Survival of freezing by hydrated tardigrades inhabiting terrestrial and freshwater habitats

The seasonality and unpredictability of environmental conditions at high altitudes and latitudes govern the life cycle patterns of organisms giving rise to stresses that cause dead or development of specific adaptations. Ice formation is a major variable affecting survival of both freshwater fauna and fauna inhabiting lichens, mosses and leaf litter. Tardigrades occupy a wide range of niches in marine, freshwater and terrestrial environments. The highest number of species is found in terrestrial habitats thanks to their ability to enter anhydrobiosis and cryobiosis. The cryobiotic ability of tardigrade species from polar regions is well known. Consequently, we focused our research on the ability to survive freezing in active hydrated state using seven tardigrade species differing in phylogenetic position and collected at various altitudes and from different habitats in a temperate area. Replicates were cooled at different cooling rates (from 0.31 °C min-1 to 3.26 °C min-1). Even though the final survival and the time required by animals to recover active life were both inversely related to the cooling rates, highly significant interspecific differences were found. Species survival ability ranges from excellent to none. Species living in xeric habitats withstand freezing better than those living in hygrophilous habitats, while true limnic species do not exhibit cryobiotic capabilities. The ability to withstand freezing seems linked to the anhydrobiotic ability. The difference in cryptobiotic performance among tardigrade species seems more influenced by selective pressures linked to local adaptation to habitat characteristics than by phylogenetic relationships.