Difference between revisions of "Tardigrade" - New World Encyclopedia

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{{Taxobox
 
{{Taxobox
 
| name = Tardigrade
 
| name = Tardigrade
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| fossil_range = Early [[Cambrian]] to Recent<ref name=Budd2001>{{cite journal
 
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| author = Budd, G.E.
 
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| year = 2001
 
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| title = Tardigrades as ‘stem-group arthropods’: the evidence from the Cambrian fauna
 
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| journal = Zool. Anz
 
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| volume = 240
 
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| pages = 265-279
 
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| doi = 10.1078/0044-5231-00034}}</ref>
 
 
| image = Hypsibiusdujardini.jpg
 
| image = Hypsibiusdujardini.jpg
 
| image_width = 288px
 
| image_width = 288px
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| phylum = '''Tardigrada'''
 
| phylum = '''Tardigrada'''
 
| phylum_authority = [[Lazzaro Spallanzani|Spallanzani]], 1777
 
| phylum_authority = [[Lazzaro Spallanzani|Spallanzani]], 1777
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| subdivision_ranks = Classes&nbsp;<ref>{{ITIS|ID=155166|taxon=Tardigrada}}</ref>
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| subdivision_ranks = Classes <small>(ITIS)</small>
 
| subdivision =  
 
| subdivision =  
 
[[Heterotardigrada]]<br />
 
[[Heterotardigrada]]<br />
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[[Eutardigrada]]
 
[[Eutardigrada]]
 
}}
 
}}
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'''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).
  
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'''Tardigrades''' (commonly known as '''water bears''') comprise the [[phylum]] '''Tardigrada'''. They are small, segmented [[animal]]s, similar and probably related to the [[arthropod]]s. 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]]. The biggest adults may reach a body length of 1.5&nbsp;[[millimetre|mm]], the smallest below 0.1&nbsp;mm. Freshly hatched [[larva]]e may be smaller than 0.05&nbsp;mm.
+
Water bears are able to survive in extreme environments that would kill almost any other [[animal]]. They can survive [[temperature]]s 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.
 +
{{toc}}
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Tardigrades reflect the remarkable diversity of [[life|living organisms]], a diversity that is integral to the delight and mystery of [[nature]] for humans.
  
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More than 1000 species of tardigrades have been described. Tardigrades occur over the entire world, from the high [[Himalayas]] (above 6,000&nbsp;[[metre|m]]), to the [[deep sea]] (below 4,000&nbsp;m) and from the [[polar region]]s to the [[equator]].
+
==Description==
 +
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 [[Europe]]an and [[Asia]]n habitats (Ramel 2008). Freshly hatched [[larva]]e may be smaller than 0.05 millimeters.
  
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The most convenient place to find tardigrades is on [[lichen]]s and [[moss]]es. Other environments are [[dune]]s, [[beach]]es, [[soil]] and [[ocean|marine]] or [[freshwater]] sediments, where they may occur quite frequently (up to 25,000 animals per [[litre]]).  Tardigrades often can be found by soaking a piece of [[moss]] in spring water.<ref>{{cite journal |quotes=no |author=Goldstein, B. and Blaxter, M. |year=2002 |title=Quick Guide: Tardigrades |journal=Current Biology |volume=12 |pages=R475}}</ref>
+
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]].
  
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Water bears are able to survive in extreme environments that would kill almost any other animal. They can survive temperatures close to absolute zero<ref>{{cite journal |quotes=no |author=Bertolani, R. et al |year=2004 |title=Experiences with dormancy in tardigrades |journal=Journal of Limnology |volume=63(Suppl 1) |pages=16-25}}</ref>, temperatures as high as 151°C (303°F), 1,000 times more radiation than any other animal<ref>[http://takabisv.taka.jaea.go.jp/jaeribio/members/Nobby/NobbyCoau/2006DH.pdf Radiation tolerance in the tardigrade Milnesium tardigradum]</ref>, nearly a decade without water, and can also survive in a vacuum like that found in space.
+
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 [[gonad]]s (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.  
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== Anatomy and morphology ==
 
