ABSTRACT: The effects of Heterocapsa circularisquama (Peridiniales, Dinophyceae) on the clearance rate of blue mussel Mytilus galloprovincialis 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 significantly reduced when exposed to H. circularisquama, even at a low dinoflagellate cell density (50 cells ml^-1). Mussels also showed extreme retraction of the mantle edge and no production of feces or pseudofeces. The reduction in clearance rate did not seem to be caused by size, density, or shape of H. circularisquama, since no inhibitory effect was observed when mussels were exposed to other morphologically similar dinoflagellates, i.e. Scrippsiella trochoidea and Heterocapsa triquetra. A filtrate of the H. circularisquama culture did not repress the clearance rate of M. galloprovincialis. '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. This is the first study to explain the specific inhibition process of H. circularisquama on the filtering activities of bivalve mollusks.

KEY WORDS: Heterocapsa circularisquama · Mytilus galloprovincialis · Clearance rate · Bivalve · Dinoflagellate · Toxic effect

Published in MEPS Vol. 146 (1997) on January 30
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997