View source for Biology:Marine invertebrates

{{Short description|Marine animals without a vertebrate column}}
[[File:Haeckel Chaetopoda-edit.jpg|thumb|upright=1.1|right|Ernst Haeckel's 96th plate, showing some marine invertebrates. Marine invertebrates have a large variety of body plans, which are currently categorised into over 30 phyla.]]

'''Marine invertebrates''' are the [[Biology:Invertebrate|invertebrate]]s that live in [[Earth:Marine habitat|marine habitat]]s. Invertebrate is a blanket term that includes all [[Biology:Animal|animal]]s apart from the [[Biology:Vertebrate|vertebrate]] members of the [[Biology:Chordate|chordate]] phylum. [[Biology:Invertebrate|Invertebrate]]s lack a [[Biology:Vertebral column|vertebral column]], and some have evolved a shell or a hard [[Biology:Exoskeleton|exoskeleton]]. As on land and in the air, marine invertebrates have a large variety of [[Biology:Body plan|body plan]]s, and have been categorised into over 30 [[Biology:Phylum|phyla]]. They make up most of the macroscopic life in the oceans.

==Evolution==
{{marine life sidebar}}

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 name=Davidson2005/> 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 animals may belong to the genus ''[[Biology:Dickinsonia|Dickinsonia]]'',<ref name="Vogel">{{cite web |last1=Vogel |first1=Gretchen |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 539 million years ago.<ref>{{Cite journal | last1 = Bobrovskiy | first1 = Ilya | title = Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals | doi = 10.1126/science.aat7228 | journal = Science | volume = 361 | issue = 6408 | pages = 1246–1249 | year = 2018 | pmid = 30237355| bibcode = 2018Sci...361.1246B | doi-access = free | 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 | author = Retallack, G.J. | 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| s2cid = 17181699 }}</ref> and spent most of their lives with their bodies firmly anchored to the sediment.<ref name=PalAss>{{PalAss2008|title=A placozoan affinity for ''Dickinsonia'' and the evolution of Late Precambrian metazoan feeding modes|
|last1=Sperling|first1=Erik|last2=Vinther|first2=Jakob|last3=Pisani|first3=Davide|last4=Peterson|first4=Kevin|page=81}}</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 | last1 = Gold | first1 = D. A. | last2 = Runnegar | first2 = B. |last3 = Gehling | first3 = J. G. |last4 = Jacobs | first4 = D. K. | year = 2015 | title = Ancestral state reconstruction of ontogeny supports a bilaterian affinity for Dickinsonia | journal = Evolution & Development | volume = 17 | issue = 6 | pages = 315–397 | doi = 10.1111/ede.12168 | pmid = 26492825 | 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:Jellyfish|jellyfish]], [[Biology:Sea anemone|sea anemone]]s and ''[[Biology:Hydra (genus)|Hydra]]''), possibly from around {{ma|580|Ma}}<ref name=Chen2015 /> The Ediacara biota, which flourished for the last 40 million years before the start of the [[Earth:Cambrian|Cambrian]],<ref name=Grazhdankin2004 /> 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 /> Others, however, have been interpreted as early [[Biology:Mollusca|mollusc]]s (''[[Biology:Kimberella|Kimberella]]''<ref name=Martin2000 /><ref name=Fedonkin1997 />), [[Biology:Echinoderm|echinoderm]]s (''[[Biology:Arkarua|Arkarua]]''<ref name=Mooi1998 />), and [[Biology:Arthropod|arthropod]]s (''[[Biology:Spriggina|Spriggina]]'',<ref name=McMenamin2003 /> ''[[Biology:Parvancorina|Parvancorina]]''<ref name=Lin2006 />). 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 />

The [[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 />

<gallery mode=packed heights=180 style=float:left>
File:DickinsoniaCostata.jpg| ''[[Biology:Dickinsonia|Dickinsonia]]'' may be the earliest animal. They appear in the fossil record 571 million to 541 million years ago.
File:Kimberella NT.jpg| ''[[Biology:Kimberella|Kimberella]]'', an early mollusc important for understanding the [[Earth:Cambrian explosion|Cambrian explosion]]. Invertebrates are grouped into different [[Biology:Phylum|phyla]] ([[Biology:Body plan|body plan]]s).
File:20191108 Opabinia regalis.png| ''[[Biology:Opabinia|Opabinia]]'', an extinct [[Biology:Stem group|stem group]] [[Biology:Arthropod|arthropod]] appeared in the [[Earth:Middle Cambrian|Middle Cambrian]]<ref name=Gould1989 />{{rp|124–136}}
</gallery>

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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 name=Gould1989 /> 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 />—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 /> 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=Marshall2006 />

==Classification==
[[File:Haeckel Bryozoa.jpg|thumb|right|Bryozoa, from Ernst Haeckel's ''Kunstformen der Natur'', 1904]]

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 name=Valentine2004a>{{cite book |last=Valentine |first=James W. |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=Valentine2004b>{{Cite book|url=https://books.google.com/books?id=DMBkmHm5fe4C |title=On the Origin of Phyla |isbn=978-0-226-84548-7 |last1=Valentine |first1=James W|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=Valentine2004b />

