Difference between revisions of "Tardigrade" - New World Encyclopedia
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| − | '''Tardigrade''', or '''water bear''', is any of the various small, segmented [[invertebrate]]s comprising the phylum '''Tardigrada'', characterized by [[symmetry#Bilateral symmetry|bilateral | + | '''Tardigrade''', or '''water bear''', is any of the various very small, segmented [[invertebrate]]s comprising the phylum '''Tardigrada''', characterized by [[symmetry (biology)#Bilateral symmetry|bilateral symmetry]], four pairs of unjointed legs, and a eutelic body (fixed number of body cells in mature adults of any one species). There are more than 700 known species (Ramel 2008). |
| + | Water bears are able to survive in extreme environments that would kill almost any other animal. They can survive temperatures close to absolute zero (Bertolani et al. 2004), temperatures as high as 151°C (303°F), one thousand times more radiation than any other animal (Horikawa 2006), nearly a decade without water, and can also survive in a vacuum like that found in space. | ||
| − | |||
| − | + | ==Description and overview== | |
| − | + | Tardigrades are small, bilaterally symmetrical, segmented [[animal]]s, similar and probably related to the [[arthropod]]s. The biggest adults may reach a body length of 1.5 millimeters and the smallest below 0.1 millimeters. ''Echiniscoides sigimunmde'' is the largest known tardigrade species and is found in European and Asian habitats (Ramel 2008). | |
| − | + | Tardigrades have a body with four [[Segment (biology)|segment]]s (not counting the head). They have eight legs, but they are not jointed as in arthropods. The feet have claws or toes. The [[cuticle]] contains [[chitin]] and is [[ecdysis|molted]]. | |
| − | + | ||
| − | Tardigrades have a body with four [[Segment (biology)|segment]]s (not counting the head) | + | Tardigrades have a [[ventral nervous system]] with one [[ganglion]] per segment, and a multilobed [[brain]]. The body cavity is partially a coelom, with a true coelom near the gonads (coelomic pouch), but most of the body cavity is a hemocoel rather than a coelom. , |
| + | |||
| + | Tardigrades lack circulatory and respiratory systems (Ramel 2008). Their digestive system is a straight through gut with an anus (Ramel 2008). The [[pharynx]] is of a triradiate, muscular, sucking kind, armed with [[stylet]]s. | ||
| + | |||
| + | Tardigrades are gonochoristic (either male or female), although in some species only females have been found, leading to the presumption that these species are [[parthenogenesis|parthenogenetic]]. Males and females are usually present, each with a single [[gonad]]. Tardigrades are oviparous. | ||
| + | |||
| + | Tardigrades are [[eutelic]]. Eutelic organisms have a fixed number of cells when they reach maturity, the exact number being constant for any one species. Development proceeds by cell division until maturity; further growth occurs via cell enlargement only. Some tardigrade species have as many as about 40,000 cells in each adult's body, others have far fewer (Seki and Toyoshima 1998; Kinchin 1994). | ||
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| + | Tardigrades occur over the entire world, from the high [[Himalayas]] (above 6,000 meters), to the [[deep sea]] (below 4,000 meters) and from the [[polar region]]s to the [[equator]]. Most live in fresh water, often in environments subject to frequent drying and re-wetting (Ramel 2008). They are found on [[lichen]]s and [[moss]]es, and in [[dune]]s, [[beach]]es, [[soil]], and [[ocean|marine]] or [[freshwater]] sediments, where they may occur quite frequently (up to 25,000 animals per [[liter]]). Tardigrades often can be found by soaking a piece of moss in spring water (Goldsteing and Blaxter 2002). | ||
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| + | Tardigrades were first described by [[Johann August Ephraim Goeze]] in 1773 (''kleiner Wasserbär'' = little water bear). The name Tardigrada means "slow walker" and was given by [[Lazzaro Spallanzani|Spallanzani]] in 1777. Freshly hatched [[larva]]e may be smaller than 0.05 mm. | ||
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| + | = | ||
