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Tardigrade

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"Tardigrada" redirects here. For the suborder of mammals, see sloth.
"Moss Piglets" redirects here. For the South Park episode, see Moss Piglets (South Park).

Tardigrade
Temporal range: Turonian –Recent Middle Cambrian stem-group fossils
Milnesium tardigradum, a eutardigrade
Echiniscus insularis, a heterotardigrade
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Superphylum: Ecdysozoa
(unranked): Panarthropoda
Phylum: Tardigrada
Spallanzani, 1777
Classes

Tardigrades (/ˈtɑːrdɪɡreɪdz/ ⓘ),[1] known colloquially as water bears or moss piglets,[2] are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär 'little water bear'. In 1776, the Italian biologist Lazzaro Spallanzani named them Tardigrada, which means 'slow walker'.

They live in diverse regions of Earth's biosphere – mountaintops, the deep sea, tropical rainforests, and the Antarctic. Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditions – such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation – that would quickly kill most other known forms of life. Tardigrades have survived exposure to outer space.

There are about 1,300 known species in the phylum Tardigrada, a part of the superphylum Ecdysozoa. The earliest known fossil is from the Cambrian, some 500 million years ago. They lack several of the Hox genes found in arthropods, and the middle region of the body corresponding to an arthropod's thorax and abdomen. Instead, most of their body is homologous to an arthropod's head.

Tardigrades are usually about 0.5 mm (0.020 in) long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction disks. Tardigrades are prevalent in mosses and lichens and can readily be collected and viewed under a low-power microscope, making them accessible to students and amateur scientists. Their clumsy crawling and their well-known ability to survive life-stopping events have brought them into science fiction and popular culture including items of clothing, statues, soft toys and crochet patterns.

Description

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Body structure

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Tardigrades have a short body with four pairs of hollow unjointed legs. Most range from 0.1 to 0.5 mm (0.004 to 0.020 in) in length, although the largest species may reach 1.3 mm (0.051 in). The body cavity is a haemocoel filled with a colorless fluid. The body covering is a cuticle that is replaced when the animal moults; it contains hardened (sclerotised) proteins and chitin but is not calcified. 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 on diffusion through the cuticle and body cavity for gas exchange.[3]

Nervous system and senses

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The tardigrade nervous system has a pair of ventral nerve cords with a pair of ganglia serving each pair of legs. The nerve cords end near the mouth at a pair of suboesophageal ganglia. These are connected by paired commissures (either side of the tube from the mouth to the pharynx) to the dorsally located cerebral ganglion or 'brain'. Also in the head are two eyespots in the brain, and several sensory cirri and pairs of hollow clavae which may be chemoreceptors.[3]

Locomotion

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Although the body is flexible and fluid-filled, locomotion does not operate mainly hydrostatically. Instead, as in arthropods, the muscles (sometimes just one or a few cells) work in antagonistic pairs that make each leg step backwards and forwards; there are also some flexors that work against hydrostatic pressure of the haemocoel. The claws help to stop the legs sliding during walking, and are used for gripping.[3]

Feeding and excretion

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Tardigrades feed by sucking animal or plant cell fluids, or on detritus. A pair of stylets pierce the prey; the pharynx muscles then pump the fluids from the prey into the gut. A pair of salivary glands secrete a digestive fluid into the mouth, and produce replacement stylets each time the animal molts.[3] Non-marine species have Malpighian tubules where the intestine joins the hindgut. Some species have excretory or other glands between or at the base of the legs.[3]

  • Video of tardigrade under the microscope
  • Living tardigrades moving around

Reproduction and life cycle

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Shed cuticle of female tardigrade, containing eggs

Most tardigrades have both male and female animals which copulate by a variety of methods. The females lay eggs. Some species appear to have no males, suggesting that parthenogenesis is common.[3]

Both sexes have a single gonad located above the intestine.[3] A pair of ducts run from the testis, opening through a single gonopore in front of the anus. Females have a single oviduct opening either just above the anus or directly into the rectum, which forms a cloaca.[3]

The male may place his sperm into the cloaca, or may penetrate the female's cuticle and place the sperm straight into her body cavity, for it to fertilise the eggs directly in the ovary. A third mechanism in some species 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, the hatchlings using their stylets to open their egg shells.[3]

