TOXIC EFFECTS OF THE DINOFLAGELLATE HETEROCAPSA-CIRCULARISQUAMA ON CLEARANCE RATE OF THE BLUE MUSSET MYTILUS-GALLOPROVINCIALIS

The effects of Heterocapsa circularisquama (Peridiniales, Dinophyceae) on the clearance rate of blue mussel Mytilus galloprovincialis were s tudied in the laboratory to clarify the mechanism involved in the toxi c effects of dinoflagellates on bivalve mollusks. The clearance rate o f blue mussels was significantly reduced when exposed to H. circularis quama, even at a low dinoflagellate cell. density (50 cells ml(-1)). M ussels also showed extreme retraction of the mantle edge and no produc tion of feces or pseudofeces. The reduction in clearance rate did not seem to be caused by size, density, or shape of H. circularisquama, si nce no inhibitory effect was observed when mussels were exposed to oth er morphologically similar dinoflagellates, i.e. Scrippsiella trochoid ea and Heterocapsa triquetra. A filtrate of the H. circularisquama cul ture did not repress the clearance rate of M. galloprovincialis. 'Nake d cells' of H. circularisquama (without cell wall), which were prepare d by centrifugation, showed no inhibitory effect on clearance rate. Th erefore, it is quite likely that the source of toxicity of H. circular isquama is localized on the cell surface. The inhibitory effect of H. circularisquama was inactivated by treatment of intact cells with tryp sin and sodium dodecylsulfate. These results indicate that the toxic r esponse of M. galloprovincialis to H. circularisquama is caused by a p rotein-like substance involving the outer cell components. This is the first study to explain the specific inhibition process of H. circular isquama on the filtering activities of bivalve mollusks.