HAPLOSPORIDIOSIS OF THE PACIFIC OYSTER, CRASSOSTREA-GIGAS

Haplosporidan parasites were observed in 10 100 spat and 1 171 adult Pacific oysters, Crassostrea gigas, reared in Matsushima Bay, Japan. Eight of the infected spat contained mild to severe plasmodial infections. The multinucleated plasmodia were 6-12 μm × 7-15 μm and were associated with an infiltration of hemocytes that occurred throughout the vesicular connective tissues of all infected oysters. Two oysters, one adult and one spat, contained advanced sporogonic infections. These were characterized by the presence of sporocysts and immature and mature operculated spores that measured 5.6-6.0 μm × 6.0-8.0 μm and were found exclusively within the digestive tubule epithelium. Electron microscopic examination revealed that mature spores contained a hinged operculum, striated and layered wall, spherule, single nucleus, and haplosporosome formative regions. Parasite morphology and infection pattern closely resemble that of Haplosporidium nelsoni, a pathogen of American oysters (C. virginica). © 1991.