Toxic effects of the dinoflagellate Heterocapsa circularisquama on clearance rate of the blue mussel Mytilus galloprovincialis

Abstract

The effects of Heterocapsa circularisquama (Peridiniales, Dinophyceae) on the clearance rate of blue mussel Mytilus galloprovjncialis were studied in the laboratory to clarify the mechanism involved in the toxic effects of dinoflagellates on bivalve mollusks The clearance rate of blue mussels was s~gnificantly reduced when exposed to H. circularisquama, even at a low dinoflagellate cell density (50 cells ml-'). Mussels also showed extreme retraction of the mantle edge and no production of feces or pseudofeces. The reduction in clearance rate did not seen1 to be caused by slze, density, or shape of H. circularisquama, since no inhibitory effect was observed when mussels were exposed to other n~orphologically similar dinoflagellates, i.e. Scrippsiella trochoidea and Heterocapsa tnquetra. A filtrate of the H. circularisquama culture d ~d not repress the clearance rate of M. galloprovincial~s. 'Naked cells' of H. circularisquama (without cell wall), which were prepared by centrifugation, showed no inhibitory effect on clearance rate. Therefore, it is quite likely that the source of toxicity of H. circularisquama is localized on the cell surface. The inhibitory effect of H. circularisquama was inactivated by treatment of intact cells with trypsin and sodium dodecylsulfate. These results indicate that the toxic response of M . galloprovincialis to H. circularisquama is caused by a protein-like substance involving the outer cell components. T h ~s is the first study to explain the specific inhibition process of H circulansquama on the filtering activities of bivalve mollusks.