Tardigrade

Body structure s with a pair of ganglia serving each pair of legs. The nerve cords end near the mouth at a pair of subpharyngeal (or 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 antenna-like clavae which may be chemoreceptors. Locomotion 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. 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 moults. Both sexes have a single gonad (an ovary or testis) located above the intestine. 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. 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 species such as H. 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. 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. 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. == Ecology and life history ==

Tardigrades as a group are cosmopolitan, 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, whether on the feet of other animals or by the wind. The majority of species live in damp habitats such as on lichens, liverworts, and mosses, and directly in soil and leaf litter. In freshwater and the sea, they live on and in the bottom, such as in between particles or around seaweeds. More specialised habitats include hot springs and as parasites or commensals 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. Many tardigrades are predatory; Milnesium lagniappe includes other tardigrades such as Macrobiotus acadianus among its prey. Tardigrades consume prey such as nematodes, and are themselves preyed upon by soil arthropods including mites, spiders and cantharid beetle larvae. 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. == Environmental tolerance ==

Tardigrades are not considered universally extremophilic 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, Dehydrated 'tun' state '' in active and 'tun' states.A↔P = anterior-posterior; mg = midgut; go = gonad;pb = pharyngeal bulb; mo = mouth; st = styletScale bars = 100 μm Tardigrades are capable of suspending their metabolism, going into a state of cryptobiosis.), lack of oxygen, and high pressure. Surviving other stresses Marine tardigrades such as Halobiotus crispae alternate each year (cyclomorphosis) between an active summer morph and a hibernating winter morph (a pseudosimplex) that can resist freezing and low salinity, but which remains active throughout. Reproduction, however, takes place only in the summer morph. Exposure to space mission carrying the BIOPAN astrobiology payload (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 and eggs were taken on the FOTON-M3 mission and exposed to vacuum, to both vacuum and UVA & B light, or to both vacuum and the full range of solar ultraviolet, for 10 days. Exposure to full solar ultraviolet was much more lethal, with only three subjects of M. tardigradum surviving. showing that they could survive microgravity and cosmic radiation, and should be suitable model organisms. In 2019, a capsule containing tardigrades in a cryptobiotic state was on board the Israeli lunar lander Beresheet which crashed on the Moon. Damage protection proteins Tardigrades' ability to stay desiccated for long periods of time was thought to depend on high levels of the sugar trehalose, common in organisms that survive desiccation. However, tardigrades do not synthesize enough trehalose for this function. The proteins may also form a glass-like matrix that protects cytoplasm from damage during desiccation. Anhydrobiosis in response to desiccation has a complex molecular basis; in Hypsibius exemplaris, 1,422 genes are upregulated during the process. Of those, 406 are specific to tardigrades, 55 being intrinsically disordered and the others globular with unknown functions. Tardigrades possess a cold shock protein; Maria Kamilari and colleagues propose (2019) that this may serve "as a RNA-chaperone involved in regulation of translation [of RNA code to proteins] following freezing." The Dsup proteins of Ramazzottius varieornatus and H. exemplaris promote survival by binding to nucleosomes and protecting chromosomal DNA from hydroxyl radicals. The Dsup protein of R. varieornatus confers resistance to ultraviolet-C by upregulating DNA repair genes. Some of these proteins are of interest to biomedical 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 prevent programmed cell death (apoptosis). == Taxonomic history ==

In 1773, Johann August Ephraim Goeze named the tardigrade , meaning 'little water-bear' in German (today, Germans often call them 'little bear-animal'). 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. In 1834, C.A.S. Schulze gave the first formal 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". This was soon followed by descriptions of species including Echiniscus testudo, Milnesium tardigradum, Hypsibius dujardini, and Ramazzottius oberhaeuseri by L.M.F. Doyère in 1840. All four of these are now the nominal species for higher tardigrade taxa. The zoologist Hartmut Greven wrote that "The unanimous opinion of all later researchers is that Doyère's 1842 dissertation is an indisputable milestone in tardigradology". 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, and in 1926 proposed the class Eutardigrada. In 1927, Ernst Marcus created the class Heterotardigrada. and in 1929 a monograph on tardigrades which Greven describes as "comprehensive" and "unsurpassed today". its validity is now doubted. In 2019, Noemi Guil and colleagues proposed to promote the order Apochela to the new class Apotardigrada. There are some 1,488 described species of tardigrades, organised into 160 genera and 36 families. File:First Recorded Image of Tardigrade.jpg|The first drawing of a tardigrade, by Johann August Ephraim Goeze, 1773 File:Echiniscus testudo Doyere 1840 Pl 12 Fig 1.png |Drawing of Echiniscus testudo on a grain of sand by L.M.F. Doyère, 1840 File:Echiniscus sp.jpg |Drawing of Echiniscus sp. by C.A.S. Schultze, 1861 File:Macrobiotus ornatus var. spinifer by Ferdinand Richters 1900.jpg|Drawing of Calohypsibius (Macrobiotus) ornatus var. spinifer by Ferdinand Richters, 1900 == Evolution ==

