Effects of the toxic dinoflagellate Alexandrium monilatum on survival, grazing and behavioral response of three ecologically important bivalve molluscs

Little is known about interactions between shellfish and the toxic dinoflagellate Alexandrium monilatum. Toxic strains produce endotoxins with hemolytic and neurotoxic properties, and have been linked to fish and invertebrate kills. We experimentally assessed the survival, grazing and behavioral responses of three shellfish species to A. monilatum. Grazing studies were conducted with two size classes of Crassostrea virginica, Mercenaria mercenaria, and Perna viridis. These species inhabit areas where blooms of A. monilatum occur. Clearance rates of each species were depressed when exposed to toxic A. monilatum alone or with nontoxic Pavlova sp., in comparison to control animals fed only nontoxic algae. Exposure to toxic A. monilatum also caused shellfish to decrease shell valve gape. Intact cells of A. monilatum were found within shellfish feces, but the cells did not re-establish growing populations following gut passage. Survival of larval M. mercenaria and C. virginica was also tested when exposed to A. monilatum cells. Survival was significantly lower for larvae exposed to sonicated A. monilatum, in comparison to control larvae tested with nontoxic A. tamarense. Overall, the data indicate that blooms of A. monilatum can adversely affect some shellfish species by reducing valve gape and clearance rate, and by inducing larval mortality.