Historically body plans were thought of as having evolved in rapidly during the [[Earth:Cambrian explosion|Cambrian explosion]],<ref name=AmSci>{{cite journal|last1=Erwin |first1=Douglas |last2=Valentine |first2=James |last3=Jablonski |first3=David|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|author=Budd, G.E. |author2=Jensen, S. |date=May 2000 |title=A critical reappraisal of the fossil record of the bilaterian phyla |journal=Biological Reviews |volume=75 |issue=2 |pages=253–295 |doi=10.1111/j.1469-185X.1999.tb00046.x |url=http://www.journals.cambridge.org/abstract_S000632310000548X
|pmid=10881389|s2cid=39772232 }}</ref>

As on land and in the air, invertebrates make up a great majority of all macroscopic life, as the vertebrates makes up a subphylum of one of over 30 known animal phyla, making the term almost meaningless for [[Biology:Taxonomy|taxonomic]] purpose. Invertebrate sea life includes the following groups, some of which are phyla:

[[File:Haeckel Actiniae.jpg|thumb|right|The 49th plate from Ernst Haeckel's ''Kunstformen der Natur'', 1904, showing various sea anemones classified as Actiniae, in the Cnidaria phylum]]
[[File:Libr0409.jpg|thumb|right|"A variety of marine worms": plate from ''Das Meer'' by M.J. Schleiden (1804–1881)]]

*Acoela, among the most primitive bilateral animals;
*[[Biology:Annelida|Annelida]], ([[Biology:Polychaete|polychaete]]s and sea [[Biology:Leech|leech]]es);
*[[Biology:Brachiopoda|Brachiopoda]], marine animals that have hard "valves" (shells) on the upper and lower surfaces ;
*[[Biology:Bryozoa|Bryozoa]], also known as moss animals or sea mats;
*[[Biology:Chaetognatha|Chaetognatha]], commonly known as arrow worms, are a phylum of predatory marine worms that are a major component of plankton;
*[[Biology:Cephalochordata|Cephalochordata]] represented in the modern oceans by the lancelets (also known as Amphioxus);
*[[Biology:Cnidaria|Cnidaria]], such as [[Biology:Jellyfish|jellyfish]], [[Biology:Sea anemone|sea anemone]]s, and [[Biology:Coral|coral]]s;
*[[Biology:Crustacean|Crustacea]], including [[Biology:Lobster|lobster]]s, [[Biology:Crab|crab]]s, shrimp, crayfish, [[Biology:Barnacle|barnacle]]s, [[Biology:Hermit crab|hermit crab]]s, [[Biology:Mantis shrimp|mantis shrimp]]s, and [[Biology:Copepod|copepod]]s;
*[[Biology:Ctenophora|Ctenophora]], also known as comb jellies, the largest animals that swim by means of cilia;
*[[Biology:Echinodermata|Echinodermata]], including sea stars, [[Biology:Brittle star|brittle star]]s, [[Biology:Sea urchin|sea urchin]]s, [[Biology:Sand dollar|sand dollar]]s, [[Biology:Holothuroidea|sea cucumber]]s, [[Biology:Crinoid|crinoid]]s, and [[Biology:Sea daisy|sea daisies]];
*[[Biology:Echiura|Echiura]], also known as spoon worms;
*[[Biology:Gnathostomulid|Gnathostomulid]]s, slender to thread-like worms, with a transparent body that inhabit sand and mud beneath shallow coastal waters;
*[[Biology:Gastrotricha|Gastrotricha]], often called hairy backs, found mostly interstitially in between sediment particles;
*[[Biology:Hemichordata|Hemichordata]], includes acorn worms, solitary worm-shaped organisms;
*Kamptozoa, goblet-shaped sessile aquatic animals, with relatively long stalks and a "crown" of solid tentacles, also called Entoprocta;
*[[Biology:Kinorhyncha|Kinorhyncha]], segmented, limbless animals, widespread in mud or sand at all depths, also called mud dragons;
*[[Biology:Loricifera|Loricifera]], very small to microscopic [[Earth:Marine sediment|marine sediment]]-dwelling animals only discovered in 1983;
*[[Biology:Mollusca|Mollusca]], including shellfish, [[Biology:Squid|squid]], [[Biology:Octopus|octopus]], [[Biology:Whelk|whelk]]s, ''Nautilus'', [[Biology:Cuttlefish|cuttlefish]], [[Biology:Nudibranch|nudibranch]]s, [[Biology:Scallop|scallop]]s, [[Biology:Sea snail|sea snail]]s, [[Biology:Aplacophora|Aplacophora]], [[Biology:Caudofoveata|Caudofoveata]], [[Biology:Monoplacophora|Monoplacophora]], [[Biology:Polyplacophora|Polyplacophora]], and [[Biology:Scaphopoda|Scaphopoda]];
*[[Biology:Myzostomida|Myzostomida]], a taxonomic group of small marine worms which are parasitic on crinoids or "sea lilies";
*Nemertinea, also known as "ribbon worms" or "proboscis worms";
*[[Biology:Orthonectida|Orthonectida]], a small phylum of poorly known parasites of marine invertebrates that are among the simplest of multi-cellular organisms;
*[[Biology:Phoronida|Phoronida]], a phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies;
*[[Biology:Placozoa|Placozoa]], small, flattened, multicellular animals around 1 millimetre across and the simplest in structure. They have no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened;
*[[Biology:Porifera|Porifera]] (sponges), multicellular organisms that have bodies full of pores and channels allowing water to circulate through them;
*[[Biology:Priapulida|Priapulida]], or penis worms, are a phylum of marine worms that live marine mud. They are named for their extensible spiny proboscis, which, in some species, may have a shape like that of a human penis;
*[[Biology:Pycnogonida|Pycnogonida]], also called sea spiders, are unrelated to spiders, or even to arachnids which they resemble;
*Sipunculida, also called peanut worms, is a group containing 144–320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms;
*Tunicata, also known as sea squirts or sea pork, are filter feeders attached to rocks or similarly suitable surfaces on the ocean floor;
*Some [[Biology:Flatworm|flatworm]]s of the classes [[Biology:Turbellaria|Turbellaria]] and [[Biology:Monogenea|Monogenea]];
*[[Biology:Xenoturbella|Xenoturbella]], a genus of bilaterian animals that contains only two marine worm-like species;
*[[Biology:Xiphosura|Xiphosura]], includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the [[Biology:Horseshoe crab|horseshoe crab]]s.

