Light characteristics are very specific in the aquatic environment. Fish vision and different light spectra perception are related to each species’ natural habit. Light is one of the main environmental conditions and can be easily manipulated in artificial rearing settings. Mucus-secreting goblet cells are the main regulators of digestion. In this study, we established whether the light spectrum (natural condition, full spectrum: green, 520 nm; red, 590 nm, and blue, 480 nm) influences growth performance and digestive activity related to mucus-secreting goblet cell activity in order to develop a good management protocol and optimal rearing system for nursery stage of Epinephelus akaara. For each light spectrum, fish (11.5 ± 0.2 g in mean initial body weight, 9.0 ± 0.1 cm mean initial total length) were reared 16 weeks under a flow-through system and fed commercial pellet diets twice daily. At the end of the experiment, the final body weights differed among the fish reared under different light spectra. The highest growth performance value and feed efficiency were observed in fish reared under the green light condition. Mucus-secreting goblet cell activity was significantly higher in the fish under green light condition than in the fish under the natural, red, and blue light conditions. Rearing of E. akaara under the green light condition had positive effects on fish growth performance and digestion. We recommend that the appropriate light spectrum for nursery stage of E. akaara is the green light condition from the perspective of growth performance and the synergistic effects of mucus-secreting goblet cells. However, longer light treatment periods are needed in future investigations to clarify the effects of light spectrum on each growing stage of E. akaara.

Melatonin has several known physiological functions, the main one being synchronization of daily and seasonal rhythms. In addition, melatonin has been reported to influence reproduction and behavioral rhythms with varying results depending on the species. To date, it remains unknown how this rhythm in locomotor activity is controlled endogenously, although there must be coordination of chemical and molecular drivers. However, the species is poorly characterized at molecular level with little sequence information available in public databases. The aim of study was to clarify involvement of endogenous melatonin rhythms and locomotor activity in day-night activity of the eel, Anguilla japonica which is an economically important but endangered species. The levels during daytime (zeitgeber time; ZT 6) were significantly (P<0.05) lower than those during nighttime (ZT 18). A similar pattern was persisted under DD conditions, whereas it disappeared under LL conditions and ocular melatonin levels remained low. Therefore, it is likely that ocular melatonin levels of the nocturnal eel reared under LD and DD conditions fluctuate in a daily/circadian manner and night-related physiological processes are dependent on eel locomotor activities which is a nocturnal species. We found that similar number of genes were differentially expressed between day (ZT6) and nighttime (ZT18), suggesting that during the nighttime also important in differential gene expression with daytime. This work also provides essential information for further studies investigating the molecular basis of daily/circadian system in this species.