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'''Tardigrades''' (commonly known as '''waterbears''' or '''moss piglets''')<ref>{{Cite news|title=Indestructible|last=Copley|first=Jon|date=1999-10-23|issue=2209|periodical=New Scientist|url=http://www.newscientist.com/article/mg16422095.100-indestructible.html|accessdate=2010-02-06|postscript=<!--None-->}}</ref> are small, water-dwelling, segmented [[animal]]s with eight legs. They are notable for being one of the most complex of all known '''[[polyextremophile]]s'''. (An '''[[extremophile]]''' is an [[organism]] that can thrive in a physically or geochemically [[extreme environment|extreme condition]] that would be detrimental to most [[life on Earth]].<ref>Rampelotto, P. H. (2010). Resistance of microorganisms to extreme environmental conditions and its contribution to Astrobiology. Sustainability, 2, 1602-1623.</ref><ref>Rothschild, L.J.; Mancinelli, R.L. Life in extreme environments. Nature 2001, 409, 1092-1101</ref> A '''[[polyextremophile]]''' such as the Tardigrade is capable of thriving in a ''variety'' of extreme conditions, any ''one'' of which would be detrimental to most life on earth.)
'''Tardigrades''' (commonly known as '''waterbears''' or '''moss piglets''' or '''mossy teddys''')<ref>{{Cite news|title=Indestructible|last=Copley|first=Jon|date=1999-10-23|issue=2209|periodical=New Scientist|url=http://www.newscientist.com/article/mg16422095.100-indestructible.html|accessdate=2010-02-06|postscript=<!--None-->}}</ref> are small, water-dwelling, segmented [[animal]]s with eight legs. They are notable for being one of the most complex of all known '''[[polyextremophile]]s'''. (An '''[[extremophile]]''' is an [[organism]] that can thrive in a physically or geochemically [[extreme environment|extreme condition]] that would be detrimental to most [[life on Earth]].<ref>Rampelotto, P. H. (2010). Resistance of microorganisms to extreme environmental conditions and its contribution to Astrobiology. Sustainability, 2, 1602-1623.</ref><ref>Rothschild, L.J.; Mancinelli, R.L. Life in extreme environments. Nature 2001, 409, 1092-1101</ref> A '''[[polyextremophile]]''' such as the Tardigrade is capable of thriving in a ''variety'' of extreme conditions, any ''one'' of which would be detrimental to most life on earth.)


Tardigrades can withstand temperatures from just above absolute zero to well above the boiling point of water. They can survive pressures greater than any found in the deepest ocean trenches and have lived through the vacuum of outer space. They can survive solar radiation, gamma radiation, ionic radiation—hundreds of times higher doses than would kill a person. They can go without food or water for nearly 10 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.
Tardigrades can withstand temperatures from just above absolute zero to well above the boiling point of water. They can survive pressures greater than any found in the deepest ocean trenches and have lived through the vacuum of outer space. They can survive solar radiation, gamma radiation, ionic radiation—hundreds of times higher doses than would kill a person. They can go without food or water for nearly 10 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.

Revision as of 15:38, 22 January 2013

Tardigrade
Temporal range: Cretaceous–Recent[1]
The tardigrade Hypsibius dujardini
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
(unranked): Panarthropoda
Phylum: Tardigrada
Spallanzani, 1777

Tardigrades (commonly known as waterbears or moss piglets or mossy teddys)[2] are small, water-dwelling, segmented animals with eight legs. They are notable for being one of the most complex of all known polyextremophiles. (An extremophile is an organism that can thrive in a physically or geochemically extreme condition that would be detrimental to most life on Earth.[3][4] A polyextremophile such as the Tardigrade is capable of thriving in a variety of extreme conditions, any one of which would be detrimental to most life on earth.)

Tardigrades can withstand temperatures from just above absolute zero to well above the boiling point of water. They can survive pressures greater than any found in the deepest ocean trenches and have lived through the vacuum of outer space. They can survive solar radiation, gamma radiation, ionic radiation—hundreds of times higher doses than would kill a person. They can go without food or water for nearly 10 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.