Evolutionary history Tardigrade fossils are rare. The only known specimens are those from mid-Cambrian deposits in Siberia (in the Orsten fauna) and a few specimens in amber from the Cretaceous of North America and the Neogene of the Dominican Republic. File:Entothryeos.png|The lobopod-like luolishaniids from the Cambrian and Ordovician are possibly the closest fossil relatives of tardigrades. Entothryeos reconstruction shown. probably a lobopodian, perhaps resembling the mid-Cambrian Aysheaia, which many analyses place close to the divergence of the tardigrade lineage. An alternative hypothesis derives tactopoda from a clade encompassing dinocaridids and Opabinia. The enigmatic panarthropodan Sialomorpha found in 30-million year old Dominican amber, while not a tardigrade, shows some apparent affinities. A 2023 morphological analysis concluded that luolishaniids, a group of Cambrian lobopodians, might be the tardigrades' closest known relatives. 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 mya. Another fossil species, Beorn leggi, is known from a Late Campanian (~72 mya) specimen of Canadian amber, belonging to the family Hypsibiidae. The related hypsibioidean Aerobius dactylus was found in the same amber piece. The youngest known fossil tadigrade genus, Paradoryphoribius, was discovered in amber dated to about 16 mya. Morphological and molecular phylogenetics studies have attempted to define how tardigrades relate to other ecdysozoan groups; alternative placements have been proposed within the Panarthropoda. The Tactopoda hypothesis holds that Tardigrada are sister to Arthropoda; the Antennopoda hypothesis is that Tardigrada are sister to (Onychophora + Arthropoda); and the Lobopodia (sensu Smith & Goldstein 2017) hypothesis is that Tardigrada are sister to Onychophora. The relationships have been debated on the basis of conflicting evidence. Genomics Tardigrade genomes vary widely in size. Hypsibius exemplaris (part of the Hypsibius dujardini group) has a compact genome of 100 megabase pairs The genome of Ramazzottius varieornatus, one of the most stress-tolerant species of tardigrades, is about half as big, at 55 Mb. 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. compared to arthropods, onychophora, and annelids. Tardigrades have lost the whole middle section of the ecdysozoan body plan, and its Hox genes. }} }} 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. }} In 2019, Noemi Guil and colleagues proposed to promote the order Apochela to the new class Apotardigrada. }} == In culture and society ==

Early 20th century beginnings Possibly the first time that tardigrades appear in non-scientific literature is in the short-story "Bathybia" by the geologist and explorer Douglas Mawson. Published in the 1908 book Aurora Australis and printed in the Antarctic, 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 enormous rotifer; another giant water bear bites a man's toe, rendering him comatose for half an hour with its anaesthetic bite. Finally, a four-foot-long tardigrade, waking from hibernation, scares the narrator from his sleep, and he realizes it was all a dream. Popularity , the Netherlands 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. Current Biology attributed their popularity to "their clumsy crawling [which] is about as adorable as can be." The zoologists James F. Fleming and Kazuhuru Arakawa called them "a charismatic phylum". They have been famous In 2015, the astrophysicist and science communicator Neil deGrasse Tyson described Earth as "the planet of the tardigrades", and they were nominated for the American Name Society's Name of the Year Award. They are popular enough to appear on merchandise like clothes, earrings, soft toys, and keychains, with crochet patterns for people to make their own tardigrades. The Dutch artist Arno Coenen created statues for St Eusebius' Church, Arnhem of microscopic organisms including a tardigrade and a coronavirus. Milnesium tardigradum was voted the winner of The Guardian "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 previous planetary extinction events". From science to popular culture '' is a recognisably tardigrade-like creature enlarged to monstrous size, with extraordinary capabilities said in the TV series to have been acquired by horizontal gene transfer. The musician Cosmo Sheldrake imagines himself as a robust tardigrade in his 2015 "Tardigrade Song". 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)". The biologists Mark Blaxter and Arakawa Kazuharu describe tardigrades' transition to science fiction and fantasy as resulting in "rare but entertaining walk-on parts". In the 2017 Star Trek: Discovery, the alien "Ripper" creature is a huge but "generally recognisable" The scholar of science in popular culture Lisa Meinecke, in Fighting 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. == See also ==