Expression of melatonin synthesis genes is controlled by a circadian clock in the pike pineal organ but not in the trout.

Melatonin production in the pineal organ of pike is regulated by a circadian clock, while in trout, it is not.

• The pineal organ of most teleosts, including pike, shows a daily rhythm in melatonin production driven by an endogenous clock.
• In trout, melatonin production responds directly to darkness rather than being regulated by a circadian rhythm.
• Expression of mRNA for two key enzymes in melatonin synthesis is controlled by a clock in pike but remains elevated in trout.
• This difference suggests that a single circadian system governs melatonin synthesis in many teleosts, but may be disrupted in trout due to a possible mutation.

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