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Tardigrades have a body with four [[Segment (biology)|segment]]s (not counting the head), four pairs of legs without joints, and feet with [[claw]]s or [[toe]]s. The [[cuticle]] contains [[chitin]] and is [[ecdysis|moulted]]. They have a [[ventral nervous system]] with one [[ganglion]] per segment, and a multilobed [[brain]]. Instead of a [[coelom]] they have a [[haemocoel]]. The only place where a true coelom can be found is around the gonad (coelomic pouch). The [[pharynx]] is of a triradiate, muscular, sucking kind, armed with [[stylet]]s. Although some species are [[parthenogenesis|parthenogenetic]], males and females are usually present, each with a single [[gonad]]. Tardigrades are [[eutelic]] (all adult tardigrades of the same species are believed to have the same number of cells) and [[oviparous]]. Some tardigrade species have as many as about 40,000 cells in each adult's body, others have far fewer. <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>
 
  
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==Ecology and life history==
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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]].
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===Feeding ecology===
 
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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>
 
  
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==Physiology==
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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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=== Extreme environments ===
 
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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.
 
  
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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.
+
====Distribution, habitat, and feeding behavior====
 +
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 moist environments, 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).
  
−
Tardigrades have been known to withstand the following extremes while in this state:
+
Most tardigrades are [[phytophagous]] or [[bacteriophagous]], but some are [[predator]]y (Lindahl 1999), such as ''Milnesium tardigradum'' and ''Macrobiotus hufelandii'' (Morgan 1977). Those feeding on plant material may feed on [[moss]]es and [[algae]], while those that are carnivorous may feed on [[nematode]]s and [[rotifer]]s (Ramel 2008).
−
* '''[[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]]''' — 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>
 
−
*'''[[Radiation]]''' — as shown by Raul M. May from the University of Paris, tardigrades can withstand 5,700&nbsp;[[Gray (unit)|gray]]s or 570,000&nbsp;[[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 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>
+
====Discovery and naming====
 +
Tardigrades were first described by [[Johann August Ephraim Goeze]] in 1773, and dubbed ''Kleiner Wasserbär'', meaning "little water bear." The name Tardigrada, which means "slow walker," was given by an Italian scientist, [[Lazzaro Spallanzani|Spallanzani]], in 1777. However, it may be that [[Anton van Leeuwenhok]] was actually the first to see tardigrades, when on September 3, 1702, he performed an experiment using dried dust from the gutter on the roof of his house (Ramel 2008). Leeuwenhok added previously boiled water to this dust and was amazed to see living organisms come into being. It was in repeating this experiment, in 1777, that Spallanzani saw tardigrades, naming them from the Greek for slow and walk (Ramel 2008).
  
−
== Evolutionary relationships and history==
+
== Extreme environments ==
−
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.
+
Tardigrades are the most hardy animals known. Scientists have reported their existence in [[hot spring]]s, on top of the [[Himalayas]], under layers of solid [[ice]], and in ocean sediments. They are the only animals known that can survive being observed in a scanning electron microscope, which involves bombarding them with electrons while in a vacuum (Ramel 2008).  
  
−
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&ndash;97}}</ref>
+
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]] (extreme desiccation), cryobiosis (decreased temperature), osmobiosis (in response to increased solute concentration in environment), or anoxybiosis (in situations lacking oxygen). Horikawa et al. (2006) report that almost all terrestrial tardigrades are able to enter an ametabolic state induced by dehydration (anhydrobiosis). While in this state, their metabolism lowers to less than 0.01 percent of what is normal and their water content can drop to one percent 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.  
  
