Tardigrade

Tardigrades
Temporal range: Cambrian–Recent[1]
Milnesium tardigradum
Scientific classification
Kingdom: Animalia
(unranked): Tactopoda
Phylum: Tardigrada
Spallanzani, 1777
Classes

Tardigrades (/ˈtɑːrdɪˌɡreɪd/; also known as water bears, space bears, or moss piglets)[2][3][4][5] are water-dwelling, eight-legged, segmented micro-animals.[2][6] They were first discovered by the German zoologist Johann August Ephraim Goeze in 1773. The name Tardigrada (meaning "slow stepper") was given three years later by the Italian biologist Lazzaro Spallanzani.[7] They have been found everywhere: from mountaintops to the deep sea and mud volcanoes;[8] from tropical rain forests to the Antarctic.[9]

Tardigrades are one of the most resilient animals known.[10][11] Individual species of tardigrades can survive extreme conditions that would be rapidly fatal to nearly all other known life forms, including complete global mass extinction events due to astrophysical events, such as supernovae, gamma-ray bursts, large asteroid impacts, or passing-by stars.[10][11] Some tardigrades can withstand temperatures down to 1 K (−458 °F; −272 °C) (close to absolute zero) while others can withstand 420 K (300 °F; 150 °C)[12] for several minutes, pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space.[13] They can go without food or water for more than 30 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.[3][14][15][16] Tardigrades, living in harsh conditions, undergo an annual process of cyclomorphosis.

They are not considered extremophilic because they are not adapted to exploit these conditions. This means that their chances of dying increase the longer they are exposed to the extreme environments,[7] whereas true extremophiles thrive in a physically or geochemically extreme environment that would harm most other organisms.[3][17][18]

Usually, tardigrades are about 0.5 mm (0.02 in; 19.69 thou) long when they are fully grown.[2] They are short and plump with four pairs of legs, each with four to eight claws also known as "disks".[2] The first three pairs of legs are directed ventrolaterally and are the primary means of locomotion, while the fourth pair is directed posteriorly on the terminal segment of the trunk and is used primarily for grasping the substrate.[19] Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a very low-power microscope, making them accessible to students and amateur scientists.[20]

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.[21] About 1,150 species of tardigrades have been described.[22][23] Tardigrades can be found throughout the world, from the Himalayas[24] (above 6,000 m (20,000 ft)), to the deep sea (below 4,000 m (13,000 ft)) and from the polar regions to the equator.

Description

Johann August Ephraim Goeze
Lazzaro Spallanzani

Johann August Ephraim Goeze originally named the tardigrade kleiner Wasserbär (Bärtierchen today), meaning "little water bear" in German. The name Tardigradum means "slow walker" and was given by Lazzaro Spallanzani in 1776.[25] 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 mm (0.059 in), the smallest below 0.1 mm. Newly hatched tardigrades may be smaller than 0.05 mm.

SEM image of Hypsibius dujardini

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, in the case of Echiniscoides wyethi,[26] may be found on barnacles.[27] Often, tardigrades can be found by soaking a piece of moss in water.[28]

Anatomy and morphology

Tardigrades have barrel-shaped bodies with four pairs of stubby legs. Most range from 0.3 to 0.5 mm (0.012 to 0.020 in) in length, although the largest species may reach 1.2 mm (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 protein and is moulted periodically.

All adult tardigrades of the same species have the same number of cells (see eutely). Some species have as many as 40,000 cells in each adult, while others have far fewer.[29][30]

The body cavity consists of a haemocoel, but the only place where a true coelom can be found is around the gonad. No respiratory organs are found, 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.[31]

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 molts, and a new pair is secreted from a pair of glands that lie on either side of the mouth. The pharynx connects to a short esophagus, and then to an intestine that occupies much of the length of the body, which is the main site of digestion. The intestine opens, via a short rectum, to an anus located at the terminal end of the body. Some species only defecate when they molt, leaving the feces behind with the shed cuticle.[31]

The brain develops in a bilaterally symmetric pattern.[32] The brain includes multiple lobes, mostly consisting of three bilaterally paired clusters of neurons.[33] The brain is attached to a large ganglion below the esophagus, from which a double ventral nerve cord runs the length of the body. The cord possesses one ganglion per segment, each of which produces lateral nerve fibres that run into the limbs. Many species possess a pair of rhabdomeric pigment-cup eyes, and numerous sensory bristles are on the head and body.[34]

Tardigrades all possess a buccopharyngeal apparatus, which, along with the claws, is used to differentiate among species.

