki S, Qu J, Cree A, Ding Y, Dinh HH, Dugan S, Holder M, Jhangiani SN, Kovar CL, Lee SL, Lewis LR, Morton D, Nazareth LV, Okwuonu G, Santibanez J, Chen R, Richards S, Muzny DM, Gillis A, Peshkin L, Wu M, Humphreys T, Su YH, Putnam NH, Schmutz J, Fujiyama A, Yu JK, Tagawa K, Worley KC, Gibbs RA, Kirschner MW, Lowe CJ, Satoh N, Rokhsar DS, Gerhart J | display-authors = 6 | title = Hemichordate genomes and deuterostome origins | journal = Nature | volume = 527 | issue = 7579 | pages = 459–65 | date = November 2015 | pmid = 26580012 | pmc = 4729200 | doi = 10.1038/nature16150 | bibcode = 2015Natur.527..459S }}</ref><ref>{{cite web | url = https://www.futurity.org/acorn-worm-pharynx-1054372-2/ | title = How humans got a pharynx from this 'ugly beast' | work = Futurity | date = 23 November 2015 }}</ref> In aquatic organisms, gill slits allow water that enters the mouth during feeding to exit. Some invertebrate chordates also use the slits to filter food from the water.<ref name=2018Clark />
}}

[[Biology:Hemichordate|Hemichordate]]s form a sister phylum to the [[Biology:Echinoderm|echinoderm]]s. They are solitary worm-shaped organisms rarely seen by humans because of their lifestyle. They include two main groups, the acorn worms and the [[Biology:Pterobranchia|Pterobranchia]]. Pterobranchia form a class containing about 30 species of small worm-shaped animals that live in secreted tubes on the ocean floor. Acorn worms form a class containing about 111 species that generally live in U-shaped burrows on the seabed, from the shoreline to a depth of 3000 metres. The worms lie there with the proboscis sticking out of one opening in the burrow, subsisting as deposit feeders or suspension feeders. It is supposed the ancestors of acorn worms used to live in tubes like their relatives, the Pterobranchia, but eventually started to live a safer and more sheltered existence in sediment burrows.<ref>{{Cite web|url=https://phys.org/news/2016-07-secret-oesia-life-prehistoric-worm.html|title=The secret to an Oesia life: Prehistoric worm built tube-like 'houses' on sea floor|website=phys.org}}</ref> Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (8&nbsp;ft 2 in), although most acorn worms are much smaller.

Acorn worms are more highly specialised and advanced than other worm-like organisms. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures they use for breathing, similar to the gills of fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates and [[Biology:Vertebrate|vertebrate]]s. Acorn worms continually form new gill slits as they grow in size, and some older individuals have more than a hundred on each side. Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft. Cilia push water through the slits, maintaining a constant flow, just as in fish.<ref name=Barnes1982>{{cite book | vauthors = Barnes RD |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 1018–1026|isbn= 978-0-03-056747-6}}</ref> Some acorn worms also have a postanal tail which may be homologous to the post-anal tail of vertebrates.

The three-section body plan of the acorn worm is no longer present in the vertebrates, except in the anatomy of the frontal neural tube, later developed into a brain divided into three parts. This means some of the original anatomy of the early chordate ancestors is still present in vertebrates even if it is not always visible. One theory is the three-part body originated from an early common ancestor of the deuterostomes, and maybe even from a common bilateral ancestor of both deuterostomes and protostomes. Studies have shown the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system,<ref>{{cite web | url = http://discovery.lifemapsc.com/library/images/secondary-organizers-of-the-early-brain-and-the-location-of-the-meso-diencephalic-dopaminergic-precursor-cells | title = Secondary organizers of the early brain and the location of the meso-diencephalic dopaminergic precursor cells | work = Life Map | archive-url = https://web.archive.org/web/20140310172544/http://discovery.lifemapsc.com/library/images/secondary-organizers-of-the-early-brain-and-the-location-of-the-meso-diencephalic-dopaminergic-precursor-cells | archive-date=10 March 2014 | access-date = 10 March 2014 }}</ref> they are controlling the development of the different body regions.<ref>{{cite journal | vauthors = Pani AM, Mullarkey EE, Aronowicz J, Assimacopoulos S, Grove EA, Lowe CJ | title = Ancient deuterostome origins of vertebrate brain signalling centres | journal = Nature | volume = 483 | issue = 7389 | pages = 289–94 | date = March 2012 | pmid = 22422262 | pmc = 3719855 | doi = 10.1038/nature10838 | publisher = ScienceLife | bibcode = 2012Natur.483..289P }}</ref>

====Marine chordates====
[[File:Figure 29 01 04.jpg|thumb| The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a 
notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.<ref>{{cite web | url = https://cnx.org/contents/VB2yhrAh@8/Chordates | title = Chordates | work = OpenStax | date = 9 May 2019 }}</ref> ]]

The [[Biology:Chordate|chordate]] phylum has three subphyla, one of which is the [[Biology:Vertebrate|vertebrate]]s (see below). The other two subphyla are marine invertebrates: the [[Biology:Tunicate|tunicate]]s ([[Biology:Salp|salp]]s and sea squirts) and the [[Biology:Cephalochordate|cephalochordate]]s (such as [[Biology:Lancelet|lancelet]]s). Invertebrate chordates are close relatives to vertebrates. In particular, there has been discussion about how closely some extinct marine species, such as Pikaiidae, [[Biology:Palaeospondylus|Palaeospondylus]], [[Biology:Zhongxiniscus|Zhongxiniscus]] and [[Biology:Vetulicolia|Vetulicolia]], might relate ancestrally to vertebrates.

<gallery caption="Invertebrate chordates are close relatives of vertebrates" mode="packed" heights="140px" style="float:left;">
File:Amphioxus.png|The [[Biology:Lancelet|lancelet]], a small translucent fish-like [[Biology:Cephalochordate|cephalochordate]], is one of the closest living invertebrate relative of the vertebrates.<ref>{{cite journal| vauthors = Gewin V |year = 2005|title = Functional genomics thickens the biological plot|journal = PLOS Biology|volume = 3|issue = 6|page = e219|doi = 10.1371/journal.pbio.0030219|pmid=15941356|pmc=1149496 | doi-access=free }}</ref><ref>{{cite web | vauthors = Timmer J | url = https://arstechnica.com/science/2008/06/lancelet-amphioxus-genome-and-the-origin-of-vertebrates/ | title = Lancelet (amphioxus) genome and the origin of vertebrates | work = Ars Technica | date = 19 June 2008 }}</ref>
File:Ascidian (Rhopalaea Crassa) (4 cm).png| Tunicates, like these [[Biology:Rhopalaea crassa|fluorescent-colored sea squirts]], may provide clues to vertebrate and therefore human ancestry.<ref>{{cite journal| vauthors = Lemaire P |year = 2011|title = Evolutionary crossroads in developmental biology: the tunicates|journal = Development|volume = 138|issue = 11|pages = 2143–2152|doi = 10.1242/dev.048975|pmid=21558365|s2cid = 40452112}}</ref>
File:Tunicate off Atauro island.jpg| [[Biology:Pyrosome|Pyrosome]]s are free-floating bioluminescent tunicates made up of hundreds of individuals.
File:23 salpchain frierson odfw (8253212250).jpg|[[Biology:Salp|Salp]] chain
</gallery>

{{clear}}

[[File:Figure 29 01 02.png|thumb|upright=1.2|left| In chordates, the four above labelled common features appear at some point during development.<ref name=2018Clark>{{cite book | vauthors = Clark MA, Choi J, Douglas M | date = 2018 | chapter-url = https://opentextbc.ca/biology2eopenstax/chapter/chordates/#fig-ch29_01_02 | chapter = Chordates | title = Biology | edition = 2nd | publisher = OpenStax | isbn = 978-1-947172-95-1}}. 50px Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License]</ref>]]

{{multiple image
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 | image1    = Features of a tunicate larva.jpg
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 | caption1  = The larval stage of the tunicate possesses all of the features characteristic of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.<ref name=2018Clark />
 | width2    = 170
 | image2    = Features of a tunicate.jpg
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 | caption2  = In the adult stage of the tunicate the notochord, nerve cord, and tail disappear.<ref name=2018Clark />
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{{clear}}

==Vertebrate animals==
{{multiple image
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 | footer    = Skeletal structures showing the vertebral column and internal skeleton running from the head to the tail.
 | image1    = Skeleton of a bass.jpg
 | caption1  = [[Biology:Actinopterygii|Ray-finned fish]]
 | image2    = SpermWhaleLyd3.jpg
 | caption2  = Marine tetrapod ([[Biology:Sperm whale|sperm whale]])
}}
{{main|Biology:Marine vertebrate}}

[[Biology:Vertebrate|Vertebrate]]s (Latin for ''joints of the spine'') are a [[Biology:Subphylum|subphylum]] of [[Biology:Chordate|chordate]]s. They are chordates that have a [[Biology:Vertebral column|vertebral column]] (backbone). The vertebral column provides the central support structure for an [[Biology:Endoskeleton|internal skeleton]] which gives shape, support, and protection to the body and can provide a means of anchoring fins or limbs to the body. The vertebral column also serves to house and protect the [[Biology:Spinal cord|spinal cord]] that lies within the vertebral column.

Marine vertebrates can be divided into marine [[Biology:Fish|fish]] and marine [[Biology:Tetrapod|tetrapod]]s.

===Marine fish===

Fish typically breathe by extracting oxygen from water through [[Biology:Fish gill|gills]] and have a skin protected by [[Biology:Fish scale|scales]] and [[Biology:Mucus|mucous]]. They use [[Biology:Fish fin|fins]] to propel and stabilise themselves in the water, and usually have a two-chambered heart and eyes well adapted to seeing underwater, as well as other [[Biology:Sensory systems in fish|sensory systems]]. Over 33,000 species of fish have been described as of 2017,<ref>{{cite web|url=http://www.fishbase.org/home.htm|title=FishBase: A Global Information System on Fishes|publisher=[[Biology:FishBase|FishBase]]|access-date=17 January 2017}}</ref> of which about 20,000 are marine fish.<ref>{{cite web|title=How Many Fish In The Sea? Census Of Marine Life Launches First Report|url=https://www.sciencedaily.com/releases/2003/10/031024064333.htm|website=Science Daily|access-date=17 January 2017}}</ref>
{{clade
|label1= ←&nbsp; vertebrates &nbsp; &nbsp;
|sublabel1= <small>(extant)</small>
|1={{clade |style = float:right;
   |label1= &nbsp; &nbsp; jawless fish &nbsp;
   |1={{clade
      |1= [[Biology:Hagfish|hagfish]]&nbsp; [[File:Eptatretus polytrema.jpg|70 px]]
      |2={{clade
         |1= [[Biology:Lamprey|lamprey]]s [[File:Eudontomyzon mariae Dunai ingola.jpg|70 px]]
         }}
      }}
   |label2=jawed fish
   |sublabel3=? mya
   |2={{clade
      |label1=  
      |1={{clade
         |1= cartilaginous fish [[File:Ginsu shark (Cretoxyrhina mantellii).jpg|70 px]]
         }}
      |2={{clade
         |1= [[Biology:Euteleostomi|bony fish]] [[File:Atlantic cod.jpg|70px]] &nbsp;→&nbsp;
         }}
      }}
   }}
}}

====Jawless fish====
[[File:Tullimonstrum NT small.jpg|thumb|upright=1.2| The Tully monster, a strange looking extinct animal with eyes like a [[Biology:Hammerhead shark|hammerhead]] protruding from its back, may be an early jawless fish.]]

Early fish had no [[Biology:Fish jaw|jaws]]. Most went extinct when they were outcompeted by jawed fish (below), but two groups survived: [[Biology:Hagfish|hagfish]] and [[Biology:Lamprey|lamprey]]s. Hagfish form a class of about 20 species of [[Biology:Eel|eel]]-shaped, [[Biology:Mucus|slime]]-producing marine fish. They are the only known living animals that have a [[Biology:Skull|skull]] but no [[Biology:Vertebral column|vertebral column]]. [[Biology:Lamprey|Lamprey]]s form a superclass containing 38 known extant species of [[Biology:Agnatha|jawless fish]].<ref>{{cite journal | vauthors = Docker MF | date = November 2006 | url = https://journal.lib.uoguelph.ca/index.php/gir/article/view/61/128 | title = Bill Beamish's Contributions to Lamprey Research and Recent Advances in the Field | journal = Guelph Ichthyology Reviews | volume = 7 }}</ref> The adult lamprey is characterized by a toothed, funnel-like sucking mouth. Although they are well known for boring into the flesh of other fish to suck their blood,<ref>{{cite book | vauthors = Hardisty MW, Potter IC | veditors = Hardisty MW, Potter IC |year=1971 |title=The Biology of Lampreys |edition=1st |publisher=Academic Press |isbn=978-0-123-24801-5 |url-access=registration |url=https://archive.org/details/biologyoflamprey0001hard }}</ref> only 18 species of lampreys are actually parasitic.<ref>{{cite journal | vauthors = Gill HS, Renaud CB, Chapleau F, Mayden RL, Potter IC |title=Phylogeny of Living Parasitic Lampreys (Petromyzontiformes) Based on Morphological Data|journal=Copeia|volume=2003|issue=4|pages=687–703|doi=10.1643/IA02-085.1|year=2003|s2cid=85969032}}</ref> Together hagfish and lampreys are the sister group to vertebrates. Living hagfish remain similar to hagfish from around 300 million years ago.<ref>{{cite web|url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html|title=Myxini|publisher=University of California Museum of Paleontology|access-date=17 January 2017|archive-url=https://web.archive.org/web/20171215173214/http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html|archive-date=15 December 2017|url-status=dead}}</ref> The lampreys are a very ancient lineage of vertebrates, though their exact relationship to [[Biology:Hagfish|hagfish]]es and [[Biology:Gnathostomata|jawed vertebrate]]s is still a matter of dispute.<ref>{{cite journal | vauthors = Green SA, Bronner ME | title = The lamprey: a jawless vertebrate model system for examining origin of the neural crest and other vertebrate traits | journal = Differentiation; Research in Biological Diversity | volume = 87 | issue = 1–2 | pages = 44–51 | year = 2014 | pmid = 24560767 | pmc = 3995830 | doi = 10.1016/j.diff.2014.02.001 }}</ref> Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,<ref>{{cite journal | vauthors = Stock DW, Whitt GS | title = Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group | journal = Science | volume = 257 | issue = 5071 | pages = 787–9 | date = August 1992 | pmid = 1496398 | doi = 10.1126/science.1496398 | bibcode = 1992Sci...257..787S }}</ref> and so also are vertebrates in a [[Biology:Monophyletic|monophyletic]] sense. Others consider them a sister group of vertebrates in the common taxon of craniata.<ref>{{cite journal | vauthors = Nicholls H | title = Evolution: Mouth to mouth | journal = Nature | volume = 461 | issue = 7261 | pages = 164–6 | date = September 2009 | pmid = 19741680 | doi = 10.1038/461164a | doi-access = free }}</ref>

