Seasonal fluctuations in the occurrence of abnormal enlargement of the
ovary of Pacific oyster Crassostrea gigas at Gokasho Bay, Mie, Japan
S.
Imanaka, N. Itoh, K. Ogawa, H. Wakabayashi-2001
Fish
Pathology, 36(2): 83-91 (from
Current Contents)
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.
(Univ
Tokyo, Grad Sch Agr & Life Sci, Tokyo 1138657, Japan, e-mail of K.
Ogawa: aogawak@mail.ecc.u-tokyo.ac.jp)