Reproduction

Shed cuticle of female Tardigrade, containing eggs.

Although some species are parthenogenic, 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.[31]

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 species attach them to nearby substrate.[31]

The eggs hatch after no more than 14 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 may molt up to 12 times.[31]

Ecology and life history

Most tardigrades are phytophagous (plant eaters) or bacteriophagous (bacteria eaters), but some are carnivorous to the extent of eating other smaller species of tardigrades (e.g., Milnesium tardigradum).[35][36] Others are cannibalistic to their own species.

Tardigrades share morphological characteristics with many species that differ largely by class. Biologists have a difficult time finding verification among tardigrade species because of this relationship. These animals are most closely related to the early evolution of arthropods.[37] Tardigrade fossils go as far back as the Cretaceous period in North America. This specific species is considered cosmopolitan and can be located in regions all over the world. The eggs and cysts of tardigrades are so resistant to other dangers that they are carried great distances, on the feet of other animals, to a different location.[38] 

Physiology

Hypsibius dujardini imaged with a scanning electron microscope

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 milder environments such as 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 suspending their metabolism (see cryptobiosis). Many species of tardigrade can survive in a dehydrated state up to five years, or in exceptional cases longer.[39] 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.[13] Their ability to remain desiccated for such long periods was thought to be largely dependent on the high levels of the nonreducing sugar trehalose, which protects their membranes, though recent research suggests that tardigrades have a unique type of disordered protein that serves a similar purpose: It replaces water in the cells and adopts a glassy, vitrified state when the animals dry out. [40] Their DNA is further protected from radiation by a protein called "Dsup" (short for damage suppressor).[41][42] In this cryptobiotic state, the tardigrade is known as a tun.[43]

Tardigrades are able to survive in extreme environments that would kill almost any other animal. Extremes at which tardigrades can survive include those of:

Irradiation of tardigrade eggs collected directly from a natural substrate (moss) showed a clear dose-related response, with a steep decline in hatchability at doses up to 4 kGy, above which no eggs hatched.[54] The eggs were more tolerant to radiation late in development. No eggs irradiated at the early developmental stage hatched, and only one egg at middle stage hatched, while eggs irradiated in the late stage hatched at a rate indistinguishable from controls.[54]

Tardigrades are one of the few groups to have survived Earth's five mass extinctions.[62]

Taxonomy

Illustration of Echiniscus sp. from 1861

Scientists have conducted morphological and molecular studies to understand how tardigrades relate to other lineages of ecdysozoan animals. Two plausible placements have been proposed: tardigrades are either most closely related to Arthropoda ± Onychophora, or tardigrades are most closely related to nematodes. Evidence for the former is a common result of morphological studies; evidence of the latter is found in some molecular analyses.

The latter hypothesis has been rejected by recent microRNA and expressed sequence tag analyses.[63] 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.[64] 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.[63][65]

Panarthropoda


Water bears (Tardigrada)


Lobopodia


Velvet worms (Onychophora)

Arthropods (Arthropoda)




The minute sizes of tardigrades and their membranous integuments make their fossilization both difficult to detect and highly unusual. The only known fossil specimens are those from mid-Cambrian deposits in Siberia and a few rare specimens from Cretaceous amber.[66]

