Effects of green light on the growth of spotted halibut, Verasper variegatus, and Japanese flounder, Paralichthys olivaceus, and on the endocrine system of spotted halibut at different water temperatures.

We have previously shown that the somatic growth of barfin flounder, Verasper moseri, was promoted by green light. The present study was undertaken to elucidate whether growth-promoting effect of green light can be observed in other flatfishes and to understand the roles of endocrine systems in green light-induced growth. Herein, we demonstrated facilitation of growth by green light in the spotted halibut, Verasper variegatus, and Japanese flounder, Paralichthys olivaceus. Blue and blue-green light showed potencies that were similar to that of green light, while the potencies of red and white light were equivalent to that of ambient light (control). We also examined the effects of green light on growth and endocrine systems of V. variegatus at various water temperatures. Growth of the fish was facilitated by green light at four different water temperatures examined; the fish were reared for 31 days at 12 and 21 °C, and 30 days at 15 and 18 °C. Increase in condition factor was observed at 15 and 18 °C. Among the genes encoding hypothalamic hormones, expression levels of melanin-concentrating hormone 1 (mch1) were enhanced by green light at the four water temperatures. Expression levels of other genes including mch2 increased at certain water temperatures. No difference was observed in the expression levels of pituitary hormone genes, including those of growth hormone and members of proopiomelanocortin family, and in plasma levels of members of the insulin family. The results suggest that green light may generally stimulate growth of flatfishes. Moreover, it is conceivable that MCH, production of which is stimulated by green light, is a key hormone; it augments food intake, which is intimately coupled with somatic growth.

Effects of different green light intensities on the growth performance and endocrine properties of barfin flounder Verasper moseri.

We previously reported that the somatic growth of barfin flounder, Verasper moseri, was effectively stimulated by the green light compared to the blue and red lights. Herein, we report the effects of different green light intensities on the growth and endocrine system of the fish. Fish were reared in a dark room with light from a light-emitting diode (LED) at a peak wavelength of 518nm under controlled photoperiod (10.5:13.5h, light:dark cycle; 06:00-16:30, light) with three levels of photon flux density (PFD)-2 (low), 7 (medium), or 21 (high) µmol·m-2·s-1 at the water surface. The average water temperature was 10.2°C, and the fish were fed until satiety. The fish reared under high PFD of green light showed the highest specific growth rates, followed by the medium PFD group. Under high PFD, the fish showed the highest amount of melanin-concentrating hormone mRNA in their brains and insulin in plasma, while the lowest amount of growth hormone was observed in their pituitary glands. These results suggest that the green light stimulated the growth of barfin flounders in a light intensity-dependent manner in association with their central and peripheral endocrine systems. However, when the fish were reared in an ordinary room where they received both ambient and green LED lights, the fish under LED and ambient light grew faster than those under ambient light only (control). Moreover, no difference was observed in the specific growth rate of the fish reared under the three different green LED light intensities, suggesting that the growth was equally stimulated by the green light within a certain range of intensities under ambient light.

Chronic effects of light irradiated from LED on the growth performance and endocrine properties of barfin flounder Verasper moseri.

We investigated the effects of specific wavelengths of light on the growth of barfin flounder. The fish, reared in white tanks in a dark room, were irradiated with light from light-emitting diodes (LEDs) with peak wavelengths of 464nm (blue), 518nm (green), and 635nm (red) under a controlled photoperiod (10.5:13.5, light-dark cycle; 06:00-16:30, light). Fish were reared for four weeks in three independent experiments at three different water temperatures (averages of 14.9°C, 8.6°C, and 6.6°C). The fish irradiated with blue and green light had higher specific growth rates (% body weight⋅day(-1)) than fish irradiated with red light. Notably, green light had the greatest effect on growth among the three light wavelengths at 6.6°C. In the brains of fish reared at 6.6°C, the amounts of melanin-concentrating hormone 1 mRNA under green light were lower than those under red light, and amounts of proopiomelanocortin-C mRNA under blue and green light were higher than those under red light. No differences were observed for other neuropeptides tested. In the pituitary, no difference was observed in growth hormone mRNA content. In plasma, higher levels of insulin and insulin-like growth factor-I were observed in fish under green light than those of fish under red light. These results suggest that the endocrine systems of barfin flounder are modulated by a specific wavelength of light that stimulates somatic growth.

Molecular and functional characterization of opsins in barfin flounder (Verasper moseri).

Green light irradiation facilitates the somatic growth of barfin flounder (Verasper moseri). However, the V. moseri visual system, which may be associated with somatic growth by acting on the endocrine system upon exposure to this particular wavelength, remains largely unexplored. Herein, we characterized the visual opsin repertoire of V. moseri to understand the molecular basis underlying this effect. The five types of visual opsins that are found in vertebrates were cloned from RNA that was extracted from the eyes of V. moseri. Notably, V. moseri possessed one pseudogene (RH2-A) and two intact (RH2-B and RH2-C) copies of "green-sensitive" opsin genes. The wavelengths of maximum absorption spectra (λmax) for each of the reconstituted photopigments were 552nm for "red-sensitive" LWS, 506nm for RH2-B, 490nm for RH2-C, 482nm and 416nm for "blue-sensitive" SWS2A and SWS2B, respectively, 367nm for "ultraviolet-sensitive" SWS1, and 494nm for "dim-light sensitive rhodopsin" RH1. The λmax of SWS2A was longer than that of any other reported vertebrate SWS2 opsin. By measuring the expression level of these opsin genes with quantitative RT-PCR in 3-, 15-, and 27-month-old fish, we found that RH2-B and SWS2A were expressed at a constant level, whereas the expression of LWS, RH2-C, SWS2B, and SWS1 opsin genes decreased, and that of RH1 increased with age. Barfin flounders inhabit inshore waters at a young age and expand their habitat to deep sea areas as they age, and green light is relatively abundant in deep water compared to the lights of other wavelengths in shallow water. Our results indicate that gene repertoire and expression profile of the opsin genes of barfin flounder are adaptive to their habitat shift that occurs during development, with some opsins acquiring a distinct λmax.