DAO 51:123-133 (2002)  -  doi:10.3354/dao051123

ABSTRACT: Haplosporidium sp. is described from rock oysters Saccostrea cuccullata Born, 1778 experiencing epizootics on the northwestern coast of Western Australia. All stages were observed as focal infections in the connective tissue of the gills, or as disseminated infections in the mantle and around digestive diverticulae. Haplosporidium sp. occurred between epithelial cells of the gut, in focal lesions in the gills, but not in the epithelium of the digestive diverticulae, and sporulation was confined to the connective tissue. Plasmodia developed into sporonts and sporocysts in a loose syncytium that gave rise to binucleate and uninucleate sporoblasts from which spores developed. Spores were flask-shaped, 5.6-6.7 x 3.3-4.0 µm, with a characteristic operculum, a few filamentous wrappings and rod-like structures in the posterior sporoplasm. Mature spores had a wall comprising inner (90 nm wide), middle (30 nm wide) and outer (130 nm wide) layers, and a surface coat of microtubules giving them a furry appearance. Oysters with empty gonad follicles were most heavily infected, and oyster condition and mortality appeared to be related to degree of infection.

KEY WORDS: Rock oysters · Saccostrea cuccullata · Haplosporidian · Haplosporidium · Minchinia · Epizootics · Histopathology · Ultrastructure