Tardigrades are found in all different kinds of habitats, from the highest elevations in the Himalayas to the deepest trenches in the deep sea, and from hot, radioactive springs to the ice cathedrals inside the Greenland ice cap. Many of the so-called terrestrial species are semi-aquatic, because all tardi-grades need a water film to be active. The arthrotardigrades are found in true marine habitats from the tidal beaches to the deep-sea mud. The terrestrial species live in mosses and lichens and tolerate desiccation for up to nine years.
Both the heterotardigrades and eutardigrades have independently invaded the terrestrial environment. One genus of eutardigrades, Halobiotus, has secondarily invaded the marine environment again. The species, H. crispae, is a strange tardigrade that cyclically changes form through the year—a transformation that usually is referred to as cyclomorphosis. The dark winter form is cyst-like and has a double cuticle, but it can still move around if it is not completely frozen. It may survive freezing for up to six months per year. The early spring form tolerates freshwater, has thin stylets, and lacks true placoids in the pharynx. The summer form is only active when the salinity is more than 30 parts per thousand, and it has a normal single layer cuticle and robust stylets with macroplacoids in the pharynx. In this stage, the gonads mature for reproduction.
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