The Tully monster is an extinct genus of soft-bodied bilaterians that lived in tropical estuaries about 300 million years ago. Since 2016 there has been controversy over whether this animal was a vertebrate or an invertebrate.<ref name="McCoy2016">{{cite journal | vauthors = McCoy VE, Saupe EE, Lamsdell JC, Tarhan LG, McMahon S, Lidgard S, Mayer P, Whalen CD, Soriano C, Finney L, Vogt S, Clark EG, Anderson RP, Petermann H, Locatelli ER, Briggs DE | display-authors = 6 | title = The 'Tully monster' is a vertebrate | journal = Nature | volume = 532 | issue = 7600 | pages = 496–9 | date = April 2016 | pmid = 26982721 | doi = 10.1038/nature16992 | s2cid = 205247805 | bibcode = 2016Natur.532..496M | url = https://ora.ox.ac.uk/objects/uuid:a6b3721a-819f-4bb6-9e78-e219362fcae9 }}</ref><ref name="Sallan2017">{{cite journal |title=The 'Tully Monster' is not a vertebrate: characters, convergence and taphonomy in Palaeozoic problematic animals |journal=Palaeontology | vauthors = Sallan L, Giles S, Sansom RS, Clarke JT, Johanson Z, Sansom IJ, Janvier P |display-authors=6 |date=February 20, 2017 |doi=10.1111/pala.12282 |volume=60 |issue=2 |pages=149–157|bibcode=2017Palgy..60..149S |url=http://pure-oai.bham.ac.uk/ws/files/39492257/Tullimonstrum_Palaeo_Revision_FINAL.pdf |doi-access=free }}</ref> In 2020 researchers found "strong evidence" that the Tully monster was a vertebrate, and was a jawless fish in the lineage of the [[Biology:Lamprey|lamprey]],<ref>{{cite web | vauthors = Geggel L | url = https://www.livescience.com/ancient-tully-monster-vertebrate.html| title = Ancient 'Tully monster' was a vertebrate, not a spineless blob, study claims | work = Live Science | date = 4 May 2020 }}</ref><ref>{{cite journal | vauthors = McCoy VE, Wiemann J, Lamsdell JC, Whalen CD, Lidgard S, Mayer P, Petermann H, Briggs DE | display-authors = 6 | title = Chemical signatures of soft tissues distinguish between vertebrates and invertebrates from the Carboniferous Mazon Creek Lagerstätte of Illinois | journal = Geobiology | volume = 18 | issue = 5 | pages = 560–565 | date = September 2020 | pmid = 32347003 | doi = 10.1111/gbi.12397 | bibcode = 2020Gbio...18..560M | s2cid = 216646333 }}</ref> while in 2023 other researchers found 3D fossils scans did not support those conclusions.<ref>{{Cite journal |last1=Mikami |first1=Tomoyuki |last2=Ikeda |first2=Takafumi |last3=Muramiya |first3=Yusuke |last4=Hirasawa |first4=Tatsuya |last5=Iwasaki |first5=Wataru |date=2023 |editor-last=Cherns |editor-first=Lesley |title=Three‐dimensional anatomy of the Tully monster casts doubt on its presumed vertebrate affinities |url=https://onlinelibrary.wiley.com/doi/10.1111/pala.12646 |journal=Palaeontology |language=en |volume=66 |issue=2 |doi=10.1111/pala.12646 |s2cid=258198566 |issn=0031-0239}}</ref>

<gallery mode="packed" heights="120px" style="float:left;">
File:Eptatretus polytrema.jpg|[[Biology:Hagfish|Hagfish]] are the only known living animals with a [[Biology:Skull|skull]] but no [[Biology:Vertebral column|vertebral column]].
File:Eudontomyzon mariae Dunai ingola.jpg|[[Biology:Lamprey|Lamprey]]s are often parasitic and have a toothed, funnel-like sucking mouth.
File:Pteraspidomorphi.gif|The extinct [[Biology:Pteraspidomorphi|Pteraspidomorphi]], ancestral to jawed vertebrates
</gallery>

{{clear left}}

[[Biology:Pteraspidomorphi|Pteraspidomorphi]] is an extinct [[Biology:Class|class]] of early jawless fish ancestral to jawed vertebrates. The few characteristics they share with the latter are now considered as primitive for all vertebrates.

Around the start of the [[Earth:Devonian|Devonian]], fish started appearing with a deep remodelling of the vertebrate skull that resulted in a [[Biology:Fish jaw|jaw]].<ref>{{cite journal | vauthors = Kimmel CB, Miller CT, Keynes RJ | title = Neural crest patterning and the evolution of the jaw | journal = Journal of Anatomy | volume = 199 | issue = Pt 1-2 | pages = 105–20 | date = 2001 | pmid = 11523812 | pmc = 1594948 | doi = 10.1017/S0021878201008068 }}</ref>
All vertebrate jaws, including the human jaw, have evolved from these early fish jaws. The appearance of the early vertebrate jaw has been described as "perhaps the most profound and radical evolutionary step in vertebrate history".<ref name=Gai2012>{{cite journal | vauthors = Gai Z, Zhu M | date = 2012 | title = The origin of the vertebrate jaw: Intersection between developmental biology-based model and fossil evidence | journal = Chinese Science Bulletin | volume = 57 | issue = 30| pages = 3819–3828 | doi = 10.1007/s11434-012-5372-z | bibcode = 2012ChSBu..57.3819G | doi-access = free }}</ref><ref name="Maisey_2000">{{cite book | vauthors = Maisey JG | date = 2000 | url = {{google books|id=gAiAPwAACAAJ|plainurl=yes}} | title = Discovering Fossil Fishes | publisher = Westview Press | pages = 1–223 | isbn = 978-0-8133-3807-1 }}</ref> Jaws make it possible to capture, hold, and chew prey. [[Biology:Agnatha|Fish without jaws]] had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period.

====Cartilaginous fish====

Jawed fish fall into two main groups: fish with bony internal skeletons and fish with cartilaginous internal skeletons. Cartilaginous fish, such as [[Biology:Shark|shark]]s and [[Biology:Batoidea|rays]], have jaws and skeletons made of [[Biology:Cartilage|cartilage]] rather than [[Biology:Bone|bone]]. [[Biology:Megalodon|Megalodon]] is an extinct species of shark that lived about 28 to 1.5&nbsp;Ma. It may looked much like a stocky version of the [[Biology:Great white shark|great white shark]], but was much larger with estimated lengths reaching {{convert|20.3|m|ft}}.<ref name="GWB" /> Found in all oceans<ref name="AN">{{cite journal | vauthors = Pimiento C, Ehret DJ, Macfadden BJ, Hubbell G | title = Ancient nursery area for the extinct giant shark megalodon from the Miocene of Panama | journal = PLOS ONE | volume = 5 | issue = 5 | pages = e10552 | date = May 2010 | pmid = 20479893 | pmc = 2866656 | doi = 10.1371/journal.pone.0010552 | veditors = Stepanova A | bibcode = 2010PLoSO...510552P | doi-access = free }}</ref> it was one of the largest and most powerful predators in vertebrate history,<ref name="GWB">{{cite journal | vauthors = Wroe S, Huber DR, Lowry M, McHenry C, Moreno K, Clausen P, Ferrara TL, Cunningham E, Dean MN, Summers AP | display-authors = 6 |title=Three-dimensional computer analysis of white shark jaw mechanics: how hard can a great white bite? |url=http://www.bio-nica.info/Biblioteca/Wroe2008GreatWhiteSharkBiteForce.pdf |journal=Journal of Zoology |volume=276 |issue=4 |pages=336–342 |year=2008 |doi=10.1111/j.1469-7998.2008.00494.x}}</ref> and probably had a profound impact on marine life.<ref name="LV">{{cite journal | vauthors = Lambert O, Bianucci G, Post K, de Muizon C, Salas-Gismondi R, Urbina M, Reumer J | title = The giant bite of a new raptorial sperm whale from the Miocene epoch of Peru | journal = Nature | volume = 466 | issue = 7302 | pages = 105–8 | date = July 2010 | pmid = 20596020 | doi = 10.1038/nature09067 | s2cid = 4369352 | bibcode = 2010Natur.466..105L }}</ref> The [[Biology:Greenland shark|Greenland shark]] has the longest known lifespan of all vertebrates, about 400 years.<ref name=Nielsen2016>{{cite journal | vauthors = Nielsen J, Hedeholm RB, Heinemeier J, Bushnell PG, Christiansen JS, Olsen J, Ramsey CB, Brill RW, Simon M, Steffensen KF, Steffensen JF | display-authors = 6 | title = Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus) | journal = Science | volume = 353 | issue = 6300 | pages = 702–4 | date = August 2016 | pmid = 27516602 | doi = 10.1126/science.aaf1703 | s2cid = 206647043 | bibcode = 2016Sci...353..702N | hdl = 2022/26597 | hdl-access = free }}
*{{cite magazine |author=Enrico de Lazaro |date=12 August 2016 |title=Greenland Sharks are Longest-Lived Vertebrates on Earth, Marine Biologists Say |magazine=Science News |url=http://www.sci-news.com/biology/greenland-sharks-longest-lived-vertebrates-04099.html}}</ref> Some sharks such as the great white are partially warm blooded and give live birth. The [[Biology:Manta ray|manta ray]], largest ray in the world, has been targeted by fisheries and is now vulnerable.<ref name="birostris">{{Cite iucn | vauthors = Marshall A, Bennett MB, Kodja G, Hinojosa-Alvarez S, Galvan-Magana F, Harding M, Stevens G, Kashiwagi T | display-authors = 6 | title = ''Manta birostris'' | volume = 2011 | page = e.T198921A9108067 | date = 2011 | doi = 10.2305/IUCN.UK.2011-2.RLTS.T198921A9108067.en }}</ref>

<gallery mode="packed" caption="Cartilaginous fishes" heights="120px" style="float:left;">
File:Acanthodes BW spaced.jpg|Cartilaginous fishes may have evolved from spiny sharks.
File:Myliobatis aquila sasrája.jpg|[[Biology:Stingray|Stingray]]
File:MantaAlfrediLCouterier.jpg| [[Biology:Manta ray|Manta ray]], the largest ray
File:Pristis clavata 2.jpg|[[Biology:Sawfish|Sawfish]], rays with long [[Biology:Rostrum (anatomy)|rostrum]]s resembling a saw. All species are now endangered.<ref name=BBC2007>{{cite news | vauthors = Black R |date=11 June 2007|title=Sawfish protection acquires teeth|work=BBC News|url=http://news.bbc.co.uk/2/hi/science/nature/6740609.stm}}</ref>
</gallery>

{{clear}}

<gallery mode="packed" heights="120px" style="float:left;">
File:Megalodon_size_chart.png|The extinct [[Biology:Megalodon|megalodon]] resembled a giant [[Biology:Great white shark|great white shark]].
File:Somniosus microcephalus1.jpg|The [[Biology:Greenland shark|Greenland shark]] lives longer than any other vertebrate.
File:Rhincodon typus (recropped).jpg| The largest extant fish, the [[Biology:Whale shark|whale shark]], is now a [[Biology:Vulnerable species|vulnerable species]].
</gallery>

{{clear}}

====Bony fish====
[[File:Guiyu BW.jpg|thumb|''Guiyu oneiros'', the earliest-known bony fish lived during the Late Silurian 419 million years ago.]]
[[File:Coelacanth-bgiu.png|thumb|Lobe fins are bedded into the body by bony stalks. They evolved into the legs of the first tetrapod land vertebrates.]]
[[File:Carassius wild golden fish 2013 G1 (2).jpg|thumb|Ray fins have spines (rays) which can be erected to stiffen the fin for better control of swimming performance.]]


Bony fish have jaws and skeletons made of [[Biology:Bone|bone]] rather than [[Biology:Cartilage|cartilage]]. Bony fish also have hard, bony plates called [[Biology:Operculum (fish)|operculum]] which help them respire and protect their gills, and they often possess a [[Biology:Swim bladder|swim bladder]] which they use for better control of their buoyancy. Bony fish can be further divided into those with lobe fins and those with ray fins. The approximate dates in the phylogenetic tree are from Near et al., 2012<ref name=PNAS>{{cite journal | vauthors = Near TJ, Eytan RI, Dornburg A, Kuhn KL, Moore JA, Davis MP, Wainwright PC, Friedman M, Smith WL | display-authors = 6 | title = Resolution of ray-finned fish phylogeny and timing of diversification | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 34 | pages = 13698–703 | date = August 2012 | pmid = 22869754 | pmc = 3427055 | doi = 10.1073/pnas.1206625109 | bibcode = 2012PNAS..10913698N | doi-access = free }}</ref> and Zhu et al., 2009.<ref name="Zhu2009" />

{{clade
|label1= ←&nbsp; [[Biology:Euteleostomi|bony fish]] &nbsp;
|sublabel1= <small>(extant)</small>
|1={{clade
   |label1= &nbsp; [[Biology:Sarcopterygii|{{nowrap|lobe fins}}]] &nbsp;
   |sublabel1= <small>419 mya</small>
   |1={{clade
      |1= &nbsp; [[Biology:Coelacanth|coelacanth]]s &nbsp; [[File:Coelacanth-bgiu.png|70 px]]
      |2={{clade
      |1= &nbsp; [[Biology:Lungfish|lungfish]] &nbsp; [[File:Barramunda.jpg|70 px]]
      |2={{clade
         |1= &nbsp; ''[[Biology:Tetrapod|tetrapod]]s'' &nbsp;→&nbsp;
         }}
      }}
     }}
   |label2= &nbsp; [[Biology:Actinopterygii|{{nowrap|ray fins}}]] &nbsp;
   |sublabel2= <small>400 mya</small>
   |2={{clade
      |label1= &nbsp; chondrosteans &nbsp;
      |1= &nbsp; [[File:Sturgeon2.jpg|70 px]] &nbsp; <small>(sturgeon, paddlefish, bichir, [[Biology:Reedfish|reedfish]])</small>
      |label2= &nbsp; [[Biology:Neopterygii|neopterygians]] &nbsp;
      |sublabel2= <small>360 mya</small>
      |2={{clade
         |1= &nbsp; [[File:Amia calva1.jpg|70 px]] &nbsp; <small>(bowfin, [[Biology:Gar|gar]]s)</small>
         |label1   = &nbsp; [[Biology:Holostei|holosteans]] &nbsp;
         |sublabel1= <small>275 mya</small>
         |2= &nbsp; <small>all remaining fish (about 14,000 marine species)</small>
         |label2= &nbsp; [[Biology:Teleost|teleost]]s &nbsp;
         |sublabel2= <small>310 mya</small>
         }}
      }}
   }}
}}

Lobe fins have the form of fleshy lobes supported by bony stalks which extend from the body.<ref>Clack, J. A. (2002) ''Gaining Ground''. Indiana University</ref> ''[[Biology:Guiyu oneiros|Guiyu oneiros]]'', the earliest-known bony fish, lived during the Late [[Earth:Silurian|Silurian]] 419 million years ago. It has the combination of both [[Biology:Actinopterygii|ray-finned]] and lobe-finned features, although analysis of the totality of its features place it closer to lobe-finned fish.<ref name="Zhu2009">{{cite journal | vauthors = Zhu M, Zhao W, Jia L, Lu J, Qiao T, Qu Q | title = The oldest articulated osteichthyan reveals mosaic gnathostome characters | journal = Nature | volume = 458 | issue = 7237 | pages = 469–74 | date = March 2009 | pmid = 19325627 | doi = 10.1038/nature07855 | s2cid = 669711 | bibcode = 2009Natur.458..469Z }}</ref> Lobe fins evolved into the legs of the first tetrapod land vertebrates, so by extension an early ancestor of humans was a lobe-finned fish. Apart from the coelacanths and the lungfishes, lobe-finned fishes are now extinct.

The remaining bony fish have ray fins. These are made of webs of skin supported by bony or horny spines (rays) which can be erected to control the fin stiffness.