==Ecology and life history== | ==Ecology and life history== | ||
===Feeding ecology=== | ===Feeding ecology=== | ||
| − | Most tardigrades are [[phytophagous]] or bacteriophagous, but some are [[predator]]y<ref>{{cite web |author=Lindahl, K. |date= | + | Most tardigrades are [[phytophagous]] or bacteriophagous, but some are [[predator]]y<ref>{{cite web |author=Lindahl, K. |date=2008-03-15 |title=Tardigrade Facts |url=http://www.iwu.edu/~tardisdp/tardigrade_facts.html}}</ref> (e.g. ''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=The Journal of Animal Ecology|volume=46|issue=1|year=1977|pages=263-279}}</ref> |
==Physiology== | ==Physiology== | ||
=== Extreme environments === | === Extreme environments === | ||
| − | Tardigrades are very hardy animals; scientists have reported their existence in [[hot spring]]s, on top of the [[Himalayas]], under layers of solid [[ice]] and in ocean sediments. Many species can be found in a milder environment like [[lake]]s, [[pond]]s and [[meadow]]s, while others can be found in stone walls and roofs. | + | Tardigrades are very hardy animals; scientists have reported their existence in [[hot spring]]s, on top of the [[Himalayas]], under layers of solid [[ice]] and in ocean sediments. Many species can be found in a milder environment like [[lake]]s, [[pond]]s and [[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 one of the few groups of species that are capable of reversibly suspending their [[metabolism]] and going into a state of [[cryptobiosis]]. Several species regularly survive in a dehydrated state for nearly ten years. Depending on the environment they may enter this state via [[anhydrobiosis]], cryobiosis, osmobiosis or anoxybiosis. While in this state their metabolism lowers to less than 0.01% of what is normal and their water content can drop to 1% of normal. Their ability to remain desiccated for such a long period is largely dependent on the high levels of the non-reducing [[sugar]] [[trehalose]], which protects their [[cell membrane|membrane]]s. | Tardigrades are one of the few groups of species that are capable of reversibly suspending their [[metabolism]] and going into a state of [[cryptobiosis]]. Several species regularly survive in a dehydrated state for nearly ten years. Depending on the environment they may enter this state via [[anhydrobiosis]], cryobiosis, osmobiosis or anoxybiosis. While in this state their metabolism lowers to less than 0.01% of what is normal and their water content can drop to 1% of normal. Their ability to remain desiccated for such a long period is largely dependent on the high levels of the non-reducing [[sugar]] [[trehalose]], which protects their [[cell membrane|membrane]]s. | ||
Tardigrades have been known to withstand the following extremes while in this state: | Tardigrades have been known to withstand the following extremes while in this state: | ||
| − | * '''[[Temperature]]''' | + | * '''[[Temperature]]'''—tardigrades can survive being heated for a few minutes to 151°C or being chilled for days at -200°C, or for a few minutes at -272°C. (1° warmer than [[absolute zero]]).<ref>{{cite web |author=Ramel, G. |date=2005-11-11 |title=The Water Bears (Phylum Tardigrada) |url=http://www.earthlife.net/inverts/tardigrada.html}}</ref> |
| − | *'''[[Pressure]]''' | + | *'''[[Pressure]]'''—they can withstand the extremely low pressure of a [[vacuum]] and also very high pressures, many times greater than [[atmospheric pressure]]. It has recently been proven that they can survive in the vacuum of space. Recent research has notched up another feat of endurability; apparently they can withstand 6,000 atmospheres pressure, which is nearly six times the pressure of water in the deepest ocean trench. <ref>{{cite journal |quotes=no |author=Seki, K & Toyoshima, M. |year=1998 |title=Preserving tardigrades under pressure |journal=[[Nature]] |volume=395 |pages=853–854}}</ref> |