[[Biology:Arthropod|Arthropod]]s total about 1,113,000 described extant species, [[Biology:Mollusc|mollusc]]s about 85,000 and [[Biology:Chordate|chordate]]s about 52,000.<ref name=Ponder2008>{{Cite book| editor=Ponder, W.F. |editor2=Lindberg, D.R.
| year=2008 | title=Phylogeny and Evolution of the Mollusca
| publisher=Berkeley: University of California Press | page=481 | isbn=978-0-520-25092-5 }}</ref>

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==Marine sponges==
[[File:Callyspongia sp. (Tube sponge).jpg|thumb|right|Sponges have no nervous, digestive or circulatory system]]

[[Biology:Sponge|Sponge]]s are animals of the [[Biology:Phylum|phylum]] Porifera (Modern Latin for ''bearing pores''<ref name=EtymologyPorifera>[http://www.etymonline.com/index.php?term=Porifera Porifera (n.)] ''Online Etymology Dictionary''. Retrieved 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}}.

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 |doi=10.1007/s00435-004-0100-0 |vauthors=Vacelet J, Duport E |title=Prey capture and digestion in the carnivorous sponge ''Asbestopluma hypogea'' (Porifera: Demospongiae) |year=2004 |journal=Zoomorphology |volume=123 |issue=4 |pages=179–190 |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:Aplysina archeri (Stove-pipe Sponge-pink variation).jpg|[[Biology:Aplysina archeri|Stove-pipe sponge]]
</gallery>

[[File:Otavia antiqua 3D reconstruction.jpg|thumb|right| Reconstruction of ''Otavia antiqua'', possibly the first animal about 760 million years ago{{hsp}}<ref>{{cite journal |author1=C. K. 'Bob' Brain |author2=Anthony R. Prave |author3=Karl-Heinz Hoffmann |author4=Anthony E. Fallick |author5=Andre Botha |author6=Donald A. Herd |author7=Craig Sturrock |author8=Iain Young |author9=Daniel J. Condon |author10=Stuart G. Allison |year=2012 |title=The first animals: ca. 760-million-year-old sponge-like fossils from Namibia |journal=South African Journal of Science |volume=108 |issue=1/2 |url=http://www.sajs.co.za/index.php/SAJS/article/view/658/966 |doi=10.4102/sajs.v108i1/2.658|doi-access=free |hdl=2263/18222 |hdl-access=free }}</ref>]]

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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=July 18, 2012}}</ref> For a long time thereafter sponges were assigned to a separate subkingdom, [[Biology:Parazoa|Parazoa]] (meaning ''beside the animals'').<ref name=Rowland2001>{{cite journal |author1=Rowland, S. M. |author2=Stephens, T. |name-list-style=amp |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 name=Sperling2007>{{cite journal|last=Sperling |first=E. A. |author2=Pisani, D. |author3=Peterson, K. J. |title=Poriferan paraphyly and its implications for Precambrian palaeobiology |journal=Geological Society, London, Special Publications |date=January 1, 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=August 22, 2012 |archive-url=https://web.archive.org/web/20090509061759/http://www.dartmouth.edu/~peterson/Sperling,%20Pisani%20and%20Peterson.pdf |archive-date=May 9, 2009 |bibcode=2007GSLSP.286..355S |s2cid=34175521 }}</ref>

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==Marine cnidarians==
[[File:Nematostella vectensis (I1419) 999 (30695685804).jpg|thumb|right|Cnidarians are the simplest animals with cells organised into tissues. Yet the starlet sea anemone contains the same genes as those that form the vertebrate head.]]

