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. The name Tardigrada means “slow walker” and was given by Spallanzani in 1777. The name water bear comes from the way they walk, reminiscent of a bear’s. 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 other animals such as humans, nearly a decade without water, and even the vacuum of space.

Tardigrades have a body with four segments (not counting the head), four pairs of legs without joints, and feet with claws or toes. The cuticle contains chitin and is moulted. They have a ventral nervous system with one ganglion per segment, and a multilobed brain. Their pigment-cup eyes are rhabdomeric. Instead of a coelom they have a haemocoel. The only place where a true coelom can be found is around the gonad (coelomic pouch). The pharynx is of a triradiate, muscular, sucking kind, armed with stylets.

Although some species are parthenogenetic, males and females are usually present, each with a single gonad. Tardigrades are eutelic (all adult tardigrades of the same species are believed to have the same number of cells) and oviparous. Some tardigrade species have as many as about 40,000 cells in each adult’s body, others have far fewer