Echiniscus testudo

The Siberian tardigrade fossils differ from living tardigrades in several ways. They have three pairs of legs rather than four, they have a simplified head morphology, and they have no posterior head appendages, but they share with modern tardigrades their columnar cuticle construction.[1] Scientists think they represent a stem group of living tardigrades.[66]

Rare specimens in Cretaceous amber have been found in two North American locations. Milnesium swolenskyi, from New Jersey, is the older of the two; its claws and mouthparts are indistinguishable from the living M. tardigradum. The other specimens from amber are from western Canada, some 15–20 million years earlier than M. swolenskyi. One of the two specimens from Canada 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.[66][67]

Aysheaia from the middle Cambrian Burgess shale has been proposed as a sister taxon to an arthropod-tardigrade clade.[68] Tardigrades have been proposed to be among the closest living relatives of the Burgess shale oddity Opabinia.[69]

Genomes and genome sequencing

Tardigrade genomes vary in size, from about 75 to 800 megabase pairs of DNA.[70] Hypsibius dujardini has a compact genome and a generation time of about two weeks; it can be cultured indefinitely and cryopreserved.[71]

The genome of Ramazzottius varieornatus, one of the most stress-tolerant species of Tardigrades, was sequenced by a team of researchers from the University of Tokyo in 2015. Analysis revealed less than 1.2% of its genes were the result of horizontal gene transfer. They also found evidence of a loss of gene pathways that are known to promote damage due to stress. This study also found a high expression of novel Tardigrade-unique proteins, including Damage suppressor (Dsup), which was shown to protect against DNA damage from X-ray radiation. The same team applied the Dsup protein to human cultured cells and found that it suppressed X-ray damage to the human cells by ~40%.[42]

Ecological importance

Many organisms that live in aquatic environments feed on species such as nematodes, tardigrades, bacteria, algae, mites, and collembolans.[72] Tardigrades work as pioneer species by inhabiting new developing environments in which to live. This movement attracts other invertebrates to populate that space, while also attracting predators.[73]