Cnidarians (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| author=Zhang, Z.-Q.| 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}}</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 |author1=Genikhovich, G. |author2=U. Technau |name-list-style=amp |year=2009 |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 |doi=10.1101/pdb.emo129 |pmid=20147257 |page=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 |author= Sinigaglia, C.|year=2013 |title=The bilaterian head patterning gene six3/6 controls aboral domain development in a cnidarian |journal=PLOS Biology |volume=11 |issue=2 |pages=e1001488 |doi=10.1371/journal.pbio.1001488 |display-authors=0 |pmid=23483856 |pmc=3586664 |doi-access=free }}</ref>

<gallery mode=packed heights=130px 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 pace with the rising water and form an [[Earth:Atoll|atoll]]
</gallery>

{{clear}}

<gallery mode=packed heights=152px style=float:left;>
File:Portuguese Man-O-War (Physalia physalis).jpg|[[Biology:Portuguese man o' war|Portuguese man o' war]]
File:Porpita porpita.jpg|''[[Biology:Porpita porpita|Porpita porpita]]''
File:Largelionsmanejellyfish.jpg|[[Biology:Lion's mane jellyfish|Lion's mane jellyfish]], largest known jellyfish<ref>Karleskint G, Richard Turner R and, James Small J (2012) [https://books.google.com/books?id=uBXTCQAAQBAJ&dq=%22lion%27s+mane+jellyfish%22+%22Introduction+to+Marine+Biology%22&pg=PA445 ''Introduction to Marine Biology''] Cengage Learning, edition 4, page 445. {{ISBN|9781133364467}}.</ref>
File:Sea walnut, Boston Aquarium (cropped).jpg| The [[Physics:Bioluminescence|bioluminescence]] sea walnut has a transient anus<ref>{{cite web| title=Animal with an anus that comes and goes could reveal how ours evolved| author=Michael Le Page| publisher=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> which forms only when it needs to defecate
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|last=Bavestrello|first=Giorgio|author2=Christian Sommer|author3=Michele Sarà|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|last=Piraino|first=Stefano|author2=F. Boero|author3=B. Aeschbach|author4=V. Schmid|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>
</gallery>

{{clear}}

==Marine worms==
{{further|Biology:Marine worm|Biology:Sea worm}}
[[File:Chaetoblack.png|thumb|300px|right|Arrow worms are predatory components of plankton worldwide.]]

Worms (Old English for ''serpent'') typically have 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 name=BBC2009>{{cite web|url=https://www.bbc.co.uk/cornwall/content/articles/2009/04/07/nature_worm_feature.shtml |title=Cornwall – Nature – Superstar Worm |publisher=BBC }}</ref> and up to {{convert|58|m|ft|}} for the marine nemertean worm (bootlace worm).<ref name=Guinness>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.

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:Hemichordata|Hemichordata]], and [[Biology:Phoronida|Phoronida]] (horseshoe worms). 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 [[Biology:Cestoda|Cestoda]] 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=time1957>{{Cite journal|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|archive-date=June 27, 2008|title=The Persistent Parasites|journal=Time Magazine}}</ref><ref name=hargis1985>{{cite journal|year=1985|author=Hargis, William J.|title=Parasitology and pathology of marine organisms of the world ocean|journal=NOAA Tech. Rep.}}</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 name=ucdavis>{{cite web|url=http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm|title=Classification of Animal Parasites|access-date=5 September 2016|archive-date=6 October 2017|archive-url=https://web.archive.org/web/20171006152348/http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm}}</ref><ref name=oxford>{{cite web |url=https://cid.oxfordjournals.org/content/29/4/734.full.pdf |title=Classification of Human Parasites, Vectors, and Similar Organisms |last=Garcia |first=Lynne |publisher=UCLA Medical Center Los Angeles California }}</ref> Over 25,000 nematode species have been described,<ref name=Hodda2011>{{cite journal | last1 = Hodda | first1 = M | year = 2011 | title = Phylum Nematoda Cobb, 1932. In: Zhang, Z.-Q. (Ed.) 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 name=Zhang2013>{{cite journal | last1 = Zhang | first1 = 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=Lambshead1993>
{{cite journal
| author = Lambshead PJD
| 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=Borgonie2011>{{cite journal
|vauthors=Borgonie G, García-Moyano A, Litthauer D, Bert W, Bester A, van Heerden E, Möller C, Erasmus M, Onstott TC | title = Nematoda from the terrestrial deep subsurface of South Africa
| journal = Nature
|date=June 2011
| volume = 474
| issue = 7349
| pages = 79–82
| doi = 10.1038/nature09974
| pmid = 21637257
| bibcode = 2011Natur.474...79B
| url = https://biblio.ugent.be/publication/1269676
| hdl = 1854/LU-1269676
| s2cid = 4399763
| hdl-access = free
}}</ref> By count they represent 90% of all animals on the [[Earth:Seabed|ocean floor]].<ref name=Danovaro2008>{{cite journal |vauthors=Danovaro R, Gambi C, Dell'Anno A, Corinaldesi C, Fraschetti S, Vanreusel A, Vincx M, Gooday AJ | title = Exponential decline of deep-sea ecosystem functioning linked to benthic biodiversity loss | journal = Curr. Biol. | 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=Platt1994>{{cite book | author = 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:Lamellibrachia luymesi1.png|''[[Biology:Lamellibrachia|Lamellibrachia luymes]]'', a [[Earth:Cold seep|cold seep]] tubeworm, lives over 250 years.
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
File:Eunice aphroditois.jpg|Bobbit worms are [[Biology:Ambush predator|ambush predator]]s that live on the seafloor
</gallery>
{{clear}}

==Echinoderms==
[[File:Haeckel Asteridea Larvae.jpg|thumb|right|upright|alt=Starfish larvae|[[Biology:Starfish|Starfish]] larvae are bilaterally symmetric, whereas the adults have fivefold symmetry]]

[[Biology:Echinoderm|Echinoderm]]s (Greek for ''spiny skin'') is a phylum which contains only marine invertebrates. The adults 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]].{{citation needed|date=August 2022}} Echinoderms are found at every ocean depth, from the [[Earth:Intertidal zone|intertidal zone]] to the [[Earth:Abyssal zone|abyssal zone]]. 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 (a superphylum), after the [[Biology:Chordata|chordates]] (which include the [[Biology:Vertebrate|vertebrate]]s, such as [[Biology:Bird|bird]]s, [[Biology:Fish|fish]]es, [[Biology:Mammal|mammal]]s, and [[Biology:Reptile|reptile]]s).

