Effects of Harmful Dinoflagellates, Gymnodinium mikimotoi and Heterocapsa circularisquama, Red-tide on Filtering Rate of Bivalve Molluscs

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Effects of two harmful dinoflagellate blooms, Gymnodinium mikimotoi and Heterocapsa cir cularisquama on the clearance rate of the blue mussel Mytilus galloprovincialis and the pacific oyster Crassostrea gigas were studied during both red tide periods occurred in Hiroshima Bay, Seto Inland Sea, Japan, 1995.The mean clearance rate of mussel and oyster decreased to 19.8% and 14.4% of con trol in the G. mikimotoi red tide, respectively.In the oyster, red tide filtrate of G. mikimotoi slightly in hibited the filtration activity.The mean clearance rate of the mussel and the oyster decreased to the 8.8% and 1.5% of the control seawater in the H. circularisquama red tide, respectively.However, filtra tion rates of both mussels and oysters were not inhibited in the filtrate of H. circularisquama red tides.Both bivalves exposed to red tide exhibited various negative responses such as retraction of their man tles and gradual valve closure.The present study demonstrated that red tides of G. mikimotoi and H. circularisquama strongly inhibited the filtration rate of bivalves.