Tardigrade

Tardigrades, commonly known as water bears or moss piglets, are a phylum of tiny, water-dwelling micro-animals. They range from about 0.3 to 0.5 millimeters in length, though some species reach up to 1.5 millimeters. They occur in moist environments worldwide, in habitats such as mosses, lichens, leaf litter, soil, freshwater, and marine sediments.

Tardigrades have a segmented body with four pairs of short, stubby legs, each ending in claws or suction cups. Their flexible cuticle allows for expansion and contraction as they move. They possess a simple digestive system and a mouth cone equipped with piercing stylets used for feeding on plant cells, algae, bacteria, and small invertebrates.

Most tardigrades require moisture to remain active and are especially common in damp terrestrial habitats. In favorable conditions they are active and reproduce, but they can endure extreme environmental changes that would devastate many other animals.

A key feature is cryptobiosis. When water is scarce or conditions become unfavorable, tardigrades can enter a tun, a desiccated, metabolically almost dormant state. In this state they can survive extreme dehydration, freezing, heat, high pressure, and ionizing radiation, and even the vacuum of outer space in some experiments. They resume normal metabolism when rehydrated. Some tardigrade species carry a protein called Dsup that appears to help protect DNA from damage under stress.

Reproduction varies by species, but most tardigrades reproduce sexually, with some capable of parthenogenesis. Eggs are often laid in protective structures or carried by the female, and juveniles resemble adults.

Tardigrades are of interest in astrobiology, cryobiology, and evolutionary biology due to their resilience and simple body plan. There are more than 1,300 described species.