Analysis of visual opsin gene expression in Japanese eels upon sexual maturation and exposure to different wavelengths of light

Fish inhabiting diverse photic environments, from the deep sea to shallow freshwater, offer a valuable model for studying visual receptor characteristics and spectral adaptation mechanisms. This study investigated changes in visual opsin transcript levels in Japanese eels (Anguilla japonica) during sexual development and exposure to light conditions simulating their life cycle. Tissue-specific expression analysis revealed the predominance of four opsin genes (dso, fwo, rh2, and sws2) in the eyes. Sexual maturation using human chorionic gonadotropin was confirmed by increased gonadosomatic index values and histological changes in the testis. Upon confirming dynamic day/night shifts in the retinal pigment epithelium layer, the effects of sexual maturation and light on opsin gene expression were analyzed via exposure to light-emitting diodes of different wavelengths. Immature eels showed increased sws2 expression under blue and green light during the day, with dominant dso, fwo, rh2, and sws2 expression in darkness and green light during the night. Conversely, mature eels showed elevated expression levels of dso and rh2 under white light and dark conditions, implicating their broad-spectrum and blue-shifted adaptation to deep-sea migration. Notably, cortisol levels in sexually mature eels were significantly lower under dark conditions, underscoring the ecological significance of low light during reproductive migration. Overall, these findings highlight the developmental plasticity of opsin expression and stress responses driven by ecological requirements and life cycle changes, enhancing our understanding of visual and physiological adaptations in migratory fish and providing insights into the evolutionary mechanisms shaping aquatic sensory systems.