Cases of shell disease in the giant oyster Crassostrea gigas (Bivalvia) cultivated in the Black Sea

This paper presents the first description of shell disease in the Crassostrea gigas oyster cultivated in the Black Sea. The shell margin, hinge and the site of adductor muscle attachment are affected by a fungus. The oyster’s exhaustion caused by excessive sedimentation results in high susceptibility to disease. It is suggested that spores of the fungus Ostracoblabe implexa were found in the silt. The disease was discovered in oysters that were lacking growth zones or that had died. Microphotos show the fungus’s ramified hyphae, 1.5–3 μm in diameter, with nodes and septa located in the fragments of affected oyster nacre matrix. They also show the close contact between two hyphae and the presence of “suckers” spreading from the mycelium to the bells of aragonite. We discuss the issue of a temporary symbiosis between the fungus and the microalga Coccolithus pelagicus, which could explain the adaptability of the fungus and the fact that its life cycle continues after the death of infected oysters.