Valve closure response to uranium exposure for a freshwater bivalve (Corbicula fluminea):: Quantification of the influence of pH

Laboratory experiments were carried out to analyze the first valve closure response of a freshwater bivalve (Corbicula fluminea) exposed to uranium during a 5-h period. Experiments were performed in a well-defined artificial water at two pH values, 5.5 and 6.5, with a noninvasive method of valve recording. Sensitivity thresholds, based on percentage of bivalve that close their valves in a given time, were determined. Response thresholds depended on the total uranium concentration, integration time of response (fast responses could only be observed for the highest concentrations), and pH. The bivalve is much more sensitive to total uranium concentration at pH 5.5 than pH 6.5. The minimal sensitivity threshold deter-mined, expressed as the uranium concentration inducing the valve closure of 50% of the bivalves, was 0.05 mumol/L at pH 5.5 after 5 h of exposure. Moreover, higher concentrations of the free ion UO22+ are required at pH 5.5 than at pH 6.5 to illicit the same response. Two hypotheses can be proposed, that UO22+ is not the only detected species or that competition exists between H+ and UO22+ for binding sites.