Stress response of a boreo-alpine species of tardigrade, Borealibius zetlandicus (Eutardigrada, Hypsibiidae)

Invertebrates living in extreme environments as well as those living under unpredictable habitat conditions must be able to survive severe environmental stresses bound to their habitats. Tardigrades represent a good animal model to analyze responses evolved by organisms to overcome extreme environmental stresses or to colonize extreme environments because they respond to desiccation or freezing in their habitats by entering cryptobiosis. The responses to environmental stresses have been evaluated almost exclusively in terrestrial tardigrades, while very little is known about the ability of limnic species to tolerate those stresses. This study evaluates the responses of the limnic boreo-alpine species Borealibius zetlandicus, under lab conditions, to stresses imposed by desiccation and temperature variation (freezing and heating). Our results indicate that active specimens are able to freeze, confirming the cryobiotic ability of this species. There is a negative correlation between survival and cooling rates. In contrast, no specimens of B. zetlandicus are able to survive desiccation. With regard to thermal tolerance, the animals show a high ability to resist heat-shock (LT50 = 33.0  0.5°C) for a short time. This wide tolerance to different environmental parameters could be the reason for the wide distribution of the species. Due to the disjunct distribution of the species and to the presence of cryptic tardigrade species that could have different ecological and physiological responses, we decided to characterize the population studied from a molecular point of view by investigating its COI mtDNA sequences.