*'''[[Dehydration]]''' - tardigrades have been shown to survive nearly one decade in a dry state.<ref>{{cite journal |quotes=no | author=Guidetti, R. & Jönsson, K.I. |year=2002 |title=Long-term anhydrobiotic survival in semi-terrestrial micrometazoans |journal=[[Journal of Zoology]] |volume=257 |pages=181-187}}</ref> Another researcher reported that a tardigrade survived over a period of 120 years in a dehydrated state, but soon died after 2 to 3 minutes.<ref>Manga Science Volume VI by Yoshitoh Asari, ISBN-05-202039-1, 1998</ref> Subsequent research has cast doubt on its accuracy since it was only a small movement in the leg.<ref>{{cite journal |quotes=no | author=Guidetti, R. & Jönsson, K.I. |year=2002 |title=Long-term anhydrobiotic survival in semi-terrestrial micrometazoans |journal=[[Journal of Zoology]] |volume=257}}</ref> | *'''[[Dehydration]]''' - tardigrades have been shown to survive nearly one decade in a dry state.<ref>{{cite journal |quotes=no | author=Guidetti, R. & Jönsson, K.I. |year=2002 |title=Long-term anhydrobiotic survival in semi-terrestrial micrometazoans |journal=[[Journal of Zoology]] |volume=257 |pages=181-187}}</ref> Another researcher reported that a tardigrade survived over a period of 120 years in a dehydrated state, but soon died after 2 to 3 minutes.<ref>Manga Science Volume VI by Yoshitoh Asari, ISBN-05-202039-1, 1998</ref> Subsequent research has cast doubt on its accuracy since it was only a small movement in the leg.<ref>{{cite journal |quotes=no | author=Guidetti, R. & Jönsson, K.I. |year=2002 |title=Long-term anhydrobiotic survival in semi-terrestrial micrometazoans |journal=[[Journal of Zoology]] |volume=257}}</ref> | ||
| − | *'''[[Radiation]]''' | + | *'''[[Radiation]]'''—as shown by Raul M. May from the University of Paris, tardigrades can withstand 5,700 [[Gray (unit)|gray]]s or 570,000 [[Rad (unit)|rad]]s of [[x-rays|x-ray radiation]]. (Ten to twenty grays or 1,000-2,000 rads could be fatal to a human). The only explanation thus far for this ability is that their lowered hydration state provides fewer reactants for the ionizing radiation. |
| − | Recent experiments conducted by Cai and Zabder have also shown that these water bears can undergo | + | Recent experiments conducted by Cai and Zabder have also shown that these water bears can undergo chemobiosis—a cryptobiotic response to high levels of environmental toxins. However, their results have yet to be verified.<ref>{{cite journal |quotes=no |author=Franceschi, T. |year=1948 |title=Anabiosi nei tardigradi |journal=[[Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova]] |volume=22 |pages=47–49}}</ref><ref>{{cite journal |quotes=no |author=Jönsson, K. I. & R. Bertolani |year=2001 |title=Facts and fiction about long-term survival in tardigrades |journal=[[Journal of Zoology]] |volume=255 |pages=121–123}}</ref> |
== Evolutionary relationships and history== | == Evolutionary relationships and history== | ||
| − | Recent [[DNA]] and [[RNA]] sequencing data indicate that tardigrades are the sister group to the [[arthropod]]s and [[Velvet worm|Onychophora]]. These groups have been traditionally thought of as close relatives of the [[annelid]]s, but newer schemes consider them [[Ecdysozoa]], together with the [[roundworm]]s (Nematoda) and several smaller phyla. The | + | Recent [[DNA]] and [[RNA]] sequencing data indicate that tardigrades are the sister group to the [[arthropod]]s and [[Velvet worm|Onychophora]]. These groups have been traditionally thought of as close relatives of the [[annelid]]s, but newer schemes consider them [[Ecdysozoa]], together with the [[roundworm]]s (Nematoda) and several smaller phyla. The Ecdysozoa-concept resolves the problem of the nematode-like [[pharynx]] as well as some data from 18S-[[rRNA]] and [[Hox gene|HOX]] ([[homeobox]]) gene data, which indicate a relation to roundworms. |
| − | The minute sizes of tardigrades and their membranous integuments make their [[fossilization]] both difficult to detect and highly unlikely. | + | The minute sizes of tardigrades and their membranous integuments make their [[fossilization]] both difficult to detect and highly unlikely. The only known fossil specimens comprise some from mid-[[Cambrian]] deposits in [[Siberia]] and a few rare specimens from [[Cretaceous]] [[amber]].<ref name=EotI>{{cite book|date=2005|publisher=Cambridge University Press|id=ISBN 0521821495|title=Evolution of the Insects|author=David A. Grimaldi and Michael S. Engel|pages=96–97}}</ref> |