* The main distinguishing feature of the chondrosteans ([[Biology:Sturgeon|sturgeon]], [[Biology:Paddlefish|paddlefish]], [[Biology:Bichir|bichir]] and [[Biology:Reedfish|reedfish]]) is the cartilaginous nature of their skeletons. The ancestors of the chondrosteans are thought to be bony fish, but the characteristic of an ossified skeleton was lost in later evolutionary development, resulting in a lightening of the frame.<ref name="chondro">{{cite web |url=http://www.palaeos.com/Vertebrates/Units/090Teleostomi/090.300.html |title=Chondrosteans: Sturgeon Relatives |publisher=paleos.com |url-status=dead |archive-url=https://web.archive.org/web/20101225152809/http://www.palaeos.com/Vertebrates/Units/090Teleostomi/090.300.html |archive-date=2010-12-25 }}</ref>
* [[Biology:Neopterygii|Neopterygians]] (from Greek for ''new fins'') appeared sometime in the Late Permian, before dinosaurs. They were a very successful group of fish, because they could move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient.<ref>{{cite journal | vauthors = López-Arbarello A | title = Phylogenetic interrelationships of ginglymodian fishes (Actinopterygii: Neopterygii) | journal = PLOS ONE | volume = 7 | issue = 7 | pages = e39370 | year = 2012 | pmid = 22808031 | pmc = 3394768 | doi = 10.1371/journal.pone.0039370 | bibcode = 2012PLoSO...739370L | doi-access = free }}</ref>

====Teleosts====
[[File:Barb gonio 080525 9610 ltn Cf.jpg|thumb| Teleosts have homocercal tails.]]
{{main|Biology:Teleost}}

About 96% of all modern fish species are teleosts,<ref>{{cite book| vauthors = Berra TM |title=Freshwater Fish Distribution|url=https://books.google.com/books?id=K-1Ygw6XwFQC&pg=PA55|year=2008|publisher=[[Organization:University of Chicago Press|University of Chicago Press]]|isbn=978-0-226-04443-9|page=55}}</ref> of which about 14,000 are marine species.<ref>{{cite journal | vauthors = Lackmann AR, Andrews AH, Butler MG, Bielak-Lackmann ES, Clark ME | title = Bigmouth Buffalo ''Ictiobus cyprinellus'' sets freshwater teleost record as improved age analysis reveals centenarian longevity | language = En | journal = Communications Biology | volume = 2 | issue = 1 | pages = 197 | date = 2019-05-23 | pmid = 31149641 | pmc = 6533251 | doi = 10.1038/s42003-019-0452-0 }}</ref> Teleosts can be distinguished from other bony fish by their possession of a homocercal tail, a tail where the upper half mirrors the lower half.<ref name=Benton/> Another difference lies in their jaw bones – teleosts have modifications in the jaw musculature which make it possible for them to protrude their jaws. This enables them to [[Earth:Predation|grab prey]] and draw it into their mouth.<ref name=Benton>{{cite book| vauthors = Benton M |title=Vertebrate Palaeontology|chapter-url=https://books.google.com/books?id=VThUUUtM8A4C&pg=PA175|year=2005|publisher={{wipe|John Wiley & Sons}}|edition=3rd|chapter=The Evolution of Fishes After the Devonian|isbn=978-1-4051-4449-0|pages=175–84}}</ref> In general, teleosts tend to be quicker and more flexible than more basal bony fishes. Their skeletal structure has evolved towards greater lightness. While teleost bones are well [[Biology:Calcification|calcified]], they are constructed from a scaffolding of struts, rather than the dense cancellous bones of [[Biology:Holostei|holostean]] fish.<ref>{{cite book| vauthors = Bone Q, Moore R |year=2008|title=Biology of Fishes|publisher=Garland Science|page=29|isbn=978-0-415-37562-7}}</ref>

Teleosts are found in almost all [[Earth:Marine habitat|marine habitat]]s.<ref name=Dorit>{{cite book|title=Zoology| vauthors = Dorit R, Walker WF, Barnes RD |year=1991|publisher=Saunders College Publishing|isbn=978-0-03-030504-7|pages=[https://archive.org/details/zoology0000dori/page/67 67–69]|url=https://archive.org/details/zoology0000dori/page/67}}</ref> They have enormous [[Biology:Diversity of fish|diversity]], and range in size from adult gobies 8mm long <ref>{{cite web|url=https://scripps.ucsd.edu/news/2645|title=Scientists Describe the World's Smallest, Lightest Fish|date=20 July 2004|publisher=[[Organization:Scripps Institution of Oceanography|Scripps Institution of Oceanography]]|access-date=9 April 2016|archive-date=5 March 2016|archive-url=https://web.archive.org/web/20160305095456/https://scripps.ucsd.edu/news/2645|url-status=dead}}</ref> to ocean sunfish weighing over 2,000&nbsp;kg.<ref>{{cite news|title=World's Heaviest Bony Fish Discovered? | vauthors = Roach J |url= http://news.nationalgeographic.com/news/2003/05/0513_030513_sunfish.html |archive-url= https://web.archive.org/web/20030517062722/http://news.nationalgeographic.com/news/2003/05/0513_030513_sunfish.html |url-status= dead |archive-date= 17 May 2003 |newspaper=National Geographic News|date=13 May 2003|access-date=9 January 2016}}</ref> The following images show something of the diversity in the shape and colour of modern marine teleosts...

<gallery mode="packed" heights="100px" style="float:left;">
File:Istiophorus platypterus.jpg| [[Biology:Sailfish|Sailfish]]
File:Coryphaena hippurus.png| [[Biology:Mahi-mahi|Mahi-mahi]]
File:Anguilla japonica 1856.jpg| [[Biology:Eel|Eel]]
File:Seepferdlein.jpg| [[Biology:Seahorse|Seahorse]]
</gallery>
<gallery mode="packed" heights="100px" style="float:left;">
File:Sunfish.jpg| Ocean sunfish
File:Humpback anglerfish.png| [[Biology:Anglerfish|Anglerfish]]
File:Tetraodon-hispidus.jpg| Pufferfish
File:Särkänniemi - fish.png| [[Biology:Clown triggerfish|Clown triggerfish]]
File:Synchiropus splendidus 2 Luc Viatour cropped.png| Mandarin dragonet
</gallery>

{{clear}}

Nearly half of all extant vertebrate species are teleosts.<ref name=IUCN1014>{{cite web | publisher = The World Conservation Union | work = IUCN Red List of Threatened Species | date = Autumn 2014 | title = Summary Statistics for Globally Threatened Species | url = http://cmsdocs.s3.amazonaws.com/summarystats/2014_3_Summary_Stats_Page_Documents/2014_3_RL_Stats_Table_1.pdf | quote = Table 1: Numbers of threatened species by major groups of organisms (1996–2014) }}</ref>

=== Marine tetrapods ===
[[File:Tiktaalik BW flopped.jpg|thumb|''[[Biology:Tiktaalik|Tiktaalik]]'', an extinct lobe-finned fish, developed limb-like fins that could take it onto land.]]

A [[Biology:Tetrapod|tetrapod]] (Greek for ''four feet'') is a vertebrate with limbs (feet). Tetrapods evolved from ancient [[Biology:Lobe-finned fish|lobe-finned fish]]es about 400 million years ago during the Devonian Period when their earliest ancestors emerged from the sea and adapted to living on land.<ref name="NarkiewiczNarkiewicz2015">{{cite journal| vauthors = Narkiewicz K, Narkiewicz M |title=The age of the oldest tetrapod tracks from Zachełmie, Poland|journal=Lethaia|volume=48|issue=1|date=January 2015|pages=10–12|issn=0024-1164|doi=10.1111/let.12083}}</ref> This change from a body plan for breathing and navigating in gravity-neutral water to a body plan with mechanisms enabling the animal to breath in air without dehydrating and move on land is one of the most profound evolutionary changes known.<ref name=Gordon&Long>{{cite journal | vauthors = Long JA, Gordon MS | title = The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition | journal = Physiological and Biochemical Zoology | volume = 77 | issue = 5 | pages = 700–19 | date = Sep–Oct 2004 | pmid = 15547790 | doi = 10.1086/425183 | s2cid = 1260442 | url = http://usf.usfca.edu/fac_staff/dever/tetrapod_review.pdf }}</ref><ref>{{cite book | vauthors = Shubin N |title=Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body |publisher=Pantheon Books |location=New York |year=2008 |isbn=978-0-375-42447-2 |url=https://archive.org/details/yourinnerfishjou00shub_0 }}</ref> Tetrapods can be divided into four classes: [[Biology:Amphibian|amphibian]]s, [[Biology:Reptile|reptile]]s, [[Biology:Bird|bird]]s and [[Biology:Mammal|mammal]]s.
{{clade
|label1=←&nbsp; [[Biology:Tetrapod|tetrapod]]s &nbsp; &nbsp;
|1={{clade
   |1=[[Biology:Amphibian|amphibian]]s <small>(there are no true marine amphibians)</small>
   |label2=[[Biology:Amniote|amniote]]s
   |2={{clade
|1=[[Biology:Mammal|mammal]]s [[File:Dolphin 2.png|40 px]]
      |label2=[[Biology:Sauropsida|sauropsids]]
      |2={{clade
         |1= lepidosaurs <small>[[File:Micrurus_fulviusHolbrookV3P10AA.jpg|20px]]</small> <small>(lizards, including snakes)</small>
         |2= [[Biology:Archosaur|archosaur]]s <small>[[File:Chinstrap_Penguin_white_background.jpg|20px]]</small> <small>(turtles, crocodiles & birds)</small>
         }}
     }}
  }}
}}

Marine tetrapods are tetrapods that returned from land back to the sea again. The first returns to the ocean may have occurred as early as the Carboniferous Period<ref>{{cite book | vauthors = Laurin M |title=How Vertebrates Left the Water |publisher=University of California Press |location=Berkeley, California, USA. |year=2010 |isbn=978-0-520-26647-6 |author-link=Michel Laurin }}</ref> whereas other returns occurred as recently as the [[Earth:Cenozoic|Cenozoic]], as in cetaceans, [[Biology:Pinniped|pinniped]]s,<ref>{{cite journal | vauthors = Canoville A, Laurin M  |year=2010 |title=Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on paleobiological inferences |journal=Biological Journal of the Linnean Society |volume=100 |pages=384–406 |doi=10.1111/j.1095-8312.2010.01431.x |issue=2 |doi-access=free }}</ref> and several [[Biology:Lissamphibia|modern amphibian]]s.<ref>{{cite journal | vauthors = Laurin M, Canoville A, Quilhac A | title = Use of paleontological and molecular data in supertrees for comparative studies: the example of lissamphibian femoral microanatomy | journal = Journal of Anatomy | volume = 215 | issue = 2 | pages = 110–23 | date = August 2009 | pmid = 19508493 | pmc = 2740958 | doi = 10.1111/j.1469-7580.2009.01104.x | author-link = Michel Laurin }}</ref> [[Biology:Amphibian|Amphibian]]s (from Greek for ''both kinds of life'') live part of their life in water and part on land. They mostly require fresh water to reproduce. A few inhabit brackish water, but there are no true marine amphibians.<ref>{{cite journal | vauthors = Hopkins GR, Brodie Jr ED | year = 2015 | title = Occurrence of Amphibians in Saline Habitats: A Review and Evolutionary Perspective | journal = Herpetological Monographs | volume = 29 | issue = 1| pages = 1–27 | doi = 10.1655/HERPMONOGRAPHS-D-14-00006 |s2cid=83659304 }}</ref> There have been reports, however, of amphibians invading marine waters, such as a Black Sea invasion by the natural hybrid ''Pelophylax esculentus'' reported in 2010.<ref>{{cite journal |vauthors=Natchev N, Tzankov N, Geme R |year=2011 |title=Green frog invasion in the Black Sea: habitat ecology of the ''Pelophylax esculentus'' complex (Anura, Amphibia) population in the region of Shablenska Tuzla lagoon in Bulgaria |journal=Herpetology Notes |volume=4 |pages=347–351 |url=http://www.herpetologynotes.seh-herpetology.org/Volume4_PDFs/Natchev_et_al_Herpetology_Notes_Volume4_pages347-351.pdf |access-date=11 August 2016 |archive-date=24 September 2015 |archive-url=https://web.archive.org/web/20150924025946/http://www.herpetologynotes.seh-herpetology.org/Volume4_PDFs/Natchev_et_al_Herpetology_Notes_Volume4_pages347-351.pdf |url-status=dead }}</ref>

====Reptiles====

Reptiles (Late Latin for ''creeping'' or ''crawling'') do not have an aquatic larval stage, and in this way are unlike amphibians. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades<ref name=S12>{{cite journal | vauthors = Sander PM | title = Paleontology. Reproduction in early amniotes | journal = Science | volume = 337 | issue = 6096 | pages = 806–8 | date = August 2012 | pmid = 22904001 | doi = 10.1126/science.1224301 | s2cid = 7041966 | bibcode = 2012Sci...337..806S }}</ref> &mdash; the fetus develops within the mother, contained in a [[Biology:Placenta|placenta]] rather than an [[Biology:Eggshell|eggshell]]. As [[Biology:Amniote|amniote]]s, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their [[Biology:Fetus|fetus]]es through various forms of placenta analogous to those of [[Biology:Mammal|mammal]]s, with some providing initial care for their hatchlings.

Some reptiles are more closely related to [[Biology:Bird|bird]]s than other reptiles, and many scientists prefer to make Reptilia a monophyletic group which includes the birds.<ref name=modestoanderson2004>{{cite journal | vauthors = Modesto SP, Anderson JS | title = The phylogenetic definition of reptilia | journal = Systematic Biology | volume = 53 | issue = 5 | pages = 815–21 | date = October 2004 | pmid = 15545258 | doi = 10.1080/10635150490503026 | doi-access = free }}{{open access}}</ref><ref name=GKT88>{{cite book | vauthors = Gauthier JA, Kluge AG, Rowe T |year=1988 |chapter=The early evolution of the Amniota |title=The Phylogeny and Classification of the Tetrapods |volume=1 | veditors = Benton MJ |publisher=Clarendon Press |location=Oxford |isbn=978-0-19-857705-8 |pages=103–155}}</ref><ref name="Laurin 95">{{Cite journal | vauthors = Laurin M, Reisz RR |year=1995 |title=A reevaluation of early amniote phylogeny |journal=Zoological Journal of the Linnean Society |volume=113 |issue=2 |pages=165–223 |doi=10.1111/j.1096-3642.1995.tb00932.x |url=http://www.iucn-tftsg.org/wp-content/uploads/file/Articles/Laurin_and_Reisz_1995.pdf |access-date=14 August 2016 |archive-url=https://web.archive.org/web/20190608150549/http://www.iucn-tftsg.org/wp-content/uploads/file/Articles/Laurin_and_Reisz_1995.pdf |archive-date=8 June 2019 |url-status=dead }}{{open access}}</ref><ref name=MSP99>{{cite journal | vauthors = Modesto SP |year=1999 |title=Observations of the structure of the Early Permian reptile ''Stereosternum tumidum'' Cope |journal=Palaeontologia Africana |volume=35 |pages=7–19 }}</ref> Extant non-avian reptiles which inhabit or frequent the sea include [[Biology:Sea turtle|sea turtle]]s, sea snakes, [[Biology:Terrapin|terrapin]]s, the [[Biology:Marine iguana|marine iguana]], and the [[Biology:Saltwater crocodile|saltwater crocodile]]. Currently, of the approximately 12,000 extant [[Biology:Reptile|reptile]] species and sub-species, only about 100 of are classed as marine reptiles.<ref>{{cite journal | vauthors = Rasmussen AR, Murphy JC, Ompi M, Gibbons JW, Uetz P | title = Marine reptiles | journal = PLOS ONE | volume = 6 | issue = 11 | pages = e27373 | date = 2011-11-08 | pmid = 22087300 | pmc = 3210815 | doi = 10.1371/journal.pone.0027373 | bibcode = 2011PLoSO...627373R | doi-access = free }}</ref>