Usually, Tardigrades are 1 millimetre (0.039 in) long when they are fully grown. They are short and plump with 4 pairs of legs, each with 4-8 claws also known as "disks." The animals are prevalent in moss and lichen and, when collected, may be viewed under a very low-power microscope, making them accessible to the student or amateur scientist as well as the professional.[5][6]

Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. It is an ancient group, with fossils dating from 530 million years ago, in the Cambrian period.[7] The first tardigrades were discovered by Johann August Ephraim Goeze in 1773. Since 1778, over 500 new tardigrade species have been found.

Description

Johann August Ephraim Goeze originally named the Tardigrade, kleiner Wasserbär, meaning 'little water bear' in German. The name Tardigrada means "slow walker" and was given by Lazzaro Spallanzani in 1777.[8] The name water bear comes from the way they walk, reminiscent of a bear's gait. The biggest adults may reach a body length of 1.5 millimetres (0.059 in), the smallest below 0.1 mm. Freshly hatched tardigrades may be smaller than 0.05 mm.

About 1,150 species of tardigrades have been described.[9][10] Tardigrades occur throughout the world, from the Himalayas[11] (above 6,000 metres (20,000 ft)), to the deep sea (below 4,000 metres (13,000 ft)) and from the polar regions to the equator.

The most convenient place to find tardigrades is on lichens and mosses. Other environments are 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.[12]

Tardigrades are able to survive in extreme environments that would kill almost any other animal. Some can survive temperatures of close to absolute zero, or 0 Kelvin (−273 °C (−459 °F)),[13] temperatures as high as 151 °C (304 °F), 1,000 times more radiation than other animals,[14] and almost a decade without water.[15] Since 2007, tardigrades have also returned alive from studies in which they have been exposed to the vacuum of space for a few days in low Earth orbit.[16][17] Tardigrades are the first known animal to survive in space.

Anatomy and morphology

Tardigrades have barrel-shaped bodies with four pairs of stubby lobopodia legs that are poorly articulated. Most range from 0.3 to 0.5 millimetres (0.012 to 0.020 in) in length, although the largest species may reach 1.2 millimetres (0.047 in). The body consists of a head, three body segments with a pair of legs each, and a caudal segment with a fourth pair of legs. The legs are without joints while the feet have four to eight claws each. The cuticle contains chitin and is moulted periodically.

Tardigrades are eutelic, with all adult tardigrades of the same species having the same number of cells. Some species have as many as 40,000 cells in each adult, while others have far fewer.[18][19]

Echiniscus testudo

The body cavity consists of a haemocoel, but the only place where a true coelom can be found is around the gonad. There are no respiratory organs, with gas exchange able to occur across the whole of the body. Some tardigrades have three tubular glands associated with the rectum; these may be excretory organs similar to the Malpighian tubules of arthropods, although the details remain unclear.[20]

The tubular mouth is armed with stylets, which are used to pierce the plant cells, algae, or small invertebrates on which the tardigrades feed, releasing the body fluids or cell contents. The mouth opens into a triradiate, muscular, sucking pharynx. The stylets are lost when the animal moults, 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 oesophagus, 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 anus located at the terminal end of the body. Some species only defecate when they molt, leaving the feces behind with the shed cuticle.[20]

The brain includes multiple lobes, mostly consisting of three bilaterally paired clusters of neurons.[21] The brain is attached to a large ganglion below the oesophagus, 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 rhabdomeric pigment-cup eyes, and there are numerous sensory bristles on the head and body.[22]

Tardigrades all possess a buccopharyngeal membrane apparatus, which, along with the claws, are used to differentiate the different species. Tardigrades are covered in cuticle which contains chitin and protein.

Reproduction

Although some species are parthenogenetic, both males and females are usually present, each with a single gonad located above the intestine. Two ducts run from the testis 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 thus forms a cloaca.[20]

Tardigrades are oviparous, and fertilization is usually external. Mating occurs during the molt with the eggs being laid inside the shed 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 attach them to the nearby substrate.[20]

The eggs hatch after no more than fourteen days, with the young already possessing their full complement of adult cells. Growth to the adult size therefore occurs by enlargement of the individual cells (hypertrophy), rather than by cell division. Tardigrades live for three to thirty months, and may moult up to twelve times.[20]

Ecology and life history

Most tardigrades are phytophagous (plant eaters) or bacteriophagous (bacteria eaters), but some are predatory (e.g., Milnesium tardigradum).[23][24]

Physiology

Scientists have reported tardigrades 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.

