View source for Biology:Tardigrade

{{short description|Phylum of microscopic animals, also known as water bears}}
{{Automatic taxobox
| fossil_range = {{fossil range|Turonian|Recent|earliest=Cambrian}}
Possible [[Earth:Precambrian|Precambrian]] and [[Earth:Cambrian|Cambrian]] Record
| image = SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png
| image_caption = ''[[Biology:Milnesium tardigradum|Milnesium tardigradum]]'', a [[Biology:Eutardigrade|eutardigrade]]
| image2 = Echiniscus insularis (10.3897-evolsyst.5.59997) Figure 6 (white background).jpg
| image2_caption = ''[[Biology:Echiniscus|Echiniscus insularis]]'', a heterotardigrade
| display_parents = 8
| taxon = Tardigrada
| authority = [[Biography:Lazzaro Spallanzani|Spallanzani]], 1777
| subdivision_ranks = Classes
| subdivision = *Eutardigrada
*[[Biology:Heterotardigrada|Heterotardigrada]]
*[[Biology:Mesotardigrada|Mesotardigrada]] (dubious)
}}

'''Tardigrades''' ({{IPAc-en|ˈ|t|ɑr|d|ᵻ|g|r|eɪ|d|z}}),<ref>{{cite Dictionary.com|tardigrade}}</ref> known colloquially as '''water bears''' or '''moss piglets''',<ref name="American Scientist"/><ref name="WRD-20140321">{{cite magazine|last=Simon|first=Matt |title=Absurd Creature of the Week: The Incredible Critter That's Tough Enough to Survive in the vacuum of Space|url=https://www.wired.com/wiredscience/2014/03/absurd-creature-week-water-bear/|date=21 March 2014|magazine=Wired|access-date=2014-03-21}}</ref><ref>{{cite news|title=Indestructible|last=Copley|first=Jon|date=23 October 1999|issue=2209|periodical=New Scientist |url=https://www.newscientist.com/article/mg16422095.100-indestructible.html|access-date=2010-02-06}}</ref><ref>{{cite web|url=https://microcosmos.foldscope.com/?p=17901|title=Stanford Tardigrade Project |publisher=Foldscope|access-date=2017-03-23|date=2016-08-10}}</ref> are a [[Biology:Phylum|phylum]] of eight-legged [[Biology:Segmentation|segmented]] micro-animals.<ref name="American Scientist"/><ref>{{cite news |url=http://www.thehindu.com/todays-paper/tp-in-school/meet-tardigrade-the-water-bear/article7630324.ece|title=Meet tardigrade, the water bear|newspaper=The Hindu|first=Cornelia|last=Dean|date=September 9, 2015 |access-date=August 9, 2019}}</ref> They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them '''Kleiner Wasserbär''' ("little water bear").<ref>{{cite journal |last1=Cross|first1=Ryan|title=Secrets of the tardigrade|journal=C&EN Global Enterprise|date=2016-11-07|volume=94|issue=44|pages=20–21|doi=10.1021/cen-09444-scitech1 |url=https://pubs.acs.org/doi/abs/10.1021/cen-09444-scitech1|access-date=31 May 2021}}</ref> In 1777, the Italian biologist [[Biography:Lazzaro Spallanzani|Lazzaro Spallanzani]] named them '''Tardigrada''' ({{IPAc-en|t|ɑr|ˈ|d|ɪ|g|r|ə|d|ə}}), which means "slow steppers".<ref name="Bordenstein">{{cite web|first=Sarah|last=Bordenstein|title=Tardigrades (Water Bears)|work=Microbial Life Educational Resources|publisher=National Science Digital Library |url=http://serc.carleton.edu/microbelife/topics/tardigrade/index.html|access-date=2014-01-24}}</ref>

They have been found in diverse regions of Earth's [[Biology:Biosphere|biosphere]]{{snd}}mountaintops, the [[Earth:Deep sea|deep sea]], [[Earth:Tropical rainforest|tropical rainforest]]s, and the Antarctic.<ref name="Bordenstein" /> Tardigrades are among the most resilient animals known,<ref name="WP-20170714">{{cite news|last=Guarino|first=Ben|title=These animals can survive until the end of the Earth, astrophysicists say|url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/07/14/these-animals-can-survive-until-the-end-of-the-earth-astrophysicists-say/|date=14 July 2017|newspaper=The Washington Post|access-date=14 July 2017}}</ref><ref name="NAT-20170714">{{cite journal|doi=10.1038/s41598-017-05796-x|pmid=28710420|pmc=5511186|title=The Resilience of Life to Astrophysical Events|journal=Scientific Reports|volume=7|issue=1|page=5419|year=2017|last1=Sloan|first1=David |last2=Alves Batista|first2=Rafael|last3=Loeb|first3=Abraham|bibcode=2017NatSR...7.5419S|arxiv=1707.04253}}</ref> with individual species able to survive extreme conditions – such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, [[Physics:Radiation|radiation]], [[Medicine:Dehydration|dehydration]], and [[Medicine:Starvation|starvation]] – that would quickly kill most other known forms of life.<ref>{{cite journal |last1=Orellana|first1=Roberto|last2=Macaya|first2=Constanza|last3=Bravo|first3=Guillermo|last4=Dorochesi|first4=Flavia|last5=Cumsille|first5=Andrés|last6=Valencia|first6=Ricardo|last7=Rojas|first7=Claudia|last8=Seeger|first8=Michael|date=2018-10-30|title=Living at the Frontiers of Life: Extremophiles in Chile and Their Potential for Bioremediation|journal=Frontiers in Microbiology|volume=9|page=2309 |doi=10.3389/fmicb.2018.02309|pmid=30425685|pmc=6218600|issn=1664-302X|doi-access=free}}</ref> Tardigrades have survived exposure to [[Astronomy:Outer space|outer space]].<ref>{{cite news |url=https://www.newscientist.com/article/dn14690-water-bears-are-first-animal-to-survive-vacuum-of-space.html|title={{'}}Water Bears' are first animal to survive vacuum of space. |magazine=New Scientist|access-date=10 September 2008|archive-url=https://web.archive.org/web/20080910062613/http://space.newscientist.com/article/dn14690-water-bears-are-first-animal-to-survive-vacuum-of-space.html|archive-date=10 September 2008|url-status=live}}</ref><ref>{{cite news|url=https://www.sciencedaily.com/releases/2008/09/080908135906.htm|title='Water Bears' Able To Survive Exposure To Vacuum Of Space|publisher=Science Daily|access-date=10 September 2008|archive-url=https://web.archive.org/web/20080911091656/https://www.sciencedaily.com/releases/2008/09/080908135906.htm|archive-date=11 September 2008|url-status=live}}</ref> There are about 1,300 known species<ref>{{cite web |first1=Peter |last1=Degma |first2=Roberto |last2=Bertolani |first3=Roberto |last3=Guidetti |title=Actual checklist of Tardigrada species (2009–2021, 40th Edition: 19-07-2021) |year=2021 |publisher=Università di Modena e Reggio Emilia 
|url=https://iris.unimore.it/retrieve/358743/Actual%20checklist%20of%20Tardigrada%2040th%20Edition%2019-07-21.pdf |access-date=2023-12-10 |doi=10.25431/11380_1178608}}</ref> in the [[Biology:Phylum|phylum]] Tardigrada, a part of the superphylum [[Biology:Ecdysozoa|Ecdysozoa]] consisting of animals that grow by [[Biology:Ecdysis|ecdysis]] (shedding an exoskeleton) such as [[Biology:Arthropod|arthropod]]s and [[Biology:Nematode|nematode]]s. The earliest known true members of the group are known from [[Earth:Cretaceous|Cretaceous]] (145 to 66 million years ago) amber, found in North America, but are essentially modern forms. Their origin is therefore likely much earlier, as they diverged from their closest relatives in the [[Earth:Cambrian|Cambrian]] over 500 million years ago.

Tardigrades are usually about {{cvt|0.5|mm}} long when fully grown.<ref name="American Scientist">{{cite web|url=https://www.americanscientist.org/article/tardigrades|title=Tardigrades|publisher=American Scientist|access-date=2018-04-13|first=William|last=Miller|date=2017-02-06}}</ref> They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction disks.<ref name="American Scientist"/><ref name="Nelson-CurrentStatus">{{cite journal|title=Current status of Tardigrada: Evolution and Ecology|last=Nelson|first=Diane|journal=Integrative and Comparative Biology |volume=42|issue=3|date=1 July 2002|pages=652–659|doi=10.1093/icb/42.3.652|pmid=21708761|doi-access=free}}</ref> Tardigrades are prevalent in [[Biology:Moss|moss]]es and [[Biology:Lichen|lichen]]s and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a low-power [[Engineering:Microscope|microscope]], making them accessible to students and amateur scientists.<ref>{{cite web|url=http://tardigrade.us/how-to-articles/how-to-find-tardigrades/|title=How to Find Tardigrades|last=Shaw|first=Michael W.|publisher=Tardigrade USA|archive-url=https://web.archive.org/web/20140210001506/http://tardigrade.us/how-to-articles/how-to-find-tardigrades/|archive-date=10 February 2014|url-status=dead|access-date=2013-01-14}}</ref>

== Naming ==
[[File:Johann August Ephraim Goeze1.jpg|thumb|upright=1.0|right|Johann August Ephraim Goeze]]

Johann August Ephraim Goeze originally named the tardigrade ''Kleiner Wasserbär'', meaning "little water-bear" in German (today, they are often referred to in German as ''Bärtierchen'' or "little bear-animal"). The name "water-bear" comes from the way they walk, reminiscent of a bear's [[Biology:Gait|gait]]. The name ''Tardigradum'' means "slow walker" and was given by [[Biography:Lazzaro Spallanzani|Lazzaro Spallanzani]] in 1777.<ref name="Bordenstein"/>

== Description ==
[[File:Waterbear.jpg|thumb|upright=1.0|right|[[Physics:Scanning electron microscope|SEM]] image of ''Hypsibius dujardini'']]

The largest adults may reach a body length of {{cvt|1.5|mm}}, the smallest below {{cvt|0.1|mm|in}}. Newly hatched tardigrades may be smaller than {{cvt|0.05|mm|in}}. For comparison, grass pollen is typically {{cvt|0.025-0.04|mm|in}}.