Except for some sea snakes, most extant marine reptiles are [[Biology:Oviparity|oviparous]] and need to return to land to lay their eggs. Apart from sea turtles, the species usually spend most of their lives on or near land rather than in the ocean. Sea snakes generally prefer shallow waters nearby land, around islands, especially waters that are somewhat sheltered, as well as near estuaries.<ref name="Sti74">{{cite book | vauthors = Stidworthy J | date = 1974 | title = Snakes of the World | publisher = Grosset & Dunlap Inc. | pages = 160 | isbn = 978-0-448-11856-7}}</ref><ref name="RAS-FAO">{{cite web | url = http://www.fao.org/3/y0870e/y0870e65.pdf | title = Sea snakes | publisher = Food and Agriculture Organization of the United Nations | access-date = 22 August 2020 }}</ref> Unlike land snakes, sea snakes have evolved flattened tails which help them swim.<ref>{{cite journal | vauthors = Rasmussen AR, Murphy JC, Ompi M, Gibbons JW, Uetz P | title = Marine reptiles | journal = PLOS ONE | volume = 6 | issue = 11 | pages = e27373 | year = 2011 | pmid = 22087300 | pmc = 3210815 | doi = 10.1371/journal.pone.0027373 | bibcode = 2011PLoSO...627373R | doi-access = free }}</ref>

<gallery mode="packed" heights="120px" style="float:left;">
File:Marine-Iguana-Espanola.jpg|[[Biology:Marine iguana|Marine iguana]]
File:Leatherback sea turtle Tinglar, USVI (5839996547).jpg|[[Biology:Leatherback sea turtle|Leatherback sea turtle]]
File:SaltwaterCrocodile('Maximo').jpg|[[Biology:Saltwater crocodile|Saltwater crocodile]]
File:Micrurus fulviusHolbrookV3P10AA.jpg|Marine snakes have flattened tails.
File:Ichthyosaurus BW.jpg|The ancient ''Ichthyosaurus communis'' independently evolved flippers similar to dolphins.
</gallery>

{{clear}}

Some [[Biology:Extinction|extinct]] marine reptiles, such as [[Biology:Ichthyosaur|ichthyosaur]]s, evolved to be viviparous and had no requirement to return to land. [[Biology:Ichthyosaur|Ichthyosaur]]s resembled dolphins. They first appeared about 245 million years ago and disappeared about 90 million years ago. The terrestrial ancestor of the ichthyosaur had no features already on its back or tail that might have helped along the evolutionary process. Yet the ichthyosaur developed a dorsal and tail fin which improved its ability to swim.<ref name="Martill, 1993">Martill D.M. (1993). "Soupy Substrates: A Medium for the Exceptional Preservation of Ichthyosaurs of the Posidonia Shale (Lower Jurassic) of Germany". ''Kaupia - Darmstädter Beiträge zur Naturgeschichte'', '''2''' : 77-97.</ref> The biologist [[Biography:Stephen Jay Gould|Stephen Jay Gould]] said the ichthyosaur was his favourite example of [[Biology:Convergent evolution|convergent evolution]].<ref>{{cite book | vauthors = Gould SJ | date = 1993 | chapter-url = https://archive.org/details/eightlittlepiggi00goul | chapter = Bent Out of Shape | title = Eight Little Piggies: Reflections in Natural History | publisher = Norton | pages = 179–94 | isbn = 978-0-393-31139-6 }}</ref> The earliest marine reptiles arose in the [[Earth:Permian|Permian]]. During the [[Earth:Mesozoic|Mesozoic]] many groups of reptiles became adapted to life in the seas, including [[Biology:Ichthyosaur|ichthyosaur]]s, [[Biology:Plesiosaur|plesiosaur]]s, [[Biology:Mosasaur|mosasaur]]s, [[Biology:Nothosaur|nothosaur]]s, [[Biology:Placodont|placodont]]s, [[Biology:Sea turtle|sea turtle]]s, [[Biology:Thalattosaur|thalattosaur]]s and [[Biology:Thalattosuchia|thalattosuchia]]ns. Marine reptiles were less numerous after mass extinction at the end of the [[Earth:Cretaceous|Cretaceous]].

====Birds====
[[File:Chesapeake Waterbird Food Web.jpg|thumb|Waterbird food web in Chesapeake Bay]]

Marine birds are [[Biology:Adaptation|adapted]] to life within the marine environment. They are often called ''seabirds''. While marine birds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking [[Biology:Convergent evolution|convergent evolution]], as the same environmental problems and feeding [[Biology:Ecological niche|niches]] have resulted in similar adaptations. Examples include [[Biology:Albatross|albatross]], [[Biology:Penguin|penguin]]s, gannets, and [[Biology:Auk|auk]]s.

In general, marine birds live longer, [[Biology:Mating|breed]] later and have fewer young than terrestrial birds do, but they invest a great deal of time in their young. Most [[Biology:Species|species]] nest in [[Biology:Bird colony|colonies]], which can vary in size from a few dozen birds to millions. Many species are famous for undertaking long annual [[Biology:Bird migration|migrations]], crossing the [[Astronomy:Equator|equator]] or circumnavigating the Earth in some cases. They feed both at the ocean's surface and below it, and even feed on each other. Marine birds can be highly pelagic, coastal, or in some cases spend a part of the year away from the sea entirely. Some marine birds plummet from heights, plunging through the water leaving vapour-like trails, similar to that of fighter planes.<ref name="baitball">{{cite web|url=http://www.lifeinthefastlane.ca/sardine-run-shark-feeding-frenzy-phenomenon-in-africa/miraculous-things |title=Sardine Run Shark Feeding Frenzy Phenomenon in Africa |url-status=dead |archive-url=https://web.archive.org/web/20081202092952/http://www.lifeinthefastlane.ca/sardine-run-shark-feeding-frenzy-phenomenon-in-africa/miraculous-things |archive-date=2 December 2008 }}</ref> Gannets plunge into the water at up to 100 kilometres per hour (60&nbsp;mph). They have air sacs under their skin in their face and chest which act like bubble-wrap, cushioning the impact with the water.

<gallery mode="packed" heights="130px" style="float:left;">
File:Goéland argenté - Julien Salmon.jpg|[[Biology:European herring gull|European herring gull]] attack herring schools from above.
File:Pygoscelis papua -Nagasaki Penguin Aquarium -swimming underwater-8a.jpg|[[Biology:Gentoo penguin|Gentoo penguin]] swimming underwater
File:Royal Albatross - east of the Tasman Peninsula, Tasmania.jpg|Albatrosses range over huge areas of ocean and some even circle the globe.
File:Morus serrator.jpg|Gannets "divebomb" at high speed
</gallery>

{{clear}}

The first marine birds evolved in the [[Earth:Cretaceous|Cretaceous]] [[Earth:Geological period|period]], and modern marine bird families emerged in the [[Earth:Paleogene|Paleogene]].

====Mammals====
[[File:sea otter with sea urchin.jpg|thumb|right| [[Biology:Sea otter|Sea otter]], a classic keystone species which controls sea urchin numbers]]
{{main| Biology:Marine mammal}}

[[Biology:Mammal|Mammal]]s (from Latin for ''breast'') are characterised by the presence of [[Biology:Mammary gland|mammary gland]]s which in [[Biology:Female#Mammalian female|females]] produce milk for feeding (nursing) their young. There are about 130 living and recently extinct marine [[Biology:Mammal|mammal]] species such as [[Biology:Pinniped|seals]], [[Biology:Dolphin|dolphin]]s, [[Biology:Whale|whale]]s, [[Biology:Manatee|manatee]]s, [[Biology:Sea otter|sea otter]]s and [[Biology:Polar bear|polar bear]]s.<ref>{{cite web|url=https://www.marinemammalscience.org/species-information/list-of-marine-mammal-species-subspecies |title=The Society for Marine Mammalogy's Taxonomy Committee List of Species and subspecies |publisher=Society for Marine Mammalogy |date=October 2015 |access-date=23 November 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150106152733/https://www.marinemammalscience.org/species-information/list-of-marine-mammal-species-subspecies/ |archive-date=6 January 2015 }}</ref> They do not represent a distinct taxon or systematic grouping, but are instead unified by their reliance on the marine environment for feeding. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Seals and sea-lions are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as [[Biology:Mating|mating]], [[Biology:Breeding in the wild|breeding]] and molting. In contrast, both otters and the polar bear are much less adapted to aquatic living. Their diet varies considerably as well: some may eat [[Earth:Zooplankton|zooplankton]]; others may eat fish, squid, shellfish, and sea-grass; and a few may eat other mammals.

In a process of [[Biology:Convergent evolution|convergent evolution]], marine mammals, especially cetaceans such as dolphins and whales, redeveloped their [[Biology:Body plan|body plan]] to parallel the streamlined fusiform body plan of [[Biology:Pelagic fish|pelagic fish]]. Front legs became [[Biology:Flipper (anatomy)|flipper]]s and back legs disappeared, a dorsal fin reappeared and the tail morphed into a powerful horizontal fluke. This body plan is an adaptation to being an active predator in a high [[Physics:Drag|drag]] environment. A parallel convergence occurred with the now extinct marine reptile [[Biology:Ichthyosaur|ichthyosaur]].<ref>{{cite book | vauthors = Romer AS, Parsons TS | date =  1986 | title = The Vertebrate Body | page = 96 | publisher = Sanders College Publishing | isbn = 978-0-03-058446-6 }}</ref>

<gallery mode="packed" heights="120px" style="float:left;">
File:Bluewhale2 noaa.jpg|Endangered [[Biology:Blue whale|blue whale]], the largest living animal<ref>{{cite web|url=http://wwf.panda.org/what_we_do/endangered_species/cetaceans/about/blue_whale/|title=Blue whale|publisher=World Wide Fund For Nature|access-date=15 August 2016}}</ref>
File:Tursiops truncatus 01.jpg| The [[Biology:Bottlenose dolphin|bottlenose dolphin]] has the highest [[Biology:Encephalization|encephalization]] of any animal after humans<ref>{{cite journal| vauthors = Marino L |title=Cetacean Brain Evolution: Multiplication Generates Complexity|journal=International Society for Comparative Psychology|issue=17|pages=1–16 |year=2004 |url=http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |access-date=15 August 2016|archive-url= https://web.archive.org/web/20180916132752/http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |archive-date=16 September 2018|url-status=dead}}</ref>
File:Beluga03.jpg|[[Biology:Beluga whale|Beluga whale]]
File:Dugong Marsa Alam.jpg|[[Biology:Dugong|Dugong]] grazing on [[Biology:Seagrass|seagrass]]
File:Noaa-walrus22.jpg|[[Biology:Walrus|Walrus]]
File:Polar Bear - Alaska (cropped).jpg|[[Biology:Polar bear|Polar bear]]
</gallery>

{{clear}}

==Primary producers==
[[File:Seawifs global biosphere.jpg|thumb|upright 1.6|Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.]]
{{main|Biology:Marine primary production}}

[[Earth:Primary producer|Primary producer]]s are the [[Biology:Autotroph|autotroph]] organisms that make their own food instead of eating other organisms. This means primary producers become the starting point in the food chain for [[Biology:Heterotroph|heterotroph]] organisms that do eat other organisms. Some marine primary producers are specialised bacteria and archaea which are [[Biology:Chemotroph|chemotroph]]s, making their own food by gathering around hydrothermal vents and cold seeps and using [[Earth:Chemosynthesis|chemosynthesis]]. However most marine [[Earth:Primary production|primary production]] comes from organisms which use [[Earth:Photosynthesis|photosynthesis]] on the carbon dioxide dissolved in the water. This process uses energy from sunlight to convert water and [[Chemistry:Carbon dioxide|carbon dioxide]]<ref name=Campbell2008>{{cite book | vauthors = Campbell NA, Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Jackson RB | title = Biology | url = https://archive.org/details/essentialbiology00camp_0 | url-access = registration | edition = 8 | year = 2008 | publisher = Pearson – Benjamin Cummings | location = San Francisco | isbn = 978-0-321-54325-7 }}</ref>{{rp|186–187}} into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells.<ref name=Campbell2008 />{{rp|1242}} Marine primary producers are important because they underpin almost all marine animal life by generating most of the [[Chemistry:Oxygen|oxygen]] and food that provide other organisms with the chemical energy they need to exist.

The principal marine primary producers are [[Biology:Cyanobacteria|cyanobacteria]], [[Biology:Algae|algae]] and marine plants. The [[Chemistry:Oxygen|oxygen]] released as a by-product of photosynthesis is needed by [[Biology:Anaerobic organism|nearly]] all living things to carry out [[Biology:Cellular respiration|cellular respiration]]. In addition, primary producers are influential in the global [[Earth:Carbon cycle|carbon]] and [[Physics:Water cycle|water]] cycles. They stabilize coastal areas and can provide habitats for marine animals. The term [[Biology:Division|division]] has been traditionally used instead of [[Biology:Phylum|phylum]] when discussing primary producers, but the International Code of Nomenclature for algae, fungi, and plants now accepts both terms as equivalents.<ref name="ICN">{{Cite book |year=2012 | veditors = Barrie FR, Buck WR, Demoulin V, Greuter W, Hawksworth DL, Herendeen PS, Knapp S, Marhold K, Prado J, Prud'homme Van Reine WF | display-editors = 6 |title=International Code of Nomenclature for algae, fungi, and plants (Melbourne Code), Adopted by the Eighteenth International Botanical Congress Melbourne, Australia, July 2011 |edition=electronic |publisher=International Association for Plant Taxonomy |url=http://www.iapt-taxon.org/nomen/main.php?page=art3 |access-date=2017-05-14 }}</ref>

===Cyanobacteria===
{{multiple image
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 | image1    = Cyanobacteria guerrero negro.jpg
 | caption1  = Cyanobacteria from a [[Biology:Microbial mat|microbial mat]]. Cyanobacteria were the first organisms to release oxygen via photosynthesis.
 | image2    = Prochlorococcus marinus (cropped).jpg
 | caption2  = The cyanobacterium genus ''[[Biology:Prochlorococcus|Prochlorococcus]]'' is a major contributor to atmospheric oxygen.
}}

[[Biology:Cyanobacteria|Cyanobacteria]] were the first organisms to evolve an ability to turn sunlight into chemical energy. They form a phylum (division) of bacteria which range from unicellular to [[Biology:Protein filament|filamentous]] and include [[Biology:Colony#Microbial colonies|colonial species]]. They are found almost everywhere on earth: in damp soil, in both freshwater and marine environments, and even on Antarctic rocks.<ref name="WalshSmith2011">{{cite book| veditors = Walsh PJ, Smith S, Fleming L, Solo-Gabriele H, Gerwick WH |title=Oceans and Human Health: Risks and Remedies from the Seas|chapter-url={{google books |plainurl=y |id=LMZPqW-PmFYC|page=271}}|date=2 September 2011|chapter=Cyanobacteria and cyanobacterial toxins|pages=271–296|publisher=Academic Press|isbn=978-0-08-087782-2}}</ref> In particular, some species occur as drifting cells floating in the ocean, and as such were amongst the first of the [[Earth:Phytoplankton|phytoplankton]].

