Effect of pH, hardness and maternal pre-exposure on the toxicity of Cd, Cu and Zn to the glochidial larvae of a freshwater clam Anodonta cygnea

Mature larvae (glochidia) stripped from the gills of the freshwater clam, Anodonta cygnea zellensis Gmelin, were exposed to cadmium, copper and zinc at slightly alkaline (pH 8.0–8.3) and acid water (pH 4.5–5.0). In addition, metal-spiked medium with different hardnesses (4.5, 9, 18, 35 and 58 mg Ca/1) was applied. To investigate the effect of sublethal maternal pre-exposure on the viability of the glochidia, preceding the glochidial harvest, some of the female clams carrying glochidia were exposed for two weeks to sublethal concentrations of Cd, Cu and Zn. The viability of the glochidia originating from the metal exposed females was studied in unspiked and metal-spiked water. Increasing water hardness did not in all cases improve the glochidial resistance to the metals. Low water pH had both a synergistic and an antagonistic effect on the toxicity of metals. Maternal pre-exposure at low metal concentrations often increased the resistance of glochidia to the metals, whereas pre-exposure at higher metal concentrations decreased glochidial resistance. The order of toxicity of the three metals studied was: Cu > Cd > Zn.