−
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 />
+
While many species survive by converting themselves into this "tun" (pulling their legs in to give their body a cylindrical shape and then shutting down their metabolism), other species do not form a tun to survive extreme conditions, including deep sea species that survive pressures as great as 6,000 atmospheres (Ramel 2008).
  
−
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&ndash;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 &ndash; Journal of Entomology|date=1964|volume=71|issue=2|pages=41}}</ref>
+
Tardigrades have been known to withstand the following extremes:
 +
* '''[[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]]) (Ramel 2008).
 +
*'''[[Pressure]].''' Tardigrades 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 (Seki and Toyoshima 1998).
 +
*'''[[Dehydration]].''' Tardigrades have been shown to survive nearly one decade in a dry state (Guidetti and Jönsson 2002). It also has been reported that a tardigrade survived over a period of 120 years in a dehydrated state, but soon died after two to three minutes (Asari 1998), but subsequent research has cast doubt on its accuracy since it was only a small movement in the leg (Guidetti and Jönsson 2002).
 +
*'''[[Radiation]].''' As shown by Raul M. May from the University of Paris, tardigrades can withstand 5,700&nbsp;[[Gray (unit)|gray]]s or 570,000&nbsp;[[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 chemobiosis&mdash;a cryptobiotic response to high levels of environmental toxins. However, their results have yet to be verified (Franceschi 1948; Jönsson and Bertolani 2001).
 +
 
 +
==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 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 only known fossil specimens comprise some from mid-[[Cambrian]] deposits in [[Siberia]] and a few rare specimens from [[Cretaceous]] [[amber]] (Grimaldi and Engel 2005).
 +
 
 +
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 (Grimaldi and Engel 2005).
 +
 
 +
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&ndash;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'' (Grimaldi and Engel 2005; Cooper 1964).
  
 
''[[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==
−
{{reflist|2}}
+
* Asari, Y. 1998. ''Manga Science,'' volume VI. Pika. ISBN 052020391.
 +
* Bertolani, R., et al. 2004. Experiences with dormancy in tardigrades. ''Journal of Limnology'' 63(Suppl 1): 16-25.
 +
* Budd, G. E. 2001. Tardigrades as "stem-group arthropods:" The evidence from the Cambrian fauna. ''Zool. Anz'' 240: 265-279.
 +
* Cooper, K. W. 1964. The first fossil tardigrade: ''Beorn leggi,'' from Cretaceous amber. ''Psyche—Journal of Entomology'' 71(2): 41.
 +
* Franceschi, T. 1948. Anabiosi nei tardigradi. ''Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova'' 22: 47–49.
 +
* Goldstein, B., and M. Blaxter. 2002. Quick guide: Tardigrades. ''Current Biology'' 12: R475.
 +
* Grimaldi, D. A., and M. S. Engel. 2005. ''Evolution of the Insects''. Cambridge University Press. ISBN 0521821495.
 +
* Guidetti, R., and K. I. Jönsson. 2002. Long-term anhydrobiotic survival in semi-terrestrial micrometazoans. ''Journal of Zoology'' 257: 181-187.
 +
* Horikawa, D. D., T. Sakashita, C. Katagiri, et al. 2006. [http://takabisv.taka.jaea.go.jp/jaeribio/members/Nobby/NobbyCoau/2006DH.pdf Radiation tolerance in the tardigrade ''Milnesium tardigradum.''] ''Int. J. Radiat. Biol.'' 82(12): 843-848. Retrieved April 19, 2008.
 +
* Integrated Taxonomic Information System (ITIS). n.d. [http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=155166 Tardigrada] ''ITIS'' Taxonomic Serial No. 155166. Retrieved April 19, 2008.
 +
* Jönsson, K. I., and R. Bertolani. 2001. Facts and fiction about long-term survival in tardigrades. ''Journal of Zoology'' 255: 121–123.
 +
* Kinchin, I. M. 1994. ''The Biology of Tardigrades''. Chapel Hill, NC: Portland Press. ISBN 1855780437.
 +
* Lindahl, K. 1999. [http://www.iwu.edu/~tardisdp/tardigrade_facts.html Tardigrade facts.] ''Illinois Wesleyan University''. Retrieved April 19, 2008.
 +
* Morgan, C. 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. 2008. [http://www.earthlife.net/inverts/tardigrada.html The phylum Tardigrada.] ''Earthlife.net''. Retrieved April 18, 2008.
 +
* Seki, K., and M. Toyoshima. 1998. Preserving tardigrades under pressure. ''Nature'' 395: 853–854.
  