The first primary producers that used photosynthesis were oceanic cyanobacteria about 2.3&nbsp;billion years ago.<ref>{{Cite web|url= http://www.astrobio.net/news-exclusive/the-rise-of-oxygen/|title= The Rise of Oxygen - Astrobiology Magazine|website= Astrobiology Magazine|date= 30 July 2003|language= en-US|access-date= 2016-04-06}}</ref><ref>{{cite journal| vauthors = Flannery DT, Walter RM |title= Archean tufted microbial mats and the Great Oxidation Event: new insights into an ancient problem|journal= Australian Journal of Earth Sciences|date= 2012|volume= 59|issue= 1|pages= 1–11|doi= 10.1080/08120099.2011.607849 |bibcode = 2012AuJES..59....1F |s2cid= 53618061}}</ref> The release of molecular [[Chemistry:Oxygen|oxygen]] by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this led to the near-extinction of [[Biology:Anaerobic organism|oxygen-intolerant organisms]], a dramatic change which redirected the evolution of the major animal and plant species.<ref name=astrobiology>{{cite web |url=http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-url=https://web.archive.org/web/20120329092237/http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-date=29 March 2012 |title=Understand the evolutionary mechanisms and environmental limits of life |access-date=13 July 2009 | vauthors = Rothschild L |date=September 2003 |url-status=dead |publisher=NASA}}</ref>

The tiny marine cyanobacterium ''[[Biology:Prochlorococcus|Prochlorococcus]]'', discovered in 1986, forms today part of the base of the ocean food chain and accounts for much of the photosynthesis of the open ocean<ref>{{cite journal | vauthors = Nadis S | title = The cells that rule the seas | journal = Scientific American | volume = 289 | issue = 6 | pages = 52–3 | date = December 2003 | pmid = 14631732 | doi = 10.1038/scientificamerican1203-52 | url = http://guowei.ccps.tp.edu.tw/nc/UploadDocument/255_02%20The%20Cells%20that%20Rules%20the%20Sea.pdf | access-date = 2 June 2019 | url-status = dead | bibcode = 2003SciAm.289f..52N | archive-url = https://web.archive.org/web/20140419222251/http://guowei.ccps.tp.edu.tw/nc/UploadDocument/255_02%20The%20Cells%20that%20Rules%20the%20Sea.pdf | archive-date = 19 April 2014 }}</ref> and an estimated 20% of the oxygen in the Earth's atmosphere.<ref name="npr">{{cite news|url=https://www.npr.org/templates/story/story.php?storyId=91448837|title=The Most Important Microbe You've Never Heard Of|website=npr.org}}</ref> It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.<ref>{{cite journal | vauthors = Flombaum P, Gallegos JL, Gordillo RA, Rincón J, Zabala LL, Jiao N, Karl DM, Li WK, Lomas MW, Veneziano D, Vera CS, Vrugt JA, Martiny AC | display-authors = 6 | title = Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 24 | pages = 9824–9 | date = June 2013 | pmid = 23703908 | pmc = 3683724 | doi = 10.1073/pnas.1307701110 | bibcode = 2013PNAS..110.9824F | doi-access = free }}</ref>

Originally, biologists classified cyanobacteria as algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria are bacteria, and hence are not even in the same [[Biology:Kingdom|Kingdom]] as algae. Most authorities today exclude all prokaryotes, and hence cyanobacteria from the definition of algae.<ref name="IntroBot">{{cite book | vauthors = Nabors MW |title= Introduction to Botany |year=2004 |publisher=Pearson Education, Inc |location=San Francisco, CA |isbn=978-0-8053-4416-5}}</ref><ref name="Allaby 92">{{cite encyclopedia | veditors = Allaby M |year=1992 |encyclopedia=The Concise Dictionary of Botany |publisher=Oxford University Press |location=Oxford |title=Algae}}</ref>
{{clear left}}

===Algae===

{{multiple image
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 | width     = 120
 | header    = Diatoms
 | footer    = Diatoms have a [[Chemistry:Silica|silica]] shell ([[Biology:Frustule|frustule]]) with radial (centric) or bilateral (pennate) symmetry.
 | image1    = Centric diatom (38258532722).jpg
 | caption1  = Centric
 | image2    = Pennate diatoms (3075304186).jpg
 | caption2  = [[Biology:Pennales|Pennate]]
}}

{{multiple image
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 | footer    = Traditionally dinoflagellates have been presented as armoured or unarmoured.
 | image1    = Peridinium digitale.jpg
 | caption1  = Armoured
 | image2    = Gymnodinium agile sp.jpg
 | caption2  = Unarmoured
}}

[[Biology:Algae|Algae]] is an informal term for a widespread and diverse group of photosynthetic protists which are not necessarily closely related and are thus polyphyletic. Marine algae can be divided into six groups:

* [[Biology:Green algae|green algae]], an informal group containing about 8,000 recognised species.<ref>{{cite journal | vauthors = Guiry MD | title =  How Many Species of Algae Are There? | journal = Journal of Phycology | volume = 48 | issue = 5 | pages = 1057–63 | date = October 2012 | pmid = 27011267 | doi = 10.1111/j.1529-8817.2012.01222.x | s2cid = 30911529 }}</ref> Many species live most of their lives as single cells or are filamentous, while others form [[Biology:Colony|colonies]] made up from long chains of cells, or are highly differentiated macroscopic seaweeds.
* [[Biology:Red algae|red algae]], a (disputed) phylum containing about 7,000 recognised species,<ref name="algaetaxo">{{Cite web |url= http://www.algaebase.org/browse/taxonomy/?id=97240 |title=Algaebase| vauthors = Guiry MD, Guiry GM |date=2016 |website=www.algaebase.org |access-date=November 20, 2016}}</ref> mostly multicellular and including many notable [[Biology:Seaweed|seaweed]]s.<ref name="algaetaxo" /><ref name="Thomas 02">{{cite book|title=Seaweeds|publisher=Life Series. [[Biology:Natural History Museum, London|Natural History Museum]], London|year=2002|isbn=978-0-565-09175-0| vauthors = Thomas D }}</ref>
* [[Biology:Brown algae|brown algae]], a [[Biology:Class|class]] containing about 2,000 recognised species,<ref>{{Cite book|url=https://books.google.com/books?id=s1P855ZWc0kC&pg=166|title=Algae: an introduction to phycology| vauthors = Hoek C, Mann D, Jahns HM, Jahns M |date=1995|publisher=Cambridge University Press|isbn=9780521316873|oclc=443576944|page=166}}</ref> mostly multicellular and including many seaweeds, including [[Biology:Kelp|kelp]]
* [[Biology:Diatom|diatom]]s, a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 percent of the oxygen produced on the planet each year,<ref name="livescience.com"/> take in over 6.7 billion metric tons of [[Chemistry:Silicon|silicon]] each year from the waters in which they live,<ref>{{cite journal | vauthors = Tréguer P, Nelson DM, Van Bennekom AJ, Demaster DJ, Leynaert A, Quéguiner B | title = The silica balance in the world ocean: a reestimate | journal = Science | volume = 268 | issue = 5209 | pages = 375–9 | date = April 1995 | pmid = 17746543 | doi = 10.1126/science.268.5209.375 | s2cid = 5672525 | bibcode = 1995Sci...268..375T }}</ref> and contribute nearly half of the organic material found in the oceans. The shells ([[Biology:Frustule|frustule]]s) of dead diatoms can reach as much as half a mile deep on the ocean floor.<ref>{{Cite web|url=https://www.kcl.ac.uk/sspp/departments/geography/people/academic/drake/Research/The-Sahara-Megalakes-Project/Lake-Megachad.aspx|title=King's College London - Lake Megachad|website=www.kcl.ac.uk|language=en-GB|access-date=2018-05-05}}</ref>
* [[Biology:Dinoflagellate|dinoflagellate]]s, a phylum of unicellular flagellates with about 2,000 marine species.<ref name="Gómez12">{{cite journal| vauthors = Gómez F |title=A checklist and classification of living dinoflagellates (Dinoflagellata, Alveolata) |journal=CICIMAR Oceánides |volume=27 |issue=1 |pages=65–140 |year=2012 |doi=10.37543/oceanides.v27i1.111 |doi-access=free }}</ref> Many dinoflagellates are known to be [[Earth:Photosynthesis|photosynthetic]], but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).<ref>{{Cite journal | vauthors = Stoecker DK | title = Mixotrophy among Dinoflagellates | doi = 10.1111/j.1550-7408.1999.tb04619.x | journal = The Journal of Eukaryotic Microbiology | volume = 46 | issue = 4 | pages = 397–401 | year = 1999 | s2cid = 83885629 | name-list-style = vanc}}</ref> Some species are [[Biology:Endosymbiont|endosymbiont]]s of marine animals and play an important part in the biology of [[Earth:Coral reef|coral reef]]s. Others predate other protozoa, and a few forms are parasitic.
* euglenophytes, a phylum of unicellular flagellates with only a few marine members

Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as ''[[Biology:Microalgae|microalgae]]'' or ''macroalgae''.
 
'''[[Biology:Microalgae|Microalgae]]''' are the microscopic types of algae, not visible to the naked eye. They are mostly unicellular species which exist as individuals or in chains or groups, though some are multicellular. Microalgae are important components of the marine protists (discussed above), as well as the phytoplankton (discussed below). They are very [[Biology:Biodiversity|diverse]]. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.<ref name = "Starckx_2012">{{cite web | vauthors = Starckx S | date = 31 October 2012 | url = http://www.flanderstoday.eu/current-affairs/place-sun | archive-url = https://web.archive.org/web/20160304105525/http://www.flanderstoday.eu/current-affairs/place-sun | archive-date = 4 March 2016 | title = A place in the sun - Algae is the crop of the future, according to researchers in Geel] Flanders Today | access-date = 8 December 2012 }}</ref> Depending on the species, their sizes range from a few micrometers (µm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.

<gallery mode="packed" heights="130" style="float:left;">
File:Chlamydomonas globosa - 400x (13263097835).jpg|''Chlamydomonas globosa'', a unicellular green alga with two flagella just visible at bottom left
File:Инфузории Ophridium versatile.jpg|''[[Biology:Chlorella vulgaris|Chlorella vulgaris]]'', a common green [[Biology:Microalgae|microalgae]], in endosymbiosis with a ciliate<ref>{{cite journal | vauthors = Duval B, Margulis L | year = 1995 | title = The microbial community of ''Ophrydium versatile'' colonies: endosymbionts, residents, and tenants | journal = Symbiosis | volume = 18 | pages = 181–210 | pmid = 11539474 }}</ref>
File:Centric diatom.jpg|Centric diatom
File:Dinoflagellates.jpg|Dinoflagellates
</gallery>

{{clear}}

'''Macroalgae''' are the larger, multicellular and more visible types of algae, commonly called [[Biology:Seaweed|seaweed]]s. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by a [[Biology:Holdfast|holdfast]]. Seaweed that becomes adrift can wash up on beaches. [[Biology:Kelp|Kelp]] is a large brown seaweed that forms large underwater [[Biology:Kelp forest|forests]] covering about 25% of the world coastlines.<ref>Wernberg, T., Krumhansl, K., Filbee-Dexter, K. and Pedersen, M.F. (2019) "Status and trends for the world's kelp forests". In: ''World seas: an environmental evaluation'', pages 57–78). Academic Press. {{doi|10.1016/B978-0-12-805052-1.00003-6}}.</ref> They are among the most productive and dynamic ecosystems on Earth.<ref name="Mann_1973">{{cite journal | vauthors = Mann KH | title = Seaweeds: Their Productivity and Strategy for Growth: The role of large marine algae in coastal productivity is far more important than has been suspected | journal = Science | volume = 182 | issue = 4116 | pages = 975–81 | date = December 1973 | pmid = 17833778 | doi = 10.1126/science.182.4116.975 | s2cid = 26764207 | bibcode = 1973Sci...182..975M }}</ref> Some ''[[Biology:Sargassum|Sargassum]]'' seaweeds are planktonic (free-floating). Like microalgae, macroalgae (seaweeds) are technically marine protists since they are not true plants.

<gallery mode="packed" heights="140px" style="float:left;">
File:Algae Pengo.svg| A seaweed is a macroscopic form of red or brown or [[Biology:Green algae|green algae]]. 
File:Sargassum on the beach, Cuba.JPG| ''[[Biology:Sargassum|Sargassum]]'' seaweed is a planktonic brown alga with air bladders that help it float.
<!-- File:Kelp forest at Taranga pinnacles Hen and Chicken Islands PA232359.JPG|[[Biology:Kelp forest|Kelp forest]] -->
File:Histrio histrio by A. H. Baldwin.jpg| [[Biology:Sargassum fish|Sargassum fish]] are camouflaged to live among drifting ''Sargassum'' seaweed.
</gallery>

[[File:Kelp forest.jpg|thumb| Kelp forests are among the most productive ecosystems on the planet.]]
{{clear left}}

{{anchor|Unicellular macroalgae}}
<gallery mode="packed" heights="140px" caption="Unicellular macroalgae (see also macroscopic protists ←)" style="float:left;">
<!-- File:Chondrus crispus - Köhler–s Medizinal-Pflanzen-034 cropped.jpg| Red alga -->
File:Ventricaria ventricosa.JPG| The unicellular [[Biology:Valonia ventricosa|bubble algae]] lives in tidal zones. It can have a 4&nbsp;cm diameter.<ref>{{cite book | vauthors = Tunnell JW, Chávez EA, Withers K |year=2007 |title=Coral reefs of the southern Gulf of Mexico |publisher=Texas A&M University Press |isbn=978-1-58544-617-9 |page=91 |url=https://books.google.com/books?id=tu0sqBp8eAAC&pg=PA91}}</ref> 
File:Acetabularia sp.jpg| The unicellular mermaid's wineglass are mushroom-shaped algae that grow up to 10&nbsp;cm high.
File:CaulerpaTaxifolia.jpg| Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80&nbsp;cm long.<ref>{{cite web | url = https://www.cabi.org/isc/datasheet/29292 | work = Invasive Species Compendium | title = ''Caulerpa taxifolia'' (killer algae) | publisher = Centre for Agriculture and Bioscience International | date = 6 November 2018 }}</ref>
</gallery>
{{clear left}}

Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions. Mermaid's wineglass, a genus of subtropical [[Biology:Green algae|green algae]], is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studying [[Biology:Cell biology|cell biology]].<ref>{{cite journal | vauthors = Mandoli DF | title = Elaboration of Body Plan and Phase Change During Development of Acetabularia: How Is the Complex Architecture of a Giant Unicell Built? | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 49 | pages = 173–198 | date = June 1998 | pmid = 15012232 | doi = 10.1146/annurev.arplant.49.1.173 | s2cid = 6241264 }}</ref> Another single celled algae, ''[[Biology:Caulerpa taxifolia|Caulerpa taxifolia]]'', has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an [[Earth:Invasive species|invasive species]] known colloquially as ''killer algae''.<ref>{{cite journal | vauthors = Madl P, Yip M | title = Literature Review of Caulerpa taxifolia | journal = BUFUS-Info | volume = 19 | issue = 31 | year = 2004 | url = http://biophysics.sbg.ac.at/ct/caulerpa.htm | access-date = 18 July 2017 | archive-date = 8 October 2022 | archive-url = https://web.archive.org/web/20221008200554/http://biophysics.sbg.ac.at/ct/caulerpa.htm | url-status = dead }}</ref>

===Origin of plants===
[[File:Evolution of seagrasses Pengo 8.png|thumb|Evolution of mangroves and seagrasses]]

Back in the [[Earth:Silurian|Silurian]], some phytoplankton evolved into [[Biology:Red algae|red]], [[Biology:Brown algae|brown]] and [[Biology:Green algae|green algae]]. These algae then invaded the land and started evolving into the land plants we know today. Later, in the [[Earth:Cretaceous|Cretaceous]], some of these land plants returned to the sea as marine plants, such as [[Earth:Mangrove|mangrove]]s and [[Biology:Seagrass|seagrass]]es.<ref>{{cite journal | vauthors = Orth RJ, Carruthers TJ, Dennison WC, Duarte CM, Fourqurean JW, Heck KL, Hughes AR, Kendrick GA, Kenworthy WJ, Olyarnik S, Short FT | display-authors = 6 | year = 2006 | title = A global crisis for seagrass ecosystems | journal = BioScience | volume = 56 | issue = 12| pages = 987–996 | doi = 10.1641/0006-3568(2006)56[987:AGCFSE]2.0.CO;2 | hdl = 10261/88476 | s2cid = 4936412 | doi-access = free }}</ref>

Marine plants can be found in [[Earth:Intertidal zone|intertidal zone]]s and shallow waters, such as [[Biology:Seagrass|seagrass]]es like [[Biology:Zostera|eelgrass]] and turtle grass, ''Thalassia''. These plants have adapted to the high salinity of the ocean environment. Plant life can also flourish in the brackish waters of estuaries, where mangroves or cordgrass or beach grass beach grass might grow.

