Shell closure and restriction of filtration are behavioral responses by which bivalve molluscs can limit exposure of soft tissues to noxious or toxic agents, including harmful microalgae. In this study, we assessed the clearance rates of five species of bivalve mollusc—the northern bay scallop Argopecten irradians irradians, the eastern oyster Crassostrea virginica, the northern quahog Mercenaria mercenaria, the softshell clam Mya arenaria, and the blue mussel Mytilus edulis—exposed for one hour to each of three harmful-algal strains: Prorocentrum minimum, Alexandrium fundyense, and Heterosigma akashiwo. Clearance rates of harmful-algal cells were compared with clearance rates of a benign microalga, Rhodomonas sp., and to a Mix of each harmful alga with Rhodomonas sp. Qualitative observations of valve closure and production of biodeposits were also assessed during the exposure experiments. Feces and pseudofeces were collected and observed with light and fluorescence microscopy for the presence or absence of intact, potentially-viable algal cells or temporary cysts. Results increase our understanding of the high variation between the different bivalve/harmful alga pairs. Responses of bivalve species to the different harmful algae were species-specific, but in most cases indicated a preferential retention of harmful algal cells, probably based upon different characteristics of the algae. Each shellfish species also reacted differently to the harmful-algal exposures; several remained open; whereas, others, such as oysters exposed to the toxic raphidophyte Heterosigma akashiwo, closed shells partially or totally. Similarly, production of feces and pseudofeces varied appreciably between the different bivalve/alga pairs; with the exception of softshell clams Mya arenaria, intact cells of most harmful-algal species tested were seen in biodeposits of the other four bivalve species. These results extend our understanding of the high species specificity in the interactions between harmful algae and bivalve molluscs and confirm that generalizations about feeding responses of bivalves to harmful algae cannot easily be made. In most cases, however, there was at least some ingestion of the harmful algae leading to exposure of soft tissues to the algal cells.
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