phylum of animals, also known as water bears
(Redirected from Tardigrada)

Tardigrades ('water bears') are microscopic, segmented animals.[1] They live in water. They are members of the tactopod phylum Tardigrada, part of the superphylum Ecdysozoa. They are also known as water bears or moss piglets.[2][3]

Temporal range: Turonian–Recent
SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2.png
Milnesium tardigradum, a eutardigrade
Echiniscus succineus (10.3897-evolsyst.3.33580) Figure 2 (cropped).jpg
Echiniscus succineus, a heterotardigrade
Scientific classification e
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Superphylum: Ecdysozoa
(unranked): Panarthropoda
Phylum: Tardigrada
Spallanzani, 1777

Tardigrades were first described in 1773.[4] Their name means "slow stepper". There are more than 1000 different species of tardigrade.[5]

Tardigrades have a cylindrical shape with four segments, each with two legs. Each leg has little claws.[5] The biggest adults may reach a body length of 1.2 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm.[4] Tardigrades feed on plant cells by penetrating the cell wall and eating what is inside. Some tardigrades are carnivores.[6][5]

Tardigrades are eutelic: all adult tardigrades of the same species have the same number of cells. Some species have as many as 40,000 cells in each adult, while others have far fewer.[7][8]

Tardigrades can be found in many habitats: in moss,[6] freshwater,[6] the Himalayas,[4] and the ocean.[4] They are one of the few animals that can be found on the highest mountains and the deepest seas.[4] About 83% of the known species live on land, the other 17% live in water.[5]


Tardigrades are able to live in environments that would kill most animals.[1]

In 2007, scientists discovered that some tardigrades were able to survive 10 days in outer space.[9][10] This meant living in a vacuum with severe radiation.[11][12][13]

Tardigrades can survive more than ten years without water.[14] Tardigrades can survive extreme temperatures. They can live for a few hours in temperatures close to absolute zero[6] and above boiling point.[14] They can survive a few minutes in temperatures as high as 151 °C (304 °F).[15][16] Some tardigrades can survive being frozen for over 30 years.[17]

Tardigrades can also survive radiation at hundreds of times a level that would be deadly to humans,[18][19] as well as toxic environments[20] and severe impact events.[21]

In 2019, a lander with Tardigrades on board crashed on the moon. It is believed that the Tardigrades survived and could live for a few years on the moon.[22][23]

The tardigrades slow their metabolism to survive these environments.[24] See rotifers for a similar account of survivability.


