An Investigation of the Pathology Associated with Mass Mortality Events in the Cultured Japanese Pearl Oyster Pinctada fucata martensii at Four Farms in Western Japan

Abstract

The epidemiological and histopathological characteristics of the mass mortality of cultured Japanese pearl oysters Pinctada fucata martensii were investigated. Rearing experiments with Japanese pearl oysters in farms revealed that the mass mortality occurs as a regular annual event in particular farms in western Japan. Diseased oysters had marked atrophy and red-brown discoloration of the soft parts of the body. Light microscopy revealed that the epithelia of the stomach, the ducts of the digestive diverticula (DD), and the DD themselves showed marked blebbing and necrosis to varying degrees during earlier stages of the disease. At advanced stages, muscle fibers of the adductor muscle, heart, mantle, and other parts of the body and the connective tissues of various organs involving the vascular system also exhibited considerable atrophy and necrosis. There were no remarkable changes in the branchial and pallial epithelia. No viral, bacterial, mycotic, or parasitic causative organisms were found in diseased oysters. The results of a case study of mass mortality at one farm suggested that there is some causal relationship between outbreaks of this disease and the existence of neighboring fish farms. These findings suggest that the mass mortality is not due to an infectious disease. We discuss pathological features and possible causes of this disease.