See also

References

  1. 1 2 Budd, G.E. (2001). "Tardigrada as 'stem-group arthropods': the evidence from the Cambrian fauna". Zoologischer Anzeiger. 240 (3–4): 265–279. doi:10.1078/0044-5231-00034.
  2. 1 2 3 4 Miller William. "Tardigrades". American Scientist. Retrieved 2013-12-02.
  3. 1 2 3 4 Simon, Matt (21 March 2014). "Absurd Creature of the Week: The Incredible Critter That's Tough Enough to Survive in the vacuum of Space". Wired. Retrieved 2014-03-21.
  4. ↑ Copley, Jon (23 October 1999). "Indestructible". New Scientist (2209). Retrieved 2010-02-06.
  5. ↑ "Stanford Tardigrade Project". Foldscope. Retrieved 2017-03-23.
  6. ↑ The Hindu SEPTEMBER 09, 2015
  7. 1 2 Bordenstein, Sarah. "Tardigrades (Water Bears)". Microbial Life Educational Resources. National Science Digital Library. Retrieved 2014-01-24.
  8. ↑ "BBC - Earth - The strange worms that live on erupting mud volcanoes". BBC. Retrieved 2017-04-15.
  9. ↑ "Tardigrades". Tardigrade. Retrieved 2015-09-21.
  10. 1 2 Guarino, Ben (14 July 2017). "These animals can survive until the end of the Earth, astrophysicists say". Washington Post. Retrieved 14 July 2017.
  11. 1 2 Sloan, David; Batista, Alves; Loeb, Abraham (14 July 2017). "The Resilience of Life to Astrophysical Events". Scientific Reports. 7 (5419). doi:10.1038/s41598-017-05796-x. Retrieved 14 July 2017.
  12. ↑ Simon, Matt. "Absurd Creature of the Week: The Incredible Critter That’s Tough Enough to Survive in Space".
  13. 1 2 Dean, Cornelia (7 September 2015). "The Tardigrade: Practically Invisible, Indestructible ‘Water Bears’". New York Times. Retrieved September 7, 2015.
  14. ↑ Brennand, Emma (17 May 2011). "Tardigrades: Water bears in space". BBC. Retrieved 2013-05-31.
  15. 1 2 Crowe, John H.; Carpenter, John F.; Crowe, Lois M. (October 1998). "The role of vitrification in anhydrobiosis". Annual Review of Physiology. 60. pp. 73–103. PMID 9558455. doi:10.1146/annurev.physiol.60.1.73.
  16. 1 2 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.
  17. ↑ Rampelotto, P. H. (2010). "Resistance of microorganisms to extreme environmental conditions and its contribution to Astrobiology". Sustainability. 2 (6): 1602–1623. doi:10.3390/su2061602.
  18. ↑ Rothschild, L.J.; Mancinelli, R.L. (22 February 2001). "Life in extreme environments". Nature. 409 (6823): 1092–1101. PMID 11234023. doi:10.1038/35059215.
  19. ↑ Romano, Frank A. (2003). "On water bears". Florida Entomologist. 86 (2): 134–137. doi:10.1653/0015-4040(2003)086[0134:OWB]2.0.CO;2.
  20. ↑ Shaw, Michael W. "How to Find Tardigrades". tardigrades.us. Retrieved 2013-01-14.
  21. ↑ "Tardigrada (water bears, tardigrades)". biodiversity explorer. Retrieved 2013-05-31.
  22. ↑ Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.
  23. ↑ Degma, P., Bertolani, R. & Guidetti, R. 2009–2011. Actual checklist of Tardigrada species. Ver. 18: 27-04-2011. tardigrada.modena.unimo.it
  24. ↑ Hogan, C. Michael. 2010. "Extremophile". eds. E.Monosson and C.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment, washington DC
  25. ↑ Bordenstein, Sarah (17 December 2008). "Tardigrades (Water Bears)". Carleton College. Retrieved 2012-09-16.
  26. ↑ Staff (29 September 2015). "Researchers discover new tiny organism, name it for Wyeths". AP News. Retrieved 2015-09-29.
  27. ↑ Perry, Emma; Miller, William (April 2015). "Echiniscoides wyethi, a new marine tardigrade from Maine, U.S.A. (Heterotardigrada: Echiniscoidea: Echiniscoididae)". Proceedings of the Biological Society of Washington. 128 (1): 103–110. doi:10.2988/0006-324X-128.1.103. Retrieved 29 December 2015.
  28. ↑ Goldstein, B. & Blaxter, M. (2002). "Quick Guide: Tardigrades". Current Biology. 12 (14): R475. doi:10.1016/S0960-9822(02)00959-4.