== Habitat ==
Tardigrades are often found on lichens and mosses; for example, by soaking a piece of moss in water.<ref>{{cite journal|doi=10.1016/S0960-9822(02)00959-4|pmid=12176341|title=Tardigrades|journal=Current Biology |volume=12|issue=14|page=R475|year=2002|last1=Goldstein|first1=Bob|last2=Blaxter|first2=Mark|doi-access=free}}</ref> Other environments in which they are found include [[Earth:Dune|dune]]s and [[Earth:Coast|coast]]s generally, [[Chemistry:Soil|soil]], [[Biology:Plant litter|leaf litter]], and [[Earth:Ocean|marine]] or freshwater sediments, where they may occur quite frequently, up to 25,000 animals per litre (95,000 animals per gallon). One tardigrade, ''Echiniscoides wyethi'',<ref name="AP-20150929">{{cite news|title=Researchers discover new tiny organism, name it for Wyeths|url=http://apnews.excite.com/article/20150929/us--new_microscopic_animal-f2f8286453.html|date=29 September 2015 |agency=AP News|access-date=2015-09-29}}</ref> may be found on [[Biology:Barnacle|barnacle]]s.<ref name=PBSW>{{cite journal|doi=10.2988/0006-324X-128.1.103|title=''Echiniscoides wyethi'', a new marine tardigrade from Maine, U.S.A. (Heterotardigrada: Echiniscoidea: Echiniscoididae)|journal=Proceedings of the Biological Society of Washington|volume=128|issue=1|pages=103–110|year=2015|last1=Perry|first1=Emma S.|last2=Miller |first2=William R.|s2cid=85893082}}</ref>

== Anatomy and morphology ==
Tardigrades have barrel-shaped bodies with four pairs of stubby legs. Most range from {{cvt|0.3|to|0.5|mm}} in length, although the largest species may reach {{cvt|1.2|mm}}.<ref name="Bordenstein"/> The body consists of a head, three body segments each with a pair of legs, and a [[Biology:Anatomical terms of location#Anterior and posterior|caudal]] segment with a fourth pair of legs. The legs are without joints, while the feet have four to eight claws each. The [[Biology:Cuticle|cuticle]] contains [[Chemistry:Chitin|chitin]] and [[Biology:Protein|protein]] and is [[Biology:Ecdysis|moulted]] periodically. The first three pairs of legs are directed downward along the sides and are the primary means of locomotion, while the fourth pair is directed backward on the last segment of the trunk and is used primarily for grasping the substrate.<ref>{{cite journal|doi=10.1653/0015-4040(2003)086[0134:OWB]2.0.CO;2|year=2003|volume=86|issue=2|pages=134–137|title=On water bears|journal=Florida Entomologist|last1=Romano|first1=Frank A.|doi-access=free}}</ref>

Tardigrades lack several [[Biology:Hox gene|Hox gene]]s and a large intermediate region of the body axis. In insects, this corresponds to the entire thorax and the abdomen. Practically the whole body, except for the last pair of legs, is made up of just the segments that are homologous to the head region in arthropods.<ref>{{cite journal|title=The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region |first1=Frank W.|last1=Smith|first2=Thomas C.|last2=Boothby|first3=Ilaria|last3=Giovannini|first4=Lorena|last4=Rebecchi|first5=Elizabeth L.|last5=Jockusch|first6=Bob|last6=Goldstein|date=1 January 2016 |journal=Current Biology|volume=26|issue=2|pages=224–229|doi=10.1016/j.cub.2015.11.059|pmid=26776737|doi-access=free|hdl=11380/1083953|hdl-access=free}}</ref>

All adult tardigrades of the same species have the same number of cells (see [[Biology:Eutely|eutely]]). Some species have as many as 40,000 cells in each adult, while others have far fewer.{{how many? |date=June 2023}}<ref name="Seki98"/><ref>Kinchin, Ian M. (1994) ''The Biology of Tardigrades'' Ashgate Publishing</ref>

The body cavity consists of a haemocoel, but the only place where a true [[Biology:Coelom|coelom]] can be found is around the gonad. No respiratory organs were found, with gas exchange able to occur across the entirety of the body. Some tardigrades have three tubular glands associated with the rectum; these may be excretory organs similar to the Malpighian tubules of [[Biology:Arthropod|arthropod]]s, although the details remain unclear.<ref name=IZ/> Also, [[Biology:Nephridium|nephridia]] are absent.<ref>[http://labs.bio.unc.edu/goldstein/SmithGoldstein2017.pdf Segmentation in Tardigrada and diversification of segmental patterns in Panarthropoda]</ref>

The tubular mouth is armed with stylets, which are used to pierce the plant cells, [[Biology:Algae|algae]], or small invertebrates on which the tardigrades feed, releasing the body fluids or cell contents. The mouth opens into a triradiate, muscular, sucking [[Biology:Pharynx|pharynx]]. The stylets are lost when the animal molts, and a new pair is secreted from a pair of glands that lie on either side of the mouth. The pharynx connects to a short [[Biology:Esophagus|esophagus]], and then to an intestine that occupies much of the length of the body, which is the main site of digestion. The intestine opens, via a short rectum, to an [[Biology:Anus|anus]] located at the terminal end of the body. Some species only defecate when they molt, leaving the feces behind with the shed cuticle.<ref name=IZ>{{cite book|last1=Barnes|first1=Robert D.|date=1982 |title=Invertebrate Zoology|publisher=Holt-Saunders International|location=Philadelphia, PA|pages=877–880|isbn=978-0-03-056747-6}}</ref>

The tardigrade nervous system consists primarily of the brain and four segmental ganglia associated with the four body segments.<ref name="pmid24152256">{{cite journal | vauthors=Mayer G, Martin C, Pflüger H, Schlegel M | title=Selective neuronal staining in tardigrades and onychophorans provides insights into the evolution of segmental ganglia in panarthropods | journal=BMC Evolutionary Biology | volume=24 | pages=230 | year=2013 | issue=1 | doi = 10.1186/1471-2148-13-230 | pmc=4015553 | pmid=24152256 | bibcode=2013BMCEE..13..230M | doi-access=free }}</ref> The  brain comprises about 1% of the total body volume.<ref name="pmid31110777">{{cite journal | vauthors=Gross V, Müller M, Pfeiffer F | title=X-ray imaging of a water bear offers a new look at tardigrade internal anatomy | journal=Zoological Letters | volume=11 | issue=5 | pages=14 | year=2019 | doi = 10.1186/s40851-019-0130-6 | pmc=6511223 | pmid=31110777 | doi-access=free }}</ref> The brain develops in a [[Biology:Symmetry in biology#Bilateral symmetry|bilaterally symmetric]] pattern.<ref name="pmid26052416">{{cite journal|doi=10.1186/s13227-015-0008-4|pmid=26052416|pmc=4458024|title=Neural development in the tardigrade ''Hypsibius dujardini'' based on anti-acetylated α-tubulin immunolabeling|journal=EvoDevo|volume=6|page=12|year=2015|last1=Gross|first1=Vladimir|last2=Mayer|first2=Georg |doi-access=free }}</ref> Tardigrades have a dorsal brain atop a paired ventral nervous system. The brain includes multiple lobes, mostly consisting of three bilaterally paired clusters of [[Biology:Neuron|neuron]]s.<ref>{{cite journal|doi=10.1007/s00435-007-0045-1|title=Three-dimensional reconstruction of the central nervous system of ''Macrobiotus hufelandi'' (Eutardigrada, Parachela): Implications for the phylogenetic position of Tardigrada|journal=Zoomorphology|volume=127|issue=1|pages=21–36|year=2007|last1=Zantke |first1=Juliane|last2=Wolff|first2=Carsten|last3=Scholtz|first3=Gerhard|s2cid=43853965}}</ref> The brain is attached to a large [[Biology:Ganglion|ganglion]] below the esophagus, from which a double ventral nerve cord runs the length of the body. The cord possesses one ganglion per segment, each of which produces lateral nerve fibres that run into the limbs. Many species possess a pair of [[Biology:Ommatidium#rhabdomere|rhabdomeric]] pigment-cup eyes, and numerous sensory bristles are on the head and body.<ref name="Greven2007">{{cite journal|doi=10.1016/j.asd.2007.06.003|pmid=18089118|title=Comments on the eyes of tardigrades|journal=Arthropod Structure & Development|volume=36|issue=4|pages=401–407|year=2007|last1=Greven|first1=Hartmut}}</ref>

Tardigrades all possess a buccopharyngeal apparatus (swallowing device made of muscles and spines that activates an inner jaw and begins digestion and movement along the throat and intestine<ref>{{cite journal |doi=10.1016/S0020-7322(98)00027-0|title=Microspines in the alimentary canal of Arthropoda, Onychophora, annelida|journal=International Journal of Insect Morphology and Embryology|volume=27|issue=4|pages=341–349|year=1998|last1=Elzinga|first1=Richard J.}}</ref>) which, along with the claws, is used to differentiate species.

