Grazing impacts of the heterotrophic dinoflagellate Polykrikos kofoidii ona bloom of Gymnodinium catenatum

In 1998, a red tide of the paralytic shellfish poisoning (PSP)-producing di noflagellate Gymnodinium catenatum Graham occurred in Yatsushiro Sea, weste rn Japan. The dramatic decline of dominant G. catenatum cells occurred duri ng the field and laboratory assessments, accompanied with growth of the het erotrophic dinoflagellate Polykrikos kofoidii Chatton. Microscopic observat ions on both field and laboratory cultured bloom water revealed that >50% o f P, kofoidii predated on the natural population of G. catenatum, and 1 to 8 G. catenatum cells were found in food vacuoles of P. kofoidii pseudocolon ies. Our results suggest that predation by P. kofoidii contributes to the c essation of a G. catenatum bloom.