Temporal range: Cambrian–Recent
Tardigrades (//; also known as water bears or moss piglets) are water-dwelling, eight-legged, segmented micro-animals. 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. They have been found everywhere from mountaintops to the deep sea, from tropical rain forests to the Antarctic.
Tardigrades are notable for being the most resilient animal: they can survive extreme conditions that would be rapidly fatal to nearly all other known life forms. They can withstand temperature ranges from 1 K (−458 °F; −272 °C) (close to absolute zero) to about 420 K (300 °F; 150 °C), 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. 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.
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, whereas true extremophiles thrive in a physically or geochemically extreme environment that would harm most other organisms.
Usually, tardigrades are about 0.5 mm (0.02 in) long when they are fully grown. They are short and plump with four pairs of legs, each with four to eight claws also known as "disks". The first three pairs of legs are directed ventrolaterally and are the primary means of locomotion (moving), while the fourth pair is directed posteriorly on the terminal segment of the trunk and is used primarily for grasping the substrate. 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.
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. About 1,150 species of tardigrades have been described. Tardigrades can be found throughout the world, from the Himalayas (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.
Johann August Ephraim Goeze originally named the tardigrade kleiner Wasserbär (Bärtierchen today), meaning "little water bear" in German. The name Tardigrada means "slow walker" and was given by Lazzaro Spallanzani in 1776. 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.
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, may be found on barnacles. Often, tardigrades can be found by soaking a piece of moss in water.
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.
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.
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.
The brain develops in a bilaterally symmetric pattern. The brain includes multiple lobes, mostly consisting of three bilaterally paired clusters of neurons. 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.
Tardigrades all possess a buccopharyngeal apparatus, which, along with the claws, is used to differentiate among species.
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.
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.
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.
Ecology and life history
|This section needs expansion. You can help by adding to it. (November 2014)|
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).
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 reversibly suspending their metabolism and going into a state of cryptobiosis. Many species of tardigrade can survive in a dehydrated state up to five years, or in exceptional cases longer. 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 long periods is largely dependent on the high levels of the nonreducing sugar trehalose, which protects their membranes. Their DNA is further protected from radiation by a protein called "Dsup" (short for damage suppressor). In this cryptobiotic state, the tardigrade is known as a tun.
Tardigrades are able to survive in extreme environments that would kill almost any other animal. Extremes at which tardigrades can survive include those of:
- Temperature – tardigrades can survive:
- 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. 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.
- Dehydration – the longest that living tardigrades have been shown to survive in a dry state is nearly 10 years, although there is one report of leg movement, not generally considered "survival", in a 120-year-old specimen from dried moss. 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.
- Radiation – tardigrades can withstand 1,000 times more radiation than other animals, 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). The only explanation found in earlier experiments for this ability was that their lowered water state provides fewer reactants for the ionizing radiation. 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.
- 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. 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.
- Environmental toxins – tardigrades are reported to undergo chemobiosis, a cryptobiotic response to high levels of environmental toxins. However, as of 2001, these laboratory results have yet to be verified.
- Outer space – tardigrades are the first known animal to survive in space. In September 2007, dehydrated tardigrades were taken into low Earth orbit on the FOTON-M3 mission carrying the BIOPAN astrobiology payload. For 10 days, groups of tardigrades were exposed to the hard vacuum of outer space, or vacuum and solar UV radiation. After being rehydrated back on Earth, over 68% of the subjects protected from high-energy UV radiation revived within 30 minutes following rehydration, but subsequent mortality was high; many of these produced viable embryos. In contrast, hydrated samples exposed to the combined effect of vacuum and full solar UV radiation had significantly reduced survival, with only three subjects of Milnesium tardigradum surviving. In May 2011, Italian scientists sent tardigrades on board the International Space Station along with extremophiles on STS-134, the final flight of Space Shuttle Endeavour. 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." In November 2011, they were among the organisms to be sent by the U.S.-based Planetary Society on the Russian Fobos-Grunt mission's Living Interplanetary Flight Experiment to Phobos; however, the launch failed. It remains unknown whether tardigrade specimens survived the failed launch.
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. 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. 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.
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.
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. Scientists think they represent a stem group of living tardigrades.
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.
Genomes and genome sequencing
Tardigrade genomes vary in size, from about 75 to 800 megabase pairs of DNA. Although a recent genome sequencing effort suggested that Hypsibius dujardini had 17% horizontal gene transfer from bacteria, fungi, and viruses, more recent sequencing efforts falsified this claim, and showed that the inflated estimations were due to contamination. Hypsibius dujardini has a compact genome and a generation time of about two weeks; it can be cultured indefinitely and cryopreserved.
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 cultures cells and found that it suppressed X-ray damage to the human cells by ~40%.
