Accumulation of 45Ca in the freshwater unionids Anodonta anatina and Unio tumidus, as influenced by water hardness, protons, and aluminum

Previous studies have revealed that freshwater bivalves subjected to acid stress suffer from severe electrolyte imbalance. A typical initial reaction is a transitory increase in concentration of Ca in the hemolymph during the first few days of exposure to such stress. Although this calcium obviously originates from the CaCO3 reserves in the clam, a possible ambient origin has never been ruled out. In the present study the uptake of Ca was studied under both circumneutral and acid conditions: two freshwater unionids, Anodonta anatina and Unio tumidus, were exposed to a bathing medium that contained 45Ca for a period of 24 hrs. In order to study the effect of the ambient Ca concentration on the efficiency of uptake of Ca, two media with different Ca concentrations (35 and 3.5 mg Ca/1; 4 × 106 cpm 45Ca/1 and 4 × 105 cpm 45Ca/1, respectively) were used. The role of the ingested food as a source of Ca was evaluated by feeding the clams with algae labeled with 45Ca. After the clams had been exposed for 24 hrs to 45Ca, the accumulation of 45Ca in the soft parts, including hemolymph, calcium concretions and glochidial larvae, was measured. Low pH (4–4.5) significantly decreased the uptake of 45Ca from the medium, whereas the addition of aluminum (900 μg/1) had only a minor effect on the uptake of 45Ca. The sites of accumulation of 45Ca under acidified conditions did not change: in all cases highest values for 45Ca concentration were measured in the gills, where the calcium concretions sequestered a large part of the 45Ca. A 90% decrease in the ambient Ca concentration caused a similar decrease in the uptake of 45Ca. Clams were able to utilize both dietary and waterborne Ca.