A Water Bear at 400x Magnification

Tardigrades (‘water bears’) are microscopic, segmented animals. They live in water. They are members of the tactopod phylum Tardigrada, part of the superphylum Ecdysozoa. They are also known as water bears or moss piglets.

Tardigrades were first described in 1773. Their name means “slow stepper”. There are more than 1000 different species of tardigrade.

Tardigrades have a cylindrical shape with four segments, each with two legs. Each leg has little claws. The biggest adults may reach a body length of 1.2 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm. Tardigrades feed on plant cells by penetrating the cell wall and eating what is inside. Some tardigrades are carnivores.

Tardigrades are eutelic: all adult tardigrades of the same species have the same number of cells. Some species have as many as 40,000 cells in each adult, while others have far fewer.

Tardigrades can be found in many habitats: in moss, freshwater, the Himalayas, and the ocean. They are one of the few animals that can be found on the highest mountains and the deepest seas. About 83% of the known species live on land, the other 17% live in water.