== Reproduction ==
[[File:TardigradeEggsInShedCuticle.jpg|thumb|upright=1.0|right|Shed cuticle of female tardigrade, containing eggs]]

Although some species are [[Biology:Parthenogenesis|parthenogenic]], both males and females are usually present, although females are frequently larger and more common. Both sexes have a single gonad located above the intestine. Two ducts run from the testes in males, opening through a single pore in front of the anus. In contrast, females have a single duct opening either just above the anus or directly into the rectum, which forms a [[Biology:Cloaca|cloaca]].<ref name=IZ/>

Tardigrades are oviparous, and fertilization is usually external. Mating occurs during the molt with the eggs being laid inside the shed [[Biology:Cuticle|cuticle]] of the female and then covered with sperm. A few species have internal fertilization, with mating occurring before the female fully sheds her cuticle. In most cases, the eggs are left inside the shed cuticle to develop, but some species attach them to a nearby substrate.<ref name=IZ/>

The eggs hatch after no more than 14 days, with the young already possessing their full complement of adult [[Biology:Cell|cells]]. Growth to adult size occurs by enlargement of the individual cells ([[Medicine:Hypertrophy|hypertrophy]]), rather than by cell division. Tardigrades may molt up to 12 times.<ref name=IZ/>

Tardigrades tend to court before mating. Courtship is an early step in mating and was first observed in tardigrades in 1895. Research shows that up to nine males aggregate around a female to mate.<ref>{{cite journal | doi=10.1080/07924259.2021.1990142 | title=Sexual reproductive behaviours of tardigrades: A review | year=2021 | last1=Sugiura | first1=Kenta | last2=Matsumoto | first2=Midori | journal=Invertebrate Reproduction & Development | volume=65 | issue=4 | pages=279–287 | bibcode=2021InvRD..65..279S | s2cid=244504291 | doi-access=free }}</ref>

== Ecology and life history ==
[[File:Tardigrade in real time.ogv|thumb|upright=1.0|right|Video of tardigrade under the microscope]]
[[File:Тихоходки, темное поле 280х.webm|thumb|upright=1.0|right|Living tardigrades moving around]]

Most tardigrades are phytophagous (plant eaters) or bacteriophagous (bacteria eaters), but some are [[Biology:Carnivore|carnivorous]] to the extent that they eat smaller species of tardigrades (for example, ''[[Biology:Milnesium tardigradum|Milnesium tardigradum]]'').<ref>{{cite journal |title=Population dynamics of two species of Tardigrada, ''Macrobiotus hufelandii'' (Schultze) and ''Echiniscus (Echiniscus) testudo'' (Doyere), in roof moss from Swansea |first=Clive I. |last=Morgan |journal=Journal of Animal Ecology |volume=46 |issue=1 |date=1977 |pages=263–79 |doi=10.2307/3960|jstor=3960|bibcode=1977JAnEc..46..263M }}</ref><ref>{{cite web |last=Lindahl |first=K. |date=15 March 2008|title=Tardigrade Facts |url=http://www.iwu.edu/~tardisdp/tardigrade_facts.html}}</ref>

Tardigrades share morphological characteristics with many species that differ largely by class. Biologists have a difficult time finding verification among tardigrade species because of this relationship.{{clarify|date=May 2019}} These animals are most closely related to the early evolution of [[Biology:Arthropod|arthropod]]s.<ref name=Brent_Nichols_2005>{{cite thesis |type=PhD |title=Tardigrade Evolution and Ecology |last=Brent Nichols |first=Phillip |publisher=University of South Florida |year=2005 |location=Tampa, FL }}</ref> Tardigrade fossils go as far back as the [[Earth:Cretaceous|Cretaceous]] period in North America. Tardigrades are considered cosmopolitan and can be located in regions all over the world. The eggs and cysts of tardigrades are so durable that they can be carried great distances on the feet of other animals.<ref name="Nelson-CurrentStatus"/>

Tardigrades have survived all five recognized mass extinctions due to their plethora of survival characteristics, including the ability to survive conditions that would be fatal to almost all other animals (see the next section).

The lifespan of tardigrades ranges from three to four months for some species, up to two years for other species, not counting their time in dormant states.<ref>{{cite book |chapter-url=http://digitalcommons.mtu.edu/bryophyte-ecology2/5/ |title=Bryophyte Ecology: Volume 2, Bryological Interaction |last=Glime |first=Janice |year=2010 |chapter=Tardigrades}}</ref>

== Physiology ==
[[File:Hypsibiusdujardini.jpg|thumb|200px|right|''Hypsibius dujardini'' imaged with a scanning electron microscope]]

[[File:Tardigrade (unknown species).jpg|thumb|200px|right|Tardigrade (unknown species, ventral view) imaged using scanning electron microscopy]]

Scientists have reported tardigrades in [[Earth:Hot spring|hot spring]]s, on top of the Himalayas<ref>{{cite encyclopedia |last=Hogan |first=C. Michael |year=2010 |url=http://www.eoearth.org/article/Extremophile?topic=49540 |title=Extremophile |editor1=E. Monosson |editor2=C. Cleveland. |encyclopedia=Encyclopedia of Earth |publisher=National Council for Science and the Environment. |location=Washington, DC }}</ref> ({{convert|6,000|m|ft|disp=semicolon|abbr=on}}, above sea level), and in the [[Earth:Deep sea|deep sea]] ({{convert|-4000|m|ft|abbr=on|disp=semicolon}}); from the polar regions to the [[Astronomy:Equator|equator]], under layers of solid [[Earth:Ice|ice]], and in ocean sediments. Many species can be found in milder environments such as lakes, ponds, and [[Earth:Meadow|meadow]]s, while others can be found in stone walls and roofs. Tardigrades are most common in moist environments but can stay active wherever they can retain at least some moisture.

Tardigrades are thought to be able to survive even complete global mass [[Biology:Extinction event|extinction event]]s caused by [[Physics:Astrophysics|astrophysical events]], such as gamma-ray bursts, or large [[Astronomy:Impact event|meteorite impacts]].<ref name="WP-20170714" /><ref name="NAT-20170714" /> Some of them can withstand extremely cold temperatures down to {{convert|0.01|K|F C|0}} (close to [[Physics:Absolute zero|absolute zero]]), while others can withstand extremely hot temperatures up to {{convert|420|K|F C|-1}}<ref>{{cite magazine|url=https://www.wired.com/2014/03/absurd-creature-week-water-bear/ |title=Absurd Creature of the Week: The Incredible Critter That's Tough Enough to Survive in Space |magazine=Wired |first=Matt |last=Simon |date=21 March 2014}}</ref><ref>{{Cite book |url=https://www.worldcat.org/oclc/340800193 |title=New Zealand inventory of biodiversity |date=2009–2012 |publisher=Canterbury University Press |others=Dennis P. Gordon |isbn=978-1-877257-72-8 |location=Christchurch, N.Z. |pages=28 |oclc=340800193}}</ref> for several minutes, pressures about six&nbsp;times greater than those found in the deepest ocean trenches, [[Physics:Ionizing radiation|ionizing radiation]] at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space.<ref name="NYT-20150907">{{cite news |last=Dean |first=Cornelia |title=The Tardigrade: Practically Invisible, Indestructible 'Water Bears' |url=https://www.nytimes.com/2015/09/08/science/the-tardigrade-water-bear.html |date=7 September 2015 |work={{wipe|The New York Times}} |access-date=7 September 2015}}</ref> Tardigrades that live in harsh conditions undergo an annual process of [[Biology:Cyclomorphosis|cyclomorphosis]], allowing for survival in subzero temperatures.<ref>{{cite journal |title=Cyclomorphosis in Tardigrada: adaptation to environmental constraints |first1=Kenneth Agerlin |last1=Halberg |first2=Dennis |last2=Persson |first3=Hans |last3=Ramløv |first4=Peter |last4=Westh |first5=Reinhardt Møbjerg |last5=Kristensen |first6=Nadja |last6=Møbjerg |date=1 September 2009 |journal=Journal of Experimental Biology |volume=212 |issue=17 |pages=2803–11 |doi=10.1242/jeb.029413 |pmid=19684214|s2cid=23429443 |doi-access= }}</ref>

They are not considered extremophilic because they are not adapted to exploit these conditions, only to endure them. This means that their chances of dying increase the longer they are exposed to the extreme environments,<ref name="Bordenstein" /> whereas true [[Earth:Extremophile|extremophile]]s thrive in a physically or geochemically [[Earth:Extreme environment|extreme environment]] that would harm most other organisms.<ref name="WRD-20140321" /><ref>{{cite journal |doi=10.3390/su2061602 |title=Resistance of Microorganisms to Extreme Environmental Conditions and Its Contribution to Astrobiology |journal=Sustainability |volume=2 |issue=6 |pages=1602–23 |year=2010 |last1=Rampelotto |first1=Pabulo Henrique |bibcode=2010Sust....2.1602R |doi-access=free }}</ref><ref>{{cite journal |doi=10.1038/35059215 |pmid=11234023 |title=Life in extreme environments |journal=Nature |volume=409 |issue=6823 |pages=1092–101 |year=2001 |last1=Rothschild |first1=Lynn J |last2=Mancinelli |first2=Rocco L |bibcode=2001Natur.409.1092R |s2cid=529873 |url=https://zenodo.org/record/1233097 }}</ref>

