Muscle Necrosis of Cultured Kuruma Shrimp (Penaeus japonicus)

Abstract

Since 1981, kuruma shrimp (about 11-14 cm in body length) exhibiting a few numbers of whitish-opaque patches in the abdominal muscles have been noticed at harvesting time, from autumn to winter, at culture farms in Yamaguchi Prefecture. These shrimp are rated a low grade, because they can not bear transportation to the market.
Histopathological examination revealed that the skeletal muscles of the white areas were necrotic. The histopathological changes in the skeletal muscles were classified into three stages. Stage I; Muscle fibers, especially of the flexor of the 2nd to 5th abdominal segments, are nectrotic with fusion and cross splitting of myofibrils or separating from myoseptum. Hemocytic infiltration is often observed at the necrotic foci. Stage II; The necrotic muscle fibers are diminished. Proliferation of fibrocytes and production of collagenous fibers are extensive. Stage III; The necrotic muscle fibers have disappeared and are replaced by connective tissue.
Environmental or physiological stress due to high water temperature, low dissolved oxygen and overcrowding has been suspected to produce muscle necrosis in shrimp. But no relation was found between the occurrence of this disease and the water quality of the kuruma shrimp culture ponds in the present study. Histopathological examination and etiological investigation suggest that the possible cause of this disease may be related to violent movement for escaping from electric net and other nets, or in the nets in overcrowding condition, used for harvest.