SURVIVAL AND GROWTH OF POST-SET OYSTERS AND CLAMS ON DIETS OF CADMIUM-CONTAMINATED MICROALGAL CULTURES

Young post-set oysters, Crassostrea virginica, and clams, Mercenaria m ercenaria, were reared in groups of 50 on diets consisting of one or m ore of the following species of microalgae: Isochrysis galbana, Phaeod actylum tricornutum, and Dunaliella tertiolecta. Strains of these spec ies that had been induced to tolerate high Cd concentrations were cult ured axenically in artificial seawater media with 15, 60, and 60 mg li ter-1 Cd, respectively. Algal diets consisting of strains of these spe cies cultured in Cd-free medium and containing no measurable Cd served as controls. Algal cells were harvested in a nitrogen deficient stati onary phase from semi-continuous cultures and analyzed for Cd uptake. Daily feeding rations of the three algal species listed above containe d known quantities of Cd: 41.0, 20.2, and 15.2 mug bivalve-1 day-1, re spectively. Daily algal rations were also analyzed for nutritional com ponents (protein, carbohydrate, and lipid), and significant effects of Cd on algal composition were found. Oysters fed cadmium-contaminated P. tricornutum, D. tertiolecta, or a 50/50 mix of these two species ex hibited varied responses. Diets including Cd-contaminated D. tertiolec ta caused mortality and weight losses of oysters whereas, oysters fed P. tricornutum cultured in Cd showed no significant effect. Clams fed unialgal diets of Cd-contaminated I. galbana, D. tertiolecta, or P. tr icornutum did not grow, but mortality was observed only in clams fed C d-contaminated P. tricornutum. Differences in bivalve responses to the various algal diets may reflect interactions between CD toxicity and nutritional factors, especially for oysters in which the toxic effects of Cd were more severe when more digestible diets were fed. These res ults demonstrate detrimental effects of pollutant-contaminated algae u pon consumer species' survival and growth, parameters of fundamental i mportance to population structure.