Meltwater effects on intertidal Antarctic limpets, Nacella concinna

Responses of the Antarctic limpet Nacella concinna (Mollusca: Gastropoda) to meltwater exposure were studied at Adelaide Island (67 degrees 34'S 68 degrees 08'W). Limpets in crevices could be bathed in pure meltwater, though animals in large pools often avoided significant exposure to lowered salinity because of marked halocline formation. Small pools containing limpets showed extremely demanding osmotic environments with fluctuations between salinities of 3 and 30 psu being recorded within 1-2 min because of alternating meltwater and wave action. Analysis of haemolymph osmolarities in animals taken from the field demonstrated significant falls in osmolality (to 600-800 mOsm kg(-1)) compared with control animals living in seawater (1000 mOsm kg(-1)). A few moribund animals had osmolarities < 600 mOsm. kg(-1). Nacella concinna are stenohaline osmoconformers with a median lower lethal salinity (96 h) of 20.9 psu and a median lethal time for freshwater exposure of 2 h 18 min. A 1 h exposure to freshwater caused a 14% increase in body volume and a 27% loss of body fluid ions. Nacella concinna responds behaviourally to meltwater by short-term, ineffective, clamp down of the shell, retraction of the head, cephalic tentacles and mantle margin tentacles. On vertical surfaces limpets respond actively to freshwater exposure by rapid detachment; 50% of animals lose adhesion within 5 min. Consideration of tidal regimes at Adelaide Island suggests that an individual intertidal limpet has a risk of being exposed to severe meltwater exposure about once per year.