Hypsibius dujardini imaged with a scanning electron microscope

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 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. In this cryptobiotic state the tardigrade is known as a tun.[25]

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 (424 K or 304 F),[26] or being chilled for days at −200 °C (73 K or -328 F),[26] or for a few minutes at −272 °C (~1 degree above absolute zero or -458 F).[27]
  • Pressure – they can withstand the extremely low pressure of a vacuum and also very high pressures, more than 1,200 times atmospheric pressure. Tardigrades can survive the vacuum of open space and solar radiation combined for at least 10 days.[27] Some species can also withstand pressure of 6,000 atmospheres, which is nearly six times the pressure of water in the deepest ocean trench, the Mariana trench.[18]
  • Dehydration – although there is one report of a leg movement in a 120-year-old specimen from dried moss,[28] this is not generally considered "survival",[29] and the longest tardigrades have been shown to survive in a dry state is nearly 10 years.[30] When exposed to extremely low temperatures, their body composition goes from 85% water to only 3%. As water expands upon freezing, dehydration ensures the tardigrades do not get ripped apart by the freezing ice (as waterless tissues cannot freeze).[31]
  • Radiation – tardigrades can withstand median lethal doses of 5,000 Gy (of gamma-rays) and 6,200 Gy (of heavy ions) in hydrated animals (5 to 10 Gy could be fatal to a human).[32] The only explanation found in earlier experiments for this ability was that their lowered water state provides fewer reactants for the ionizing radiation.[33] 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 ability to efficiently repair damage to their DNA resulting from that exposure.[34]
  • Environmental toxins – tardigrades can undergo chemobiosis—a cryptobiotic response to high levels of environmental toxins. However, these laboratory results have yet to be verified.[28][29]
  • Outer space – In September 2007, tardigrades were taken into low Earth orbit on the FOTON-M3 mission and for 10 days were exposed to the vacuum of space. After being rehydrated back on Earth, over 68% of the subjects protected from high-energy UV radiation survived and many of these produced viable embryos, and a handful had survived full exposure to solar radiation.[27][35] In May 2011, Italian scientists sent tardigrades into space along with other extremophiles on STS-134, the final flight of Space Shuttle Endeavour.[36][37][38] Their conclusion was that microgravity and cosmic radiation "did not significantly affect survival of tardigrades in flight, confirming that tardigrades represent a useful animal for space research."[39] In November 2011, they were among the organisms to be sent by the US-based Planetary Society on the Russian Fobos-Grunt mission's Living Interplanetary Flight Experiment to Phobos; however, the launch failed.

Evolutionary relationships and history

Illustration of Echiniscus sp. from 1861

A number of morphological and molecular studies have sought to resolve the relationship of tardigrades to other lineages of ecdysozoan animals. Two plausible placements have been recovered: tardigrades most closely related to arthropods (a common result of morphological studies) or tardigrades most closely related to nematodes (found in some molecular analyses).

The latter hypothesis has been rejected by recent microRNA and expressed sequence tag analyses.[40] Apparently, the grouping of tardigrades with nematodes found in a number of molecular studies is a long branch attraction artifact. Within the arthropod group (called panarthropoda and comprising onychophora, tardigrades and euarthropoda), three patterns of relationship are possible: tardigrades sister to onychophora plus arthropods (the lobopodia hypothesis); onychophora sister to tardigrades plus arthropods (the tactopoda hypothesis); and onychophora sister to tardigrades.[41] Recent analyses indicate that the panarthropoda group is monophyletic, and that tardigrades are a sister group of lobopodia, the lineage consisting of arthropods and Onychophora.[40][42]

Panarthropoda

Arthropods (Arthropoda)

Lobopoda

Velvet worms (Onychophora)

Water bears (Tardigrada)

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.[43]

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.[43]

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 Hypsibiidae.[43][44]

Aysheaia from the middle Cambrian Burgess shale has been proposed as a sister-taxon to an arthropod-tardigrade clade.[45]

Tardigrades have sometimes been linked to the prehistoric oddity Opabinia as a close living relative.[46]

Genomes and genome sequencing

Tardigrade genomes vary in size, from about 75 to 800 megabase pairs of DNA.[47] The genome of a tardigrade species, Hypsibius dujardini, is being sequenced at the Broad Institute.[48] Hypsibius dujardini has a compact genome and a generation time of about two weeks, and it can be cultured indefinitely and cryopreserved.[49]

