ANHYDROBIOSIS AND COLD TOLERANCE IN TARDIGRADES

A review of the literature regarding anhydrobiosis and cold tolerance in tardigrades is presented. During increasing desiccation, invertebra tes like tardigrades, rotifers, nematodes and some collembolans are ab le to shut down metabolism to undetectable levels. When tardigrades ar e entering anhydrobiosis, a tun-like structure is formed, facilitated by structural adaptations of the cuticle. Slow dehydration is essentia l for tun formation, and the accumulation of trehalose during this pro cess may help to stabilize phospholipids and proteins. Wax extrusion o n the cuticle surface reduces transpiration. A fraction of 5-15% of th e initial body water is retained during anhydrobiosis. Tardigrades are principally aquatic organisms, but anhydrobiosis makes it possible fo r some species to live in habitats with changing moisture conditions. Tardigrades in anhydrobiosis may tolerate exposure to freezing tempera tures of liquid gases, and some species also survive such temperatures in their hydrated state. Few investigations are available on the rela tion of tardigrades to temperatures more representative to their natur al environments. Experimental studies, however, from Greenland and the Antarctic Continent suggest that some species overwinter both in a hy drated frozen state and in anhydrobiosis. During the summer, a number of tardigrade species have been recorded from cryoconite holes, formed on the surface of glaciers. These species are freeze tolerant since t heir habitats are permanently frozen during the winter.