 
==External links==
 
==External links==
−
{{Commonscat|Tardigrada}}
+
All links retrieved February 26, 2023.
 
*[http://www.tardigrada.net/ Tardigrada Newsletter]
 
*[http://www.tardigrada.net/ Tardigrada Newsletter]
 
*[http://tardigrades.bio.unc.edu/ Tardigrades - Pictures and Movies]
 
*[http://tardigrades.bio.unc.edu/ Tardigrades - Pictures and Movies]
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*[http://www.tardigrades.org/ The Edinburgh Tardigrade project]
+
*[http://www.tardigrades.de Tardigrades (English/German)]
−
*[http://www.extrapersonality.com/tardigrades.html NJ Tardigrade Survey]
 
−
*[http://www.q7.com/~vvv/tardigrade/ Tardigrade Appreciation Headquarters]
 
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*[http://www.tardigrades.de Tardigrades (English/German)]
 
 
*[http://www.microscopy-uk.org.uk/mag/artjun00/mmbearp.html The incredible water bear!]
 
*[http://www.microscopy-uk.org.uk/mag/artjun00/mmbearp.html The incredible water bear!]
−
*[http://tardigrade.acnatsci.org/ Tardigrade Reference Center]
 
−
*[http://tardigradesinspace.blogspot.com/ Tardigrades in space]
 
−
*[http://tardigrade.bioapps.biozentrum.uni-wuerzburg.de/ Tardigrade data and analysis]
 
  
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
  
 
{{credit|Tardigrade|205755191}}
 
{{credit|Tardigrade|205755191}}

Latest revision as of 04:32, 27 February 2023

Tardigrade
The tardigrade Hypsibius dujardini
The tardigrade Hypsibius dujardini
Scientific classification
Kingdom: Animalia
Subkingdom: Ecdysozoa
(unranked) Panarthropoda
Phylum: Tardigrada
Spallanzani, 1777
Classes (ITIS)

Heterotardigrada
Mesotardigrada
Eutardigrada

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.

Tardigrades reflect the remarkable diversity of living organisms, a diversity that is integral to the delight and mystery of nature for humans.

Description

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). Freshly hatched larvae may be smaller than 0.05 millimeters.

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).

Distribution, habitat, and feeding behavior

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 moist environments, 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).

Most tardigrades are phytophagous or bacteriophagous, but some are predatory (Lindahl 1999), such as Milnesium tardigradum and Macrobiotus hufelandii (Morgan 1977). Those feeding on plant material may feed on mosses and algae, while those that are carnivorous may feed on nematodes and rotifers (Ramel 2008).

Discovery and naming

Tardigrades were first described by Johann August Ephraim Goeze in 1773, and dubbed Kleiner Wasserbär, meaning "little water bear." The name Tardigrada, which means "slow walker," was given by an Italian scientist, Spallanzani, in 1777. However, it may be that Anton van Leeuwenhok was actually the first to see tardigrades, when on September 3, 1702, he performed an experiment using dried dust from the gutter on the roof of his house (Ramel 2008). Leeuwenhok added previously boiled water to this dust and was amazed to see living organisms come into being. It was in repeating this experiment, in 1777, that Spallanzani saw tardigrades, naming them from the Greek for slow and walk (Ramel 2008).