<gallery mode="packe"" heights="140px" style="float:left;">
File:Mangrove-Keti Bundar.jpg|[[Earth:Mangrove|Mangrove]]s
File:Floridian seagrass bed.jpg|[[Biology:Seagrass meadow|Seagrass meadow]]
File:Leafy Sea Dragon SA.jpg| Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.<ref>{{FishBase |genus=Phycodurus |species=eques |year=2009 |month=July}}</ref>
</gallery>

{{clear}}

The total world area of mangrove forests was estimated in 2010 as {{convert|134,257|km2}} (based on satellite data).<ref>{{cite journal | vauthors = Giri C, Ochieng E, Tieszen LL, Zhu Z, Singh A, Loveland T, Masek J, Duke N | display-authors = 6 | year = 2011 | title = Status and distribution of mangrove forests of the world using earth observation satellite data | journal = Global Ecology and Biogeography | volume = 20 | issue = 1| pages = 154–159 | doi = 10.1111/j.1466-8238.2010.00584.x }}</ref><ref>{{cite journal | vauthors = Thomas N, Lucas R, Bunting P, Hardy A, Rosenqvist A, Simard M | title = Distribution and drivers of global mangrove forest change, 1996-2010 | journal = PLOS ONE | volume = 12 | issue = 6 | pages = e0179302 | year = 2017 | pmid = 28594908 | pmc = 5464653 | doi = 10.1371/journal.pone.0179302 | bibcode = 2017PLoSO..1279302T | doi-access = free }}</ref> The total world area of seagrass meadows is more difficult to determine, but was conservatively estimated in 2003 as {{convert|177000|km2}}.<ref>{{cite book |vauthors=Short FT, Frederick T |title=World atlas of seagrasses |date=2003 |publisher=University of California Press |location=Berkeley, Calif. |isbn=978-0-520-24047-6 |url=https://www.unep-wcmc.org/resources-and-data/world-atlas-of-seagrasses |page=24 |access-date=10 July 2019 |archive-date=10 July 2019 |archive-url=https://web.archive.org/web/20190710075836/https://www.unep-wcmc.org/resources-and-data/world-atlas-of-seagrasses |url-status=dead }}</ref>

Mangroves and seagrasses provide important nursery habitats for marine life, acting as hiding and foraging places for larval and juvenile forms of larger fish and invertebrates.<ref>{{cite book | vauthors = Spalding M | date = 2010 | url = https://www.taylorfrancis.com/books/9781849776608 | title = World atlas of mangroves | publisher = Routledge | isbn = 978-1-84977-660-8 | doi = 10.4324/9781849776608}}</ref>

==Plankton and trophic interactions==
[[File:Plankton collage.jpg|thumb|upright=1.16|right| Plankton are drifting or floating organisms that cannot swim against a current, and include organisms from most areas of life: bacteria, archaea, algae, protozoa and animals.|alt=Six relatively large variously shaped organisms with dozens of small light-colored dots all against a dark background. Some of the organisms have antennae that are longer than their bodies.]]
{{Further|Earth:Plankton|Biology:Bacterioplankton|Biology:Ichthyoplankton|Earth:Mycoplankton}}

[[Earth:Plankton|Plankton]] (from Greek for ''wanderers'') are a diverse group of organisms that live in the [[Earth:Water column|water column]] of large bodies of water but cannot swim against a current. As a result, they wander or drift with the currents.<ref>{{cite book | vauthors = Lalli C, Parsons T | year = 1993 | title = Biological Oceanography: An Introduction | publisher = Butterworth-Heinemann | isbn = 0-7506-3384-0 }}</ref> Plankton are defined by their [[Biology:Ecological niche|ecological niche]], not by any [[Biology:Phylogenetics|phylogenetic]] or [[Biology:Taxonomy|taxonomic]] classification. They are a crucial source of food for many marine animals, from [[Biology:Forage fish|forage fish]] to [[Biology:Whale|whale]]s. Plankton can be divided into a plant-like component and an animal component.

===Phytoplankton===
[[Earth:Phytoplankton|Phytoplankton]] are the plant-like components of the plankton community ("phyto" comes from the Greek for ''plant''). They are autotrophic (self-feeding), meaning they generate their own food and do not need to consume other organisms.

Phytoplankton consist mainly of microscopic photosynthetic [[Biology:Eukaryote|eukaryote]]s which inhabit the upper sunlit layer in all oceans. They need sunlight so they can photosynthesize. Most phytoplankton are single-celled algae, but other phytoplankton are bacteria and some are [[Biology:Protist|protist]]s.<ref>{{cite web | vauthors = Lindsey R, Scott M, Simmon R | date = 2010 | url = https://earthobservatory.nasa.gov/features/Phytoplankton | title = What are phytoplankton | work = NASA Earth Observatory }}</ref> Phytoplankton groups include cyanobacteria (above), [[Biology:Diatom|diatom]]s, various other types of [[Biology:Algae|algae]] (red, green, brown, and yellow-green), [[Biology:Dinoflagellate|dinoflagellate]]s, [[Biology:Euglenid|euglenoids]], coccolithophorids, cryptomonads, chrysophytes, chlorophytes, [[Biology:Prasinophyte|prasinophyte]]s, and silicoflagellates. They form the base of the [[Earth:Primary production|primary production]] that drives the ocean [[Biology:Food web|food web]], and account for half of the current global primary production, more than the terrestrial forests.<ref>{{cite journal | vauthors = Field CB, Behrenfeld MJ, Randerson JT, Falkowski P | title = Primary production of the biosphere: integrating terrestrial and oceanic components | journal = Science | volume = 281 | issue = 5374 | pages = 237–40 | date = July 1998 | pmid = 9657713 | doi = 10.1126/science.281.5374.237 | bibcode = 1998Sci...281..237F | url = https://escholarship.org/uc/item/9gm7074q }}</ref>

{{multiple image
 | align = right
|  caption_align = center
 | direction = horizontal
 | header    = Coccolithophores
 | footer    = [[Biology:Coccolithophore|Coccolithophore]]s build calcite skeletons important to the marine carbon cycle.<ref>{{cite book | vauthors = Rost B, Riebesell U | date = 2004 | chapter = Coccolithophores and the biological pump: responses to environmental changes | title = Coccolithophores: From Molecular Processes to Global Impact | pages = 99–125 | publisher = Springer | isbn = 978-3-662-06278-4 }}</ref>
 | image1    = 9Calcidiscus leptoporus, diploid, SEM, showing coccoliths.tif
 | width1    = 107
 | caption1  = ...have plates called [[Chemistry:Coccolith|coccolith]]s
 | image2    = Discoaster surculus 01.jpg
 | width2    = 133
 | caption2  = ...extinct fossil
}}

<gallery caption="Phytoplankton" mode="packed" heights="130px" style="float:left;">
File:Phytoplankton Lake Chuzenji.jpg| Phytoplankton are the foundation of the ocean food chain.
File:Phytopla.jpg|Phytoplankton come in many shapes and sizes.
File:Diatoms (248 05) Various diatoms.jpg|[[Biology:Diatom|Diatom]]s are one of the most common types of phytoplankton.
File:Phytoplankton - the foundation of the oceanic food chain.jpg| [[Biology:Colony|Colonial]] phytoplankton
</gallery>

<gallery mode="packed" heights="130px" style="float:left;">
File:Prochlorococcus marinus 2.jpg| The cyanobacterium ''[[Biology:Prochlorococcus|Prochlorococcus]]'' accounts for much of the ocean's primary production.
File:Cyanobacterial Scum.JPG|Green [[Biology:Cyanobacteria|cyanobacteria]] scum washed up on a rock in California
File:Gyrodinium dinoflagellate.jpg| ''[[Biology:Gymnodinium|Gyrodinium]]'', one of the few naked dinoflagellates which lack armour
File:Stichotricha secunda - 400x (14974779356).jpg| Zoochlorellae (green) living inside the ciliate ''Stichotricha secunda''
</gallery>

[[File:Pinnularia major.jpg|thumb|There are over 100,000 species of diatoms which account for 50% of the ocean's primary production.]]
[[File:Super Blooms.ogv|thumb|Red, orange, yellow and green represent areas where algal blooms abound. Blue areas represent nutrient-poor zones where phytoplankton exist in lower concentrations.]]

<gallery mode="packed" heights="130px" style="float:left;">
File:JRYSEM-247-05-azurapl.jpg| Coccolithophores named after the BBC documentary series. ''The Blue Planet''
File:Emiliania huxleyi.jpg| The [[Biology:Coccolithophore|coccolithophore]] ''Emiliania huxleyi''
File:Cwall99 lg.jpg| [[Biology:Algae bloom|Algae bloom]] of ''Emiliania huxleyi'' off the southern coast of England
File:Fjouenne sbrmvr012w 20070924163039 small.jpg| ''Guinardia delicatula'', a diatom responsible for [[Biology:Algal bloom|algal bloom]]s in the North Sea and the English Channel<ref>{{cite journal | vauthors = Arsenieff L, Simon N, Rigaut-Jalabert F, Le Gall F, Chaffron S, Corre E, Com E, Bigeard E, Baudoux AC | title = First Viruses Infecting the Marine Diatom Guinardia delicatula | journal = Frontiers in Microbiology | volume = 9 | issue = | pages = 3235 | date = 2018 | pmid = 30687251 | pmc = 6334475 | doi = 10.3389/fmicb.2018.03235 | doi-access = free }}</ref>
</gallery>

{{clear}}

===Zooplankton===
{{multiple image
 | align = right
 | direction = horizontal
 | header            = Radiolarians
 | header_align      = center
 | footer            = Drawings by [[Biography:Ernst Haeckel|Haeckel]] 1904
 | footer_align      = center
 | image1    = Haeckel Phaeodaria 1.jpg
 | width1    = 160
 | image2    = Haeckel Stephoidea edit.jpg
 | width2    = 160
}}

[[Earth:Zooplankton|Zooplankton]] are the animal component of the planktonic community ("zoo" comes from the Greek for ''animal''). They are heterotrophic (other-feeding), meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.

{{multiple image
 | align     = left
 | direction = horizontal
 | header    = Foraminiferans
 | footer    = Foraminiferans are important unicellular zooplankton protists, with calcium shells.
 | image1    = Foram-globigerina hg.jpg
 | width1    = 136
 | caption1  = ...can have more than one nucleus
 | image2    = G bulloides Brady 1884.jpg
 | width2    = 104
 | caption2  = ...and defensive spines
}}
{{multiple image
 | align     = right
 | direction = horizontal
 | header    = Turing and radiolarian morphology
 | footer    = Computer simulations of [[Turing pattern]]s on a sphere closely replicate some radiolarian shell patterns.<ref>{{cite journal | vauthors = Varea C, Aragón JL, Barrio RA | title = Turing patterns on a sphere | journal = Physical Review E | volume = 60 | issue = 4 Pt B | pages = 4588–92 | date = October 1999 | pmid = 11970318 | doi = 10.1103/PhysRevE.60.4588 | bibcode = 1999PhRvE..60.4588V }}</ref>
 | image1    = Spherical radiolarian.jpg
 | width1    = 208
 | caption1  = Shell of a spherical radiolarian
 | image2    = Radiolarians - Actinomma sol (33732012006).jpg
 | width2    = 111
 | caption2  = Shell micrographs
}}

Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Many protozoans (single-celled protists that prey on other microscopic life) are zooplankton, including zooflagellates, [[Biology:Foraminifera|foraminifera]]ns, radiolarians and some [[Biology:Dinoflagellate|dinoflagellate]]s. Other dinoflagellates are mixotrophic and could also be classified as phytoplankton; the distinction between plants and animals often breaks down in very small organisms. Other zooplankton include pelagic cnidarians, ctenophores, [[Biology:Mollusc|mollusc]]s, [[Biology:Arthropod|arthropod]]s and [[Biology:Tunicate|tunicate]]s, as well as planktonic [[Biology:Chaetognatha|arrow worms]] and [[Biology:Polychaete|bristle worms]].

Radiolarians are unicellular protists with elaborate silica shells

{{clear left}}

'''Microzooplankton: major grazers of the plankton'''
<gallery mode="packed" heights="144px" style="float:left;">
File:Mikrofoto.de-Radiolarien 6.jpg|Radiolarians come in many shapes.
File:Planktic Foraminifera of the northern Gulf of Mexico.jpg| Group of planktic foraminiferans
File:Copepod 2 with eggs.jpg|[[Biology:Copepod|Copepod]]s eat phytoplankton. This one is carrying eggs.
File:Protoperidinium dinoflagellate.jpg| The dinoflagellate, ''Protoperidinium'' extrudes a large feeding veil to capture prey.
</gallery>

{{clear}}

Larger zooplankton can be predatory on smaller zooplankton.

'''Macrozooplankton'''
----
<gallery mode="packed" heights="144px" style="float:left;">
File:Aurelia aurita (Cnidaria) Luc Viatour.jpg| Moon jellyfish
File:Cestum veneris in Hawaii.png| [[Biology:Venus girdle|Venus girdle]], a ctenophore 
File:Chaetoblack 2.png| Arrow worm
File:Tomopteriskils.jpg|''[[Biology:Tomopteris|Tomopteris]]'', a planktonic segmented worm with unusual yellow [[Physics:Bioluminescence|bioluminescence]]<ref>{{cite book| vauthors = Harvey EN |title = Bioluminescence|publisher = Academic Press|year = 1952 }}</ref>
File:Amphipodredkils.jpg|Marine amphipod
File:Antarctic krill (Euphausia superba).jpg| [[Biology:Krill|Krill]]
File:Enypniastes sp.jpg| Pelagic sea cucumber
</gallery>
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Many marine animals begin life as zooplankton in the form of eggs or larvae, before they develop into adults. These are meroplanktic, that is, they are planktonic for only part of their life.

<gallery caption="Larvae and juveniles" mode="packed" heights="130px" style="float:left;">
File:Salmonlarvakils 2.jpg| Salmon larva hatching from its egg
File:Molalavdj.jpg|Ocean sunfish larva
File:Squidu.jpg|Juvenile planktonic [[Biology:Squid|squid]]
File:Larva de phyllosoma.jpg| [[Biology:Phyllosoma|Larva stage]] of a spiny lobster
</gallery>

{{clear}}

===Mixotrophic plankton===
{{multiple image
 | align = right
 | direction = vertical
 | width     = 300
 | image1    = Dinoflagellate lumincescence 2.jpg
 | caption1  = A surf wave at night sparkles with blue light due to the presence of a bioluminescent dinoflagellate, such as ''Lingulodinium polyedrum''
 | image2    = Potential Mechanism for Dazzling Blue Flashes of Light in Oceans Identified (6300345394).jpg
 | caption2  = A suggested explanation for glowing seas<ref>{{cite web | url = https://www.nsf.gov/news/news_summ.jsp?org=NSF&cntn_id=122037 | title = Suggested Explanation for Glowing Seas--Including Currently Glowing California Seas | work = National Science Foundation | date = 18 October 2011 }}</ref>
}}

Dinoflagellates are often mixotrophic or live in [[Earth:Symbiosis|symbiosis]] with other organisms.