Echinoderms 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=Fox>{{cite web | url=http://lanwebs.lander.edu/faculty/rsfox/invertebrates/asterias.html | title=''Asterias forbesi'' | author=Fox, Richard | work=Invertebrate Anatomy OnLine | publisher=Lander University | access-date=14 June 2014}}</ref>

The 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. Further, it is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution.

<gallery mode=packed heights=140px 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:Espardenya (animal).jpg|[[Biology:Sea cucumber|Sea cucumber]]s filter feed on plankton and suspended solids
File:Enypniastes sp.jpg|[[Biology:Elasipodida|Benthopelagic sea cucumber]]s can lift off the seafloor and journey as much as {{convert|1000|m|abbr=on}} up the water column
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=Holsinger2005>Holsinger, K. (2005). Keystone species. Retrieved May 10, 2010, from {{cite web |url=http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html |title=Keystone species |access-date=2010-05-12 |archive-url=https://web.archive.org/web/20100630134633/http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html |archive-date=2010-06-30 }}</ref>
File:Colorful crinoids at shallow waters of Gili Lawa Laut.JPG|Colorful [[Biology:Crinoid|sea lilies]] in shallow waters
</gallery>

{{clear}}

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]].

==Marine molluscs==
[[File:Dactylioceras NT.jpg|thumb|right| Reconstruction of an [[Biology:Ammonite|ammonite]], a highly successful early cephalopod that first appeared in the Devonian (about 400 mya). They became extinct during the same extinction event that killed the land dinosaurs (about 66 mya).]]

[[Biology:Mollusc|Mollusc]]s (Latin for ''soft'') form a [[Biology:Phylum|phylum]] with about 85,000 extant recognized [[Biology:Species|species]].<ref name=Chapman2009>Chapman, A.D. (2009). [http://www.environment.gov.au/biodiversity/abrs/publications/other/species-numbers/2009/04-02-groups-invertebrates.html#mollusca Numbers of Living Species in Australia and the World, 2nd edition]. Australian Biological Resources Study, Canberra. Retrieved 12 January 2010. {{ISBN|978-0-642-56860-1}} (printed); {{ISBN|978-0-642-56861-8}} (online).</ref> By species count they are the largest [[Biology:Marine biology|marine]] phylum, comprising 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|author=Hancock, Rebecca|archive-url=https://web.archive.org/web/20090530042720/http://www.austmus.gov.au/display.cfm?id=2897|archive-date=2009-05-30}}</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. The majority of species still live in the oceans, from the seashores to the [[Earth:Abyssal zone|abyssal zone]], but some form a significant part of the freshwater fauna and the terrestrial [[Earth:Ecosystem|ecosystem]]s.

The mollusc phylum is divided into 9 or 10 [[Biology:Taxonomy|taxonomic]] [[Biology:Class|classes]], two of which are extinct. 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 ([[Biology:Sea snail|sea snail]]s), whereas gastropods without protective shells are referred to as [[Biology:Slug|slug]]s ([[Biology:Sea slug|sea slug]]s). Gastropods are by far the most numerous molluscs in terms of classified species, accounting for 80% of the total.<ref name=Ponder2008 /> 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), and about 1,200 freshwater species. [[Biology:Cephalopod|Cephalopod]] include [[Biology:Octopus|octopus]], [[Biology:Squid|squid]] and [[Biology:Cuttlefish|cuttlefish]]. They are found in all oceans, and neurologically are the most advanced of the invertebrates.<ref name=Barnes2001>{{Cite book
| vauthors = ((Barnes, R.S.K)), Calow P, ((Olive, P.J.W.)), Golding DW, Spicer JI | year=2001
| title=The Invertebrates, A Synthesis | edition=3| publisher=Blackwell Science | location =UK
}}</ref> 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/ }}</ref> and an estimated 11,000 extinct [[Biology:Taxon|taxa]] have been described.<ref name=mollusca12>{{The Mollusca|volume=12}}</ref> 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=16 January 2013}}</ref>