  1. 1.0 1.1 Neuman, Yair (October 2006). "Cryptobiosis: A new theoretical perspective". Progress in Biophysics and Molecular Biology. Elsevier. 92 (2): 258–267. doi:10.1016/j.pbiomolbio.2005.11.001. PMID 16380155.
  2. Miller, William (2017-02-06). "Tardigrades". American Scientist. Archived from the original on 2018-04-14. Retrieved 2018-04-13.
  3. Simon, Matt (21 March 2014). "Absurd Creature of the Week: the incredible Ccritter that's tough enough to survive in the vacuum of space". Wired. Archived from the original on 2014-03-25. Retrieved 2014-03-21.
  4. 4.0 4.1 4.2 4.3 4.4 Riffenburgh, Beau (2007). Encyclopedia of the Antarctic. Encyclopedia of the Antarctic. Vol. 1 (illustrated ed.). CRC Press. p. 983. ISBN 978-0-415-97024-2.
  5. 5.0 5.1 5.2 5.3 Margulis, Lynn; Schwartz, Karlene V. (1998). Five kingdoms: an illustrated guide to the phyla of life on earth (illustrated ed.). Elsevier. p. 324. ISBN 978-0-7167-3027-9.
  6. 6.0 6.1 6.2 6.3 "Tardigrade (animal)". Encyclopædia Britannica. Archived from the original on 2011-01-23. Retrieved 2011-01-02.
  7. Seki, Kunihiro; Toyoshima, Masato 1998. Preserving tardigrades under pressure. Nature 395 (6705): 853–854. Preserving tardigrades under pressure | Nature Archived 2021-02-14 at the Wayback Machine
  8. Kinchin, Ian M. 1994. The biology of tardigrades. Ashgate Publishing.
  9. Jönsso, K. Ingema; et al. (2008). "Tardigrades survive exposure to space in low Earth orbit". Current Biology. 18 (17): R729–R731. doi:10.1016/j.cub.2008.06.048. PMID 18786368. S2CID 8566993. Archived from the original on 9 April 2011. Retrieved 1 January 2011.
  10. Whalen, Joann K.; Sampedro, Luis (2010). Soil Ecology and Management (illustrated ed.). CABI. p. 73. ISBN 978-1-84593-563-4.
  11. "Creature survives naked in space". Space.com. 8 September 2008. Archived from the original on 2018-10-11. Retrieved 2011-12-22.
  12. Jönsson, K. Ingemar; Rabbow, Elke; Schill, Ralph O; Harms-Ringdahl, Mats; Rettberg, Petra (2008). "Tardigrades survive exposure to space in low Earth orbit". Current Biology. 18 (17): R729–R731. doi:10.1016/j.cub.2008.06.048. PMID 18786368. S2CID 8566993.
  13. Courtland, Rachel (8 September 2008). "'Water bears' are first animal to survive space vacuum". New Scientist. Archived from the original on 2011-05-11. Retrieved 2011-05-22.
  14. 14.0 14.1 Grimaldi, David; Engel, Michael S. (2005). Michael S. Engel (ed.). Evolution of the insects. Cambridge Evolution Series (illustrated, reprint ed.). Cambridge University Press. p. 97. ISBN 978-0-521-82149-0.
  15. Horikawa, Daiki D (2012). "Survival of tardigrades in extreme environments: a model animal for astrobiology". In Altenbach, Alexander V.; Bernhard, Joan M.; Seckbach, Joseph (eds.). Anoxia. Cellular Origin: life in extreme habitats and astrobiology. Vol. 21. pp. 205–17. doi:10.1007/978-94-007-1896-8_12. ISBN 978-94-007-1895-1.
  16. "Extreme animals - Astrobiology Magazine". Astrobiology Magazine. 2002-09-01. Archived from the original on 2017-05-31. Retrieved 2018-02-12.
  17. BBC News: News from elewhere. Japan: 'Water bear' reproduces after 30 years on ice - BBC News Archived 2021-02-23 at the Wayback Machine
  18. "Radiation tolerance in the tardigrade Milnesium tardigradum" (PDF).[permanent dead link]
  19. Horikawa, Daiki D.; Sakashita, Tetsuya; Katagiri, Chihiro; Watanabe, Masahiko; Kikawada, Takahiro; Nakahara, Yuichi; Hamada, Nobuyuki; Wada, Seiichi; Funayama, Tomoo (2006-01-01). "Radiation tolerance in the tardigrade Milnesium tardigradum". International Journal of Radiation Biology. 82 (12): 843–848. doi:10.1080/09553000600972956. ISSN 0955-3002. PMID 17178624. S2CID 25354328. Archived from the original on 2021-10-31. Retrieved 2021-10-31.
  20. Jönsson, K. Ingemar; Bertolani, Roberto (2001). "Facts and fiction about long-term survival in tardigrades". Journal of Zoology. 255 (1): 121–3. doi:10.1017/S0952836901001169. Archived from the original on 2021-10-31. Retrieved 2021-10-31.
  21. Sloan, David; Alves Batista, Rafael; Loeb, Abraham (2017). "The Resilience of Life to Astrophysical Events". Scientific Reports. 7 (1): 5419. arXiv:1707.04253. Bibcode:2017NatSR...7.5419S. doi:10.1038/s41598-017-05796-x. PMC 5511186. PMID 28710420.
  22. Sample, Ian (6 August 2019). "Tardigrades may have survived spacecraft crashing on moon". The Guardian. Archived from the original on 6 August 2019. Retrieved 7 August 2019.
  23. Resnick, Brian (6 August 2019). "Tardigrades, the toughest animals on Earth, have crash-landed on the moon". Vox. Archived from the original on 29 November 2019. Retrieved 7 August 2019.
  24. "The almost indestructible water bear". Awake!. 88 (3): 30. March 2007. ISSN 0005-237X. Archived from the original on 7 August 2019. Retrieved 7 August 2019.{{cite journal}}: CS1 maint: date and year (link)