Survival and growth of post-set oysters and clams on diets of cadmium-contaminated microalgal cultures

Abstract

Young post-set oysters, Crassostrea virginica, and clams, Mercenaria mercenaria, were reared in groups of 50 on diets consisting of one or more of the following species of microalgae: Isochrysis galbana, Phaeodactylum tricornutum, and Dunaliella tertiolecta. Strains of these species that had been induced to tolerate high Cd concentrations were cultured axenically in artificial seawater media with 15, 60, and 60 mg liter −1 Cd, respectively. Algal diets consisting of strains of these species cultured in Cd free medium and containing no measurable Cd served as controls. Algal cells were harvested in a nitrogen deficient stationary phase from semi-continuous cultures and analyzed for Cd uptake. Daily feeding rations of the three algal species listed above contained known quantities of Cd: 41·0, 20·2, and 15·2 μg bivalve −1 day −1, respectively. Daily algal rations were also analyzed for nutritional components (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 exhibited varied responses. Diets including Cd-contaminated D. tertiolecta 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. tricornutum did not grow, but mortality was observed only in clams fed Cd-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 results demonstrate detrimental effects of pollutant-contaminated algae upon consumer species' survival and growth, parameters of fundamental importance to population structure.