Tardigrades are one of the few groups of species that are capable of suspending their metabolism (see [[Biology:Cryptobiosis|cryptobiosis]]). While in this state, their metabolism lowers to less than 0.01% of normal and their water content can drop to 1% of normal,<ref name="NYT-20150907" /> and they can go without food or water for more than 30&nbsp;years, only to later rehydrate, forage, and reproduce.<ref name="WRD-20140321" /><ref>{{cite web|last=Brennand |first=Emma |url=https://www.bbc.co.uk/nature/12855775 |title=Tardigrades: Water bears in space |publisher=BBC |date=17 May 2011 |access-date=2013-05-31}}</ref><ref name="auto">{{Cite news |last1=Crowe |first1=John H. |last2=Carpenter |first2=John F. |last3=Crowe |first3=Lois M. |date=October 1998 |title=The role of vitrification in anhydrobiosis |periodical=Annual Review of Physiology |volume=60 |pages=73–103 |pmid=9558455 |doi=10.1146/annurev.physiol.60.1.73 }}</ref><ref name="Guidetti, R. & Jönsson, K.I. 2002 181–187">{{cite journal |doi=10.1017/S095283690200078X |title=Long-term anhydrobiotic survival in semi-terrestrial micrometazoans |journal=Journal of Zoology |volume=257 |issue=2 |pages=181–87 |year=2002 |last1=Guidetti |first1=Roberto |last2=Jönsson |first2=K. Ingemar |citeseerx=10.1.1.630.9839 }}</ref><ref>{{Cite book |url=http://www.hup.harvard.edu/catalog.php?isbn=9780674975910 |title=Life at the Edge of Sight: A Photographic Exploration of the Microbial World |last1=Chimileski |first1=Scott |last2=Kolter |first2=Roberto |publisher=Belknap Press: An Imprint of Harvard University Press |year=2017 |isbn=978-0674975910 |location=Cambridge, MA }}</ref> Many species of tardigrade can survive in a dehydrated state up to five&nbsp;years, or longer in exceptional cases.<ref>{{cite journal |doi=10.1098/rspb.2015.2547 |pmid=26763705 |title=Experimental macroevolution |journal=Proceedings of the Royal Society B: Biological Sciences |volume=283 |issue=1822 |pages=20152547 |year=2016 |last1=Bell |first1=Graham|pmc=4721102 }}</ref><ref>{{cite web |last1=Anderson |first1=David |title=Humans are just starting to understand this nearly invincible creature – and it's fascinating |url=http://www.businessinsider.com/tardigrades-facts-water-bear-science-outer-space-2017-10 |website=BusinessInsider.com |publisher=Business Insider Inc. |access-date=26 October 2017}}</ref> Depending on the environment, they may enter this state via anhydrobiosis, allowing tardigrades, along with some other micro-metazoans (such as worms, rotifers, and crustaceans), protozoans and plants, that ability to survive in inhospitable habitats, as opposed to other living things. In addition to offering protection from desiccation and freezing under normal circumstances, anhydrobiosis also permits resistance to unnatural abiotic extremes such as subzero temperatures,<ref>Jönsson, K., & Bertolani, R. (2001). Facts and fiction about long-term survival in tardigrades. Journal of Zoology, 255(1), 121-123. doi:10.1017/S0952836901001169</ref> cryobiosis, osmobiosis, or anoxybiosis.

Their ability to remain desiccated for such long periods of time was thought to be dependent on high levels of the nonreducing disaccharide [[Chemistry:Trehalose|trehalose]],<ref name="hibshman">{{cite journal | last1=Hibshman | first1=Jonathan D. | last2=Clegg | first2=James S. | last3=Goldstein | first3=Bob | title=Mechanisms of Desiccation Tolerance: Themes and Variations in Brine Shrimp, Roundworms, and Tardigrades | journal=Frontiers in Physiology | volume=11 | date=2020-10-23 | page=592016 | issn=1664-042X | pmid=33192606 | pmc=7649794 | doi=10.3389/fphys.2020.592016| doi-access=free }}</ref> which is commonly seen in other organisms that survive desiccation, and tardigrades have [[Biology:Trehalase|trehalase]] [[Biology:Gene|gene]]s.<ref name="kamilari">{{cite journal | last1=Kamilari | first1=Maria | last2=Jørgensen | first2=Aslak | last3=Schiøtt | first3=Morten | last4=Møbjerg | first4=Nadja | title=Comparative transcriptomics suggest unique molecular adaptations within tardigrade lineages | journal=BMC Genomics | volume=20 | issue=1 | date=2019-07-24 | page=607 | issn=1471-2164 | pmid=31340759 | pmc=6652013 | doi=10.1186/s12864-019-5912-x | doi-access=free }}</ref> However, it has been seen that in both tardigrades and [[Biology:Bdelloidea|bdelloid rotifers]], there is only a partial capability to synthesize trehalose in quantities that may contribute to desiccation tolerance.<ref name=hibshman/><ref>{{Cite journal|last1=Lapinski|first1=Jens|last2=Tunnacliffe|first2=Alan|date=2003|title=Anhydrobiosis without trehalose in bdelloid rotifers|journal=FEBS Letters|language=en|volume=553|issue=3|pages=387–390|doi=10.1016/S0014-5793(03)01062-7|pmid=14572656|s2cid=1692056 |issn=1873-3468|doi-access=}}</ref>

In response to this finding, more research was done on how these animals survived such extreme conditions. It was found that [[Biology:Intrinsically disordered proteins|intrinsically disordered proteins]] (IDPs) were highly expressed in response to desiccation in tardigrades. Additionally, three new IDPs were found to be specific to tardigrades and coined [[Biology:Tardigrade specific proteins|tardigrade specific proteins]] (TDPs). These TDPs may maintain the structure of membranes by associating with the polar heads of the [[Biology:Lipid bilayer|phospholipids bilayers]], avoiding structural damage upon rehydration.<ref>{{cite journal |doi=10.1016/j.molcel.2017.02.018 |pmid=28306513 |pmc=5987194 |title=Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation |journal=Molecular Cell |volume=65 |issue=6 |pages=975–984.e5 |year=2017 |last1=Boothby |first1=Thomas C |last2=Tapia |first2=Hugo |last3=Brozena |first3=Alexandra H |last4=Piszkiewicz |first4=Samantha |last5=Smith |first5=Austin E |last6=Giovannini |first6=Ilaria |last7=Rebecchi |first7=Lorena |last8=Pielak |first8=Gary J |last9=Koshland |first9=Doug |last10=Goldstein |first10=Bob }}</ref> Also, TDPs, being highly hydrophilic, are thought to be involved in a vitrification mechanism, where a glass-like matrix forms within cells to protect the cellular contents upon desiccation.<ref>{{Cite journal|last1=Boothby|first1=Thomas C.|last2=Piszkiewicz|first2=Samantha|last3=Holehouse|first3=Alex|last4=Pappu|first4=Rohit V.|last5=Pielak|first5=Gary J.|date= December 2018|title=Tardigrades use intrinsically disordered proteins to survive desiccation|url=https://linkinghub.elsevier.com/retrieve/pii/S0011224018304024|journal=Cryobiology|language=en|volume=85|pages=137–138|doi=10.1016/j.cryobiol.2018.10.077|hdl=11380/1129511|s2cid=92411591|hdl-access=free}}</ref> Their [[Biology:DNA|DNA]] is further protected from radiation by a protein called "[[Biology:Dsup|dsup]]" (short for ''damage suppressor'').<ref>{{cite news |last1=Tauger |first1=Nathan |last2= Gill |first2=Victoria |url=https://www.bbc.com/news/science-environment-37384466 |title=Survival secret of 'Earth's hardiest animal' revealed |work=BBC News |date=20 September 2016 |access-date=2016-09-21 }}</ref><ref name="Hashimoto et al. 2015">{{cite journal |doi=10.1038/ncomms12808 |pmid=27649274 |pmc=5034306 |title=Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein |journal=Nature Communications |volume=7 |pages=12808 |year=2016 |last1=Hashimoto |first1=Takuma |last2=Horikawa |first2=Daiki D |last3=Saito |first3=Yuki |last4=Kuwahara |first4=Hirokazu |last5=Kozuka-Hata |first5=Hiroko |last6=Shin-i |first6=Tadasu |last7=Minakuchi |first7=Yohei |last8=Ohishi |first8=Kazuko |last9=Motoyama |first9=Ayuko |last10=Aizu |first10=Tomoyuki |last11=Enomoto |first11=Atsushi |last12=Kondo |first12=Koyuki |last13=Tanaka |first13=Sae |last14=Hara |first14=Yuichiro |last15=Koshikawa |first15=Shigeyuki |last16=Sagara |first16=Hiroshi |last17=Miura |first17=Toru |last18=Yokobori |first18=Shin-Ichi |last19=Miyagawa |first19=Kiyoshi |last20=Suzuki |first20=Yutaka |last21=Kubo |first21=Takeo |last22=Oyama |first22=Masaaki |last23=Kohara |first23=Yuji |last24=Fujiyama |first24=Asao |last25=Arakawa |first25=Kazuharu |last26=Katayama |first26=Toshiaki |last27=Toyoda |first27=Atsushi |last28=Kunieda |first28=Takekazu |bibcode=2016NatCo...712808H }}</ref> In this cryptobiotic state, the tardigrade is known as a tun.<ref>Piper, Ross (2007), ''Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals'', Greenwood Press. p.&nbsp;277. {{ISBN|978-0-313-33922-6}}.</ref>

Tardigrades can survive in extreme environments that would kill almost any other animal.<ref name=kamilari/> Extremes at which tardigrades can survive include those of:
* [[Physics:Temperature|Temperature]] – tardigrades can survive:
** A few minutes at {{cvt|151|C|F}}<ref name="survival">{{cite book |doi=10.1007/978-94-007-1896-8_12 |chapter=Survival of Tardigrades in Extreme Environments: A Model Animal for Astrobiology |editor1-first=Alexander V. |editor1-last=Altenbach |editor2-first=Joan M. |editor2-last=Bernhard |editor3-first=Joseph |editor3-last=Seckbach |title=Anoxia |volume=21 |pages=205–17 |series=Cellular Origin, Life in Extreme Habitats and Astrobiology |year=2012 |last1=Horikawa |first1=Daiki D |isbn=978-94-007-1895-1 }}</ref>
** 30&nbsp;years at {{cvt|-20|C|F}}<ref name="CB-201602">{{cite journal|doi=10.1016/j.cryobiol.2015.12.003|title=Recovery and reproduction of an Antarctic tardigrade retrieved from a moss sample frozen for over 30 years|journal=Cryobiology|date=February 2015|last1=Tsujimoto|first1=Megumu|last2=Imura|first2=Satoshi|last3=Kanda|first3=Hiroshi|pmid=26724522|volume=72|issue=1|pages=78–81|doi-access=free}}</ref>
** A few days at {{cvt|-200|C|F K|0}}<ref name="survival"/>
** A few minutes at {{cvt|-272|C|F K|0}}<ref>{{cite journal |last1=Becquerel |first1=Paul |year=1950 |title=La suspension de la vie au dessous de 1/20&nbsp;K absolu par demagnetization adiabatique de l'alun de fer dans le vide les plus eléve |trans-title=The suspension of life below 1/20&nbsp;K absolute by adiabatic demagnetization of iron alum in the highest vacuum |language=fr |journal=Comptes Rendus des Séances de l'Académie des Sciences |volume=231 |issue=4 |pages=261–63 |url=http://gallica.bnf.fr/ark:/12148/bpt6k3183z/f261.item }}</ref>