See also

References

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  2. ^ Copley, Jon (1999-10-23). "Indestructible". New Scientist. No. 2209. Retrieved 2010-02-06.
  3. ^ Rampelotto, P. H. (2010). Resistance of microorganisms to extreme environmental conditions and its contribution to Astrobiology. Sustainability, 2, 1602-1623.
  4. ^ Rothschild, L.J.; Mancinelli, R.L. Life in extreme environments. Nature 2001, 409, 1092-1101
  5. ^ Shaw, Michael W. ""How to Find Tardigrades"". tardigrades.us/. Retrieved 2013-01-14.
  6. ^ Viral; et al. "How to Find and Care for a Pet Tardigrade ( Water Bear )". wikihow.com/. Retrieved 2013-01-14. {{cite web}}: Explicit use of et al. in: |last= (help)
  7. ^ biodiversity explorer: Tardigrada (water bears, tardigrades)
  8. ^ Bordenstein, Sarah (December 17, 2008). "Tardigrades (Water Bears)". Carleton College. Retrieved September 16, 2012.
  9. ^ Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.
  10. ^ Degma, P., Bertolani, R. & Guidetti, R. 2009-2011. Actual checklist of Tardigrada species. Ver. 18: 27-04-2011. http://www.tardigrada.modena.unimo.it/miscellanea/Actual%20checklist%20of%20Tardigrada.pdf
  11. ^ C.Michael Hogan. 2010. Extremophile. eds. E.Monosson and C.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment, washington DC
  12. ^ Goldstein, B. and Blaxter, M. (2002). "Quick Guide: Tardigrades". Current Biology. 12 (14): R475. doi:10.1016/S0960-9822(02)00959-4.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ Becquerel P. (1950). "La suspension de la vie au dessous de 1/20 K absolu par demagnetization adiabatique de l'alun de fer dans le vide les plus eléve". C. R. Hebd. Séances Acad. Sci. Paris (in French). 231: 261–263.
  14. ^ Radiation tolerance in the tardigrade Milnesium tardigradum
  15. ^ Crowe, John H.; Carpenter, John F.; Crowe, Lois M. (1998). "The role of vitrification in anhydrobiosis". Annual Review of Physiology. Vol. 60. pp. 73–103. doi:10.1146/annurev.physiol.60.1.73. PMID 9558455. {{cite news}}: Unknown parameter |month= ignored (help)
  16. ^ Staff Space.com (8 September 2008). "Creature Survives Naked in Space". Space.com. Retrieved 2011-12-22.
  17. ^ Mustain, Andrea (22 December 2011). "Weird wildlife: The real land animals of Antarctica". MSNBC. Retrieved 2011-12-22.
  18. ^ a b Seki, Kunihiro; Toyoshima, Masato (1998-10-29). "Preserving tardigrades under pressure". Nature. 395 (6705): 853–854. doi:10.1038/27576.
  19. ^ Ian M. Kinchin (1994) The Biology of Tardigrades, Ashgate Publishing
  20. ^ a b c d e Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 877–880. ISBN 0-03-056747-5.
  21. ^ Juliane Zantke, Carsten Wolff, Gerhard Scholtz (2008). "Three-dimensional reconstruction of the central nervous system of Macrobiotus hufelandi (Eutardigrada, Parachela): implications for the phylogenetic position of Tardigrada" (PDF). ZOOMORPHOLOGY. 127 (1): 21–26. doi:10.1007/s00435-007-0045-1.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/j.asd.2007.06.003, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/j.asd.2007.06.003 instead.
  23. ^ 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). British Ecological Society: 263–279. doi:10.2307/3960. JSTOR 3960.
  24. ^ Lindahl, K. (2008-03-15). "Tardigrade Facts".
  25. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  26. ^ a b Horikawa, Daiki D. (2012). Alexander V. Altenbach, Joan M. Bernhard & Joseph Seckbach (ed.). Anoxia Evidence for Eukaryote Survival and Paleontological Strategies (21 ed.). Netherlands: Springer Netherlands. pp. 205–217. ISBN 978-94-007-1895-1. Retrieved 21 January 2012. {{cite book}}: More than one of |author= and |last= specified (help)
  27. ^ a b c Jönsson, K. Ingemar; Rabbow, Elke; Schill, Ralph O.; Harms-Ringdahl, Mats; Rettberg, Petra (2008-09-09). "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.
