PHYSIOLOGICAL-RESPONSES TO SEVERE ACID STRESS IN 4 SPECIES OF FRESH-WATER CLAMS (UNIONIDAE)

Four species of freshwater clam, Anodonta anatina, A. cygnea, Unio pictorum, and U. tumidus were exposed for 2 weeks to acidified soft water (pH 4.0-4.5, Ca 4.6 mg/L) and for 4 weeks to acid in hard water conditions (Ca 18.5 mg/L). The exposures caused a decrease in Na+, K+, and Cl- ion and a rapid increase of Ca2+ in the hemolymph. The elevation of the hemolymph Ca2+ was positively correlated with the decrease in the hemolymph pH in all species studied. Low ambient [Ca2+] level accelerated the pH decrease and Ca2+ increase in the hemolymph. Na+ and Cl- ion concentrations changed less rapidly in the soft conditions. Although there were minor changes in the mineral composition of the calcium concretions in the gills, the amount of calcium in the concretions did not change during the exposure. There was no correlation between the thickness of the shell and the ionic response, but all four species responded to low ambient pH in the same way. © 1990 Springer-Verlag New York Inc.