Dormancy in Freshwater Tardigrades

For more than two centuries, tardigrades have been well known for their ability to undergo dormancy. However, this capability has been well studied mainly in the so-called limnoterrestrial species, i.e., in the species colonizing moist terrestrial habitats, such as mosses, lichens, and leaf litter. In these kinds of substrates, tardigrades are active only when a film of water is available around their body so in this condition they behave like aquatic animals. When the substrate dries or freezes, tardigrades achieve dormancy (quiescence) by entering cryptobiosis, specifically anhydrobiosis or cryobiosis, respectively. In freshwater habitats, both forms of cryptobiosis have been verified only in species able to live both in freshwater and terrestrial habitats. In the truly freshwater (or limnic) species, anhydrobiosis has not been verified, while cryobiosis has been confirmed in a few species. Another dormancy phenomenon bound to diapause is frequent in freshwater species: encystment (sometimes found even in limnoterrestrial species). The cyst state, which involves deep structural and physiological modifications, has been known from the beginning of the past century, but only recently has its morphology and inducing factors been studied in depth. Although data on molecular mechanisms allowing cryptobiosis are available, this information does not exist for encystment.