Research published in 2020 shows that tardigrades are sensitive to high temperatures. Researchers showed it takes 48 hours at {{Cvt|37.1|C|F|abbr=}} to kill half of active tardigrades that have not been acclimated to heat. Acclimation boosted the temperature needed to kill half of active tardigrades to {{Cvt|37.6|C|F|abbr=}}. Tardigrades in the tun state fared a bit better, tolerating higher temperatures. It took heating to {{Cvt|82.7|C|F|abbr=}} to kill half of tun-state tardigrades within one hour. Longer exposure time decreased the temperature needed for lethality, though. For 24 hours of exposure, {{Cvt|63.1|C|F|abbr=}} was enough to kill half of the tun-state tardigrades.<ref>{{cite journal |last1=Neves |first1=Ricardo Cardoso |last2=Hvidepil |first2=Lykke K. B. |last3=Sørensen-Hygum |first3=Thomas L. |last4=Stuart |first4=Robyn M. |last5=Møbjerg |first5=Nadja |title=Thermotolerance experiments on active and desiccated states of Ramazzottius varieornatus emphasize that tardigrades are sensitive to high temperatures |journal=Scientific Reports |date=9 January 2020 |volume=10 |issue=1 |page=94 |doi=10.1038/s41598-019-56965-z |pmid=31919388 |pmc=6952461 |bibcode=2020NatSR..10...94N }}</ref>

* [[Physics:Pressure|Pressure]]&nbsp;– they can withstand the extremely low pressure of a [[Physics:Vacuum|vacuum]] and also very high pressures, more than 1,200&nbsp;times [[Physics:Atmospheric pressure|atmospheric pressure]]. Some species can also withstand pressures of 6,000&nbsp;atmospheres, which is nearly six times the pressure of water in the deepest ocean trench, the Mariana Trench.<ref name="Seki98">{{cite journal |doi=10.1038/27576 |title=Preserving tardigrades under pressure |journal=Nature |volume=395 |issue=6705 |pages=853–54 |year=1998 |last1=Seki |first1=Kunihiro |last2=Toyoshima |first2=Masato |bibcode=1998Natur.395..853S |s2cid=4429569 }}</ref> Tardigrades can survive at altitudes of over {{convert|19,600|ft|m|abbr=off|sp=us}} and depths of over {{convert|15,000|ft|m}} below the surface.{{Citation needed|date=June 2023}}
* [[Physics:Impact (mechanics)|Impact]]s&nbsp;– tardigrades can survive impacts up to about 900 meters per second, and momentary shock pressures up to about 1.14 gigapascals.<ref name="Science-20210518"/>
* [[Medicine:Dehydration|Dehydration]]&nbsp;– the longest that living tardigrades have been shown to survive in a dry state is nearly 10&nbsp;years,<ref name="auto" /><ref name="Guidetti, R. & Jönsson, K.I. 2002 181–187" /> although there is one report of leg movement, not generally considered "survival",<ref name="Jönsson, K. Ingemar & R. Bertolani 2001 121–123">{{cite journal |doi=10.1017/S0952836901001169 |title=Facts and fiction about long-term survival in tardigrades |journal=Journal of Zoology |volume=255 |issue=1 |pages=121–23 |year=2001 |last1=Jönsson |first1=K. Ingemar |last2=Bertolani |first2=Roberto |url=http://lup.lub.lu.se/record/147519 }}</ref> in a 120-year-old specimen from dried moss.<ref name="Franceschi, T. 1948 47–49">{{cite journal |last= Franceschi |first=T. |year=1948 |title=Anabiosi nei tardigradi |trans-title=Anabiosis in Tardigrades |language=it |journal=Bollettino dei Musei e Degli Istituti Biologici dell'Università di Genova |volume=22 |pages=47–49 }}</ref> When exposed to extremely low temperatures, their body composition goes from 85%&nbsp;water to only 3%. Because water expands upon freezing, dehydration ensures the tardigrades' tissues are not ruptured by the expansion of freezing ice.<ref>{{citation|first= Michael |last= Kent |year=2000| title= Advanced Biology| publisher=Oxford University Press}}</ref>
* [[Physics:Radiation|Radiation]]&nbsp;– tardigrades can withstand 1,000 times more radiation than other animals,<ref>{{cite journal| title=Radiation tolerance in the tardigrade Milnesium tardigradum| year=2006| doi=10.1080/09553000600972956| last1=Horikawa| first1=Daiki D.| last2=Sakashita| first2=Tetsuya| last3=Katagiri| first3=Chihiro| last4=Watanabe| first4=Masahiko| last5=Kikawada| first5=Takahiro| last6=Nakahara| first6=Yuichi| last7=Hamada| first7=Nobuyuki| last8=Wada| first8=Seiichi| last9=Funayama| first9=Tomoo| last10=Higashi| first10=Seigo| last11=Kobayashi| first11=Yasuhiko| last12=Okuda| first12=Takashi| last13=Kuwabara| first13=Mikinori| journal=International Journal of Radiation Biology| volume=82| issue=12| pages=843–848| pmid=17178624| s2cid=25354328}}</ref> median lethal doses of 5,000&nbsp;[[Physics:Gray (unit)|Gy]] (of gamma rays) and 6,200&nbsp;Gy (of heavy ions) in hydrated animals (5 to 10&nbsp;Gy could be fatal to a human).<ref name="Horikawa et al. 2006"/> The only explanation found in earlier experiments for this ability was that their lowered water state provides fewer reactants for [[Physics:Ionizing radiation|ionizing radiation]].<ref name="Horikawa et al. 2006">{{cite journal|last1=Horikawa|first1=Daiki D|last2=Sakashita|first2=Tetsuya|last3=Katagiri|first3=Chihiro|last4=Watanabe|first4=Masahiko|last5=Kikawada|first5=Takahiro|last6=Nakahara|first6=Yuichi|last7=Hamada|first7=Nobuyuki|last8=Wada|first8=Seiichi|last9=Funayama|first9=Tomoo|year=2009|title=Radiation tolerance in the tardigrade ''Milnesium tardigradum''|journal=International Journal of Radiation Biology|volume=82|issue=12|pages=843–48|doi=10.1080/09553000600972956|pmid=17178624|last10=Higashi|first10=Seigo|last11=Kobayashi|first11=Yasuhiko|last12=Okuda|first12=Takashi|last13=Kuwabara|first13=Mikinori|s2cid=25354328}}</ref> However, subsequent research found that tardigrades, when hydrated, still remain highly resistant to shortwave UV radiation in comparison to other animals, and that one factor for this is their efficient ability to repair damage to their DNA resulting from that exposure.<ref>{{cite web|last=Horikawa|first=Daiki D.|title=UV Radiation Tolerance of Tardigrades|url=https://astrobiology.nasa.gov/seminars/featured-seminar-channels/early-career-seminars/2012/04/24/uv-radiation-tolerance-of-tardigrades/|publisher=NASA.com|access-date=2013-01-15|archive-url=https://web.archive.org/web/20130218052059/https://astrobiology.nasa.gov/seminars/featured-seminar-channels/early-career-seminars/2012/04/24/uv-radiation-tolerance-of-tardigrades/|archive-date=2013-02-18|url-status=dead}}</ref>

:Irradiation of tardigrade eggs collected directly from a natural substrate (moss) showed a clear dose-related response, with a steep decline in hatchability at doses up to 4&nbsp;kGy, above which no eggs hatched.<ref name="Jönsson 2013">{{cite journal |doi=10.4081/jlimnol.2013.s1.e9 |title=Tolerance to gamma-irradiation in eggs of the tardigrade Richtersius coronifer depends on stage of development |journal=Journal of Limnology |volume=72 |issue=1 |pages=9 |year=2013 |last1=Jönsson |first1=Ingemar |last2=Beltran-Pardo |first2=Eliana |last3=Haghdoost |first3=Siamak |last4=Wojcik |first4=Andrzej |last5=Bermúdez-Cruz |first5=Rosa María |last6=Bernal Villegas |first6=Jaime E |last7=Harms-Ringdahl |first7=Mats |doi-access=free }}</ref> The eggs were more tolerant to radiation late in development. No eggs irradiated at the early developmental stage hatched, and only one egg at middle stage hatched, while eggs irradiated in the late stage hatched at a rate indistinguishable from controls.<ref name="Jönsson 2013"/>
* [[Chemistry:Toxicant|Environmental toxins]]&nbsp;– tardigrades are reported to undergo chemobiosis, a [[Biology:Cryptobiosis|cryptobiotic]] response to high levels of environmental toxins. However, as of 2001, these laboratory results have yet to be verified.<ref name="Jönsson, K. Ingemar & R. Bertolani 2001 121–123" /><ref name="Franceschi, T. 1948 47–49" />

