Photoperiod modulates ontogenetic opsin expression in the endangered salmonid Brachymystax tsinlingensis during early development

Photoperiod orchestrates key developmental events in salmonids, like smoltification. The endangered, light-sensitive Brachymystax tsinlingensis faces a juvenile production bottleneck, possibly mediated by visual opsins. To optimize light conditions from an opsin plasticity perspective, we quantified ontogenetic expression of all five visual opsins ( RH1 , RH2 , SWS1 , SWS2 , LWS ) from embryos to juveniles (59 days) under six photoperiods. Results revealed distinct, photoperiod-sensitive expression patterns: RH1 , SWS2 , and LWS expression was delayed, and opsin dynamics varied significantly across treatments. Despite this, all opsins increased throughout development regardless of photoperiod, with RH1 , SWS1 , and LWS being most abundant, indicating a strong genetic component. This demonstrates precise illumination requirements for early development. Rearing under complete darkness or natural-like cycles supports full opsin repertoire development, aiding conservation of this endangered species.