Seasonal fluctuations in the occurrence of abnormal enlargement of the ovary of Pacific oyster Crassostrea gigas at Gokasho Bay, Mie, Japan

Abstract

Seasonal occurrences of the abnormally enlarged ovary of Pacific oysters Crassostrea gigas caused by an unidentified intracellular protozoan parasite was investigated in Gokasho Bay, Mie Prefecture, Japan. Cultured and wild oysters were collected bimonthly for one year from July 1996, and seasonal fluctuations in the prevalence of infection, parasite development and histological changes of the infected tissue were examined. Parasites, very similar to Marteilioides chungmuensis infecting Pacific oysters in Korea, had 8 forms, 4 of which were frequently observed, while the other 4 forms, which were not described in M. chungmuensis, were rare. The total prevalence of infection (percentage of infected oysters positive for at least one of the two examinations: gross and smear inspections of the ovary) was usually higher than the gross prevalence of infection, indicating that infected oysters included ones without visible legions. In cultured oysters, the prevalence of infection increased in summer, suggesting that active multiplication of the parasite takes place in warm water months. From autumn through spring, no substantial change in the total prevalence of infection was observed in cultured oysters (18-20%), while it continued to decline in wild oysters during that period. These differences between cultured and wild oysters could be attributed to the oyster size and environmental factors such as salinity. Histologically, the number of infected oocytes increased in summer, but decreased in winter. From the variations in the parasite morphology and maturation of oocytes, the life cycle of the parasite and the mechanism of the legion formation are postulated.