===Survival after exposure to outer space===
Tardigrades are the first known animal to survive after exposure to outer space.<ref name="NS-20080908" /> In September 2007, dehydrated tardigrades were taken into [[Astronomy:Low Earth orbit|low Earth orbit]] on the FOTON-M3 mission carrying the [[Biology:BIOPAN|BIOPAN]] astrobiology payload. For 10&nbsp;days, groups of tardigrades, some of them previously dehydrated, some of them not, were exposed to the hard vacuum of outer space, or vacuum and solar UV&nbsp;radiation.<ref name="low-earth-orbit" /><ref name="WRD-20140321" /><ref name="Science-20080908">{{cite web |title=Creature Survives Naked in Space |url=http://www.space.com/5817-creature-survives-naked-space.html |date=8 September 2008 |publisher=Space.com |access-date=2011-12-22 }}</ref><ref name="MSNBC-20111222">{{cite web |last=Mustain |first=Andrea |title=Weird wildlife: The real land animals of Antarctica |url=http://www.nbcnews.com/id/45766560 |date=22 December 2011 |publisher=NBC News |access-date=2011-12-22 }}</ref> Back on Earth, more than 68% of the subjects protected from solar UV&nbsp;radiation were reanimated within 30&nbsp;minutes following rehydration; although subsequent mortality was high, many of these produced viable embryos.<ref name="low-earth-orbit">{{cite journal |doi=10.1016/j.cub.2008.06.048 |pmid=18786368 |title=Tardigrades survive exposure to space in low Earth orbit |journal=Current Biology |volume=18 |issue=17 |pages=R729–R731 |year=2008 |last1=Jönsson |first1=K. Ingemar |last2=Rabbow |first2=Elke |last3=Schill |first3=Ralph O |last4=Harms-Ringdahl |first4=Mats |last5=Rettberg |first5=Petra |s2cid=8566993 |doi-access=free }}</ref><ref name="NS-20080908">{{Cite news |last=Courtland |first=Rachel |date=8 September 2008 |url=https://www.newscientist.com/article/dn14690-water-bears-are-first-animal-to-survive-space-vacuum.html |title='Water bears' are first animal to survive space vacuum |work=New Scientist |access-date=2011-05-22 }}</ref> In contrast, hydrated samples exposed to the combined effect of vacuum and full solar UV&nbsp;radiation had significantly reduced survival, with only three subjects of ''[[Biology:Milnesium tardigradum|Milnesium tardigradum]]'' surviving.<ref name="low-earth-orbit" /> Also, it was found that the space vacuum did not have a significant effect on egg-laying in either ''R. coronifer'' or ''M. tardigradum''. However, ''M. tardigradum'' exposed to UV radiation had a lower egg laying rate.<ref>{{Cite journal|last1=Jönsson|first1=K. Ingemar|last2=Rabbow|first2=Elke|last3=Schill|first3=Ralph O.|last4=Harms-Ringdahl|first4=Mats|last5=Rettberg|first5=Petra|date= September 2008 |title=Tardigrades survive exposure to space in low Earth orbit|journal=Current Biology|language=en|volume=18|issue=17|pages=R729–R731|doi=10.1016/j.cub.2008.06.048|pmid=18786368|s2cid=8566993|doi-access=free}}</ref> In May 2011, Italian scientists sent tardigrades on board the International Space Station along with extremophiles on [[Engineering:STS-134|STS-134]], the final flight of {{OV|105}}.<ref>{{cite web |author=NASA Staff |url=http://www.nasa.gov/mission_pages/station/research/experiments/BIOKIS.html |archive-url=https://web.archive.org/web/20110417085459/http://www.nasa.gov/mission_pages/station/research/experiments/BIOKIS.html |url-status=dead |archive-date=17 April 2011 |title=BIOKon In Space (BIOKIS) |date=17 May 2011 |publisher=[[Organization:NASA|NASA]] |access-date=2011-05-24}}</ref><ref>{{cite web |last=Brennard |first=Emma |title=Tardigrades: Water bears in space |url=https://www.bbc.co.uk/nature/12855775 |date=17 May 2011 |publisher=BBC |access-date=2011-05-24}}</ref><ref>{{cite web|date=17 May 2011|title=Tardigrades: Water bears in space|publisher=BBC Nature|url=https://www.bbc.co.uk/nature/12855775}}</ref> Their conclusion was that microgravity and cosmic radiation "did not significantly affect survival of tardigrades in flight, and stated that tardigrades represent a useful animal for space research."<ref>{{cite book |hdl=2434/239127 |first1=L. |last1=Rebecchi |first2=T. |last2=Altiero |first3=A. M. |last3=Rizzo |first4=M. |last4=Cesari |first5=G. |last5=Montorfano |first6=T. |last6=Marchioro |first7=R. |last7=Bertolani |first8=R. |last8=Guidetti |chapter=Two tardigrade species on board of the STS-134 space flight |page=89 |title=12th&nbsp;International Symposium on Tardigrada |chapter-url=http://www.tardigrada.net/newsletter/images/symposia/12_Booklet.pdf |isbn=978-989-96860-7-6 |year= 2012 }}</ref><ref>{{Cite web|url=https://news.harvard.edu/gazette/story/2019/07/harvard-study-suggests-asteroids-might-play-key-role-in-spreading-life/|title=Harvard study suggests asteroids might play key role in spreading life|last=Reuell|first=Peter|date=2019-07-08|website=Harvard Gazette|language=en-US|access-date=2019-11-30}}</ref> In November 2011, they were among the organisms to be sent by the U.S.-based Planetary Society on the Russian [[Astronomy:Fobos-Grunt|Fobos-Grunt]] mission's [[Astronomy:Living Interplanetary Flight Experiment|Living Interplanetary Flight Experiment]] to [[Astronomy:Phobos (moon)|Phobos]]; however, the launch failed. In August 2019, scientists reported that a capsule containing tardigrades in a cryptobiotic state may have survived for a while on the [[Astronomy:Moon|Moon]] after the April 2019 crash landing of ''[[Astronomy:Beresheet|Beresheet]]'', a failed Israeli [[Astronomy:Lunar lander|lunar lander]], but in May 2021 it was reported that they were unlikely to have survived the impact.<ref name="WRD-20190805">{{cite news|url=https://www.wired.com/story/a-crashed-israeli-lunar-lander-spilled-tardigrades-on-the-moon/|title=A Crashed Israeli Lunar Lander Spilled Tardigrades On The Moon|last=Oberhaus|first=Daniel|date=5 August 2019|magazine=Wired|access-date=6 August 2019}}</ref><ref name="VOX-20190806">{{cite news|url=https://www.vox.com/science-and-health/2019/8/6/20756844/tardigrade-moon-beresheet-arch-mission|title=Tardigrades, the toughest animals on Earth, have crash-landed on the moon – The tardigrade conquest of the solar system has begun|last=Resnick|first=Brian|date=6 August 2019|work=Vox|access-date=6 August 2019}}</ref><ref name="Science-20210518">{{cite journal  |doi=10.1126/science.abj5282|title=Hardy water bears survive bullet impacts—up to a point|year=2021|last1=O'Callaghan|first1=Jonathan|journal=Science|s2cid=236376996}}</ref>

In recent years, there has also been increased speculation regarding tardigrades' ability to survive on Mars without any life support systems,<ref>Łukasz Kaczmarek. Can Tardigrades Theoretically Survive on Mars? Conference: Early Earth and ExoEarths: origin and evolution of lifeAt: Warszawa, Poland. April 2017. https://www.researchgate.net/publication/319213582_Can_tardigrades_theoretically_survive_on_Mars. Accessed on Oct 16, 2021</ref> but it would still "need stuff to eat" to survive.<ref>{{cite journal |last1=Ledford |first1=Heidi |title=Spacesuits optional for 'water bears' |journal=Nature |date=8 September 2008 |doi=10.1038/news.2008.1087 }}</ref>

== Taxonomy ==
{{multiple image
 | align = right
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 | width     = 180
 | image1    = Echiniscus sp.jpg
 | alt1      =
 | caption1  = Illustration of ''[[Biology:Echiniscus|Echiniscus]]'' sp. from 1861
 | image2    = Echiniscus testudo Doyere 1840 Pl 12 Fig 1.png
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 | caption2  = Drawing of ''Echiniscus testudo'' on a grain of sand
}}

Scientists have conducted [[Biology:Morphology|morphological]] and molecular studies to understand how tardigrades relate to other lineages of ecdysozoan animals. Two plausible placements have been proposed: tardigrades are either most closely related to [[Biology:Arthropoda|Arthropoda]] and [[Biology:Onychophora|Onychophora]], or to nematodes. Evidence for the former is a common result of [[Biology:Morphology|morphological studies]]; evidence for the latter is found in genomic analysis.<ref>Yoshida et al. (2017) Comparative genomics of the tardigrades Hypsibius dujardini and Ramazzottius varieornatus. PLOS.</ref>

{{clade
    |label1=[[Biology:Panarthropoda|Panarthropoda]]
    |1={{clade
    |1={{nowrap|'''Water bears''' ('''Tardigrada''')}} [[File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png|60px]]
    |label2=Antennopoda
    |2={{clade
    |1=Velvet worms ([[Biology:Onychophora|Onychophora]]) [[File:Velvet worm.png|80px]]
    |2=Arthropods ([[Biology:Arthropoda|Arthropoda]]) [[File:Aptostichus simus Monterey County.jpg|70px]]
    }}
  }}
}}

The minute sizes of tardigrades and their membranous integuments make their [[Biology:Fossil|fossil]]ization both difficult to detect and highly unusual. The only known fossil specimens are those from mid-[[Earth:Cambrian|Cambrian]] deposits in Siberia (Orsten fauna) and a few rare specimens from [[Earth:Cretaceous|Cretaceous]] [[Earth:Amber|amber]].<ref name="EotI">{{cite book |date=2005 |publisher=Cambridge University Press |isbn=978-0-521-82149-0 |title=Evolution of the Insects |url=https://archive.org/details/evolutioninsects00grim_110 |url-access=limited |first1=David A. |last1=Grimaldi |first2=Michael S. |last2=Engel |pages=[https://archive.org/details/evolutioninsects00grim_110/page/n110 96]–97}}</ref>

The Siberian tardigrade fossils differ from living tardigrades in several ways. They have three pairs of legs rather than four, they have a simplified head morphology, and they have no posterior head appendages, but they share with modern tardigrades their columnar cuticle construction.<ref name=Budd2001>{{cite journal |doi=10.1078/0044-5231-00034 |title=Tardigrades as 'Stem-Group Arthropods': The Evidence from the Cambrian Fauna |journal=Zoologischer Anzeiger |volume=240 |issue=3–4 |pages=265–79 |year=2001 |last1=Budd |first1=Graham E | url=https://www.researchgate.net/publication/223465290}}</ref> Scientists think they represent a stem group of living tardigrades.<ref name="EotI" />

In October 2021, a new species, ''[[Biology:Paradoryphoribius|Paradoryphoribius chronocaribbeus]]'', was discovered as a fossil in amber that was dated to be 16 million years old.<ref name="LS-20211005">{{cite news |last=Lanese |first=Nicoletta |title=Tardigrade trapped in amber is a never-before-seen species |url=https://www.livescience.com/new-tardigrade-species-found-in-amber |date=5 October 2021 |work=Live Science |accessdate=6 October 2021 }}</ref>

