Light is a key environmental factor that synchronizes all life-stages of fish, from embryo development to sexual maturation. The present review covers information gathered in recent years to emphasize the importance of photoperiod and melatonin in fish physiology. It is well known that melatonin is a much conserved feature in vertebrates that plays a central role in the entrainment of daily and annual physiological rhythms. In this article we have reviewed about the photoperiod and its importance in fish reproduction, pineal gland structure and function, sites of melatonin production in fish, role of AANAT enzyme in melatonin production, endocrine regulation of melatonin production and its role in neuroendocrine regulations of reproduction, growth, feeding behavioural responses and its importance in aquaculture.