<gallery mode=packed heights=140px style=float:left;>
File:Calmarcolossal.jpg|[[Biology:Colossal squid|Colossal squid]], largest of all invertebrates<ref name=BBC2008>{{cite news|author=Black, Richard|title=Colossal squid out of the freezer|publisher=BBC News|date=April 26, 2008|url=http://news.bbc.co.uk/1/hi/sci/tech/7367774.stm }}</ref>
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 web|url=https://www.newscientist.com/article/dn14033-simpleminded-nautilus-reveals-flash-of-memory.html|title=Simple-Minded Nautilus Shows Flash of Memory|author=Ewen Callaway|date=2 June 2008|publisher=New Scientist|access-date=7 March 2012}}</ref><ref name=Phillips2008>{{cite journal|title=Living Fossil Memories|author=Kathryn Phillips|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|author1=Robyn Crook|author2=Jennifer Basil|name-list-style=amp|year=2008|title=A biphasic memory curve in the chambered nautilus, ''Nautilus pompilius'' L. (Cephalopoda: Nautiloidea)|journal=The Journal of Experimental Biology|volume=211|pages=1992–1998|doi=10.1242/jeb.018531|issue=12|pmid=18515730|s2cid=6305526}}</ref>
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:Syrinx aruanus shell.jpg|The sea snail ''[[Biology:Syrinx aruanus|Syrinx aruanus]]'' has the largest shell 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}}

[[File:Archimollusc-en.svg|thumb|upright=1.5| {{center|Generalized or ''hypothetical ancestral mollusc''}}]]

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.<ref name=Ruppert2004 /><ref name=Hayward1996>{{Cite book|last=Hayward|first=PJ|title=Handbook of the Marine Fauna of North-West Europe|year=1996|publisher=Oxford University Press|isbn=978-0-19-854055-7}}</ref>{{rp|484–628}} 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.<ref name=Hayward1996/> 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. The simplest type of molluscan [[Biology:Reproductive system|reproductive system]] relies on [[Biology:External fertilization|external fertilization]], but more complex variations occur. All produce eggs, from which may emerge [[Biology:Trochophore|trochophore]] larvae, more complex [[Biology:Veliger|veliger]] larvae, or miniature adults. The depiction is rather similar to modern [[Biology:Monoplacophora|monoplacophora]]ns, and some suggest it may resemble very early molluscs.<ref name=Ruppert2004 />{{rp|284–291}}<ref name=Ruppert2004 />{{rp|298–300}}<ref name=Giribet2006>{{Cite journal| last3=Lindgren, A.R.| last1=Giribet | first3=A. R. | first1=G.| last4=Huff, S.W. | first4=S. W. | first2=A.| last5 = Schrödl, M | first5=M.| last6 = Nishiguchi, M.K. | first6=M. K.| title = Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons| last2= Okusu, A| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 103| issue = 20| pages = 7723–7728| date=May 2006 | pmid = 16675549| pmc = 1472512| doi = 10.1073/pnas.0602578103|bibcode = 2006PNAS..103.7723G | doi-access=free }}</ref><ref name=Healy2001>{{Cite book
| author=Healy, J.M. | chapter=The Mollusca | pages=120–171
| title=Invertebrate Zoology | year=2001 | edition=2 | editor=Anderson, D.T.
| publisher=Oxford University Press | isbn=978-0-19-551368-4
}}</ref>

Good evidence exists for the appearance of marine gastropods, cephalopods and bivalves in the [[Earth:Cambrian|Cambrian]] period {{ma|Cambrian|{{period end|Cambrian}}}}. However, the evolutionary history both of molluscs' emergence from the ancestral [[Biology:Lophotrochozoa|Lophotrochozoa]] and of their diversification into the well-known living and [[Biology:Fossil|fossil]] forms are still subjects of vigorous debate among scientists.

{{clear}}

==Marine arthropods==
{{Annotated image |float=right| image=HemimerusHanseni.jpg |caption=[[Biology:Segmentation|Segmentation]] and [[Biology:Tagma|tagmata]] of an arthropod<ref name=Ruppert2004 />{{rp|518–522}} |width=150 |image-width=80 |height=180
| annotations=
{{Annotation|95|5|[[Biology:Cephalon (arthropod head)|<small>Head</small>]]}}
{{Annotation|90|17|_______________________|color=green}}
{{Annotation|95|37|[[Biology:Thorax#In arthropods|<small>Thorax</small>]]}}
{{Annotation|90|50|_______________________|color=black}}
{{Annotation|95|80|[[Biology:Abdomen#Other animals|<small>Abdomen</small>]]}}
{{Annotation|90|100|_______________________|color=green}}
}}

[[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.

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.

Their 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, but the main eyes of [[Biology:Spider|spider]]s are ocelli that can form images and, in a few cases, can swivel to track prey. 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. Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Marine species all lay eggs and use 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.

<gallery mode=packed heights=150px style=float:left;>
File:Olenoides typicalis.jpg|[[Biology:Trilobite|Trilobite]]s, now extinct, roamed oceans for 270 million years.<ref name=firstlife>{{cite web|url=http://firstlifeseries.com/learn-more/|title=Learn more about First Life - David Attenborough's First Life|first=Yes We|last=Work|archive-url=https://web.archive.org/web/20110126162514/http://firstlifeseries.com/learn-more/|archive-date=2011-01-26}}</ref>
File:Limule(dD) rotated.jpg|[[Biology:Horseshoe crab|Horseshoe crab]], a [[Biology:Living fossil|living fossil]] arthropod from 450 million years ago
</gallery>
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<gallery caption=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:Stenopus hispidus (high res).jpg|The [[Biology:Stenopus hispidus|banded cleaner shrimp]] is a crustacean common in the tropics.
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|author1=D. R. Currie|author2=T. M. Ward|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}}</ref>
File:Macrocheira kaempferi.jpg|The [[Biology:Japanese spider crab|Japanese spider crab]] has the longest leg span of any arthropod.
</gallery>
{{clear}}

The evolutionary ancestry of arthropods dates back to the [[Earth:Cambrian|Cambrian]] period. The group is generally regarded as [[Biology:Monophyletic|monophyletic]], and many analyses support the placement of arthropods with [[Biology:Cycloneuralia|cycloneuralia]]ns (or their constituent clades) in a superphylum [[Biology:Ecdysozoa|Ecdysozoa]]. Overall however, the basal relationships of Metazoa are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated.

