White background reduces the occurrence of staining, activates melanin-concentrating hormone and promotes somatic growth in barfin flounder

A pleuronectiform fish, the barfin flounder, Verasper moseri, is promising for aquaculture and resource enhancement in Northern Japan due to its high commercial value. A major problem in the aquaculture is staining (pigmentation of the skin on non-eyed side) under culture conditions since it diminishes the commercial value of the fish. Melanin-concentrating hormone (MCH) is a peptide hormone produced in the hypothalamus which turns the body of teleosts pale. Here, we examined the employment of MCH to prevent staining. Bolus injection of MCH (1 μg/g body weight (BW)) in barfin flounder turned the stained area pale. The occurrence of staining was suppressed by injection of MCH (1 μg/g BW, once a week, total 10 times). These results indicated that MCH affected the non-eyed side color in addition to the physiological color change on the eyed side. Then, we prepared juvenile fish having no staining by rearing with a tank containing Microceramic on the bottom and transferred them to white, yellow, or black tanks without Microceramic. Eight months later, the percent area of each fish that is stained in white or yellow tanks was significantly smaller than that of fish reared in a black tank. The number of MCH-immunoreactive cell bodies in the brain was significantly greater in the white tank fish than that of the black tank fish, suggesting that white background enhanced the secretion of endogenous MCH, by which occurrence of staining was prevented. Interestingly, both the total length and body weight of the white-reared fish were greater than those of the black-reared fish, respectively. The enhanced production of MCH might augment the food intake of barfin flounders as in mammals, so that the somatic growth of the fish is promoted.