Water Bears: Tardigrada

Water bears are mostly microscopic, measuring 0.00787 to 0.0472 inches (0.2 to 1.2 millimeters) in length. They have bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree) and can only be divided into similar halves along one plane. The outside of the body, or cuticle (KYU-tih-kuhl), may have platelike scales, spines, and other appendages. Most species are whitish or clear, but some land-dwelling, or terrestrial (te-REH-stree-uhl), species are yellow, orange, green, red, or greenish black.

Their bodies are made up of five indistinct segments, including the head and four trunk segments. Each trunk segment bears a pair of stumpy, segmented legs. The first three pairs are used for walking, while the last pair is used for clinging. Terrestrial and freshwater species have legs with fewer segments that end in two or four claws. Most marine water bears have telescoping legs that can be withdrawn inside their bodies. Depending on the species, these legs have up to 13 claws or four toelike structures with varying numbers of claws. In some species, the toes are tipped with suction cups or sticky, rod-shaped discs, like the feet of a gecko.

Water bears feed through a mouth with bristlelike jaws called stylets (STAI-lehts). The stylets are used to puncture cell walls so fluids are sucked into the mouth. Because they are small and live in moist or wet environments, water bears do not have a circulatory or respiratory system. They breathe directly through the body wall. Some species have kidneylike organs that remove waste and regulate body salts, while others rely on special glands located at the bases of the legs.

Both young and adult water bears shed the cuticle, or molt, just like insects, spiders, and other arthropods. The mouthparts and surrounding structures, as well as the toes and claws, are produced by special glands after the new cuticle has formed.