Tardigrades in extreme habitats: morphological and molecular aspects in anhydrobiotic and encysted Amphibolus volubilis.

In terrestrial ecosystems tardigrades often inhabit transient systems that can support life only for a fraction of the year (e.g. arctic tundra, deserts, temporary ponds, mosses, lichens and leaf litter). Persistence of tardigrades in these harsh habitats is due to their ability to enter dormant states such as cryptobiosis and/or encystment. Cryptobiosis is directly induced by exogenous stimuli (desiccation: anhydrobiosis; freezing: cryobiosis) and it is promptly broken when the adverse conditions are removed. Encystment is under endogenous control and could be only indirectly induced by environment stimuli. Breaking encystment requires a specific cue that may or may not correspond to favourable environmental conditions.Amphibolus volubilis is a moss-dwelling tardigrade with a boreo-alpine distribution. Therefore, it represents a good model species to study the survival strategies in unpredictable environments. We verified that it is able both to enter cryptobiosis and to form cysts. Morphological changes of the body have been evidenced in anhydrobiotic tuns and cysts collected in nature from moss collected in a post-glacial valley (Northern Apennines, Modena, Italy, 1700 m a.s.l.) and/or induced in lab. Entering anhydrobiosis specimens contract longitudinally turning their body into a tun, lose most of their free and bound water and reduce or suspend their metabolism. The cyst is the result of a series of successive and continued morphological changes that are more complex than those involved in tun formation. Encystment involves repeated de novo synthesis of new cuticular structures (several cuticles with different ultrastructure, modified and unmodified buccal-pharyngeal apparatuses and claws) and a reduction of metabolism and loss of water lower than those involved in the anhydrobiotic state. The expression of Heat shock proteins (Hsps) have also been investigated in A. volubilis. Hsp 70 has been detected in active, anhydrobiotic and encysted specimens, evidencing that the synthesis of Hsp 70 in this species is not only related to desiccation stress. The synthesis of Hsp 70 was evidenced only in the anhydrobiotic state of the tardigrade Richtersius coronifer and related to tun formation (Ramløv and Westh, 2001).