TOXICITY AND LD(50) LEVELS OF THE RED TIDE DINOFLAGELLATE HETEROCAPSA-CIRCULARISQUAMA ON JUVENILE PEARL OYSTERS

Laboratory experiments were conducted to clarify the mechanism of the mass mortality of pearl oysters (Pinctada fucata) associated with the red tide dinoflagellate Heterocapsa circularisquama such as occurred i n Ago Bay, Japan, in 1992. Two-month-old pearl oysters were treated wi th various densities of H. circularisquama, Heterosigma akashiwo and a sterile control. Immediately after the exposure to H. circularisquama cells, juvenile pearl oysters rapidly contracted their mantles and cl osed their shell valves. Then, they contracted their gills and experie nced irregular heartbeat patterns until the heart stopped permanently. The mortality of the juveniles was closely correlated with the cell d ensity of H. circularisquama. The LD(50) was approximately 20000 cells ml(-1) after 24 h and 10000 cells ml(-1) after 48 h from the start of the experiments. However, in the presence of either H. akashiwo at 10 0 000 cells ml(-1) or supernatant of H. circularisquama culture, juven ile pearl oysters did not die within 72 h. During the red tide in Ago Bay, 1992, the maximum cell density of N. circularisquama reached over 80 000 cells ml(-1). Therefore, we consider that the mass mortality o f cultured pearl oysters during this red tide was caused by the direct action of H. circularisquama cells.