Shusaku Takagi

Hemolytic suppression roles of taurine in yellowtail Seriola quinqueradiata fed non‐fishmeal diet based on soybean protein

To elucidate the hemolytic suppression roles of taurine and the necessity of dietary taurine supplementation in yellowtail Seriola quinqueradiata fed a diet without fishmeal, juvenile fish with an initial body weight of 250 g were fed for 40 weeks in floating net cages on soybean protein diets supplemented with 0, 3.0, 4.5 and 6.0% taurine. Taurine concentration of the experimental diets were 0.03, 33.9, 52.8 and 71.6 mg/g, respectively. On the 21st week, fish fed the taurine unsupplemented diet had inferior growth and feed performances, higher death, and there were incidences of green liver and hemolytic anemia. In this group, hepatic and plasma taurine concentrations, serum osmolality and osmotic tolerance of erythrocytes (EC50 value) were significantly lower, and plasma hydroperoxide concentration was markedly higher than in the taurine supplemented groups. These conditions markedly improved corresponding with the increase of dietary taurine concentration. These results indicate that taurine plays a role in hemolytic suppression through osmoregulation and biomembrane stabilization in fish. In addition, it is suggested that yellowtail requires dietary taurine as an essential nutrition for maintaining physiological condition normally.

Efficacy of taurine supplementation for preventing green liver syndrome and improving growth performance in yearling red sea bream Pagrus major fed low-fishmeal diet

This study was performed to evaluate the efficacy of taurine supplementation for preventing green liver syndrome and improving growth performance in red sea bream Pagrus major fed a low-fishmeal (FM) diet. Yearling red sea bream were fed for 34 weeks on low-FM diets either supplemented with taurine, or without taurine, and the tissue taurine and bile pigment concentrations were measured. Compared to the fish fed the FM diet, fish fed the low-FM diet without taurine supplementation resulted in inferior feed performances and higher incidence of green liver related to the morphological transformation of the erythrocytes. In these fish, the hepatopancreatic taurine concentration was significantly lower and hepatopancreatic biliverdin concentration was high compared to the fish fed the FM diet. These parameters were markedly improved by taurine supplementation of the low-FM diet and were similar in levels to the fish fed the FM diet. These results indicate that green liver appearance and inferior feed performances of red sea bream fed the low-FM diet without taurine supplementation were caused by dietary taurine deficiency, and indicate the requirement of taurine supplementation to low-FM diets for red sea bream.

Recently Emerged Myxosporean Encephalomyelitis of Cultured Yellowtail Seriola quinqueradiata in Japan

Encephalomyelitis associated with developmental stages of a myxosporean parasite has recently occurred in cultured yellowtail Seriola quinqueradiata from western Japan. Although mature spore stages of the parasite were not found in the spinal cord, the myxosporean was molecularly identified as Myxobolus spirosulcatus by the 18S rDNA analysis and in situ hybridization and has been previously reported as a non-pathogenic parasite infecting the bile duct of yellowtail. However, the causal relationship between the encephalomyelitis and M. spirosulcatus is still controversial. Typical signs of the disease were reduced feeding, erratic and abnormal swimming, skin ulceration from the jaw to the abdomen, and redness of the brain. Histopathologically, this disease was characterized by 1) gliosis and multi-focal glial nodules, 2) nerve cell necrosis with neurophagia, 3) blood congestion and hemorrhages in the central nervous tissue, 4) swollen eosinophilic nerve fibers and degenerative axons. In the present study, we conducted further histopathological observations and tried to determine the disease mechanisms in relation to water temperature and feeding regimes. It was indicated that developing plasmodia directly caused the gliosis and glial nodules in the nervous system. Fluctuating water temperature was suggested to be partly responsible for disease outbreak. Detection rates of M. spirosulcatus spores in the bile were significantly higher in the constant temperature groups than those in the fluctuating temperature groups. Further studies on the biology of M. spirosulcatus are required to implement a management strategy to the myxosporean encephalomyelitis of yellowtail.