Extreme environments

Tardigrades are the most hardy animals known. Scientists have reported their existence in hot springs, on top of the Himalayas, under layers of solid ice, and in ocean sediments. They are the only animals known that can survive being observed in a scanning electron microscope, which involves bombarding them with electrons while in a vacuum (Ramel 2008).

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 (extreme desiccation), cryobiosis (decreased temperature), osmobiosis (in response to increased solute concentration in environment), or anoxybiosis (in situations lacking oxygen). Horikawa et al. (2006) report that almost all terrestrial tardigrades are able to enter an ametabolic state induced by dehydration (anhydrobiosis). While in this state, their metabolism lowers to less than 0.01 percent of what is normal and their water content can drop to one percent 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.

While many species survive by converting themselves into this "tun" (pulling their legs in to give their body a cylindrical shape and then shutting down their metabolism), other species do not form a tun to survive extreme conditions, including deep sea species that survive pressures as great as 6,000 atmospheres (Ramel 2008).

Tardigrades have been known to withstand the following extremes:

  • 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) (Ramel 2008).
  • Pressure. Tardigrades 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 (Seki and Toyoshima 1998).
  • Dehydration. Tardigrades have been shown to survive nearly one decade in a dry state (Guidetti and Jönsson 2002). It also has been reported that a tardigrade survived over a period of 120 years in a dehydrated state, but soon died after two to three minutes (Asari 1998), but subsequent research has cast doubt on its accuracy since it was only a small movement in the leg (Guidetti and Jönsson 2002).
  • 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 (Franceschi 1948; Jönsson and Bertolani 2001).

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 (Grimaldi and Engel 2005).

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 (Grimaldi and Engel 2005).

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 (Grimaldi and Engel 2005; Cooper 1964).

Aysheaia from the middle Cambrian Burgess shale might be related to tardigrades.

References
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  • Asari, Y. 1998. Manga Science, volume VI. Pika. ISBN 052020391.
  • Bertolani, R., et al. 2004. Experiences with dormancy in tardigrades. Journal of Limnology 63(Suppl 1): 16-25.
  • Budd, G. E. 2001. Tardigrades as "stem-group arthropods:" The evidence from the Cambrian fauna. Zool. Anz 240: 265-279.
  • Cooper, K. W. 1964. The first fossil tardigrade: Beorn leggi, from Cretaceous amber. Psyche—Journal of Entomology 71(2): 41.
  • Franceschi, T. 1948. Anabiosi nei tardigradi. Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova 22: 47–49.
  • Goldstein, B., and M. Blaxter. 2002. Quick guide: Tardigrades. Current Biology 12: R475.
  • Grimaldi, D. A., and M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press. ISBN 0521821495.
  • Guidetti, R., and K. I. Jönsson. 2002. Long-term anhydrobiotic survival in semi-terrestrial micrometazoans. Journal of Zoology 257: 181-187.
  • Horikawa, D. D., T. Sakashita, C. Katagiri, et al. 2006. Radiation tolerance in the tardigrade Milnesium tardigradum. Int. J. Radiat. Biol. 82(12): 843-848. Retrieved April 19, 2008.
  • Integrated Taxonomic Information System (ITIS). n.d. Tardigrada ITIS Taxonomic Serial No. 155166. Retrieved April 19, 2008.
  • Jönsson, K. I., and R. Bertolani. 2001. Facts and fiction about long-term survival in tardigrades. Journal of Zoology 255: 121–123.
  • Kinchin, I. M. 1994. The Biology of Tardigrades. Chapel Hill, NC: Portland Press. ISBN 1855780437.
  • Lindahl, K. 1999. Tardigrade facts. Illinois Wesleyan University. Retrieved April 19, 2008.
  • Morgan, C. 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. 2008. The phylum Tardigrada. Earthlife.net. Retrieved April 18, 2008.
  • Seki, K., and M. Toyoshima. 1998. Preserving tardigrades under pressure. Nature 395: 853–854.

External links

All links retrieved February 26, 2023.

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