Ecology and life history

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Tardigrades are cosmopolitan, living in all Earth's biomes (biogeographical regions). Among marine habitats, they range from the intertidal zone to abyssal depths, in every ocean. In freshwater, they live in a variety of cave, lake, and river habitats. On land, they frequent lichens, liverworts, and mosses which grow on soil, tree bark, and rocks; they are also found directly in soil and leaf litter.[4][5]

A few extant species, such as Tetrakentron synaptae, alongside the Cambrian 'Orsten' tardigrade, are parasitic.[6][7]

The eggs and cysts of tardigrades are durable enough to be carried great distances on the feet of other animals.[8] The lifespan of tardigrades ranges from three to four months for some species, up to two years for other species, not counting their time in dormant states.[9] Many aquatic invertebrate predators feed on tardigrades as part of their diet.[10] One species, Echiniscoides wyethi, lives on barnacles.[11]

With the exception of 62 exclusively freshwater species, all non-marine tardigrades are found in terrestrial environments. Because the majority of the marine species belongs to Heterotardigrada, the most ancestral class, the phylum evidently has a marine origin.[12]

Environmental tolerance

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Tardigrades inhabit extreme habitats as varied as hot springs, glacial cryoconite holes, and on top of the Himalayas.[13][14] They are not considered extremophilic because they are not adapted to exploit these conditions, only to endure them. This means that their chances of dying increase the longer they are exposed to the extreme environments,[15] whereas true extremophiles thrive in a physically or geochemically extreme environment.[16]

Dehydrated 'tun' state

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Tardigrades are capable of suspending their metabolism. While in this state, they can go without food or water for around five years, sometimes longer, only to later rehydrate, forage, and reproduce.[17][18][19] Depending on the environment, they may enter this state via anhydrobiosis, allowing tardigrades to survive in habitats that might otherwise be fatal. Anhydrobiosis also permits resistance to unnatural abiotic extremes such as subzero temperatures.[20] In this cryptobiotic state with suspended metabolism, the tardigrade is known as a "tun".[21] Mitochondria and muscle contraction due to mitochondria are essential for entering the dehydrated "tun" state.[22]

Damage protection proteins

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Tardigrades' ability to remain desiccated for long periods of time was thought to depend on high levels of the sugar trehalose,[23] common in organisms that survive desiccation.[24] However, tardigrades do not only synthesize enough trehalose for this function.[23][25] Instead, tardigrades produce intrinsically disordered proteins in response to desiccation. Three of these are specific to tardigrades and have been called tardigrade specific proteins. These may protect membranes from damage by associating with the polar heads of lipid molecules.[26] They may also form a glass-like matrix that protects cytoplasm from damage during desiccation.[27]

Tardigrade DNA is protected from radiation by the dsup ("damage suppressor") protein.[28] In a test with human cells, dsup reduced X-ray damage by around 40%.[28] The shielding likely involves strong electrostatic attraction and high protein flexibility, forming a molecular aggregate with DNA.[29] The Dsup proteins of Ramazzottius varieornatus and Hypsibius exemplaris promote survival by binding to nucleosomes and protecting chromosomal DNA from hydroxyl radicals.[30] The Dsup protein of R. varieornatus confers resistance to ultraviolet-C by upregulating DNA repair genes that protect the genomic DNA from the damages introduced by UV irradiation.[31]

Extremes of temperature

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Tardigrades can survive in extreme environments that would kill almost any other animal, including:[24][32][33]

Tardigrades can survive:

  • A few minutes at 151 °C (304 °F)[34]
  • 30 years at −20 °C (−4 °F)[35]
  • A few days at −200 °C (−328 °F; 73 K)[34]
  • A few minutes at −272 °C (−458 °F; 1 K)[36]

Tardigrades are however sensitive to high temperatures: 48 hours at 37.1 °C (98.8 °F) kills half of unacclimitized active tardigrades. Acclimation boosts the lethal temperature to 37.6 °C (99.7 °F). Those in the tun state fare better, half surviving 82.7 °C (180.9 °F) for one hour. Longer exposure decreases the lethal temperature. For 24 hours of exposure, 63.1 °C (145.6 °F) kills half of the tun state tardigrades.[37]

Extremes of pressure

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Tardigrades can withstand the extremely low pressure of a vacuum, and very high pressures, more than 1,200 times atmospheric pressure. Some species can also withstand pressures of 6,000 atmospheres, nearly six times the pressure of water in the deepest ocean trench.[38]

