Tardigrades (commonly known as water bears) form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Spallanzani in 1777. The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm.
More than 1000 species of tardigrades have been described. Tardigrades occur over the entire world, from the high Himalayas (above 6,000 m), to the deep sea (below 4,000 m) 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 litre). Tardigrades often can be found by soaking a piece of moss in spring water.
Tardigrades are polyextremophiles and are able to survive in extreme environments that would kill almost any other animal. Some can survive temperatures close to absolute zero, temperatures as high as 151 °C (303 °F), 1,000 times more radiation than any other animal, nearly a decade without water, and even the vacuum of space.
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.
Tardigrades have been known to withstand the following extremes while in this state:
Recent experiments conducted by Cai and Zabder have also shown that these tardigrades can undergo chemobiosis — a cryptobiotic response to high levels of environmental toxins. However, their results have yet to be verified. In September 2008, a space launch showed that tardigrades can survive the extreme environment of outer space for 10 days. 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 survived full exposure to the sun.
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 Hipsiblidae.