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Tardigrades are the most resilient animal known: 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 geochemicallyextreme 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, 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.
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 was thought to be largely dependent on the high levels of the nonreducing sugar trehalose, which protects their membranes, though recent research suggests that tardigrades have a unique protein that serves a similar purpose as trehalose. 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.