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In fish, the circadian clock represents a key regulator of many aspects of biology and is controlled by combinations of abiotic and biotic factors. These environmental factors are frequently manipulated in fish farms as part of strategies designed to maximize productivity. The flatfish turbot, Scophthalmus maximus, represents one of the most important species within the aquaculture sector in Asia and Europe. Despite the strategic importance of this species, the function and regulation of the turbot circadian system remains poorly understood. Here, we have characterized the core circadian clock genes, clock1, per1, per2 and cry1 in turbot and have studied their daily expression in various tissues under a range of lighting conditions and feeding regimes. We have also explored the influence of light and feeding time on locomotor activity. Rhythmic expression of the four core clock genes was observed in all tissues studied under light dark (LD) cycle conditions. Rhythmicity of clock gene expression persisted upon transfer to artificial free running, constant conditions confirming their endogenous circadian clock control. Furthermore, turbot showed daily cycles of locomotor activity and food anticipatory activity (FAA) under LD and scheduled-feeding, with the activity phase as well as FAA coinciding with and being dependent upon exposure to light. Thus, while FAA was absent under constant dark (DD) conditions, it was still detected in constant light (LL). In contrast, general locomotor activity was arrhythmic in both constant darkness and constant light, pointing to a major contribution of light, in concert with the circadian clock, in timing locomotor activity in this species. Our data represents an important contribution to our understanding of the circadian timing system in the turbot and thereby the optimization of rearing protocols and the improvement of the well-being of turbot within fish farming environments. [ABSTRACT FROM AUTHOR]