  28. ^ a b Franceschi, T. (1948). "Anabiosi nei tardigradi". Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova. 22: 47–49.
  29. ^ a b Jönsson, K. Ingemar & R. Bertolani (2001). "Facts and fiction about long-term survival in tardigrades". Journal of Zoology. 255: 121–123. doi:10.1017/S0952836901001169.
  30. ^ Guidetti, R. & Jönsson, K.I. (2002). "Long-term anhydrobiotic survival in semi-terrestrial micrometazoans". Journal of Zoology. 257 (2): 181–187. doi:10.1017/S095283690200078X.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  31. ^ Michael Kent (2000), Advanced Biology, Oxford University Press.
  32. ^ Horikawa DD, Sakashita T, Katagiri C, Watanabe M, Kikawada T, Nakahara Y, Hamada N, Wada S, Funayama T, Higashi S, Kobayashi Y, Okuda T, Kuwabara M. (2006). "Radiation tolerance in the tardigrade Milnesium tardigradum". International Journal of Radiation Biology. 82 (12): 843–8. doi:10.1080/09553000600972956. PMID 17178624.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  33. ^ Horikawa, Daiki D. (1 January 2006). "Radiation tolerance in the tardigrade". International Journal of Radiation Biology. 82 (12): 843–848. doi:10.1080/09553000600972956. PMID 17178624. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  34. ^ Horikawa, Daiki D. "UV Radiation Tolerance of Tardigrades". NASA.com. Retrieved 15 January 2013. {{cite journal}}: Cite journal requires |journal= (help)
  35. ^ Courtland, Rachel (2008-09-08). "'Water bears' are first animal to survive space vacuum". New Scientist. Retrieved 2011-05-22.
  36. ^ NASA Staff (2011-05-17). "BIOKon In Space (BIOKIS)". NASA. Retrieved 2011-05-24.
  37. ^ Brennard, Emma (2011-05-17). "Tardigrades: Water bears in space". BBC. Retrieved 2011-05-24.
  38. ^ "Tardigrades: Water bears in space". BBC Nature. 2011-05-17.
  39. ^ Rebecchi, L.; et al. "Two Tardigrade Species On Board the STS-134 Space Flight" in "International Symposium on Tardigrada, 23-26 July 2012" (PDF). p. 89. Retrieved 2013-01-14. {{cite web}}: Explicit use of et al. in: |first= (help)
  40. ^ a b Campbell, Lahcen (2011). "MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of Arthropoda". PNAS Early Edition. doi:10.1073/pnas.1105499108. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  41. ^ Telford, Maximilian (2008). "The evolution of the Ecdysozoa". Phil. Trans. R. Soc. B. 363 (1496): 1529–1537. doi:10.1098/rstb.2007.2243. Retrieved 2013-09-09. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  42. ^ Sequencing of Tardigrade Genome
  43. ^ a b c David A. Grimaldi and Michael S. Engel (2005). Evolution of the Insects. Cambridge University Press. pp. 96–97. ISBN 0-521-82149-5.
  44. ^ Kenneth W. Cooper (1964). "The first fossil tardigrade: Beorn leggi, from Cretaceous Amber". Psyche – Journal of Entomology. 71 (2): 41. doi:10.1155/1964/48418.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  45. ^ Richard A. Fortey and Richard H. Thomas (2001). Arthropod Relationships. Chapman & Hall. p. 383. ISBN [[Special:BookSources/04127542075 |04127542075 [[Category:Articles with invalid ISBNs]]]]. {{cite book}}: Check |isbn= value: invalid character (help)
  46. ^ Budd, G.E. (1996). "The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group". Lethaia. 29 (1): 1–14. doi:10.1111/j.1502-3931.1996.tb01831.x.
  47. ^ "Genome Size of Tardigrades".
  48. ^ Entrez. "Genome Projects for Hypsibius dujardini".
  49. ^ Gabriel, W.; et al. (2007). "The tardigrade Hypsibius dujardini, a new model for studying the evolution of development". Developmental Biology. 312 (2): 545–559. doi:10.1016/j.ydbio.2007.09.055. PMID 17996863. {{cite journal}}: Explicit use of et al. in: |author= (help)
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