- List of microorganisms tested in outer space
- Living Interplanetary Flight Experiment – study of selected microorganisms in outer space
- Mopsechiniscus franciscae – tardigrade found in Victoria Land Antarctica
- Budd, G.E. (2001). "Tardigrades as 'stem-group arthropods': the evidence from the Cambrian fauna". Zool. Anz. 240 (3–4): 265–279. doi:10.1078/0044-5231-00034.
- Miller William. "Tardigrades". American Scientist. Retrieved 2013-12-02.
- 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.
- Copley, Jon (23 October 1999). "Indestructible". New Scientist (2209). Retrieved 2010-02-06.
-  The Hindu SEPTEMBER 09, 2015
- Bordenstein, Sarah. "Tardigrades (Water Bears)". Microbial Life Educational Resources. National Science Digital Library. Retrieved 2014-01-24.
- "Tardigrades". Tardigrade. Retrieved 2015-09-21.
- Wired, Water bear
- Dean, Cornelia (7 September 2015). "The Tardigrade: Practically Invisible, Indestructible 'Water Bears'". New York Times. Retrieved September 7, 2015.
- Brennand, Emma (17 May 2011). "Tardigrades: Water bears in space". BBC. Retrieved 2013-05-31.
- 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. doi:10.1146/annurev.physiol.60.1.73. PMID 9558455.
- 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.
- 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.
- Rothschild, L.J.; Mancinelli, R.L. (22 February 2001). "Life in extreme environments". Nature. 409 (6823): 1092–1101. doi:10.1038/35059215. PMID 11234023.
- 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.
- Shaw, Michael W. "How to Find Tardigrades". tardigrades.us. Retrieved 2013-01-14.
- "Tardigrada (water bears, tardigrades)". biodiversity explorer. Retrieved 2013-05-31.
- Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.
- Degma, P., Bertolani, R. & Guidetti, R. 2009–2011. Actual checklist of Tardigrada species. Ver. 18: 27-04-2011. tardigrada.modena.unimo.it
- Hogan, C. Michael. 2010. "Extremophile". eds. E.Monosson and C.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment, washington DC
- Bordenstein, Sarah (17 December 2008). "Tardigrades (Water Bears)". Carleton College. Retrieved 2012-09-16.
- Staff (29 September 2015). "Researchers discover new tiny organism, name it for Wyeths". AP News. Retrieved 2015-09-29.
- 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.
- Goldstein, B. & Blaxter, M. (2002). "Quick Guide: Tardigrades". Current Biology. 12 (14): R475. doi:10.1016/S0960-9822(02)00959-4.
- Seki, Kunihiro; Toyoshima, Masato (1998-10-29). "Preserving tardigrades under pressure". Nature. 395 (6705): 853–854. doi:10.1038/27576.
- Kinchin, Ian M. (1994) The Biology of Tardigrades, Ashgate Publishing
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 877–880. ISBN 0-03-056747-5.
- Gross V, Mayer G (2015). "Neural development in the tardigrade Hypsibius dujardini based on anti-acetylated α-tubulin immunolabeling". EvoDevo. 6: 12. doi:10.1186/s13227-015-0008-4. PMC . PMID 26052416.
- 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.
- Greven, H. (Dec 2007). "Comments on the eyes of tardigrades". Arthropod Structure & Development. 36 (4): 401–407. doi:10.1016/j.asd.2007.06.003. ISSN 1467-8039. PMID 18089118.
- 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. British Ecological Society. 46 (1): 263–279. doi:10.2307/3960. JSTOR 3960.
- Lindahl, K. (2008-03-15). "Tardigrade Facts".
- Bell, Graham (2016). "Experimental macroevolution". Proceedings of the Royal Society B: Biological Sciences. 283 (1822): 20152547. doi:10.1098/rspb.2015.2547.
- Tauger, Nathan; Gill, Victoria (20 September 2016). "Survival secret of 'Earth's hardiest animal' revealed". BBC News. Retrieved 2016-09-21.
- 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. doi:10.1038/ncomms12808. Retrieved 2016-09-21.
- Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- 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.
- 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. doi:10.1016/j.cryobiol.2015.12.003. PMID 26724522.
- 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.
- 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. doi:10.1016/j.cub.2008.06.048. PMID 18786368.
- 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.
- Franceschi, T. (1948). "Anabiosi nei tardigradi". Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova. 22: 47–49.
- Kent, Michael (2000), Advanced Biology, Oxford University Press
- Radiation tolerance in the tardigrade Milnesium tardigradum
- Horikawa DD; Sakashita T; Katagiri C; Watanabe M; Kikawada T; Nakahara Y; Hamada N; Wada S; et al. (2006). "Radiation tolerance in the tardigrade Milnesium tardigradum". International Journal of Radiation Biology. 82 (12): 843–8. doi:10.1080/09553000600972956. PMID 17178624.
- 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". International Journal of Radiation Biology. 82 (12): 843–848. doi:10.1080/09553000600972956. PMID 17178624.
