Tardigrades live in diverse regions of Earth'sbiosphere – mountaintops, thedeep sea,tropical rainforests, and theAntarctic. They are among the most resilient animals known, with individual species able to survive severe conditions, such as exposure to extreme temperatures, extremepressures (both high and low), air deprivation,radiation,dehydration, andstarvation – that would quickly kill most other forms oflife. Tardigrades have survived exposure toouter space.
There are about 1,500 known species in the phylum Tardigrada, a part of the superphylumEcdysozoa. The earliest known fossil is from theCambrian, some 500 million years ago. They lack several of theHox genes found in arthropods, and the middle region of the body corresponding to an arthropod's thorax and abdomen. Instead, most of their body ishomologous to an arthropod's head.
Tardigrades are usually about 0.5 mm (0.02 in) long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or sticky pads. Tardigrades are prevalent inmosses andlichens and can readily be collected and viewed under a low-powermicroscope, making them accessible to students and amateur scientists. Their clumsy crawling and their well-known ability to survive extreme conditions have brought them intoscience fiction and popular culture including items of clothing, statues, soft toys andcrochet patterns.
Tardigrades have a short plump body with four pairs of hollow unjointed legs. Most range from 0.05 to 0.5 mm (0.002 to 0.02 in) in length, although the largest species may reach 1.3 mm (0.051 in). The body cavity is ahaemocoel, an open circulatory system, filled with a colourless fluid. The body covering is acuticle that is replaced when the animalmoults; it contains hardened (sclerotised) proteins andchitin but is notcalcified. Each leg ends in one or more claws according to the species; in some species, the claws are modified as sticky pads. In marine species, the legs are telescopic. There are no lungs, gills, or blood vessels, so tardigrades rely ondiffusion through the cuticle and body cavity forgas exchange.[3] They are made up of only about 1000 cells.[4]
The tardigrade nervous system has a pair ofventral nerve cords with a pair ofganglia serving each pair of legs. The nerve cords end near the mouth at a pair of subpharyngeal (or suboesophageal) ganglia. These are connected by pairedcommissures (either side of the tube from the mouth to thepharynx) to the dorsally located cerebral ganglion or 'brain'. Also in the head are twoeyespots in the brain, and several sensorycirri and pairs of hollow antenna-like clavae which may bechemoreceptors.[3]
The tardigradeDactylobiotus dispar can be trained byclassical conditioning to curl up into the defensive 'tun' state in response to a blue light associated with a small electric shock, an aversive stimulus. This demonstrates that tardigrades are capable of learning.[5]
Although the body is flexible and fluid-filled, locomotion does not operate mainlyhydrostatically. Instead, as inarthropods, the muscles (sometimes just one or a few cells) work in antagonistic pairs that make each leg step backwards and forwards; there are also someflexors that work against hydrostatic pressure of the haemocoel. The claws help to stop the legs sliding during walking, and are used for gripping.[3]
Tardigrades feed by sucking animal or plant cell fluids, or ondetritus. A pair ofstylets pierce the prey; the pharynx muscles then pump the fluids from the prey into the gut. A pair ofsalivary glands secrete a digestive fluid into the mouth, and produce replacement stylets each time the animal moults.[3] Non-marine species have excretoryMalpighian tubules where the intestine joins thehindgut. Some species have excretory or other glands between or at the base of the legs.[3]
Shed cuticle of female tardigrade, containing eggs, each 50μm across
Most tardigrades have both male and female animals, which copulate by a variety of methods. The females lay eggs; those ofAusteruseus faeroensis are spherical, 80μm in diameter, with a knobbled surface. In other species, the eggs can be ovoid, as inHypsibius annulatus, or may be spherical with pyramidal or bottle-shaped surface ornamentation. Some species appear to have no males, suggesting thatparthenogenesis is common.[3]
Both sexes have a singlegonad (an ovary or testis) located above the intestine.[3] A pair ofducts run from the testis, opening through a singlegonopore in front of the anus. Females have a single oviduct opening either just above the anus or directly into the rectum, which forms acloaca.[3]
