Temporal range: Cretaceous–Recent
|The tardigrade Hypsibius dujardini|
Tardigrades are notable for being one of the most complex of all known polyextremophiles. (An extremophile is an organism that can thrive in a physically or geochemically extreme condition that would be detrimental to most life on Earth.) For example, tardigrades can withstand temperatures from just above absolute zero to well above the boiling point of water, as well as pressures greater than any found in the deepest ocean trenches, ionizing radiation — at doses hundreds of times higher than would kill a person and have lived through the vacuum of outer space. They can go without food or water for nearly 120 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.
Usually, tardigrades are 1 millimetre (0.039 in) long when they are fully grown. They are short and plump with 4 pairs of legs, each with 4-8 claws also known as "disks." The animals are prevalent in moss and lichen and, when collected, may be viewed under a very low-power microscope, making them accessible to the student or amateur scientist as well as the professional.
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. The first tardigrades were discovered by Johann August Ephraim Goeze in 1773. Since 1778, over 500 tardigrade species have been found.
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 1773. 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 millimetres (0.059 in), the smallest below 0.1 mm. Freshly hatched tardigrades may be smaller than 0.05 mm.
About 1,150 species of tardigrades have been described. Tardigrades occur throughout the world, from the Himalayas (above 6,000 metres (20,000 ft)), to the deep sea (below 4,000 metres (13,000 ft)) and from the polar regions to the equator.
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 often can be found by soaking a piece of moss in spring water.
Anatomy and morphology 
Tardigrades have barrel-shaped bodies with four pairs of stubby lobopodia legs that are poorly articulated. Most range from 0.3 to 0.5 millimetres (0.012 to 0.020 in) in length, although the largest species may reach 1.2 millimetres (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. There are no respiratory organs, 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 moults, 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 oesophagus, 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 includes multiple lobes, mostly consisting of three bilaterally paired clusters of neurons. The brain is attached to a large ganglion below the oesophagus, 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 there are numerous sensory bristles on the head and body.
Tardigrades all possess a buccopharyngeal membrane apparatus, which, along with the claws, is used to differentiate among species.
Although some species are parthenogenetic, 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 attach them to nearby substrate.
The eggs hatch after no more than fourteen 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 moult up to twelve times.
Ecology and life history 
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 a milder environment like 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. Several species regularly survive in a dehydrated state for nearly ten years. 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 a long period is largely dependent on the high levels of the non-reducing sugar, trehalose, which protects their membranes. 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. The following are extremes states Tradigrades can survive:
- Temperature – Tardigrades can survive being heated for a few minutes to 151 °C (424 K or 304 F), or being chilled for days at −200 °C (73 K or -328 F), or some can survive temperatures for a few minutes at −273 °C (~1 degree above absolute zero/0 Kelvin or -458 F).
- 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 – although there is one report of a leg movement in a 120-year-old specimen from dried moss, this is not generally considered "survival", and the longest tardigrades have been shown to survive in a dry state is nearly 10 years. 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.
- Environmental toxins – tardigrades can undergo chemobiosis—a cryptobiotic response to high levels of environmental toxins. However, these laboratory results have yet to be verified.
- Outer space – Tardigrades are the first known animal to survive in Space. Since September 2007, Tardigrades were taken into low Earth orbit on the FOTON-M3 mission and for 10 days were exposed to the vacuum of space from which they returned alive. After being rehydrated back on Earth, over 68% of the subjects protected from high-energy UV radiation survived and many of these produced viable embryos, and a handful had survived full exposure to solar radiation. In May 2011, Italian scientists sent tardigrades into space along with other 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 US-based Planetary Society on the Russian Fobos-Grunt mission's Living Interplanetary Flight Experiment to Phobos; however, the launch failed.
Evolutionary relationships and history 
A number of morphological and molecular studies have sought to resolve the relationship of tardigrades to other lineages of ecdysozoan animals. Two plausible placements have been recovered: tardigrades most closely related to arthropods (a common result of morphological studies) or tardigrades most closely related to nematodes (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 unlikely. The only known fossil specimens comprise some from mid-Cambrian deposits in Siberia and a few rare specimens from Cretaceous amber.
The Siberian tardigrades 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. It is considered that they probably represent a stem group of living tardigrades.
The rare specimens in Cretaceous amber comprise Milnesium swolenskyi, from New Jersey, the oldest, whose claws and mouthparts are indistinguishable from the living M. tartigradum; and two specimens from western Canada, some 15–20 million years younger than M. swolenskyi. Of the two latter, one 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. The genome of a tardigrade species, Hypsibius dujardini, is being sequenced at the Broad Institute. Hypsibius dujardini has a compact genome and a generation time of about two weeks, and it can be cultured indefinitely and cryopreserved.
See also 
- Living Interplanetary Flight Experiment, aimed to test whether selected microorganisms could survive a few years in outer space
- Oncopoda, a hypothetical group of animals of which the Tardigrada would be part
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