Exploration of the daily circadian clock genes rhythmicity during early development of Eurasian perch

Circadian clocks provide organisms with an internal timing mechanism to synchronize their physiology and behaviour with the day–night cycle. Despite extensive research in model species, the ontogeny of circadian rhythms in non-model teleosts remains poorly understood. Here, we investigated the early development of daily core circadian clock gene rhythmicity in Eurasian perch using high-resolution sampling across the embryonic and larval stages under a 14 L:10 D photoperiod. We analysed the expression of five core clock genes ( clockb , arntl1a , per1b , cry1b , and cry2 ) using qPCR and assessed rhythmicity with cosinor analysis. During embryogenesis, no persistent rhythmic expression was observed, although several genes exhibited temporally coincident expression peaks and transient co-expression patterns shortly before hatching. Rhythmic expression emerged at 3 days post-hatching and persisted throughout larval development, becoming more consistent in early juveniles. Among the analysed genes, per1b , cry1b , arntl1a , and clockb maintained the longest rhythmic expression, whereas cry2 remained largely arrhythmic at the whole-body level. These findings provide the first evidence that daily rhythmic expression of core clock genes emerges after hatching in Eurasian perch under light-dark conditions, providing fundamental insights into the ontogeny of clock gene expression in a non-model teleost and a baseline for future ecological and aquaculture-related studies.