<gallery caption="Mixoplankton" mode="packed" heights="144px" style="float:left;">
File:Tintinnid ciliate Favella.jpg| [[Biology:Tintinnid|Tintinnid]] ciliate ''Favella''
File:Euglena mutabilis - 400x - 1 (10388739803) (cropped).jpg|''[[Biology:Euglena|Euglena mutabilis]]'', a photosynthetic [[Biology:Flagellate|flagellate]]
File:Noctiluca scintillans unica.jpg| ''[[Biology:Noctiluca scintillans|Noctiluca scintillans]]'', a bioluminescence dinoflagellate
</gallery>
{{clear left}}

Some dinoflagellates are [[Biology:Dinoflagellate#Bioluminescence|bioluminescent]]. At night, ocean water can light up internally and [[Biology:Dinoflagellate#Bioluminescence|sparkle with blue light]] because of these dinoflagellates.<ref>{{cite book | vauthors = Castro P, Huber ME |title=Marine Biology |url=https://archive.org/details/marinebiology00cast_419 |url-access=limited |publisher=McGraw Hill |year=2010 |isbn=978-0071113021 |pages=[https://archive.org/details/marinebiology00cast_419/page/n110 95] |edition=8th }}</ref><ref>{{cite journal | vauthors = Hastings JW | title = Chemistries and colors of bioluminescent reactions: a review | journal = Gene | volume = 173 | issue = 1 Spec No | pages = 5–11 | year = 1996 | pmid = 8707056 | doi = 10.1016/0378-1119(95)00676-1 }}</ref> Bioluminescent dinoflagellates possess [[Biology:Scintillons|scintillons]], individual cytoplasmic bodies which contain [[Biology:Dinoflagellate luciferase|dinoflagellate luciferase]], the main enzyme involved in the luminescence. The luminescence, sometimes called ''the phosphorescence of the sea'', occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.<ref name="Haddock_2009">{{cite journal | vauthors = Haddock SH, Moline MA, Case JF | title = Bioluminescence in the sea | journal = Annual Review of Marine Science | volume = 2 | pages = 443–93 | date = 2009 | pmid = 21141672 | doi = 10.1146/annurev-marine-120308-081028 | s2cid = 3872860 | bibcode = 2010ARMS....2..443H }}</ref>

===Marine food web===
[[File:Oceanic Food Web.jpg|thumb|upright=1.4|Pelagic food web]]

Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which tend to be r-strategists that grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers, such as mature forests, are often K-strategists that grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production.

Because of this inversion, it is the zooplankton that make up most of the marine animal [[Earth:Biomass (ecology)|biomass]]. As primary consumers, they are the crucial link between the primary producers (mainly phytoplankton) and the rest of the marine food web (secondary consumers).<ref>{{cite web  | date = 2008 | url = https://public.ornl.gov/site/gallery/detail.cfm?id=326 | title = Carbon Cycling and Biosequestration | page = 81 | quote = Workshop report DOE/SC-108 | publisher = US Department of Energy Department of Energy Office of Science }}</ref>

If phytoplankton dies before it is eaten, it descends through the euphotic zone as part of the [[Biology:Marine snow|marine snow]] and settles into the depths of sea. In this way, phytoplankton sequester about 2 billion tons of carbon dioxide into the ocean each year, causing the ocean to become a sink of carbon dioxide holding about 90% of all sequestered carbon.<ref name="EarthTimes">{{cite web|url=http://www.earthtimes.org/scitech/role-marine-plankton-sequestration-carbon/1062/|title=The role of marine plankton in sequestration of carbon | vauthors = Campbell M |date=22 June 2011 |website=EarthTimes|access-date=22 August 2014}}</ref>

In 2010 researchers found whales carry nutrients from the depths of the ocean back to the surface using a process they called the whale pump.<ref name="Roman_2010">{{cite journal | vauthors = Roman J, McCarthy JJ | title = The whale pump: marine mammals enhance primary productivity in a coastal basin | journal = PLOS ONE | volume = 5 | issue = 10 | pages = e13255 | date = October 2010 | pmid = 20949007 | pmc = 2952594 | doi = 10.1371/journal.pone.0013255 | bibcode = 2010PLoSO...513255R | id = e13255 | doi-access = free }}</ref> Whales feed at deeper levels in the ocean where [[Biology:Krill|krill]] is found, but return regularly to the surface to breathe. There whales defecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface where [[Earth:Phytoplankton|phytoplankton]] consume it. In the Gulf of Maine the whale pump provides more nitrogen than the rivers.<ref name="ScienceDaily">{{cite web|url=https://www.sciencedaily.com/releases/2010/10/101012101255.htm|title=Whale poop pumps up ocean health| vauthors = Brown JE |date=12 Oct 2010|website=Science Daily|access-date=18 August 2014}}</ref>

{{clear}}

==Other interactions==

===Biogeochemical cycles===
{{multiple image
 | header   = Marine biogeochemical cycles
 | width    = 360px
 | image1   = Earth Pacific jul 30 2010.jpg
 | alt1     = 
 | caption1 = The dominant feature of the planet viewed from space is water – oceans of liquid water flood most of the surface while water vapour swirls in atmospheric clouds and the poles are capped with ice. 
}}
{{further|Biology:Marine biogeochemical cycles|Biology:Biological pump|Earth:Blue carbon}}

Taken as a whole, the oceans form a single marine system where water – the "universal solvent" <ref>{{cite web |title=Water, the Universal Solvent |url=http://water.usgs.gov/edu/solvent.html |website=USGS |access-date=27 June 2017 |archive-url=https://web.archive.org/web/20170709141251/https://water.usgs.gov/edu/solvent.html |archive-date=9 July 2017 |url-status=live  }}</ref> – dissolves nutrients and substances containing elements such as oxygen, carbon, nitrogen and phosphorus. These substances are endlessly cycled and recycled, chemically combined and then broken down again, dissolved and then precipitated or evaporated, imported from and exported back to the land and the atmosphere and the ocean floor. Powered both by the biological activity of marine organisms and by the natural actions of the sun and tides and movements within the Earth's crust, these are the [[Biology:Marine biogeochemical cycles|marine biogeochemical cycles]].<ref>{{cite conference | vauthors = Brum JR, Morris JJ, Décima M, Stukel MR | date = 2014 | title = Chapter 2: Mortality in the oceans: Causes and consequences | conference = Eco-DAS IX Symposium Proceedings | pages = 16–48 | publisher = Association for the Sciences of Limnology and Oceanography | isbn = 978-0-9845591-3-8}}.</ref><ref>{{Cite book |title=Campbell Biology | vauthors = Reece JB |year=2013 |publisher=Pearson |isbn=978-0-321-77565-8 |edition=10th }}</ref>

<gallery mode="packed" heights="140px" style="float:left;">
File:OceanCarbonCycle.jpg|[[Earth:Oceanic carbon cycle|Marine carbon cycle]]<ref name="Prentice_2001">{{Cite web| vauthors = Prentice IC |title=The carbon cycle and atmospheric carbon dioxide|url =http://ir.anet.ua.ac.be/irua/handle/10067/381670151162165141?show=full|publisher=Climate change 2001: the scientific basis: contribution of Working Group I to the Third Assessment Report of the Intergouvernmental Panel on Climate Change / Houghton, J.T. [edit.]|year=2001|access-date=31 May 2012 }}</ref>
File:Oxygen cycle.svg|[[Earth:Oxygen cycle|Oxygen cycle]]
</gallery>

{{clear left}}

<gallery mode="packed" heights="105px" style="float:left;">
File:Marine Nitrogen Cycle.jpg|[[Biology:Nitrogen cycle#Marine nitrogen cycle|Marine nitrogen cycle]]
File:Phosphorus cycle.png|Marine [[Biology:Phosphorus cycle|phosphorus cycle]]
</gallery>

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===Sediments and biogenic ooze===
[[File:Marine sediment thickness (cropped).jpg|thumb|upright=1.4| Thickness of marine sediments]]

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous. [[Earth:Terrigenous sediment|Terrigenous sediment]]s account for about 45% of the total marine sediment, and originate in the erosion of [[Earth:Rock (geology)|rocks]] on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains of marine protists (single-celled plankton and benthos organisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. ''Ooze'', in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by John Murray, the "father of modern oceanography", who proposed the term ''radiolarian ooze'' for the silica deposits of radiolarian shells brought to the surface during the Challenger Expedition.<ref>{{cite book | vauthors = Thomson CW | date = 2014 | url = https://books.google.com/books?id=zcFkAwAAQBAJ&q=radiolarian+ooze | title = Voyage of the Challenger: The Atlantic | publisher = Cambridge University Press | page = 235 | isbn = 978-1-108-07475-9 }}</ref> A ''biogenic ooze'' is a [[Earth:Pelagic sediment|pelagic sediment]] containing at least 30 percent from the skeletal remains of marine organisms.

{{clear}}

{|class="wikitable"
! colspan=8 |{{centre|Main types of biogenic ooze}}
|-
! type
! mineral<br />forms
! protist<br />responsible
!
! name of<br />skeleton
! description
|-
! width=90px rowspan=2 | [[Earth:Siliceous ooze|Siliceous ooze]]
| rowspan=2 align=center | SiO<sub>2</sub><br />[[Chemistry:Quartz|quartz]]<br />[[Chemistry:Glass|glass]]<br />[[Chemistry:Opal|opal]]<br />[[Earth:Chert|chert]]
| [[Biology:Diatom|diatom]]s
| style="background:#000000;"| [[File:Lyrella hennedy 1600x contrast invertion.jpg|90px]]
| [[Biology:Frustule|frustule]]
| Individual diatoms range in size from 0.002 to 0.2&nbsp;mm.<ref name="Hasle_1996">{{cite book| vauthors = Hasle GR, Syvertsen EE, Steidinger, Tangen K | veditors = Tomas CR |title=Identifying Marine Diatoms and Dinoflagellates|chapter-url=https://books.google.com/books?id=KQxPtwonlqoC|access-date=2013-11-13|date=1996-01-25|publisher=Academic Press|isbn=978-0-08-053441-1|pages=5–385|chapter=Marine Diatoms}}</ref>
|-
| radiolarians
| style="background:#000000;"| [[File:Calocycloma sp. - Radiolarian (32163186535).jpg|90px]]
| skeleton
| Radiolarians are protozoa with diameters typically between 0.1 and 0.2&nbsp;mm that produce intricate mineral skeletons, usually made of silica
|-
! rowspan=3 | Calcareous ooze
| rowspan=3 align=center | [[Chemistry:Calcium carbonate|CaCO<sub>3</sub>]]<br />[[Chemistry:Calcite|calcite]]<br />[[Chemistry:Aragonite|aragonite]]<br />[[Earth:Limestone|limestone]]<br />[[Earth:Chalk|chalk]]
| foraminiferans
| style="background:#000000;"| [[File:Foram-globigerina hg.jpg|90px]]
| [[Biology:Test|test]]
| There are about 10,000 living species of foraminiferans,<ref name="Adl_2007">{{cite journal | vauthors = Adl SM, Leander BS, Simpson AG, Archibald JM, Anderson OR, Bass D, Bowser SS, Brugerolle G, Farmer MA, Karpov S, Kolisko M, Lane CE, Lodge DJ, Mann DG, Meisterfeld R, Mendoza L, Moestrup Ø, Mozley-Standridge SE, Smirnov AV, Spiegel F | display-authors = 6 | title = Diversity, nomenclature, and taxonomy of protists | journal = Systematic Biology | volume = 56 | issue = 4 | pages = 684–9 | date = August 2007 | pmid = 17661235 | doi = 10.1080/10635150701494127 | doi-access = free }}</ref> usually under 1&nbsp;mm in size.
|-
| [[Biology:Coccolithophore|coccolithophore]]s
| style="background:#000000;"| [[File:Coccolithus pelagicus 2.jpg|90px]]
| [[Chemistry:Coccolith|coccolith]]
| Coccolithophores are spherical cells usually less than 0.1&nbsp;mm across, enclosed by calcareous plates called coccoliths.<ref name="Moheimani_2012">{{cite journal | vauthors = Moheimani NR, Webb JP, Borowitzka MA | title = Bioremediation and other potential applications of coccolithophorid algae: a review. | journal = Algal Research | date = October 2012 | volume = 1 | issue = 2 | pages = 120–33 | doi = 10.1016/j.algal.2012.06.002}}</ref> Coccoliths are important microfossils. They are the largest global source of biogenic calcium carbonate, and make significant contributions to the global carbon cycle.<ref>{{cite journal | vauthors = Taylor AR, Chrachri A, Wheeler G, Goddard H, Brownlee C | title = A voltage-gated H+ channel underlying pH homeostasis in calcifying coccolithophores | journal = PLOS Biology | volume = 9 | issue = 6 | pages = e1001085 | date = June 2011 | pmid = 21713028 | pmc = 3119654 | doi = 10.1371/journal.pbio.1001085 | doi-access = free }}</ref> They are the main constituent of chalk deposits such as the white cliffs of Dover.
|-
|}

<gallery mode="packed" heights="130px" style="float:left;">
File:Radiolarian - Heliodiscus umbonatus (Ehr.), Haeckel (28187768550).jpg| An elaborate mineral skeleton of a radiolarian made of silica.
File:Marine diatoms SEM2.jpg| Diatoms, major components of marine plankton, also have silica skeletons called [[Biology:Frustule|frustule]]s. 
File:CSIRO ScienceImage 7202 SEM Coccolithophorid.jpg| [[Biology:Coccolithophore|Coccolithophore]]s have plates or scales made with calcium carbonate called [[Chemistry:Coccolith|coccolith]]s
File:Globigerina.png| Calcified [[Biology:Test|test]] of a planktic foraminiferan
</gallery>

{{clear}}

<gallery mode="packed" heights="160px" style="float:left;">
File:Stephanopyxis grunowii.jpg| A diatom microfossil from 40 million years ago
File:Diatomaceous Earth BrightField.jpg|[[Earth:Diatomaceous earth|Diatomaceous earth]] is a soft, siliceous, [[Earth:Sedimentary rock|sedimentary rock]] made up of microfossils in the form of the [[Biology:Frustule|frustule]]s (shells) of single cell diatoms (click to magnify).
File:PSM V44 D483 Globigerina ooze.jpg| Illustration of a ''[[Biology:Globigerina|Globigerina]]'' ooze
File:FMIB 47660 Shells from Globigerina Ooze.jpeg| Shells ([[Biology:Test|tests]]), usually made of calcium carbonate, from a [[Biology:Foraminifera|foraminifera]]l ooze on the deep ocean floor
</gallery>

{{clear}}

===Land interactions===
[[File:Ocean drainage.png|thumb|upright=1.5|The drainage basins of the principal oceans and seas of the world are marked by continental divides. The grey areas are endorheic basins that do not drain to the ocean.]]

Land interactions impact marine life in many ways. Coastlines typically have continental shelves extending some way from the shore. These provide extensive shallows sunlit down to the seafloor, allowing for photosynthesis and enabling habitats for seagrass meadows, coral reefs, kelp forests and other [[Earth:Benthos|benthic life]]. Further from shore the continental shelf slopes towards deep water. Wind [[Physics:Ekman transport|blowing at the ocean surface]] or deep ocean currents can result in cold and nutrient rich waters from abyssal depths moving up the continental slopes. This can result in [[Earth:Upwelling|upwelling]]s along the outer edges of continental shelves, providing conditions for phytoplankton blooms.

Water evaporated by the sun from the surface of the ocean can precipitate on land and eventually return to the ocean as [[Earth:Surface runoff|runoff]] or discharge from rivers, enriched with nutrients as well as [[Earth:Marine pollution|pollutants]]. As rivers discharge into estuaries, freshwater mixes with saltwater and becomes brackish. This provides another shallow water habitat where [[Biology:Mangrove forest|mangrove forest]]s and estuarine fish thrive. Overall, life in inland lakes can evolve with greater diversity than happens in the sea, because freshwater habitats are themselves diverse and compartmentalised in a way marine habitats are not. Some aquatic life, such as salmon and [[Biology:Eel|eel]]s, [[Biology:Fish migration|migrate]] back and forth between freshwater and marine habitats. These migrations can result in exchanges of pathogens and have impacts on the way life evolves in the ocean.