== Evolutionary history ==
[[File:20210000 Luolishaniids Luolishaniidae Luolishaniida.png|thumb|Schematic reconstruction of four luolishaniids, possibly the closest known fossil relatives of modern tardigrades.]]
[[File:OrstenTardigrade.jpg|thumb|Reconstruction of the unnamed "Orsten" tardigrade, from the Cambrian Kuonamka Formation]]
[[File:Fossil_Tardigrade_Dominican_Amber.png|thumb|Reconstruction of ''[[Biology:Paradoryphoribius|Paradoryphoribius]]'', a miocene tardigrade]]
There are multiple lines of evidence that tardigrades are secondarily miniaturized from a larger ancestor,<ref>{{cite journal|doi = 10.1016/j.asd.2018.11.006|pmid = 30447338|title = Miniaturization of tardigrades (water bears): Morphological and genomic perspectives|journal = Arthropod Structure & Development|volume = 48|pages = 12–19|year = 2018|last1 = Gross|first1 = Vladimir|last2 = Treffkorn|first2 = Sandra|last3 = Reichelt|first3 = Julian|last4 = Epple|first4 = Lisa|last5 = Lüter|first5 = Carsten|last6 = Mayer|first6 = Georg| s2cid=53669741 |doi-access = }}</ref> probably a lobopodian and perhaps resembling ''[[Biology:Aysheaia|Aysheaia]]'', which many analyses place close to the divergence of the tardigrade lineage.<ref name="ArtRels">{{cite book|date=2001|publisher=Chapman & Hall|isbn=978-0-412-75420-3|title=Arthropod Relationships|first1=Richard A. |last1=Fortey |first2=Richard H. |last2=Thomas|page=383}}</ref><ref name=Smith2014>{{Cite journal | doi=10.1038/nature13576 | pmid=25132546| title=Hallucigenia's onychophoran-like claws and the case for Tactopoda| journal=Nature| volume=514| issue=7522| pages=363–66| year=2014| last1=Smith| first1=Martin R.| last2=Ortega-Hernández| first2=Javier| bibcode=2014Natur.514..363S| s2cid=205239797| url=http://dro.dur.ac.uk/19108/1/19108.pdf}}</ref> An alternative hypothesis derives [[Biology:Tactopoda|tactopoda]] from a clade encompassing dinocaridids and ''[[Biology:Opabinia|Opabinia]]''.<ref name="Budd1996">{{cite journal|last1=Budd|first1=Graham E|year=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|bibcode=1996Letha..29....1B }}</ref> A 2023 analysis concluded, on the basis of numerous morphological similarities, that [[Biology:Luolishaniidae|luolishaniids]], a group of Cambrian lobopodians, might be the closest known relatives of Tardigrada.<ref name="Kihm2023">{{cite journal |doi=10.1073/pnas.2211251120 |title=Cambrian lobopodians shed light on the origin of the tardigrade body plan |year=2023 |last1=Kihm |first1=Ji-Hoon |last2=Smith |first2=Frank W. |last3=Kim |first3=Sanghee |last4=Rho |first4=Hyun Soo |last5=Zhang |first5=Xingliang |last6=Liu |first6=Jianni |last7=Park |first7=Tae-Yoon S. |journal=Proceedings of the National Academy of Sciences |volume=120 |issue=28 |pages=e2211251120 |pmid=37399417 |pmc=10334802 |bibcode=2023PNAS..12011251K |doi-access=free }}</ref>

The oldest remains of modern tardigrades are those of ''[[Biology:Milnesium swolenskyi|Milnesium swolenskyi]],'' belonging to the living genus ''[[Biology:Milnesium|Milnesium]]'' known from a [[Earth:Late Cretaceous|Late Cretaceous]] ([[Earth:Turonian|Turonian]]) aged specimen of New Jersey amber, around 90 million years old. Another fossil, ''[[Biology:Beorn (tardigrade)|Beorn leggi]]'', is known from a Late [[Earth:Campanian|Campanian]] (~72 million years old) specimen of Canadian amber<ref>{{cite journal |doi=10.1155/1964/48418 |first=Kenneth W. |last=Cooper|title=The first fossil tardigrade: ''Beorn leggi'', from Cretaceous Amber |journal=Psyche: A Journal of Entomology |date=1964 |volume=71 |issue=2 |pages=41–48|doi-access=free }}</ref> and has been placed in its own family ([[Biology:Beornidae|Beornidae]]), but was subsequently suggested as belonging to [[Biology:Hypsibiidae|Hypsibiidae]]. An indeterminate heterotardigrade was also noted from the same deposit.<ref>{{Citation|last1=Guidetti|first1=Roberto|title=Paleontology and Molecular Dating|date=2018|url=http://link.springer.com/10.1007/978-3-319-95702-9_5|work=Water Bears: The Biology of Tardigrades|volume=2|pages=131–143|editor-last=Schill|editor-first=Ralph O.|place=Cham|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-95702-9_5|isbn=978-3-319-95701-2|access-date=2020-11-24|last2=Bertolani|first2=Roberto|series=Zoological Monographs }}</ref>

The enigmatic [[Biology:Panarthropoda|panarthropod]] ''[[Biology:Sialomorpha|Sialomorpha]]'' found in 30-million year old [[Earth:Dominican amber|Dominican amber]], while not classifiable as a tardigrade, shows some apparent affinities.<ref name="Poinar2019">{{Cite journal|last1=Poinar|first1=George|last2=Nelson|first2=Diane R.|date=September 28, 2019|title=A new microinvertebrate with features of mites and tardigrades in Dominican amber|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/ivb.12265|journal=Invertebrate Biology |volume=138|issue=4|doi=10.1111/ivb.12265|s2cid=204157733|issn=1077-8306}}</ref><ref name="GIZMODO2019">{{cite web|url=https://www.gizmodo.com.au/2019/10/youve-heard-of-water-bears-but-how-about-these-ancient-mold-pigs/|title=You've Heard Of Water Bears, But How About These Ancient Mould Pigs?|author=Dvorsky, George|date=October 9, 2019|publisher=Gizmodo|accessdate=October 9, 2019}}</ref>

== Genomes and genome sequencing ==
Tardigrade [[Biology:Genome|genome]]s vary in size, from about 75 to 800 mega[[Biology:Base pair|base pair]]s of DNA.<ref>{{cite web|url=http://www.genomesize.com/search.php?search=type&value=Tardigrades&display=100|title=Genome Size of Tardigrades}}</ref> ''Hypsibius exemplaris'' (formerly ''Hypsibius dujardini'') has a compact genome of 100 megabase pairs<ref>{{cite journal|last1=Yoshida|first1=Yuki|last2=Koutsovoulos|first2=Georgios|last3=Laetsch|first3=Dominik R.|last4=Stevens|first4=Lewis|last5=Kumar|first5=Sujai|last6=Horikawa|first6=Daiki D.|last7=Ishino|first7=Kyoko|last8=Komine|first8=Shiori|last9=Kunieda|first9=Takekazu|last10=Tomita|first10=Masaru|last11=Blaxter|first11=Mark|last12=Arakawa|first12=Kazuharu|last13=Tyler-Smith|first13=Chris|title=Comparative genomics of the tardigrades Hypsibius dujardini and Ramazzottius varieornatus|journal=PLOS Biology|date=27 July 2017|volume=15|issue=7|pages=e2002266|doi=10.1371/journal.pbio.2002266|pmid=28749982|pmc=5531438 |doi-access=free }}</ref> and a generation time of about two weeks; it can be cultured indefinitely and cryopreserved.<ref>{{cite journal |doi=10.1016/j.ydbio.2007.09.055 |pmid=17996863 |title=The tardigrade Hypsibius dujardini, a new model for studying the evolution of development |journal=Developmental Biology |volume=312 |issue=2 |pages=545–559 |year=2007 |last1=Gabriel |first1=Willow N |last2=McNuff |first2=Robert |last3=Patel |first3=Sapna K |last4=Gregory |first4=T. Ryan |last5=Jeck |first5=William R |last6=Jones |first6=Corbin D |last7=Goldstein |first7=Bob |doi-access= }}</ref>

The genome of ''Ramazzottius varieornatus'', one of the most stress-tolerant species of tardigrades, was sequenced by a team of researchers from the University of Tokyo in 2015. While previous research had claimed that around one-sixth of the genome had been acquired from other organisms,<ref>Fiona Macdonald (7 December 2015). "[https://www.sciencealert.com/new-research-casts-doubt-on-the-claim-that-tardigrades-get-one-sixth-of-dna-from-other-species New Research Casts Doubt on The Claim That Tardigrades Get 1/6 of DNA From Other Species]". ''ScienceAlert''.</ref> it is now known that less than 1.2% of its genes were the result of [[Biology:Horizontal gene transfer|horizontal gene transfer]]. They also found evidence of a loss of gene pathways that are known to promote damage due to stress. This study also found a high expression of novel tardigrade-unique proteins, including [[Biology:Dsup|Damage suppressor (Dsup)]], which was shown to protect against DNA damage from [[Physics:X-ray|X-ray]] radiation. The same team applied the Dsup protein to human cultured cells and found that it suppressed X-ray damage to the human cells by around 40%.<ref name="Hashimoto et al. 2015"/> While the exact mechanism of DNA protection is largely unknown, the results from an August 2020 study suggest that strong electrostatic attractions along with high protein flexibility help form a molecular aggregate, which allows Dsup to shield DNA.<ref>{{Cite journal|last1=Mínguez-Toral|first1=Marina|last2=Cuevas-Zuviría|first2=Bruno|last3=Garrido-Arandia|first3=María|last4=Pacios|first4=Luis F.|date= December 2020|title=A computational structural study on the DNA-protecting role of the tardigrade-unique Dsup protein|url= |journal=Scientific Reports|language=en|volume=10|issue=1|pages=13424|doi=10.1038/s41598-020-70431-1|issn=2045-2322|pmc=7414916|pmid=32770133|bibcode=2020NatSR..1013424M}}</ref>