==Other phyla==
* [[Biology:Tardigrade|Tardigrade]], [[Biology:Lobopodia|Lobopodia]], ([[Biology:Onychophora|Onychophora]])
* Non-craniate (non-vertebrate) chordates: [[Biology:Cephalochordate|Cephalochordate]], Tunicata and ''[[Biology:Haikouella|Haikouella]]''. These invertebrates are close relatives of the vertebrates.

<gallery caption= "Non-craniate 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 the closest living invertebrate relative of the vertebrates.<ref name=Gewin2005>{{cite journal|last1 = Gewin|first1 = 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 name=ArsTechnica2008>[https://arstechnica.com/science/2008/06/lancelet-amphioxus-genome-and-the-origin-of-vertebrates/ Lancelet (amphioxus) genome and the origin of vertebrates] ''Ars Technica'', 19 June 2008.</ref>
File:Ascidian (Rhopalaea Crassa) (4 cm).png|Fluorescent-colored sea squirts, ''[[Biology:Rhopalaea crassa|Rhopalaea crassa]]''. Tunicates may provide clues to vertebrate (and therefore human) ancestry.<ref>{{cite journal|last1 = Lemaire|first1 = 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:23 salpchain frierson odfw (8253212250).jpg|[[Biology:Salp|Salp]] chain
File:Gill slits.png|Gill slits in an acorn worm (left) and tunicate (right)
</gallery>

{{clear}}

==Minerals from sea water==
There are a number of marine invertebrates that use minerals that are present in the sea in such minute quantities that they were undetectable until the advent of [[Physics:Spectroscopy|spectroscopy]]. [[Chemistry:Vanadium|Vanadium]] is concentrated by some [[Biology:Tunicate|tunicate]]s for use in their blood cells to a level ten million times that of the surrounding seawater. Other tunicates similarly concentrate [[Chemistry:Niobium|niobium]] and [[Chemistry:Tantalum|tantalum]].<ref name=Ruppert2004>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 }}</ref>{{rp|947}} Lobsters use [[Chemistry:Copper|copper]] in their respiratory pigment [[Biology:Hemocyanin|hemocyanin]], despite the proportion of this metal in seawater being minute.<ref name=Ruppert2004 />{{rp|638}} Although these elements are present in vast quantities in the ocean, their extraction by man is not economic.<ref name=Carson1997>{{cite book |title=The Sea Around Us |last=Carson |first=Rachel |year=1997 |publisher=Oxford Paperbacks |isbn=978-0-19-506997-6 |pages=190–191 }}</ref>

==Glass models==

<gallery mode=packed style=float:left caption="Blaschka glass marine invertebrates, 1863–1880">
File:Anthomedusae - Lymnorea trieda.JPG
File:Museum Histoire Naturelle Geneva Blaschka Aurelia aurita 21102014.jpg| ''Aurelia aurita''
File:Museum Histoire Naturelle Geneva Blaschka Cnidaria 21102014.jpg| Cnidarian
File:Blaschka Natural History Museum Londres 28072013 2.jpg| [[Biology:Octopus|Octopus]]
File:Museum Histoire Naturelle Geneva Blaschka Glaucus atlanticus 21102014.jpg| ''[[Biology:Glaucus atlanticus|Glaucus atlanticus]]''
File:BlaschkaSW (4).JPG| [[Biology:Sea anemone|Sea anemone]]
File:BlaschkaPisaCU (7).JPG
</gallery>

{{clear}}

==See also==
* [[Biology:Marine life|Marine life]]
* [[Biology:Marine biology|Marine biology]]
* [[Biology:Marine vertebrate|Marine vertebrate]]
* [[Biology:List of marine aquarium invertebrate species|List of marine aquarium invertebrate species]]
* [[Biology:Wiwaxia|Wiwaxia]]

== Notes ==
{{reflist|
| 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 |last1=Jímenez-Guri |first1=Eva |last2=Philippe |first2=Hervé |last3=Okamura |first3=Beth |last4=Holland |first4=Peter W. H. |date=July 6, 2007 |title=''Buddenbrockia'' Is a Cnidarian Worm |journal=Science |volume=317 |issue=5834 |pages=116–118 |bibcode=2007Sci...317..116J |doi=10.1126/science.1142024 |issn=0036-8075 |pmid=17615357 |s2cid=5170702 }}</ref>
}}

==References==
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<ref name=Davidson2005>{{cite web |url=http://micro.magnet.fsu.edu/cells/animalcell.html |title=Animal Cell Structure |last=Davidson |first=Michael W. |date=May 26, 2005 |website=Molecular Expressions |publisher=[[Organization:Florida State University|Florida State University]] |location=Tallahassee, FL |access-date=2008-09-03}}</ref>

