The intracellular ovarian parasite causing unaesthetic appearance in the ovary of Pacific oysters Crassostrea gigas in Japan has remained unidentified since the first report of its occurrence in the 1930s. The parasite was observed through transmission electron microscopy, and identified as the paramyxean Marteilioides chungmuensis, which was first reported from Pacific oysters in Korea. In order to obtain information on the development of the parasite, the diameters of both the parasite and the infected ova were measured. The growth of the parasite was highly correlated with the growth and maturation of host gonad cells. Histological observations suggest that M. chungmuensis invades immature ova, which move to the center of the follicle along with its development, and infected ova are finally released outside through the genital canal.
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