Clock and clock-related gene expression is light responsive in the Atlantic salmon () embryo.

Light and photoperiod vary in a predictable manner throughout the daily and annual cycle that is utilized by organisms to direct processes of living. The aquaculture industry applies light to manipulate salmon development, but the effects have not yet been thoroughly investigated in early ontogeny. Here, salmon eggs and larvae were subjected to three different light regimes (continuous dark, continuous light and compressed simulated natural photoperiod [LD] to provide calendar time information). The expression of eight clock- and melatonin-related genes (, , , , , , ) was examined through one daily cycle before eyeing, after eyeing, and before start-feeding. and showed indications of being maternally deposited, and expression increased for most genes through development. All genes showed clear differences in expression between light regimes, and rhythmically expressed genes were more abundant and with stronger rhythms under LD regime. was rhythmically expressed before eyeing, and all genes were rhythmically expressed under LD at start-feeding. Interestingly, at this time, the positive, negative, and stabilizing arm of the clock peaked simultaneously along with the melatonin-related genes mid-photophase. These results implore greater attention to the lighting conditions used during early development, as different lightings could have lasting effects.