| − | The Siberian tardigrades differ from living tardigrades in several ways. | + | The Siberian tardigrades 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. It is considered that they probably represent a stem group of living tardigrades.<ref name=EotI /> |
| − | The rare specimens in Cretaceous amber comprise ''Milnesium swolenskyi'', from [[New Jersey]], the oldest, whose claws and mouthparts are indistinguishable from the living ''M. tartigradum''; and two specimens from western [[Canada]], some 15–20 million years younger than ''M. swolenskyi''. | + | The rare specimens in Cretaceous amber comprise ''Milnesium swolenskyi'', from [[New Jersey]], the oldest, whose claws and mouthparts are indistinguishable from the living ''M. tartigradum''; and two specimens from western [[Canada]], some 15–20 million years younger than ''M. swolenskyi''. Of the two latter, one has been given its own genus and family, ''Beorn leggi'' (the genus named by Cooper after the character [[Beorn]] from ''[[The Hobbit]]'' by [[J. R. R. Tolkien]] and the species named after his student William M. Legg), however it bears a strong resemblance to many living specimens in the family ''Hipsiblidae''.<ref name=EotI /><ref>{{cite journal|author=Kenneth W. Cooper|title=The first fossil tardigrade: ''Beorn leggi'', from Cretaceous Amber|journal=Psyche – Journal of Entomology|date=1964|volume=71|issue=2|pages=41}}</ref> |
''[[Aysheaia]]'' from the middle [[Cambrian]] [[Burgess shale]] might be related to tardigrades. | ''[[Aysheaia]]'' from the middle [[Cambrian]] [[Burgess shale]] might be related to tardigrades. | ||
==References== | ==References== | ||
| − | + | * Bertolani, R. et al |year=2004 |title=Experiences with dormancy in tardigrades |journal=Journal of Limnology |volume=63(Suppl 1) |pages=16-25}}</ref> | |
| + | |||
| + | (<ref>[http://takabisv.taka.jaea.go.jp/jaeribio/members/Nobby/NobbyCoau/2006DH.pdf Radiation tolerance in the tardigrade Milnesium tardigradum]</ref> | ||
| + | |||
| + | * Goldstein, B. and Blaxter, M. |year=2002 |title=Quick Guide: Tardigrades |journal=Current Biology |volume=12 |pages=R475}}</ref> | ||
| + | |||
| + | . <ref>Seki, K & Toyoshima, M. (1998). Preserving tardigrades under pressure. Nature 395: 853–854.</ref><ref>Ian M. Kinchin (1994) The Biology of Tardigrades, Ashgate Publishing</ref> | ||
| − | Ramel, G. 2008. http://www.earthlife.net/inverts/tardigrada.html The phylum Tardigrada]. ''Earthlife.net''. Retrieved April 18, 2008. | + | * Ramel, G. 2008. http://www.earthlife.net/inverts/tardigrada.html The phylum Tardigrada]. ''Earthlife.net''. Retrieved April 18, 2008. |
==External links== | ==External links== | ||
Revision as of 18:53, 19 April 2008
| Tardigrade
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 The tardigrade Hypsibius dujardini
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| Scientific classification | ||||||||
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|
Heterotardigrada |
Tardigrade, or water bear, is any of the various very small, segmented invertebrates comprising the phylum Tardigrada, characterized by bilateral symmetry, four pairs of unjointed legs, and a eutelic body (fixed number of body cells in mature adults of any one species). There are more than 700 known species (Ramel 2008).
Water bears are able to survive in extreme environments that would kill almost any other animal. They can survive temperatures close to absolute zero (Bertolani et al. 2004), temperatures as high as 151°C (303°F), one thousand times more radiation than any other animal (Horikawa 2006), nearly a decade without water, and can also survive in a vacuum like that found in space.
Description and overview
Tardigrades are small, bilaterally symmetrical, segmented animals, similar and probably related to the arthropods. The biggest adults may reach a body length of 1.5 millimeters and the smallest below 0.1 millimeters. Echiniscoides sigimunmde is the largest known tardigrade species and is found in European and Asian habitats (Ramel 2008).
Tardigrades have a body with four segments (not counting the head). They have eight legs, but they are not jointed as in arthropods. The feet have claws or toes. The cuticle contains chitin and is molted.