  29. 1 2 Seki, Kunihiro; Toyoshima, Masato (1998-10-29). "Preserving tardigrades under pressure". Nature. 395 (6705): 853–854. doi:10.1038/27576.
  30. ↑ Kinchin, Ian M. (1994) The Biology of Tardigrades, Ashgate Publishing
  31. 1 2 3 4 5 Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 877–880. ISBN 0-03-056747-5.
  32. ↑ Gross V, Mayer G (2015). "Neural development in the tardigrade Hypsibius dujardini based on anti-acetylated α-tubulin immunolabeling". EvoDevo. 6: 12. PMC 4458024 Freely accessible. PMID 26052416. doi:10.1186/s13227-015-0008-4.
  33. ↑ Zantke, Juliane; Wolff, Carsten; Scholtz, Gerhard (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.
  34. ↑ Greven, H. (Dec 2007). "Comments on the eyes of tardigrades". Arthropod Structure & Development. 36 (4): 401–407. ISSN 1467-8039. PMID 18089118. doi:10.1016/j.asd.2007.06.003.
  35. ↑ Morgan, Clive I. (1977). "Population dynamics of two species of Tardigrada, Macrobiotus hufelandii (Schultze) and Echiniscus (Echiniscus) testudo (Doyere), in roof moss from Swansea". Journal of Animal Ecology. British Ecological Society. 46 (1): 263–279. JSTOR 3960. doi:10.2307/3960.
  36. ↑ Lindahl, K. (2008-03-15). "Tardigrade Facts".
  37. ↑ Brent Nichols, Phillip (2005). Tardigrade Evolution and Ecology (Ph.D.). University of South Florida.
  38. ↑ Nelson, Diane (2002). "Current status of Tardigrada:Evolution and Ecology".
  39. ↑ Bell, Graham (2016). "Experimental macroevolution". Proceedings of the Royal Society B: Biological Sciences. 283 (1822): 20152547. doi:10.1098/rspb.2015.2547.
  40. ↑ Boothby, T.C.; Tapia, H.; et al. (2017). "Tardigrades use intrinsically disordered proteins to survive desiccation". Molecular Cell. 65 (6): 975–984. doi:10.1016/j.molcel.2017.02.018.
  41. ↑ Tauger, Nathan; Gill, Victoria (20 September 2016). "Survival secret of 'Earth's hardiest animal' revealed". BBC News. Retrieved 2016-09-21.
  42. 1 2 Hashimoto, Takuma; et al. (20 September 2015). "Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein". Nature Communications. 7: 12808. PMC 5034306 Freely accessible. PMID 27649274. doi:10.1038/ncomms12808. Retrieved 2016-09-21.
  43. ↑ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  44. 1 2 Horikawa, Daiki D. (2012). Alexander V. Altenbach; Joan M. Bernhard; Joseph Seckbach, eds. Anoxia Evidence for Eukaryote Survival and Paleontological Strategies (21st ed.). Springer Netherlands. pp. 205–217. ISBN 978-94-007-1895-1. Retrieved 2012-01-21.
  45. ↑ Tsujimoto, Megumu; Imura, Satoshi; Kanda, Hiroshi (February 2015). "Recovery and reproduction of an Antarctic tardigrade retrieved from a moss sample frozen for over 30 years". Cryobiology. 72: 78–81. PMID 26724522. doi:10.1016/j.cryobiol.2015.12.003.
  46. ↑ 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". Comptes rendus hebdomadaires des séances de l'Académie des Sciences (in French). 231: 261–263.
  47. 1 2 3 Jönsson, K. Ingemar; Rabbow, Elke; Schill, Ralph O.; Harms-Ringdahl, Mats & Rettberg, Petra (9 September 2008). "Tardigrades survive exposure to space in low Earth orbit". Current Biology. 18 (17): R729–R731. PMID 18786368. doi:10.1016/j.cub.2008.06.048.
  48. 1 2 Jönsson, K. Ingemar & Bertolani, R. (September 2001). "Facts and fiction about long-term survival in tardigrades". Journal of Zoology. 255: 121–123. doi:10.1017/S0952836901001169.
  49. 1 2 Franceschi, T. (1948). "Anabiosi nei tardigradi". Bollettino dei Musei e degli Istituti Biologici dell'Università di Genova. 22: 47–49.
  50. ↑ Kent, Michael (2000), Advanced Biology, Oxford University Press
  51. ↑ "Radiation tolerance in the tardigrade Milnesium tardigradum" (PDF).
