1Nagasaki Prefectural Institute of Fisheries, Nagasaki, Nagasaki 851-2213, 2Institute for East China Sea Research, Nagasaki University, Nagasaki, Nagasaki 851-2213, Japan
Heterocapsa circularisquama is a red-tide forming dinoflagellate causing mass mortality of bivalve molluscs in Western Japan. Bioassay on bivalve molluscs was made using two strains of H. circularisquama isolated from Omura Bay, Nagasaki Prefecture in 1999 and 2002. Pearl oyster Pinctada fucata died within 2-3 days after being exposed to 5,000 cells/mL of H. circularisquama. Short-neck clam Ruditapes philippinarum died within 4-6 days after being exposed to 5,000 cells/mL of H. circularisquama. The growth characteristics of H. circularisquama isolated from Omura Bay in 1999 were examined in 54 different combinations of temperature (0-30°C) and salinity (6-36) under a light intensity of 80 μmol/m2/s. H. circularisquama reproduced at 12.5-30°C and 16-36 in salinity. The highest growth rate was obtained at 30°C and 32 in salinity. Under the conditions, maximum specific growth rate was 0.91 day-1. The effect of light intensity on the growth of H. circularisquama was examined at seven different conditions ranging from 10 to 140 μmol/m2/s under 14 h light and 10 h dark cycle and 25°C. H. circularisquama grew well at a light intensity of 20 μmol/m2/s and higher, then the growth was saturated at 80 μmol/m2/s. Under the conditions of this experiment, the half-saturation constant for growth of light intensity (Ks) and threshold value (Io) were 24.0 μmol/m2/s and 15.5 μmol/m2/s, respectively. The occurrence of H. circularisquama in Omura Bay, Imari Bay and Tachibana Bay was confirmed in the present study by observation of body scale structures.