Tardigrades have a ventral nervous system with one ganglion per segment, and a multilobed brain. The body cavity is partially a coelom, with a true coelom near the gonads (coelomic pouch), but most of the body cavity is a hemocoel rather than a coelom. ,
Tardigrades lack circulatory and respiratory systems (Ramel 2008). Their digestive system is a straight through gut with an anus (Ramel 2008). The pharynx is of a triradiate, muscular, sucking kind, armed with stylets.
Tardigrades are gonochoristic (either male or female), although in some species only females have been found, leading to the presumption that these species are parthenogenetic. Males and females are usually present, each with a single gonad. Tardigrades are oviparous.
Tardigrades are eutelic. Eutelic organisms have a fixed number of cells when they reach maturity, the exact number being constant for any one species. Development proceeds by cell division until maturity; further growth occurs via cell enlargement only. Some tardigrade species have as many as about 40,000 cells in each adult's body, others have far fewer (Seki and Toyoshima 1998; Kinchin 1994).
Tardigrades occur over the entire world, from the high Himalayas (above 6,000 meters), to the deep sea (below 4,000 meters) and from the polar regions to the equator. Most live in fresh water, often in environments subject to frequent drying and re-wetting (Ramel 2008). They are found on lichens and mosses, and in dunes, beaches, soil, and marine or freshwater sediments, where they may occur quite frequently (up to 25,000 animals per liter). Tardigrades often can be found by soaking a piece of moss in spring water (Goldsteing and Blaxter 2002).
Tardigrades were first described by Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Spallanzani in 1777. Freshly hatched larvae may be smaller than 0.05 mm.
=
Ecology and life history
Feeding ecology
Most tardigrades are phytophagous or bacteriophagous, but some are predatory[3] (e.g. Milnesium tardigradum).[4]
Physiology
Extreme environments
Tardigrades are very hardy animals; scientists have reported their existence in hot springs, on top of the Himalayas, under layers of solid ice and in ocean sediments. Many species can be found in a milder environment like lakes, ponds and meadows, 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 one of the few groups of species that are capable of reversibly suspending their metabolism and going into a state of cryptobiosis. Several species regularly survive in a dehydrated state for nearly ten years. Depending on the environment they may enter this state via anhydrobiosis, cryobiosis, osmobiosis or anoxybiosis. While in this state their metabolism lowers to less than 0.01% of what is normal and their water content can drop to 1% of normal. Their ability to remain desiccated for such a long period is largely dependent on the high levels of the non-reducing sugar trehalose, which protects their membranes.
Tardigrades have been known to withstand the following extremes while in this state:
- Temperature—tardigrades can survive being heated for a few minutes to 151°C or being chilled for days at -200°C, or for a few minutes at -272°C. (1° warmer than absolute zero).[5]
- Pressure—they can withstand the extremely low pressure of a vacuum and also very high pressures, many times greater than atmospheric pressure. It has recently been proven that they can survive in the vacuum of space. Recent research has notched up another feat of endurability; apparently they can withstand 6,000 atmospheres pressure, which is nearly six times the pressure of water in the deepest ocean trench. [6]
- Dehydration - tardigrades have been shown to survive nearly one decade in a dry state.[7] Another researcher reported that a tardigrade survived over a period of 120 years in a dehydrated state, but soon died after 2 to 3 minutes.[8] Subsequent research has cast doubt on its accuracy since it was only a small movement in the leg.[9]
- Radiation—as shown by Raul M. May from the University of Paris, tardigrades can withstand 5,700 grays or 570,000 rads of x-ray radiation. (Ten to twenty grays or 1,000-2,000 rads could be fatal to a human). The only explanation thus far for this ability is that their lowered hydration state provides fewer reactants for the ionizing radiation.
Recent experiments conducted by Cai and Zabder have also shown that these water bears can undergo chemobiosis—a cryptobiotic response to high levels of environmental toxins. However, their results have yet to be verified.[10][11]
Evolutionary relationships and history
Recent DNA and RNA sequencing data indicate that tardigrades are the sister group to the arthropods and Onychophora. These groups have been traditionally thought of as close relatives of the annelids, but newer schemes consider them Ecdysozoa, together with the roundworms (Nematoda) and several smaller phyla. The Ecdysozoa-concept resolves the problem of the nematode-like pharynx as well as some data from 18S-rRNA and HOX (homeobox) gene data, which indicate a relation to roundworms.
