Tardigrades, also known as water bears or moss piglets, are aquatic, eight-legged, microscopic animals. They were discovered by the German zoologist Johann August Ephraim Goeze in 1773. The name Tardigrada (meaning "slow stepper") was given three years later by the Italian biologist Lazzaro Spallanzani. They have been found everywhere, being capable of living in most enviroments. Tardigrades are one of the most resilient known animals, with individual species able to survive conditions that would be fatal to nearly all other life, even being able to survive in space. About 1,150 species of tardigrade are known. The group includes fossils dating from 530 million years ago (The Cambrian period).

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". Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates.