DAO 50:137-144 (2002)  -  doi:10.3354/dao050137

ABSTRACT: Four intracellular prokaryotes are reported from the scallops Pecten novaezelandiae Reeve, 1853 and Chlamys delicatula Hutton, 1873. Elongated (1025 x 110 nm), irregular (390 × 200 nm), or toroidal (410 x 200 nm) mollicute-like organisms (M-LOs) occurred free in the cytoplasm in the digestive diverticular epithelial cells of both scallop species. Those in P. novaezelandiae bore osmiophilic blebs that sometimes connected the organisms together, and some had a rod-like protrusion, both of which resemble the blebs and tip structures of pathogenic mycoplasmas. The M-LOs in C. delicatula had a slightly denser core than periphery. Round M-LOs, 335 x 170 nm, occurred free in the cytoplasm of agranular haemocytes in P. novaezelandiae, without apparent harm to the host cell. In P. novaezelandiae, 2 types of highly prevalent (95 to 100%) basophilic inclusions in the branchial epithelium contained Rickettsia-like organisms (R-LOs). Type 1 inclusions occurred in moderately hypertrophied, intensely basophilic cells, 8 to 10 µm in diameter, containing elongate intracellular R LOs, 2000 x 500 nm. Type 2 inclusions were elongated and moderately basophilic in markedly hypertrophic branchial epithelial cells, 50 x 20 µm in diameter, containing intracellular organisms 500 × 200 nm in diameter. The possible roles of these organisms in pathogenesis is discussed.

KEY WORDS: Scallops · Pecten · Chlamys · Digestive epithelium · Branchial epithelium · Haemocytes · Rickettsias · Mollicutes · Mycoplasmas · Tip structure · Ultrastructure