Light sensing in fish embryonic cells: A circadian analysis of visual and non-visual opsin expression using a seabream embryonic stem cell line

Daily and seasonal light-dark cycles are key environmental cues that have driven the evolution of circadian clocks, allowing them to synchronise with local time and anticipate responses to predictable events. This synchronisation requires efficient light sensing and transduction mechanisms operated by photosensitive cells located in central structures such as the retina and/or the pineal gland. However, fish embryos exhibit light sensitivity even before these structures develop and fish peripheral tissues also contain opsin photopigments and circadian pacemakers. Building on this, we previously showed that SAEC-H7, a monoclonal embryonic stem cell line from gilthead seabream, responds directly to light and possesses a light-entrainable clock. In this study, we aimed to identify the repertoire of visual and non-visual opsin expressed in SAEC-H7 and analyse their daily and circadian expression patterns. SAEC-H7 express five out of ten visual opsins ( sws1 , sws2a , sws2b , rh2.4 and lws ) and eleven out of twenty-five non-visual opsins ( opn3 , opn4m1 , opn4m2 , opn4m3 , opn5 , opn6a , opn7a , tmt1a , tmt1b , tmt2a and tmt2b ) identified in seabream transcriptomic and genomic databases. Most of these opsins show daily variations and/or rhythms in expression, with peaks at day onset, and a subset of them are under clock control, exhibiting robust circadian expression rhythms even in the absence of external light cues. These findings highlight the intrinsic light sensitivity and rhythmicity of seabream embryonic cells, suggesting that fish embryos can detect light and initiate circadian responses at very early stages. This temporal regulation of light sensitivity should be considered in aquaculture protocols and fish cell research. • SAEC-H7 cells express five visual and eleven non-visual opsins. • Ten of the sixteen opsins present in SAEC-H7 are sensitive to blue light. • Most of these opsins exhibit daily variations and/or rhythms in expression. • A subset of these opsins showed robust circadian rhythms under 24-h dark conditions. • Fish embryonic cells can detect light and initiate circadian responses at early stages.