Tardigrada

Tardigrada is a phylum of tiny, water-dwelling animals commonly known as water bears or moss piglets. They are microfauna usually between 0.3 and 0.5 millimeters in length and occur in a wide range of moist environments, including water films in mosses, lichens, leaf litter, soil, as well as freshwater and marine habitats. They are widely distributed worldwide and can be found from tropical to polar regions.

An overview of their biology shows a compact body with four pairs of short, stubby legs, each ending in claws or adhesive pads. The body is encased in a flexible cuticle that is molted as they grow. They feed primarily by using piercing mouthparts called stylets to suck fluids from plant cells, algae, or small invertebrates. Tardigrades belong to the superphylum Ecdysozoa, which includes arthropods and nematodes, and they share an ecdysozoan characteristic of molting their outer cuticle.

A notable feature of tardigrades is their ability to enter cryptobiosis, a reversible ametabolic state. In cryptobiosis, they lose most or all body water and become a tun, a compact, protective form that can endure extreme environmental stresses. In this state they can survive desiccation, freezing, high radiation, and even the vacuum of space; normal activity resumes when conditions improve.

Reproduction is predominantly sexual, though parthenogenesis occurs in some species. Eggs hatch into juveniles that resemble small adults, and development includes molt cycles. Taxonomically, tardigrades are placed within the phylum Tardigrada in the Ecdysozoa and are divided into groups such as Eutardigrada and Heterotardigrada, among others, reflecting their diversity.