Acute physiological responses of the freshwater snail Elimia flava (Mollusca: Pleuroceridae) to environmental pH and calcium

The individual and interactive effects of environmental pH (7 [control], 6, 5, and 4) and calcium (0, 5, and 50 mg/L) were studied on hemolymph ions (pH, Ca2+, total CO2, Na+, K+) and osmolality in the freshwater snail, Elimia flava, over a 72-h exposure. All hemolymph factors strongly differed with environmental pH. Snails exposed to pH 4 were inactive and experienced more dramatic ionic disturbances than snails at pH 5, 6, and 7, including reduced hemolymph pH, depressed Na+ concentrations, and increased Ca2+ and total CO2 concentrations. There was an initial but transient increase in hemolymph K+ over the 72 h exposure period. Environmental calcium ameliorated effects of acidification on hemolymph pH and Na+, reducing the degree of depression in both variables irrespective of environmental pH or exposure time. In a separate experiment, effects of acidification on snail respiration were examined in which VO2 was measured over 24 h in snails exposed to pH 7 and 4. Exposure to pH 4 caused a 64% reduction in oxygen uptake at 2 h and a maximum reduction (81%) at 11 h. Our results suggest that snails exposed to pH 4 cease interacting with the surrounding medium and use internal stores of CaCO3 to buffer hemolymph acidification, whereas snails at pH 5 and higher appear to use environmental calcium as a buffer source. These results suggest an important role of environmental calcium in ameliorating the impacts of short-term, sublethal acidification.