Tardigrades (commonly known as waterbears or moss piglets) are small, water-dwelling, segmented animals with eight legs. They form the phylum Tardigrada, part of the superphylum Ecdysozoa.
Tardigrades were first described in 1773 by Johann August Ephraim Goeze, who called them kleiner Wasserbär, meaning ‘little water bear’ in German. The name Tardigrada means «slow walker» and was given by Lazzaro Spallanzani in 1777. 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.
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 able to survive in extreme environments that would kill almost any other animal. Some can survive temperatures of close to absolute zero, or 0 Kelvin (−273 °C (−459 °F)), temperatures as high as 151 °C (304 °F), 1,000 times more radiation than other animals, and almost a decade without water.
Since 2007, tardigrades have also returned alive from studies in which they have been exposed to the vacuum of space for a few days in low Earth orbit.