The male may place his sperm into the cloaca, or may penetrate the female'scuticle and place the sperm straight into her body cavity, for it tofertilise the eggs directly in the ovary. A third mechanism in species such asH. annulatus is for the male to place the sperm under the female's cuticle; when she moults, she lays eggs into the cast cuticle, where they are fertilised.[3] Courtship occurs in some aquatic tardigrades, with the male stroking his partner with his cirri to stimulate her to lay eggs; fertilisation is then external.[3]
Up to 30 eggs are laid, depending on the species. Terrestrial tardigrade eggs have drought-resistant shells. Aquatic species either glue their eggs to a substrate or leave them in a cast cuticle. The eggs hatch within 14 days, and the hatchlings use their stylets to open their egg shells.[3]
Tardigrades as a group arecosmopolitan, living in many environments on land, in freshwater, and in the sea. Their eggs and resistant life-cycle stages (cysts and tuns) are small and durable enough to enable long-distance transport, whetheron the feet of other animals orby the wind.[3]
Individual species have more specialised distributions, many being both regional and limited to a single type of habitat, such as mountains.[6] Some species have wide distributions: for instance,Echiniscus lineatus ispantropical.[6]Halobiotus is restricted to coldHolarctic seas.[6] Species such asBorealibius andEchiniscus lapponicus have a discontinuous distribution, being both polar and on tall mountains. This could be a result of long-distance transport by the wind or the remains of an ancient geographic range when the climate was colder.[6] A small percentage of species may be cosmopolitan.[6]
The majority of species live in damp habitats such as onlichens,liverworts, andmosses, and directly in soil andleaf litter. In freshwater and the sea, they live on and in the bottom, such as in between particles or aroundseaweeds. More specialised habitats include hot springs and asparasites orcommensals of marine invertebrates. In soil, there can be as many as 300,000 per square metre; on mosses, they can reach a density of over 2 million per square metre.[3]
Except for 62 exclusively freshwater species, all non-marine tardigrades are found in terrestrial environments. Because the majority of the marine species belong to Heterotardigrada, the most ancestral class, the phylum evidently has a marine origin.[10]
Tardigrades are not considered universallyextremophilic because they are not adapted to exploit many of the extreme conditions that their environmental tolerance has been measured in, only to endure them. This means that their chances of dying increase the longer they are exposed to these extreme environments,[11] whereas true extremophiles thrive there.[12]
Richtersius coronifer in active and 'tun' states. A↔P = anterior-posterior; mg = midgut; go = gonad; pb = pharyngeal bulb; mo = mouth; st = stylet Scale bars = 100 μm
Tardigrades are capable of suspending theirmetabolism, going into a state ofcryptobiosis.[3] Terrestrial and freshwater tardigrades are able to tolerate long periods when water is not available, such as when the moss or pond they are living in dries out, by drawing their legs in and forming a desiccated cyst, the cryptobiotic 'tun' state, where no metabolic activity takes place.[3] In this state, they can go without food or water for several years.[3] Further, in that state they become highly resistant toenvironmental stresses, including temperatures from as low as −272 °C (−458 °F) to as much as +149 °C (300 °F) (at least for short periods of time[13]), lack ofoxygen,[3]vacuum,[3]ionising radiation,[3][14] and high pressure.[15]
Marine tardigrades such asHalobiotus crispae alternate each year (cyclomorphosis) between an active summermorph and a hibernating winter morph (a pseudosimplex) that can resist freezing and lowsalinity, but which remains active throughout. Reproduction, however, takes place only in the summer morph.[3]
Tardigrades can surviveimpacts up to about 900 metres per second (3,000 ft/s), and momentary shock pressures up to about 1.14 gigapascals (165,000 psi).[16]
The 2007FOTON-M3 mission carrying theBIOPAN astrobiologypayload (illustrated) exposed tardigrades to vacuum, solar ultraviolet, or both, showing their ability to survive in the space environment.
