Effects of selected light wavelengths on the transcript levels of photoreceptors and growth-related hormones and peptides in the Malabar grouper Epinephelus malabaricus

Fish growth is influenced by environmental cues, including light conditions. The duration of exposure (photo -period), the intensity of exposure (illumination), and quality (light spectrum) play crucial roles in maximizing growth. This study aimed to clarify how selected light wavelengths are perceived and transduced as an internal signal to stimulate growth of the Malabar grouper Epinephelus malabaricus, an important aquaculture species in Asian countries. Four opsins (RH1, RH2, SWS2, and LWS) were selected from the database of photoreceptors of groupers, and reverse-transcription polymerase chain reaction revealed that these opsins were expressed in the eyes and brain. When fish were reared under conditions of long-day (light:dark = 14:10) with red (peak at 632 nm), green (519 nm), and blue (465 nm) LED and natural light, the specific growth rate was higher (P < 0.05) in fish exposed to blue LED light than to red LED light. Rearing fish under shorter light wavelengths (blue or green) resulted in high transcript levels (P < 0.05) of RH1 and RH2 in the eyes (retina) and of RH1 and SWS2 in the brain (diencephalon), suggesting that certain light wavelengths activate photoreceptors in light perceptive or-gans. The transcript levels of growth hormone (GH) and insulin-like growth factor (IGF-I) increased significantly (P < 0.05) in the pituitary and liver of fish reared under blue LED light, respectively. Moreover, similar increases (P < 0.05) in the transcripts of neuropeptide Y (NPY) and pro-opiomelanocortin were observed in the brain of fish reared under blue LED light. We concluded that blue light perceived by visual opsins in the eyes and brain activates the growth of the endocrine axis and its interaction with NPY. Consequently, rearing the Malabar grouper under specific light wavelengths helps maximize their growth.