Ecophysiology of two intertidal mites at South Georgia
The resistance of Halozetes marinus (Cryptostigmata) and Hyadesia maxima (Astigmata) to cold and heat, to submergence in water and to anoxia was studied to determine their adaptations to the intertidal environment of the sub-Antarctic. (2) Supercooling capacities to lower than -20°C exist in both species, and they remain unchanged during acclimation at temperatures around 0°C for up to 46 d. More than 77% of all samples were low group individuals (supercooling points ≤ -15°C). Glycerol and myoinositol were found in maximum concentrations of ca. 10-20 μ g mg-1 live weight, which together with four other polyols and sugars did not influence supercooling. (3) The probability of inoculative freezing increased with lowered temperature and increased time at subzero temperatures, with higher mortality in freshwater than in seawater. (4) Tolerance to heat (35°C) was greatest in moist rather than dry conditions, and losses of 25-30% body weight resulted in ca. 50% mortality in both mites. (5) Mortality during submersion in freshwater was greater than in seawater, suggesting that normal tidal submersion has no effect. Both species also survived 16 d under anoxia. (6) These mites are well adapted to the South Georgian intertidal environment in respect of the ecophysiological features examined, and inoculative freezing may be the main mortality factor during severe winters