Tardigrades have survived exposure to space. In 2007, dehydrated tardigrades were taken on theFOTON-M3 mission and exposed to vacuum, or to both vacuum and solar ultraviolet, for 10 days.[17] Back on Earth, more than 68% of the subjects protected from ultraviolet were reanimated by rehydration, and many produced viable embryos.[17]
Tardigrades' ability to stay desiccated for long periods of time was thought to depend on high levels of the sugartrehalose,[25] common in organisms that survive desiccation.[26] However, tardigrades do not synthesize enough trehalose for this function.[25] Instead, tardigrades produceintrinsically disordered proteins in response to desiccation. Three of these are specific to tardigrades and have been calledtardigrade specific proteins. These may protectmembranes from damage by associating with the polar heads of lipid molecules.[27] The proteins may also form a glass-like matrix that protects cytoplasm from damage during desiccation.[28] Anhydrobiosis in response to desiccation has a complex molecular basis; inHypsibius exemplaris, 1,422 genes areupregulated during the process. Of those, 406 are specific to tardigrades, 55 being intrinsically disordered and the others globular with unknown functions.[29]
Tardigrades possess a cold shock protein; Maria Kamilari and colleagues propose (2019) that this may serve "as a RNA-chaperone involved in regulation oftranslation [ofRNA code to proteins] following freezing."[26]
TardigradeDNA is protected from radiation by theDsup ("damage suppressor") protein.[30] The Dsup proteins ofRamazzottius varieornatus andH. exemplaris promote survival by binding tonucleosomes and protectingchromosomal DNA fromhydroxyl radicals.[31] The Dsup protein ofR. varieornatus confers resistance toultraviolet-C by upregulatingDNA repair genes.[32]
Some of these proteins are of interest tobiomedical research. Potential is seen in Dsup's ability to protect against damage; in CAHS and LEA's ability to protect from desiccation; and some CAHS proteins could serve to preventprogrammed cell death (apoptosis).[33]
In 1773,Johann August Ephraim Goeze named the tardigradeKleiner Wasserbär, meaning 'little water-bear' in German (today, Germans often call themBärtierchen 'little bear-animal').[34][35] The namewater bear comes from the way they walk, reminiscent of abear'sgait. The nameTardigradum means 'slow walker' and was given byLazzaro Spallanzani in 1776.[36][11] In 1834,C.A.S. Schulze gave the firstformal description of a tardigrade,Macrobiotus hufelandi, in a work subtitled "a new animal from the crustacean class, capable of reviving after prolonged asphyxia and dryness".[37][38] This was soon followed by descriptions of species includingEchiniscus testudo,Milnesium tardigradum,Hypsibius dujardini, andRamazzottius oberhaeuseri byL.M.F. Doyère in 1840. All four of these are now thenominal species for higher tardigrade taxa.[39] The zoologistHartmut Greven wrote that "The unanimous opinion of all later researchers is that Doyère's 1842 dissertationMemoire sur les Tardigrades is an indisputable milestone in tardigradology".[40]
Ferdinand Richters worked on the taxonomy of tardigrades from 1900 to 1913, with studies of Nordic, Arctic, marine, and South American species; he described many species at this time,[41][42] and in 1926 proposed the classEutardigrada.[43][44] In 1927,Ernst Marcus created the classHeterotardigrada.[45][46] and in 1929 a monograph on tardigrades[47] which Greven describes as "comprehensive" and "unsurpassed today".[40] In 1937 Gilbert Rahm, studying the fauna of Japan's hot springs, distinguished the classMesotardigrada, with a single speciesThermozodium esakii;[48] its validity is now doubted.[49] In 1962,Giuseppe Ramazzotti proposed thephylum Tardigrada.[50]In 2019, Noemi Guil and colleagues proposed to promote the orderApochela to the new class Apotardigrada.[51] There are some 1,488 described species of tardigrades, organised into 160 genera and 36 families.[52]
Tardigradefossils are rare. The only known specimens are those from mid-Cambrian deposits inSiberia (in theOrsten fauna) and a few specimens inamber from theCretaceous of North America and theNeogene of the Dominican Republic.[3][53] The Siberian fossils differ from living tardigrades in several ways. They have three pairs of legs rather than four, they have a simplified headmorphology, and they have no posterior head appendages, but they share with modern tardigrades their columnar cuticle construction. Scientists think they represent astem group of living tardigrades.[54]
Multiple lines of evidence show that tardigrades are secondarily miniaturised from a larger ancestor,[58] probably alobopodian, perhaps resembling the mid-CambrianAysheaia, which many analyses place close to thedivergence of the tardigrade lineage.[56][57] An alternative hypothesis derivestactopoda from aclade encompassingdinocaridids andOpabinia.[59] The enigmaticpanarthropodanSialomorpha found in 30-million year oldDominican amber, while not a tardigrade, shows some apparent affinities.[60] A 2023 morphological analysis concluded thatluolishaniids, a group of Cambrianlobopodians, might be the tardigrades' closest known relatives.[55]
Morphological andmolecular phylogenetics studies have attempted to define how tardigrades relate to other ecdysozoan groups; alternative placements have been proposed within thePanarthropoda.[64] TheTactopoda hypothesis holds that Tardigrada aresister to Arthropoda; theAntennopoda hypothesis is that Tardigrada are sister to (Onychophora + Arthropoda); and theLobopodia (sensu Smith & Goldstein 2017) hypothesis is that Tardigrada are sister to Onychophora. The relationships have been debated on the basis of conflicting evidence.[65]
Tardigradegenomes vary widely in size.[66]Hypsibius exemplaris (part of theHypsibius dujardini group) has a compact genome of 100 megabase pairs[64] and ageneration time of about two weeks; it can be cultured indefinitely andcryopreserved.[22] The genome ofRamazzottius varieornatus, one of the most stress-tolerant species of tardigrades, is about half as big, at 55 Mb.[64] About 1.6% of its genes are the result ofhorizontal gene transfer from other species, not implying any dramatic effect.[64]
