Shedding light on the functional significance of opsins: Insights from fish

Since the cloning of the first opsin in 1983, numerous opsin transcripts have been discovered across a variety of tissues and species. However, the functional significance of many of these opsins remains unclear, particularly for non-visual opsins that function outside the visual system, with their downstream processes largely unexplored. Fish offer a promising model to bridge this knowledge gap, due to their extensive opsin repertoire, genetic tractability, and evolutionary relevance. This review critically synthesises current understanding of the downstream processes regulated by opsins in fish, with a focus on evolutionary adaptations, the circadian clock and dermal colour change. Rather than being residual photoreceptors, non-visual opsins appear to be active mediators of physiological and behavioural responses. By connecting molecular mechanisms with development, behaviour, and environmental sensing, we propose that opsins serve as key components of sensory integration and adaptation. In doing so, this review reframes the study of opsins beyond vision, offering new perspectives on their widespread expression and functional versatility, ultimately advancing our understanding of photoreceptor biology.