GROWTH, SURVIVAL, AND GLYCOGEN-CONTENT OF TRIPLOID PACIFIC OYSTER CRASSOSTREA-GIGAS IN THE WATERS OF HIROSHIMA, JAPAN

Triploidy was induced in the Pacific oyster Crassostrea gigas by treating newly fertifiized eggs with cytochalasin B in July 1987 and 1988. Growth, survival, glycogen content, and moisture content of triploid oysters cultured in the waters of Hiroshima were investigated. In the spawning season (July to September) shell height, total weight and soft body weight of diploids decreased or failed to increase, while those of triploids continued to increase. At the end of September the mean values of shell height, total weight and soft body weight of triploids were 1.0 to 1.2, 1.4 to 1.9, and 1.8 to 2.5 times respectively as much as those of diploids. The difference of soft body weight between diploids and triploids suggests that diploids lost 44 to 60% of wet soft body weight or 56 to 67% of dry soft body weight as gametes spawned. Condition index, which means the ratio of soft body weight against total body weight including shells, decreased sharply in diploids in the spawning season owing to mass spawning, while this sharp decrease was not observed in triploids. In diploids, the moisture content of the soft body, which was around 80% before the spawning season, increased 83 to 84% in the spawning season. But in the case of triploids, moisture content was at a low level of 77 to 82%. The glycogen content of diploids decreased to a low level of 2.7 to 5.4% on a dry basis according to gametogenesis, while that of triploids was at a high level of 8.5 to 16.9%. These results are due to the retardation of gonado- and gametogenesis. Mortality of triploids was about half of diploids from September to November, which suggests that the decrease in physiological activities must be smaller in triploids than in diploids because of the retardation of gonado- and gametogenesis.