Thermotolerance and thermal acclimation in active tardigrades.

The ability of desiccated (anhydrobiotic) tardigrades to survive and to resist high temperature stresses (up to 100°C) is well-known, while the tolerance of active (hydrated) tardigrades to heat-shocks is still very little known. In order to increase our knowledge on thermal adaptations of active tardigrades, we have carried out lab experiments on three eutardigrade species to evaluate the ability to survive to heat-shock (considering lethal temperature - LT50 - and critical thermal maximum - CTmax), and the possibility of thermotolerance induction. We considered two terrestrial species, the moss-dwelling Amphibolus volubilis and the lichen-dwelling Ramazzottius oberhaeuseri, and one limnic species, Borealibius zetlandicus. These species differ each other in anhydrobiotic and cryobiotic abilities, substrate colonized.Starting from a uniform condition of tardigrades maintained for 24 h in water at 16°C, groups of hydrated tardigrades have been exposed for 1 hour to a heat-shock (different experiments from 26°C to 42°C). The presence of active animals (body movements) was evaluated immediately after heat-shock (t0), after 1h (t1) and after 24 h (t24) from heat-shock. Survival was represented by active animals at t24. For A. volubilis and R. oberhaeuseri the presence of active animals and survival was evaluated also after acclimation of 1 h (the first species at 28°C, the latter at 30°C) and subsequent heat-shocking to temperatures higher than 33°C. All species look thermotolerant, even though their survival significantly decreases with the increase of the stress temperatures. Both CTmax and LT50 are species-specific. The first one is 39.0°C for A. volubilis and 37.0°C for R. oberhaeuseri and B. zetlandicus. The latter is 35.1°C for A. volubilis, 33.6°C for R. oberhaeuseri and 33.0° for B. zetlandicus. The percentage of active animals changes according to the shock temperature and differs among the species. The number of animals with active movements often significantly increases between t0 and t1 for some temperatures, apart R. oberhaeuseri which shows a decrease at 28°C. There are not significant differences between t1 and t24 in the number of active animals, with the exception of an increase in A. volubilis at 30°C and 33°C. Both in A. volubilis and R. oberhaeuseri the acclimation produces significant survival increases of 10-40% for heat-shock temperature between 33°C–39°C, while at 40°C and 42°C there is not survival. These results demonstrate that tardigrades, even when active, have evident ability to survive heath stresses, even though sampled in different habitats and characterized by quite different cryptobiotic performances.