Radiation

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Tardigrades can withstand 1,000 times more radiation than other animals,[39] 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).[40] Earlier experiments attributed this to their lowered water content, providing fewer reactants for ionizing radiation.[40] However, tardigrades, when hydrated, remain much more resistant to shortwave UV radiation than other animals; one reason is their ability to repair damage to their DNA.[41]

Exposure to space (vacuum and ultraviolet)

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Tardigrades have survived exposure to space. In 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, some of them previously dehydrated, some of them not, were exposed to the hard vacuum of space, or vacuum and solar ultraviolet radiation.[42][43] Back on Earth, more than 68% of the subjects protected from solar ultraviolet radiation were reanimated within 30 minutes following rehydration; although subsequent mortality was high, many produced viable embryos.[42]

In contrast, hydrated samples exposed to the combined effect of vacuum and full solar ultraviolet radiation had significantly reduced survival, with only three subjects of Milnesium tardigradum surviving.[42] The space vacuum did not have a significant effect on egg-laying in either R. coronifer or M. tardigradum. However, M. tardigradum exposed to UV radiation had a lower egg laying rate.[44] In 2011, Italian scientists sent tardigrades on board the International Space Station along with extremophiles on STS-134, the final flight of Space Shuttle Endeavour.[45] Their conclusion was that microgravity and cosmic radiation "did not significantly affect survival of tardigrades in flight" and that tardigrades could be useful in space research.[46][47]

In 2011, tardigrades were to be sent on the Russian Fobos-Grunt mission's Living Interplanetary Flight Experiment to Phobos, but the launch failed. In 2019, a capsule containing tardigrades in a cryptobiotic state was on board the Israeli lunar lander Beresheet which crashed on the Moon; they were described as unlikely to have survived the impact.[48] Despite tardigrades' ability to survive in space, tardigrades on Mars would still need food.[49]

Other stresses

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The longest that living tardigrades have been shown to survive in a dry state is nearly 10 years.[18]

They can survive Impacts up to about 900 meters per second, and momentary shock pressures up to about 1.14 gigapascals.[48]

Taxonomy

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Johann August Ephraim Goeze originally named the tardigrade Kleiner Wasserbär, meaning 'little water-bear' in German (today, Germans often call them Bärtierchen 'little bear-animal').[50][51] The name water bear comes from the way they walk, reminiscent of a bear's gait. The name Tardigradum means 'slow walker' and was given by Lazzaro Spallanzani in 1776.[52][15] 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,[53][54] and in 1926 set up the class Eutardigrada.[55][56] In 1927, Ernst Marcus set up the class Heterotardigrada.[57] In 1937 Gilbert Rahm, studying the fauna of Japan's hot springs, distinguished the class Mesotardigrada, with a single species Thermozodium esakii;[58] its validity is now doubted.[59] In 1962, Giuseppe Ramazzotti [it] proposed the phylum Tardigrada.[60] In 2019, Noemi Guil and colleagues proposed to promote the order Apochela to the new class Apotardigrada.[61] There are about 1,300 described species in the phylum Tardigrada, a part of the superphylum Ecdysozoa.[62]

Evolution

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Evolutionary history

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Further information: Lobopodia

The minute sizes of tardigrades and their membranous integuments make fossils difficult to detect and highly unusual. The only known fossil specimens are those from mid-Cambrian deposits in Siberia (in the Orsten fauna) and a few specimens from Cretaceous and Neogene amber.[63][64] 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.[65] Scientists think they represent a stem group of living tardigrades.[63]

Multiple lines of evidence show that tardigrades are secondarily miniaturized from a larger ancestor,[69] probably a lobopodian, perhaps resembling the mid-Cambrian Aysheaia, which many analyses place close to the divergence of the tardigrade lineage.[67][68] An alternative hypothesis derives tactopoda from a clade encompassing dinocaridids and Opabinia.[70] The enigmatic panarthropodan Sialomorpha found in 30-million year old Dominican amber, while not a tardigrade, shows some apparent affinities.[71] A 2023 morphological analysis concluded that luolishaniids, a group of Cambrian lobopodians, might be the tardigrades' closest known relatives.[66]

Tardigrades lack several of the Hox genes found in arthropods, and a large intermediate region of the body axis. In insects, this corresponds to the entire thorax and abdomen. Practically the whole body, except for the last pair of legs, is made up of just the segments that are homologous to the head region in arthropods. This implies that tardigrades evolved from an ancestral ecdysozoan with a longer body and more segments.[72]