The [[Biology:Dsup|Dsup]] proteins of tardigrades ''Ramazzottius varieornatus'' and ''Hypsibius exemplaris'' promote survival by binding to [[Biology:Nucleosome|nucleosome]]s and protecting [[Biology:Chromosome|chromosomal]] [[Biology:DNA|DNA]] from [[Chemistry:Hydroxyl radical|hydroxyl radical]]s.<ref>Chavez C, Cruz-Becerra G, Fei J, Kassavetis GA, Kadonaga JT. The tardigrade damage suppressor protein binds to nucleosomes and protects DNA from hydroxyl radicals. Elife. 2019 Oct 1;8:e47682. doi: 10.7554/eLife.47682. PMID 31571581; PMCID: PMC6773438</ref>  The Dsup protein of ''R. varieornatus'' also confers resistance to [[Physics:Ultraviolet#Subtypes|ultraviolet-C]] by upregulating [[Biology:DNA repair|DNA repair]] genes that protect the genomic DNA from the damages introduced by UV irradiation.<ref>Ricci C, Riolo G, Marzocchi C, Brunetti J, Pini A, Cantara S. The Tardigrade Damage Suppressor Protein Modulates Transcription Factor and DNA Repair Genes in Human Cells Treated with Hydroxyl Radicals and UV-C. Biology (Basel). 2021 Sep 27;10(10):970. doi: 10.3390/biology10100970. PMID 34681069; PMCID: PMC8533384</ref>

== Ecological importance ==
Many organisms that live in aquatic environments feed on species such as nematodes, tardigrades, bacteria, algae, mites, and collembolans.<ref>{{Cite journal |last=Kinchin |first=IM |date=1987 |title=The moss fauna 1; Tardigrades. |journal=Journal of Biological Education |volume=21 |issue=4 |pages=288–90|doi=10.1080/00219266.1987.9654916 }}</ref> Tardigrades work as pioneer species by inhabiting new developing environments. This movement attracts other invertebrates to populate that space, while also attracting predators.<ref name=Brent_Nichols_2005/>

== In popular culture ==
*The short-story "Bathybia" by [[Biography:Douglas Mawson|Douglas Mawson]], published in the book ''Aurora Australis'' in 1909, deals with an expedition to the South Pole where the team encounters giant mushrooms and arthropods, and a four-foot-long tardigrade waking from hibernation; this scares the narrator from his sleep, and he realizes it was all a dream.<ref>{{cite book |last1=Blum |first1=Hester |title=The News at the Ends of the Earth: The Print Culture of Polar Exploration |date=2019 |publisher=Duke University Press |isbn=9781478004486 |page=170 |url=https://ia803005.us.archive.org/13/items/newsatendsofeart00blum/newsatendsofeart00blum.pdf}}</ref>
* When the characters in the superhero films ''Ant-Man'' (2015) and ''Ant-Man and the Wasp'' (2018) shrink themselves to enter the "Quantum Realm", they encounter tardigrades.<ref>{{cite web|url=https://www.dailydot.com/parsec/quantum-realm-ant-man/|title=How the Quantum Realm could play into future Marvel films|website=[[Company:The Daily Dot|The Daily Dot]]|date=10 July 2018|access-date=29 July 2018}}</ref><ref>{{cite news|url=https://www.nytimes.com/2018/07/06/movies/antman-and-the-wasp-science.html|title=The Science (and the Scientists) Behind 'Ant-Man'|access-date=29 July 2018|newspaper=The New York Times|date=6 July 2018|last1=King|first1=Darryn}}</ref><ref>{{cite web|url=https://nationalpost.com/entertainment/movies/ant-man-and-the-wasp-needs-a-little-help|title=Ant-Man and the Wasp needs a little help|quote=Ant-Man and the Wasp is still intermittent fun, particularly for fans of tardigrades, the water-dwelling micro-fauna that had a brief cameo in the first Ant-Man, and get their well-deserved close-up in this one.|date=4 July 2018|access-date=29 July 2018}}{{Dead link|date=July 2020 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
* In the 2015 sci-fi horror film ''Harbinger Down'', the characters have to deal with deadly mutated tardigrades.<ref>{{cite magazine|url=https://ew.com/article/2014/06/10/lance-henriksen-harbinger-down/|title='Harbinger Down': New trailer for creature feature|magazine=Entertainment Weekly|access-date=3 October 2018}}</ref><ref>{{cite web|url=https://bloody-disgusting.com/reviews/3351171/review-harbinger-bleak-tiring-vanilla-creature-feature/|title='Harbinger Down' Review: A Bleak & Vanilla Creature Feature|website=bloody-disgusting.com|date=31 July 2015|access-date=3 October 2018}}</ref>
* The second arc of the comic book ''Paper Girls'' (2015) features a pair of tardigrades that have been enlarged to a massive size as a side effect of time travel.<ref>{{cite news |last1=Raftery |first1=Brian |title=If You Only Read One Comic This Month, Make It 'Paper Girls' |url=https://www.wired.com/2016/10/paper-girls-must-read/ |access-date=8 August 2019 |magazine=Wired |date=5 October 2016}}</ref>
* Musician Cosmo Sheldrake imagines himself a tardigrade in his 2015 "Tardigrade Song".<ref>{{cite web |title=Cosmo Sheldrake shares new single and tour dates |url=http://diymag.com/2015/02/02/cosmo-sheldrake-shares-new-single-and-tour-dates |website=DIY |date=2 February 2015 |access-date=21 September 2019 |language=en}}</ref><ref>{{cite web |title=Cosmo Sheldrake - Tardigrade Song {{!}} Folk Radio |url=https://www.folkradio.co.uk/2015/02/cosmo-sheldrake-tardigrade-song/ |website=Folk Radio UK - Folk Music Magazine |access-date=21 September 2019 |date=2 February 2015}}</ref>
* In ''Star Trek: Discovery'' (2017), the alien "Ripper" creature who is used to "navigate" through a galactic [[Biology:Mycelium|mycelium]] network and instantly move the ship from one location in the galaxy to another is referred to as a "giant space tardigrade" and said to be a cousin of the tardigrade.<ref>{{cite web|url=https://www.inverse.com/article/37284-tardigrade-star-trek-ripper|title=The Scientific Truth About Ripper the 'Star Trek' Tardigrade Is a Huge Relief|date=10 October 2017 |access-date=5 September 2018}}</ref><ref>{{cite web|url=https://www.forbes.com/sites/stevensalzberg/2017/10/30/new-star-trek-series-makes-massive-science-blunder/#47c2d5631b37|title=New 'Star Trek' Series Makes Massive Science Blunder|first=Steven|last=Salzberg|website={{wipe|Forbes}}|access-date=5 September 2018}}</ref>
* The 2017 ''South Park'' episode "Moss Piglets" involves a science experiment in which tardigrades learn to dance to the music of Taylor Swift.<ref>{{cite web|url=https://www.forbes.com/sites/danidiplacido/2017/11/16/south-park-review-cartman-creates-a-monster-in-moss-piglets/#4bec978e3dca|title='South Park' Review: Cartman Creates A Monster In 'Moss Piglets'|first=Dani Di|last=Placido|website={{wipe|Forbes}}|access-date=29 July 2018}}</ref><ref>{{cite web|url=http://www.ibtimes.com.au/south-park-season-21-episode-8-live-stream-moss-piglets-1561712|title='South Park' season 21 episode 8 live stream: 'Moss Piglets'|date=15 November 2017|access-date=29 July 2018}}</ref>
* The 2018 ''[[Religion:Family Guy|Family Guy]]'' episode "Big Trouble in Little Quahog" features Stewie and Brian being shrunk to a microscopic level, during which they meet a group of friendly tardigrades or "water bears" who help them.<ref>{{cite web |author1=Nicole Yang |title=Kyrie Irving got a credit in the most recent episode of 'Family Guy'. Meet "Vernon The Water Bear." |url=https://www.boston.com/sports/boston-celtics/2018/10/22/kyrie-irving-family-guy |publisher=Boston Globe Media Partners, LLC |access-date=22 October 2018 |date=22 October 2018}}</ref>
* The plot in the 2021 ''Sam & Max: This Time It's Virtual'' video game includes an abandoned tardigrade amusement park, "Cap'N Aquabear's Funtime Park".<ref>{{cite magazine|author1=Marcus Stewart|title=How Sam & Max: This Time It's Virtual! Brings The Comedic Crime-Fighting Duo To VR|magazine=Game Informer |url=https://www.gameinformer.com/exclusive/2021/03/31/how-sam-max-this-time-its-virtual-brings-the-comedic-crime-fighting-duo-to-vr|access-date=21 March 2021|date=21 March 2021}}</ref>

== See also ==
* [[Biology:List of tardigrades of South Africa|List of tardigrades of South Africa]]
* [[Astronomy:List of microorganisms tested in outer space|List of microorganisms tested in outer space]]
* [[Astronomy:Living Interplanetary Flight Experiment|Living Interplanetary Flight Experiment]], study of selected microorganisms in outer space
* [[Biology:Panspermia|Panspermia]]

== References ==
{{Reflist}}

== External links ==
{{Refbegin|30em}}
* [http://www.tardigrada.net/ Tardigrada Register] {{Webarchive|url=https://web.archive.org/web/20150320234913/http://www.tardigrada.net/ |date=2015-03-20 }}
*{{APOD |date=6 March 2013 |title=Tardigrade in Moss}}
{{refend}}

{{Animalia}}
{{Extremophiles}}
{{Life on Earth}}
{{Taxonbar|from=Q5194}}

[[Category:Tardigrades| ]]
[[Category:Extant Cambrian first appearances]]
[[Category:Polyextremophiles]]
[[Category:Psychrophiles]]
[[Category:Radiodurants]]
[[Category:Thermophiles]]
[[Category:Xerophiles]]

{{Sourceattribution|Tardigrade}}