Expression of circadian rhythm-related hormones in juvenile olive flounders (Paralichthys olivaceus) following exposure to total residual oxidant

Some substances used in aquaculture may be toxic to fish depending on their concentration. Here, we investigated the changes in circadian rhythm in olive flounders (Paralichthys olivaceus) exposed to total residual oxidant (TRO), a substance produced by the reaction of O-3 with Br- ions, at two different concentrations (20 and 40 mu g/L) for different periods (2, 6, 10, 14, 18, 22, 30, and 42 h) to identify the optimum concentration of TRO for aquaculture. We analysed mRNA expression and the activity of circadian rhythm-related hormones (period 2 (Per2), cryptochrome 1 (Cry1), melatonin receptor 1 (MT-R1), and melatonin) in the diencephalon and plasma. The levels of circadian rhythm-related hormones in fish exposed to 40 mu g/L TRO were significantly lower than those in the other groups. However, there were no significant changes in circadian rhythm-related hormone levels in fish exposed to 20 mu g/L TRO. These results indicate that 20 mu g/L TRO does not significantly affect the circadian rhythms in the olive flounder and that it is suitable for sterilisation purposes in aquaculture.