Tardigrade body plan compared to arthropods, onychophora, and annelids. Tardigrades have lost the whole middle section of the ecdysozoan body plan, and its Hox genes.[72]

The oldest remains of modern tardigrades are those of Milnesium swolenskyi, belonging to the living genus Milnesium known from a Late Cretaceous (Turonian) aged specimen of New Jersey amber, around 90 million years old. Another fossil species, Beorn leggi, is known from a Late Campanian (~72 mya) specimen of Canadian amber, belonging to the family Hypsibiidae.[73] The related hypsibioidean Aerobius dactylus was found in the same amber piece.[74][75] The youngest known fossil tadigrade genus, Paradoryphoribius, was discovered in amber dated to about 16 mya.[64]

External phylogeny

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Morphological and molecular (genomic) studies have attempted to define how tardigrades relate to other ecdysozoan groups. Two plausible placements have been proposed: tardigrades are either most closely related to Arthropoda and Onychophora, or to nematodes.[76]

Panarthropoda

Water bears (Tardigrada)

Antennopoda

Velvet worms (Onychophora)

Arthropods (Arthropoda)

Internal phylogeny

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In 2012, the internal phylogeny of the phylum was studied using molecular markers (ribosomal RNA), finding that the Heterotardigrada and Arthrotardigrada seemed to be paraphyletic.[77]

Tardigrada

"Arthrotardigrada"

Echiniscoidea

Eutardigrada
Apochela

Milnesiidae

Parachela

Isohypsibiodea

Macrobiotoidea

Hypsibioidea

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 reorganized, but the major groupings remain in place.[78]

Tardigrada
Heterotardigrada
Eutardigrada
Apochela

Milnesiidae

Parachela

Isohypsibiodea

Macrobiotoidea

Hypsibioidea

Genomics

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Tardigrade genomes vary in size, from about 75 to 800 megabase pairs of DNA.[79] Hypsibius exemplaris (formerly Hypsibius dujardini) has a compact genome of 100 megabase pairs[76] and a generation time of about two weeks; it can be cultured indefinitely and cryopreserved.[80] The genome of Ramazzottius varieornatus, one of the most stress-tolerant species of tardigrades, was sequenced in 2015. Less than 1.2% of its genes are the result of horizontal gene transfer from other species.[81]

In culture and society

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Popularity

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Tardigrades are common in mosses and lichens on walls and roofs, and can readily be collected and viewed under a low-power microscope. 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.[82] Current Biology attributed their popularity to "their clumsy crawling [which] is about as adorable as can be."[5] The zoologists James F. Fleming and Kazuhuru Arakawa called them "a charismatic phylum".[59] They have been famous[83] 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.[83] These traits have made them appear in various kinds of science fiction an other pop culture.[84][85] Live Science notes that they are popular enough to appear on merchandise like clothes, earrings, and keychains, with crochet patterns for people to make their own tardigrade.[86] The Dutch artist Arno Coenen [nl] created statues for St Eusebius' Church, Arnhem of microscopic organisms including a tardigrade and a coronavirus.[87]

In books and music

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The short-story "Bathybia" by Douglas Mawson, published in the 1908 book Aurora Australis, printed in the Antarctic, 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 enormous rotifer; another giant water bear bites a man's toe, rendering him comatose for half an hour. Finally, a four-foot-long tardigrade, waking from hibernation, scares the narrator from his sleep, and he realizes it was all a dream.[88][89]

Musician Cosmo Sheldrake imagines himself a tardigrade in his 2015 "Tardigrade Song".[90][91]

In film and TV

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When the characters in the superhero films Ant-Man (2015) and Ant-Man and the Wasp (2018) shrink themselves to enter the "Quantum Realm", they encounter tardigrades.[92][93][94] In the 2015 sci-fi horror film Harbinger Down, the characters have to deal with deadly mutated tardigrades, the result of Cold War experiments.[95][96] In Star Trek: Discovery (2017), the alien "Ripper" creature, who is used to "navigate" through a galactic mycelium network and instantly reposition the ship, is a huge version of a terrestrial tardigrade.[97][96] The 2017 South Park episode "Moss Piglets" involves a science experiment in which tardigrades learn to dance to the music of Taylor Swift.[98] The 2018 Family Guy episode "Big Trouble in Little Quahog" features Stewie and Brian being shrunk to a microscopic level, during which they meet a group of friendly tardigrades or "water bears" who help them.[99]

See also

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References

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