Abstract
Effects of two harmful dinoflagellate blooms, Gymnodinium mikimotoi and Heterocapsa circularisquama 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 control in the G. mikimotoi red tide, respectively. In the oyster, red tide filtrate of G. mikimotoi slightly inhibited 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, filtration 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 mantles and gradual valve closure. The present study demonstrated that red tides of G. mikimotoi and H. circularisquama strongly inhibited the filtration rate of bivalves.