The minute sizes of tardigrades and their membranous integuments make their fossilization both difficult to detect and highly unlikely. The only known fossil specimens comprise some from mid-Cambrian deposits in Siberia and a few rare specimens from Cretaceous amber.[12]
The Siberian tardigrades 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. It is considered that they probably represent a stem group of living tardigrades.[12]
The rare specimens in Cretaceous amber comprise Milnesium swolenskyi, from New Jersey, the oldest, whose claws and mouthparts are indistinguishable from the living M. tartigradum; and two specimens from western Canada, some 15–20 million years younger than M. swolenskyi. Of the two latter, one has been given its own genus and family, Beorn leggi (the genus named by Cooper after the character Beorn from The Hobbit by J. R. R. Tolkien and the species named after his student William M. Legg), however it bears a strong resemblance to many living specimens in the family Hipsiblidae.[12][13]
Aysheaia from the middle Cambrian Burgess shale might be related to tardigrades.
ReferencesISBN links support NWE through referral fees
- Bertolani, R. et al |year=2004 |title=Experiences with dormancy in tardigrades |journal=Journal of Limnology |volume=63(Suppl 1) |pages=16-25}}</ref>
([14]
- Goldstein, B. and Blaxter, M. |year=2002 |title=Quick Guide: Tardigrades |journal=Current Biology |volume=12 |pages=R475}}</ref>
- Ramel, G. 2008. http://www.earthlife.net/inverts/tardigrada.html The phylum Tardigrada]. Earthlife.net. Retrieved April 18, 2008.
External links
- Tardigrada Newsletter
- Tardigrades - Pictures and Movies
- The Edinburgh Tardigrade project
- NJ Tardigrade Survey
- Tardigrade Appreciation Headquarters
- Tardigrades (English/German)
- The incredible water bear!
- Tardigrade Reference Center
- Tardigrades in space
- Tardigrade data and analysis
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- ↑ Budd, G.E. (2001). Tardigrades as ‘stem-group arthropods’: the evidence from the Cambrian fauna. Zool. Anz 240: 265-279.
- ↑ Tardigrada (TSN 155166). Integrated Taxonomic Information System.
- ↑ Lindahl, K. (2008-03-15). Tardigrade Facts.
- ↑ Morgan, Clive I. (1977). Population Dynamics of two Species of Tardigrada, Macrobiotus hufelandii (Schultze) and Echiniscus (Echiniscus) testudo (Doyere), in Roof Moss from Swansea. The Journal of Animal Ecology 46 (1): 263-279.
- ↑ Ramel, G. (2005-11-11). The Water Bears (Phylum Tardigrada).
- ↑ Seki, K & Toyoshima, M. (1998). Preserving tardigrades under pressure. Nature 395: 853–854.
- ↑ Guidetti, R. & Jönsson, K.I. (2002). Long-term anhydrobiotic survival in semi-terrestrial micrometazoans. Journal of Zoology 257: 181-187.
- ↑ Manga Science Volume VI by Yoshitoh Asari, ISBN-05-202039-1, 1998
- ↑ Guidetti, R. & Jönsson, K.I. (2002). Long-term anhydrobiotic survival in semi-terrestrial micrometazoans. Journal of Zoology 257.
- ↑ Franceschi, T. (1948). Anabiosi nei tardigradi. Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova 22: 47–49.
- ↑ Jönsson, K. I. & R. Bertolani (2001). Facts and fiction about long-term survival in tardigrades. Journal of Zoology 255: 121–123.
- ↑ 12.0 12.1 12.2 David A. Grimaldi and Michael S. Engel (2005). Evolution of the Insects. Cambridge University Press, 96–97. ISBN 0521821495.
- ↑ Kenneth W. Cooper (1964). The first fossil tardigrade: Beorn leggi, from Cretaceous Amber. Psyche – Journal of Entomology 71 (2): 41.
- ↑ Radiation tolerance in the tardigrade Milnesium tardigradum
- ↑ Seki, K & Toyoshima, M. (1998). Preserving tardigrades under pressure. Nature 395: 853–854.
- ↑ Ian M. Kinchin (1994) The Biology of Tardigrades, Ashgate Publishing
