Behavioural thermoregulation modulates circadian clock gene expression in juvenile Atlantic salmon (Salmo salar)

Environmental temperature is a key factor regulating physiological processes and circadian organization in ectothermic vertebrates. In fish, behavioural thermoregulation optimize physiological performance and may contribute to the synchronization of biological clocks, however, its effects on central and peripheral circadian oscillators remain poorly understood. In the present study, we investigated the daily rhythms of thermal preference and their influence on circadian organization in juvenile Atlantic salmon (Salmo salar). Fish were maintained either at a constant temperature or in a multi-chamber thermal gradient, allowing free thermal selection throughout a 24 h light-dark cycle. Daily thermal preference patterns were quantified through video analysis, while the expression of key clock genes were analysed in the brain, proximal intestine, and distal intestine using absolute quantitative PCR and cosinor analysis. Fish exposed to the thermal gradient displayed robust daily rhythms of thermal preference, together with significant circadian oscillations in clock gene expression across all tissues examined. However, the effects of thermal selection were tissue-specific. In both intestinal regions, thermal choice enhanced clock gene expression, whereas the brain exhibited more complex responses, including phase shifts and differential modulation among molecular clock components. These findings suggest a high degree of plasticity in the central circadian network in response to self-selected thermal environments. Overall, our results demonstrate that behavioural thermal selection is an important modulator of both central and peripheral circadian clocks in juvenile Atlantic salmon. This study highlights the pivotal role of temperature as a biological timing cue and advances our understanding of how thermoregulatory behaviour shapes circadian physiology in ectothermic vertebrates.