  52. 1 2 Horikawa, Daiki D.; Sakashita, Tetsuya; Katagiri, Chihiro; Watanabe, Masahiko; Kikawada, Takahiro; Nakahara, Yuichi; Hamada, Nobuyuki; Wada, Seiichi; et al. (1 January 2006). "Radiation tolerance in the tardigrade Milnesium tardigradum". International Journal of Radiation Biology. 82 (12): 843–848. PMID 17178624. doi:10.1080/09553000600972956.
  53. ↑ Horikawa, Daiki D. "UV Radiation Tolerance of Tardigrades". NASA.com. Retrieved 2013-01-15.
  54. 1 2 Jönsson, Ingemar; Beltran-Pardo, Eliana; Haghdoost, Siamak; Wojcik, Andrzej; Bermúdez-Cruz, Rosa Maria; Bernal Villegas, Jaime E.; Harms-Ringdahl, Mats (2013). "Tolerance to gamma-irradiation in eggs of the tardigrade Richtersius coronifer depends on stage of development". Journal of Limnology. 71 (12th International Symposium on Tardigrada). Retrieved 2013-08-05.
  55. ↑ "Creature Survives Naked in Space". Space.com. 8 September 2008. Retrieved 2011-12-22.
  56. ↑ Mustain, Andrea (22 December 2011). "Weird wildlife: The real land animals of Antarctica". MSNBC. Retrieved 2011-12-22.
  57. ↑ Courtland, Rachel (8 September 2008). "'Water bears' are first animal to survive space vacuum". New Scientist. Retrieved 2011-05-22.
  58. ↑ NASA Staff (17 May 2011). "BIOKon In Space (BIOKIS)". NASA. Retrieved 2011-05-24.
  59. ↑ Brennard, Emma (17 May 2011). "Tardigrades: Water bears in space". BBC. Retrieved 2011-05-24.
  60. ↑ "Tardigrades: Water bears in space". BBC Nature. 17 May 2011.
  61. ↑ 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.
  62. ↑ Cosmos:A Spacetime Odyssey. Dir. Bill Pope. Perf. Neil DeGrasse Tyson. Cosmos Studios, 2014. Netflix.
  63. 1 2 Campbell, Lahcen; Omar Rota-Stabelli; Gregory D. Edgecombe; Trevor Marchioro; Stuart J. Longhorn; Maximilian J. Telford; Hervé Philippe; Lorena Rebecchi; et al. (2011). "MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of Arthropoda". Proceedings of the National Academy of Sciences of the United States of America. 108 (38): 15920–4. PMC 3179045 Freely accessible. PMID 21896763. doi:10.1073/pnas.1105499108.
  64. ↑ Telford, Maximilian; Sarah J Bourlat; Andrew Economou; Daniel Papillon & Omar Rota-Stabelli (April 2008). "The evolution of the Ecdysozoa". Philosophical Transactions of the Royal Society B: Biological Sciences. 363 (1496): 1529–1537. PMC 2614232 Freely accessible. PMID 18192181. doi:10.1098/rstb.2007.2243. Retrieved 2013-09-09.
  65. ↑ "Sequencing of Tardigrade Genome" (PDF). The Royal Society. 2003. Retrieved 2013-05-31.
  66. 1 2 3 Grimaldi, David A.; Engel, Michael S. (2005). Evolution of the Insects. Cambridge University Press. pp. 96–97. ISBN 0-521-82149-5.
  67. ↑ Cooper, Kenneth W. (1964). "The first fossil tardigrade: Beorn leggi, from Cretaceous Amber". Psyche – Journal of Entomology. 71 (2): 41–48. doi:10.1155/1964/48418.
  68. ↑ Fortey, Richard A.; Thomas, Richard H. (2001). Arthropod Relationships. Chapman & Hall. p. 383. ISBN 0-412-75420-7.
  69. ↑ Budd, G.E. (March 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.
  70. ↑ "Genome Size of Tardigrades".
  71. ↑ Gabriel, W; McNuff, Robert; Patel, Sapna K.; Gregory, T. Ryan; Jeck, William R.; Jones, Corbin D.; Goldstein, Bob (15 December 2007). "The tardigrade Hypsibius dujardini, a new model for studying the evolution of development". Developmental Biology. 312 (2): 545–559. PMID 17996863. doi:10.1016/j.ydbio.2007.09.055.
  72. ↑ Kinchin, IM (1987). "The moss fauna 1; Tardigrades.". Journal of Biological Education. 21(4): 288–290.
  73. ↑ Brent Nichols, Phillip (2005). Tardigrade Evolution and Ecology (Ph.D.). University of Florida.

NASA Astronomy Picture of the Day: Tardigrade in Moss (6 March 2013)

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.