Genomic studies across different tardigrade groups help reconstruct the evolution of their genome, such as the relationship of tardigrade body segments to those of other Panarthropoda. A 2023 review concludes that despite the diversity of body plan among the Panarthropoda, the tardigrade body plan maps best with "a simple one-to-one alignment of anterior segments".[67] Such studies may eventually reveal how they miniaturised themselves from larger ecdysozoans.[67]
Tardigrades lack several of theHox genes found in arthropods, and a large intermediate region of the body axis. In insects, this corresponds to the entirethorax andabdomen. Practically the whole body, except for the last pair of legs, is made up of just the segments that arehomologous to the head region in arthropods. This implies that tardigrades evolved from an ancestral ecdysozoan with a longer body and more segments.[68]
In 2018, a report integrating multiple morphological and molecular studies concluded that while the Arthrotardigrada appear to be paraphyletic, the Heterotardigrada is an accepted clade. All the lower-level taxa have been much reorganised, but the major groupings remain in place.[70]
Possibly the first time that tardigrades appear in non-scientific literature is in the short-story "Bathybia" by the geologist and explorerDouglas Mawson. Published in the 1908 bookAurora Australis and printed in theAntarctic, it deals with an expedition to the South Pole where the team encounters giant mushrooms and arthropods. The team watches a giant tardigrade fighting a similarly enormousrotifer; another giant water bear bites a man's toe, rendering him comatose for half an hour with itsanaesthetic bite. Finally, a four-foot-long tardigrade, waking from hibernation, scares the narrator from his sleep, and he realizes it was all a dream.[71][72]
Tardigrades are common inmosses andlichens on walls and roofs, and can readily be collected and viewed under a low-powermicroscope. If they are dry, they can be reanimated on a microscope slide by adding a little water, making them accessible to beginning students and amateur scientists.[73]Current Biology attributed their popularity to "their clumsy crawling [which] is about as adorable as can be."[74] The zoologists James F. Fleming and Kazuhuru Arakawa called them "a charismatic phylum".[49] They have been famous[75] for their ability to survive life-stopping events such as being dried out since Spallanzani first resuscitated them from some dry sediment in a gutter in the 18th century.[75] In 2015, the astrophysicist and science communicatorNeil deGrasse Tyson described Earth as "the planet of the tardigrades", and they were nominated for theAmerican Name Society's Name of the Year Award.[76]Live Science notes that they are popular enough to appear on merchandise like clothes, earrings, and keychains, withcrochet patterns for people to make their own tardigrade.[77] The Dutch artistArno Coenen [nl] created statues forSt Eusebius' Church, Arnhem of microscopic organisms including a tardigrade and acoronavirus.[78]
Milnesium tardigradum was voted the winner ofThe Guardian's "2025 invertebrate of the year" competition, from a shortlist of ten. The article describing the conclusion of the contest stated that the species had "endured all five previousplanetary extinction events".[79]
The 'Ripper' inStar Trek: Discovery is a recognisably tardigrade-like creature enlarged to monstrous size, with extraordinary capabilities said in the TV series to have been acquired byhorizontal gene transfer.[80]
The tardigrades' traits, including their ability to survive extreme conditions,[81] have earned them a place in science fiction and other pop culture.[81][82] The musicianCosmo Sheldrake imagines himself as a robust tardigrade in his 2015 "Tardigrade Song".[83][84] He sings "If I were a tardigrade ... Pressure wouldn't squash me and fire couldn't burn ... I can live life in vacuums for years with no drink (A ha)".[85]
The biologists Mark Blaxter and Arakawa Kazuharu describe tardigrades' transition to science fiction and fantasy as resulting in "rare but entertaining walk-on parts".[86] They note that in the 2015 sci-fi horror filmHarbinger Down, the protagonists have to deal with tardigrades that havemutated throughCold War experiments into intelligent and deadly shapeshifters.[86]
In the 2017Star Trek: Discovery, the alien "Ripper" creature is a huge but "generally recognisable"[80] version of a terrestrial tardigrade. The protagonist, thexeno-anthropologistMichael Burnham, explains that the Ripper can "incorporate foreign DNA into its own genome viahorizontal gene transfer. When Ripper borrows DNA from themycelium [of its symbiotic fungi[87][86]], he's granted an all-access travel pass".[80] The scholar of science in popular culture Lisa Meinecke, inFighting for the Future: Essays on Star Trek: Discovery, writes that the animal shares some of the real tardigrade's characteristics, including "its physical resilience to extreme environmental" stresses.[88] She adds that while taking on fungal DNA is "ostensibly grounded" in science, it equally carries a "mystical impetus of what [the French philosophers]Deleuze andGuattari call abecoming",[88] an entanglement of species that changes those involved "and ties together all life".[88] The border of that symbiosis is the "Outsider or Anomalous", which stabilises the system and embodies its future possibilities. The characters Burnham and Stamets see that the tardigrade plays this 'Outsider' role.[88]
^Hyvonen, R.; Persson, T. (1996). "Effects of fungivorous and predatory arthropods on nematodes and tardigrades in microcosms with coniferous forest soil".Biology and Fertility of Soils.21 (1–2):121–127.Bibcode:1996BioFS..21..121H.doi:10.1007/BF00336003.