<ref name=Chen2015>{{cite journal |author1=Jun-Yuan Chen |last2=Oliveri |first2=Paola |author3=Feng Gao |last4=Dornbos |first4=Stephen Q. |author5=Chia-Wei Li |last6=Bottjer |first6=David J. |last7=Davidson |first7=Eric H. |date=August 1, 2002 |title=Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China |url=https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf |journal=Developmental Biology |volume=248 |issue=1 |pages=182–196 |doi=10.1006/dbio.2002.0714 |issn=0012-1606 |pmid=12142030 |access-date=2015-02-04 |display-authors=3 |archive-url=https://web.archive.org/web/20130526203745/https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf |archive-date=May 26, 2013 }}</ref>

<ref name=Grazhdankin2004>{{cite journal |last=Grazhdankin |first=Dima |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>

<ref name=Seilacher1992>{{cite journal |last=Seilacher |first=Adolf |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>

<ref name=Martin2000>{{cite journal |last1=Martin |first1=Mark W. |last2=Grazhdankin |first2=Dmitriy V. |last3=Bowring |first3=Samuel A. |last4=Evans |first4=David A. D. |last5=Fedonkin |first5=Mikhail A. |last6=Kirschvink |first6=Joseph L. |date=May 5, 2000 |title=Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution |journal=Science |volume=288 |issue=5467 |pages=841–845 |doi=10.1126/science.288.5467.841 |bibcode=2000Sci...288..841M |issn=0036-8075 |pmid=10797002 |s2cid=1019572 |display-authors=3 }}</ref>

<ref name=Fedonkin1997>{{cite journal |last1=Fedonkin |first1=Mikhail A. |last2=Waggoner |first2=Benjamin M. |date=August 28, 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>

<ref name=Mooi1998>{{cite journal |last1=Mooi |first1=Rich |last2=David |first2=Bruno |date=December 1998 |title=Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms |url=http://icb.oxfordjournals.org/content/38/6/965.full.pdf |journal=American Zoologist |volume=38 |issue=6 |pages=965–974 |doi=10.1093/icb/38.6.965 |issn=1540-7063 |access-date=2015-02-05 |doi-access=free }}</ref>

<ref name=McMenamin2003>{{cite conference |url=https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |title=''Spriggina'' is a trilobitoid ecdysozoan |first=Mark A. S. |last=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, CO |page=105 |oclc=249088612 |access-date=2007-11-24 |archive-date=12 April 2016 |archive-url=https://web.archive.org/web/20160412064305/https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm }} Paper No. 40-2 presented at the Geological Society of America's 2003 Seattle Annual Meeting (November 2–5, 2003) on November 2, 2003, at the Washington State Convention Center.</ref>

<ref name=Lin2006>{{cite journal |author1=Jih-Pai Lin |last2=Gon |first2=Samuel M. III |last3=Gehling |first3=James G. |last4=Babcock |first4=Loren E. |author5=Yuan-Long Zhao |author6=Xing-Liang Zhang |author7=Shi-Xue Hu |author8=Jin-Liang Yuan |author9=Mei-Yi Yu |author10=Jin Peng |display-authors=3 |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>

<ref name=Butterfield2006>{{cite journal |last=Butterfield |first=Nicholas J. |date=December 2006 |title=Hooking some stem-group 'worms': fossil lophotrochozoans in the Burgess Shale |journal=[[Biology:BioEssays|BioEssays]] |volume=28 |issue=12 |pages=1161–1166 |doi=10.1002/bies.20507 |issn=0265-9247 |pmid=17120226 |s2cid=29130876 }}</ref>

<ref name=Bengtson2004>{{cite book |last=Bengtson |first=Stefan |year=2004 |chapter=Early Skeletal Fossils |editor1-last=Lipps |editor1-first=Jere H. |editor1-link=Jere H. Lipps |editor2-last=Waggoner |editor2-first=Benjamin M. |title=Neoproterozoic-Cambrian Biological Revolutions: Presented as a Paleontological Society Short Course at the Annual Meeting of the Geological Society of America, Denver, Colorado, November 6, 2004 |series=Paleontological Society Papers |volume=10 |location=New Haven, CT |publisher=Yale University Reprographics & Imaging Service; [[Organization:Paleontological Society|Paleontological Society]] |oclc=57481790 |pages=67–78}}</ref>

<ref name=Budd2003>{{cite journal |last=Budd |first=Graham E. |date=February 2003 |title=The Cambrian Fossil Record and the Origin of the Phyla |url=http://icb.oxfordjournals.org/content/43/1/157.full.pdf+html |format=PDF |journal=Integrative and Comparative Biology |volume=43 |issue=1 |pages=157–165 |doi=10.1093/icb/43.1.157 |issn=1557-7023 |pmid=21680420 |access-date=2015-02-06 |doi-access=free }}</ref>

<ref name=Budd1996>{{cite journal |last=Budd |first=Graham E. |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>

<ref name=Marshall2006>{{cite journal |last=Marshall |first=Charles R. |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>
}}

==Other references==
*[http://www.earthlife.net/inverts/an-phyla.html List of Animal Phyla]

{{aquatic ecosystem topics|expanded=marine}}

[[Category:Marine animals|Invertebrates]]

{{Sourceattribution|Marine invertebrates}}