{{clear}}

==Anthropogenic impacts==
[[File:Global cumulative human impact on the ocean.png|thumb|upright=1.5|Global cumulative human impact on the ocean<ref name="Halpern_2019">{{cite journal | vauthors = Halpern BS, Frazier M, Afflerbach J, Lowndes JS, Micheli F, O'Hara C, Scarborough C, Selkoe KA | display-authors = 6 | title = Recent pace of change in human impact on the world's ocean | journal = Scientific Reports | volume = 9 | issue = 1 | pages = 11609 | date = August 2019 | pmid = 31406130 | pmc = 6691109 | doi = 10.1038/s41598-019-47201-9 | bibcode = 2019NatSR...911609H }}</ref>]]
{{main|Biology:Human impact on marine life}}

Human activities affect marine life and [[Earth:Marine habitat|marine habitat]]s through overfishing, [[Earth:Marine pollution|pollution]], [[Biology:Ocean acidification|acidification]] and the introduction of [[Earth:Invasive species|invasive species]]. These impact [[Earth:Marine ecosystem|marine ecosystem]]s and [[Biology:Marine food web|food webs]] and may result in consequences as yet unrecognised for the [[Biology:Biodiversity|biodiversity]] and continuation of marine life forms.<ref>{{cite web | url = https://www.geomar.de/en/research/human-impacts-on-marine-ecosystems/ | title = Human impacts on marine ecosystems | work = GEOMAR Helmholtz Centre for Ocean Research | access-date = 22 October 2019 | archive-date = 31 October 2020 | archive-url = https://web.archive.org/web/20201031110957/https://www.geomar.de/en/research/human-impacts-on-marine-ecosystems/ | url-status = dead }}</ref>

{{clear}}

==Biodiversity and extinction events==
[[File:Phanerozoic Biodiversity.svg|thumb|upright=1.5|Apparent marine fossil diversity during the Phanerozoic<ref name="Rosing_2010">{{cite journal | vauthors = Rosing MT, Bird DK, Sleep NH, Bjerrum CJ | title = No climate paradox under the faint early Sun | journal = Nature | volume = 464 | issue = 7289 | pages = 744–7 | date = April 2010 | pmid = 20360739 | doi = 10.1038/nature08955 | s2cid = 205220182 | bibcode = 2010Natur.464..744R }}</ref>]]
{{annotated image/Extinction|caption= Apparent extinction intensity, i.e. the fraction of [[Biology:Genus|genera]] going extinct at any given time as reconstructed from the fossil record (excluding the current [[Biology:Holocene extinction event|Holocene extinction event]])}}

[[Biology:Biodiversity|Biodiversity]] is the result of over three billion years of [[Biology:Evolution|evolution]]. Until approximately 600 million years ago, all life consisted of [[Biology:Archaea|archaea]], [[Biology:Bacteria|bacteria]], protozoans and similar single-celled organisms. The history of biodiversity during the [[Earth:Phanerozoic|Phanerozoic]] (the last 540 million years), starts with rapid growth during the [[Earth:Cambrian explosion|Cambrian explosion]] – a period during which nearly every phylum of [[Biology:Multicellular organism|multicellular organism]]s first appeared. Over the next 400 million years or so, invertebrate diversity showed little overall trend and vertebrate diversity shows an overall exponential trend.<ref name="Sahney_2010">{{cite journal | vauthors = Sahney S, Benton MJ, Ferry PA | title = Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land | journal = Biology Letters | volume = 6 | issue = 4 | pages = 544–7 | date = August 2010 | pmid = 20106856 | pmc = 2936204 | doi = 10.1098/rsbl.2009.1024 }}</ref>

However, more than 99 percent of all species that ever lived on Earth, amounting to over five billion species,<ref name="Book-Biology">{{cite book | vauthors = McKinney ML | chapter = How do rare species avoid extinction? A paleontological view | title = The Biology of Rarity | date = 1997 | pages = 110–29 | doi = 10.1007/978-94-011-5874-9_7 | isbn = 978-94-010-6483-5 | chapter-url = https://books.google.com/books?id=4LHnCAAAQBAJ&pg=PA110 }}</ref> are estimated to be [[Biology:Extinction|extinct]].<ref name="StearnsStearns1999">{{cite book | vauthors = Stearns BP, Stearns SC |title=Watching, from the Edge of Extinction |date=1999 |publisher=Yale University Press |location=New Haven, CT |isbn=978-0-300-08469-6 | url = https://books.google.com/books?id=0BHeC-tXIB4C&pg=PA1921 | page = x }}</ref><ref name="NYT-20141108-MJN">{{cite news | vauthors = Novacek MJ |date=8 November 2014 |title=Prehistory's Brilliant Future |url=https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |newspaper={{wipe|The New York Times}} |location=New York |issn=0362-4331 |access-date=2014-12-25}}</ref> These extinctions occur at an uneven rate. The dramatic rise in diversity has been marked by periodic, massive losses of diversity classified as mass extinction events.<ref name="Sahney_2010"/> Mass extinction events occur when life undergoes precipitous global declines. Most diversity and [[Earth:Biomass (ecology)|biomass]] on earth is found among the [[Biology:Microorganism|microorganism]]s, which are difficult to measure. Recorded extinction events are therefore based on the more easily observed changes in the diversity and abundance of larger [[Biology:Multicellular organism|multicellular organism]]s, rather than the total diversity and abundance of life.<ref>{{cite journal | vauthors = Nee S | title = Extinction, slime, and bottoms | journal = PLOS Biology | volume = 2 | issue = 8 | pages = E272 | date = August 2004 | pmid = 15314670 | pmc = 509315 | doi = 10.1371/journal.pbio.0020272 | doi-access = free }}</ref> Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.

Based on the fossil record, the [[Background extinction rate|background rate of extinctions]] on Earth is about two to five taxonomic families of marine animals every million years. The [[Earth:Great Oxygenation Event|Great Oxygenation Event]] was perhaps the first major extinction event. Since the [[Earth:Cambrian explosion|Cambrian explosion]] five [[Biology:Extinction event#Major extinction events|major mass extinctions]] have significantly exceeded the background extinction rate.<ref name="Ward_2006">{{cite journal | vauthors = Ward PD | title = Impact from the deep | journal = Scientific American | volume = 295 | issue = 4 | pages = 64–71 | date = October 2006 | pmid = 16989482 | doi = 10.1038/scientificamerican1006-64 | bibcode = 2006SciAm.295d..64W }}</ref> The worst was the Permian-Triassic extinction event, 251 million years ago. One generally estimates that the Big Five mass extinctions of the Phanerozoic (the last 540 million years) wiped out more than 40% of marine genera and probably more than 70% of marine species.<ref>Marine Extinctions: Patterns and Processes - an overview. 2013. ''CIESM Monograph 45'' [https://www.researchgate.net/publication/271767063]</ref> The current [[Earth:Holocene extinction|Holocene extinction]] caused by human activity, and now referred to as the "sixth extinction", may prove ultimately more devastating.

==Investigating and Exploring Marine Life==

===Research and study===
In order to perform research and enrich Marine Life knowledge, Scientists use various methods in-order to reach and explore the depths of the ocean. several Hi-tech instruments and vehicles are used for this purpose. <ref name=":0">{{Cite web |title=Investigating Marine Life {{!}} Census of Marine Life |url=http://www.coml.org/investigating/home.html |access-date=2023-12-27 |website=www.coml.org}}</ref>

* [[Engineering:Autonomous underwater vehicle|Autonomous Underwater Vehicles]] (AUVs)- Underwater robots used to explore the ocean. AUVs are independent robots and can explore unmanned.  They are released from a ship and are operated from the surface. <ref>{{Cite web |title=Exploration Tools: AUVs: NOAA Office of Ocean Exploration and Research |url=https://oceanexplorer.noaa.gov/technology/subs/auvs/auvs.html |access-date=2023-12-27 |website=oceanexplorer.noaa.gov |language=en-US}}</ref>
* Deep-Towed Vehicles (DTVs)- vehicles towed behind research vessels, offering a simpler alternative to more advanced underwater vehicles. They serve as versatile platforms for deploying oceanographic instruments to measure various ocean parameters, with specific models like the DTV BRIDGET used for studying hydrothermal vent plumes by moving near the ocean floor.<ref>{{Cite web |title=Deep-Towed Vehicles (DTVs) {{!}} Census of Marine Life |url=http://www.coml.org/investigating/transport/dtvs.html |access-date=2023-12-27 |website=www.coml.org}}</ref>
* Manned Submersibles- an manned underwater vehicle used for exploring, experimenting and is often used by army. <ref name=":0" />
* [[Earth:Research vessel|Research vessels]] (R/Vs)- a boat or ship used to conduct research over a ling period of time. It is capable of transporting a diverse range of sampling and surveying equipment. Research vessels typically feature on-board laboratory space, allowing researchers to promptly analyze the materials collected during cruises.
* Remotely Operated Vehicles (ROVs)- unmanned vehicles. able to reach greater depths under water in order to collect a wider variety of information.  <ref name=":0" /><ref>{{Cite web |title=The Drone Revolution underwater |url=https://www.advancednavigation.com/robotics/micro-auv/hydrus/ |access-date=2023-12-27 |website=Advanced Navigation |language=en-AU}}</ref>
{{clear}}

== See also ==
* {{annotated link|Organization:Census of Marine Life}}
* [[Biology:Marine larval ecology|Marine larval ecology]]

==Notes==
{{Reflist|30em
 | group = note
 | refs =
<ref name=Myxozoa>[[Biology:Myxozoa|Myxozoa]] were thought to be an exception, but are now thought to be heavily modified members of the [[Biology:Cnidaria|Cnidaria]]. {{cite journal | vauthors = Jiménez-Guri E, Philippe H, Okamura B, Holland PW | title = Buddenbrockia is a cnidarian worm | journal = Science | volume = 317 | issue = 5834 | pages = 116–8 | date = July 2007 | pmid = 17615357 | doi = 10.1126/science.1142024 | bibcode = 2007Sci...317..116J | s2cid = 5170702 }}</ref>
<ref name=trench_depth>This is the measurement taken by the vessel ''Kaikō'' in March 1995 and is considered the most accurate measurement to date. See the [[Earth:Challenger Deep|Challenger Deep]] article for more details.</ref>
}}
{{Notelist}}

== References ==
{{Reflist|colwidth=30em|
| refs =
<ref name=kaiko7000>{{cite web |title=7,000&nbsp;m Class Remotely Operated Vehicle ''KAIKO 7000'' |url=http://www.jamstec.go.jp/e/about/equipment/ships/kaiko7000.html |publisher=Japan Agency for Marine-Earth Science and Technology (JAMSTEC) |access-date=7 June 2008 |archive-date=10 April 2020 |archive-url=https://web.archive.org/web/20200410211118/http://www.jamstec.go.jp/e/about/equipment/ships/kaiko7000.html |url-status=dead }}</ref>

<ref name="Kennish_2001">{{cite book | vauthors = Kennish KJ |date=2001 |title=Practical handbook of marine science |page=35 |edition=3rd |publisher=CRC Press |series=Marine science series |isbn=978-0-8493-2391-1}}</ref>

<ref name="LeCalvez_2009">{{cite journal | vauthors = Le Calvez T, Burgaud G, Mahé S, Barbier G, Vandenkoornhuyse P | title = Fungal diversity in deep-sea hydrothermal ecosystems | journal = Applied and Environmental Microbiology | volume = 75 | issue = 20 | pages = 6415–21 | date = October 2009 | pmid = 19633124 | pmc = 2765129 | doi = 10.1128/AEM.00653-09 | bibcode = 2009ApEnM..75.6415L }}</ref>

<ref name="Michon_2006">{{cite web | vauthors = Scott M |date=24 April 2006 |url=http://earthobservatory.nasa.gov/Study/HeatBucket/ |title=Earth's Big heat Bucket |publisher=NASA Earth Observatory |access-date=14 March 2007}}</ref>

<ref name="Mullen_2002">{{cite web| vauthors = Mullen L |date=11 June 2002 |url=http://www.astrobio.net/news/article223.html |archive-url=https://web.archive.org/web/20070630122335/http://www.astrobio.net/news/article223.html |archive-date=30 June 2007 |title=Salt of the Early Earth |publisher=NASA Astrobiology Magazine |access-date=14 March 2007 |url-status=dead }}</ref>

<ref name="Charette_2010">{{Cite journal| vauthors = Charette MA, Smith WH |title=The Volume of Earth's Ocean |journal=Oceanography |volume=23 |issue=2 |pages=112–14 |date=June 2010 |doi=10.5670/oceanog.2010.51 |doi-access=free |hdl=1912/3862 |hdl-access=free }}</ref>

<ref name="Morris_2007">{{cite web | vauthors = Morris RM |url=http://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html |title=Oceanic Processes |publisher=NASA Astrobiology Magazine |access-date=14 March 2007 |url-status=dead |archive-url=https://web.archive.org/web/20090415082741/http://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html |archive-date=15 April 2009}}</ref>

<ref name="Sample_2005">{{cite web | vauthors = Sample S |date=21 June 2005 |url=http://science.hq.nasa.gov/oceans/physical/SST.html |title=Sea Surface Temperature |publisher=NASA |access-date=21 April 2007 |url-status=dead |archive-url=https://web.archive.org/web/20130406163412/http://science.nasa.gov/earth-science/oceanography/ |archive-date=6 April 2013 |df=dmy-all }}</ref>
}}

==Further reading==
{{Wikivoyage|Marine life}}
{{refbegin}}
* {{cite journal | vauthors = Halpern BS, Walbridge S, Selkoe KA, Kappel CV, Micheli F, D'Agrosa C, Bruno JF, Casey KS, Ebert C, Fox HE, Fujita R, Heinemann D, Lenihan HS, Madin EM, Perry MT, Selig ER, Spalding M, Steneck R, Watson R | display-authors = 6 | title = A global map of human impact on marine ecosystems | journal = Science | volume = 319 | issue = 5865 | pages = 948–52 | date = February 2008 | pmid = 18276889 | doi = 10.1126/science.1149345 | s2cid = 26206024 | bibcode = 2008Sci...319..948H }}
* {{cite journal | vauthors = Paleczny M, Hammill E, Karpouzi V, Pauly D | title = Population Trend of the World's Monitored Seabirds, 1950-2010 | journal = PLOS ONE | volume = 10 | issue = 6 | pages = e0129342 | year = 2015 | pmid = 26058068 | pmc = 4461279 | doi = 10.1371/journal.pone.0129342 | bibcode = 2015PLoSO..1029342P | doi-access = free }}
* {{Cite book | ref = Ruppert | vauthors = Ruppert EE, Fox RS, Barnes RD | title = Invertebrate Zoology | publisher = Brooks / Cole | edition = 7th | isbn = 978-0-03-025982-1 | year = 2004 | url = https://archive.org/details/isbn_9780030259821 }}
* {{cite web | url = https://www.theguardian.com/environment/radical-conservation/2015/sep/22/after-60-million-years-of-extreme-living-seabirds-are-crashing | title = After 60 million years of extreme living, seabirds are crashing | work = The Guardian | date = 22 September 2015 }}
{{refend}}

{{Aquatic ecosystem topics|expanded=marine}}
{{Animalia}}
{{Life on Earth}}
[[Category:Marine organisms| ]]
[[Category:Marine biology| ]]
[[Category:Biological oceanography|*]]

{{Sourceattribution|Marine life}}