- Horikawa, Daiki D. "UV Radiation Tolerance of Tardigrades". NASA.com. Retrieved 2013-01-15.
- 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.
- "Creature Survives Naked in Space". Space.com. 8 September 2008. Retrieved 2011-12-22.
- Mustain, Andrea (22 December 2011). "Weird wildlife: The real land animals of Antarctica". MSNBC. Retrieved 2011-12-22.
- Courtland, Rachel (8 September 2008). "'Water bears' are first animal to survive space vacuum". New Scientist. Retrieved 2011-05-22.
- NASA Staff (17 May 2011). "BIOKon In Space (BIOKIS)". NASA. Retrieved 2011-05-24.
- Brennard, Emma (17 May 2011). "Tardigrades: Water bears in space". BBC. Retrieved 2011-05-24.
- "Tardigrades: Water bears in space". BBC Nature. 17 May 2011.
- 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.
- 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". PNAS. 108 (38): 15920–4. doi:10.1073/pnas.1105499108. PMC . PMID 21896763.
- Telford, Maximilian; Sarah J Bourlat; Andrew Economou; Daniel Papillon & Omar Rota-Stabelli (April 2008). "The evolution of the Ecdysozoa". Phil. Trans. R. Soc. B. 363 (1496): 1529–1537. doi:10.1098/rstb.2007.2243. PMC . PMID 18192181. Retrieved 2013-09-09.
- "Sequencing of Tardigrade Genome" (PDF). The Royal Society. 2003. Retrieved 2013-05-31.
- Grimaldi, David A.; Engel, Michael S. (2005). Evolution of the Insects. Cambridge University Press. pp. 96–97. ISBN 0-521-82149-5.
- Budd, G. (2001). "Tardigrades as 'Stem-Group Arthropods': The Evidence from the Cambrian Fauna". Zoologischer Anzeiger. 240 (3–4): 265–279. doi:10.1078/0044-5231-00034. ISSN 0044-5231.
- 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.
- Fortey, Richard A.; Thomas, Richard H. (2001). Arthropod Relationships. Chapman & Hall. p. 383. ISBN 0-412-75420-7.
- 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.
- "Genome Size of Tardigrades".
- Boothby, Thomas C.; Tenlen, Jennifer R.; Smith, Frank W.; Wang, Jeremy R.; Patanella, Kiera A.; Nishimura, Erin Osborne; Tintori, Sophia C.; Li, Qing; Jones, Corbin D. (2015-12-29). "Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade". Proceedings of the National Academy of Sciences. 112 (52): 15976–15981. doi:10.1073/pnas.1510461112. ISSN 0027-8424. PMC . PMID 26598659.
- "Rival Scientists Cast Doubt Upon Recent Discovery About Invincible Animals".
- Koutsovoulos, Georgios; Kumar, Sujai; Laetsch, Dominik R.; Stevens, Lewis; Daub, Jennifer; Conlon, Claire; Maroon, Habib; Thomas, Fran; Aboobaker, Aziz A.; Blaxter, Mark (2016). "No evidence for extensive horizontal gene transfer in the genome of the tardigrade Hypsibius dujardini". Proceedings of the National Academy of Sciences. 113: 201600338. doi:10.1073/pnas.1600338113. ISSN 0027-8424.
- Delmont, Tom O.; Eren, A. Murat (2016-01-01). "Identifying contamination with advanced visualization and analysis practices: metagenomic approaches for eukaryotic genome assemblies". PeerJ. 4: e1839. doi:10.7717/peerj.1839. PMC . PMID 27069789.
- 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. doi:10.1016/j.ydbio.2007.09.055. PMID 17996863.
- Hashimoto T; et al. (20 September 2016). "Extremotolerant tardigrade genome and improvedradiotolerance of human cultured cells bytardigrade-unique protein.". Nature Communications. 7 (7:12808 e64793.): 12808. doi:10.1038/ncomms12808.
|Wikispecies has information related to: Tardigrada|
|Wikimedia Commons has media related to Tardigrada.|
- Tardigrade water bear NYTimes, 2015
- Tardigrada Newsletter
- Tardigrades – Pictures and Movies
- The Edinburgh Tardigrade project
- Instructions for finding tardigrades
- The incredible water bear!
- Tardigrade Reference Center
- Tardigrades in space
- Tardigrade data and analysis
- A short film about tardigrade research from NPR's Science Friday
- Tardigrada at the Tree of Life Web Project
- Swiss Center of Tardigrade Research – Ecology, Physiology and Evolutionary Biology of Tardigrades
- NASA Astronomy Picture of the Day: Tardigrade in Moss (6 March 2013)
- Video (07:54) – First Animal to Survive in Space
- Video (00:38) – Tardigrade Movement in Water
- Tardigrades are so tough, they can survive outer space (March 2015). BBC
- The International Society of Tardigrade Hunters