^Horikawa, Daiki D. (2012). "Survival of Tardigrades in Extreme Environments: A Model Animal for Astrobiology". In Altenbach, Alexander V.; Bernhard, Joan M.; Seckbach, Joseph (eds.).Anoxia. Cellular Origin, Life in Extreme Habitats and Astrobiology. Vol. 21. pp. 205–217.doi:10.1007/978-94-007-1896-8_12.ISBN978-94-007-1895-1.
^Horikawa, Daiki D.; Sakashita, Tetsuya; Katagiri, Chihiro; Watanabe, Masahiko; Kikawada, Takahiro; et al. (2006). "Radiation tolerance in the tardigradeMilnesium tardigradum".International Journal of Radiation Biology.82 (12):843–848.Bibcode:2006IJRB...82..843H.doi:10.1080/09553000600972956.PMID17178624.S2CID25354328.
^abGabriel, Willow N.; McNuff, Robert; Patel, Sapna K.; Gregory, T. Ryan; Jeck, William R.; Jones, Corbin D.; Goldstein, Bob (2007). "The tardigradeHypsibius dujardini, a new model for studying the evolution of development".Developmental Biology.312 (2):545–559.doi:10.1016/j.ydbio.2007.09.055.PMID17996863.
^Guidetti, Roberto; Rizzo, Angela Maria; Altiero, Tiziana; Rebecchi, Lorena (2012). "What can we learn from the toughest animals of the Earth? Water bears (tardigrades) as multicellular model organisms in order to perform scientific preparations for lunar exploration".Planetary and Space Science.74 (1):97–102.Bibcode:2012P&SS...74...97G.doi:10.1016/j.pss.2012.05.021.hdl:11380/738549.
^Bertolani, Roberto; Rebecchi, Lorena; Giovannini, Ilaria; Cesari, Michele (17 August 2011). "DNA barcoding and integrative taxonomy ofMacrobiotus hufelandi C.A.S. Schultze 1834, the first tardigrade species to be described, and some related species".Zootaxa.2997 (1):19–36.doi:10.11646/zootaxa.2997.1.2.hdl:11380/663445.
^Mach, Martin."Prof. Ferdinand Richters".Water Bear web base. Retrieved15 December 2024. (with full Richters bibliography; first published inBärtierchen-Journal, issue 62)
^Richters, Ferdinand; Krumbach, T.H. (1926). "Tardigrada". In Kŭkenthal, W.; Krumbach, T.H. (eds.).Handbook of Zoology. Vol. 3. Berlin and Leipzig. pp. 1–68.{{cite book}}: CS1 maint: location missing publisher (link)
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^Jørgensen, Aslak; Kristensen, Reinhardt M.; Møbjerg, Nadja (2018). "Phylogeny and Integrative Taxonomy of Tardigrada".Water Bears: The Biology of Tardigrades. Vol. 2. Springer International Publishing. pp. 95–114.doi:10.1007/978-3-319-95702-9_3.ISBN978-3-319-95701-2.
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![The lobopod-like luolishaniids from the Cambrian and Ordovician are possibly the closest fossil relatives of tardigrades. Entothryeos reconstruction shown.[55]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aEntothryeos.png)
![Tardigrades may be descended from a lobopod that resembled Aysheaia.[56][57]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAysheaia_pedunculata2021.jpg)
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