Seasonal Reproduction in Fish:A Functional Interplay between the Pineal Organ and Photoperiods

Reproduction in most fishes is discontinuous or seasonal. Rhythms with a periodicity of one year represent major components in the adaptation of concerned fish to their environment. Environmental factors, such as light and temperature, play an important role in the synchronization of this rhythmic activity of reproduction. In vertebrates, synchronization is mediated through the system which is composed of sensors and circadian oscillators like the pineal organ, the lateral eyes and the suprachiasmatic nuclei of the hypothalamus. The fish pineal seems to be involved in the timing and control of reproduction. On the basis of several experimental evidences, it is believed that the pineal is able to translate environmental information (photoperiod, temperature) into rhythmic messages. To date, two kinds of signals have been clearly identified, a nervous signal and a neurohormonal signal, i.e., melatonin. Whereas the functional significance of the nervous signal is not yet clearly understood, there is increasing evidence that in at least temperate zone vertebrates, melatonin is the chemical messenger of photoperiod (Zeitgeber). In contrast to the neuronal message, elaborated mainly (if not exclusively) by the pineal of ectotherms, melatonin appears to be a conservative signal in the vertebrate phylum. In all the species investigated so far, melatonin production is high during nighttime and low during daytime. Despite this apparent homogeneity, the involvement of the pineal in the temporal organization of reproduction, as well as the mechanisms by which the photic information is processed in the control of gonadal germ maturation in fish have been the matter of great interest. Molecular mechanism involved in photoreceptor-induced pathway in fish is yet to be known, without which benefit from such information remains unveiled.