Osmotic stress tolerance in semi-terrestrial tardigrades

Little is known about ionic and osmotic stress tolerance in tardigrades. Here, we examine salt stress tolerance in Ramazzottius oberhaeuseri and Echiniscus testudo from Niva (Denmark) and address whether limno-terrestrial tardigrades can enter a state of quiescence (osmobiosis) in the face of high external osmolyte concentrations. Direct transfers into NaCl solutions showed an upper tolerance level of around 600mOsmkg(-1) in R.oberhaeuseri and 200mOsmkg(-1) in E.testudo. During salt exposures, R.oberhaeuseri contracted into a tun', whereas E.testudo remained active leaving it more susceptible to acute effects of the ions. Further experiments focused on the more resilient R.oberhaeuseri, which entered a tun and readily regained activity when directly exposed to polyethylene glycol and sucrose of up to 872 +/- 0 and 813 +/- 3mOsmkg(-1), respectively, revealing a higher tolerance towards non-ionic osmolytes as compared to NaCl. Ramazzottius oberhaeuseri furthermore readily regained activity following gradual increases in non-ionic osmolytes and NaCl of up to 2434 +/- 28 and 1905 +/- 3mOsmkg(-1), respectively, showing that short-term acclimation promoted salt stress tolerance. Our results suggest that the limno-terrestrial R.oberhaeuseri enters a state of quiescence in the face of high external osmotic pressure and that it, in this state, is highly tolerant of ionic and osmotic stress.