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The rhythmic production of melatonin is governed by intrapineal oscillators in all fish species so far investigated except the rainbow trout. To determine whether the latter represents an exception among fish, we measured in vitro melatonin secretion in pineal organs of nine wild freshwater and six marine teleost species cultured at constant temperature and under different photic conditions. The results demonstrate that pineal organs of all species maintain a rhythmic secretion of melatonin under light:dark cycles and complete darkness, and strongly suggest that most fish possess endogenous intrapineal oscillators driving the rhythm of melatonin production, with the exception of the rainbow trout.

Serotonin is believed to play an important role in the neuronal development of various invertebrates and mammals, but only one study has yet investigated the development of serotoninergic neurons in the brain of a teleost. In the present study, we investigate the development of serotoninergic neurons in the brain of the brook trout, Salvelinus fontinalis, by immunohistochemistry, from fertilisation to the resorption of the yolk sac and in juveniles for reference. Maintaining trout embryos and larvae in 9L: 15D or in complete darkness made no difference in the appearance and distribution of serotonin immunoreactive (5-HTir) neurons. On day 56, the first 5-HTir perikarya, indicative of the primordia of the nuclei raphe medialis/dorsalis, have stained in the presumptive isthmus region. At hatching time (days 95–115), all the 5-HTir nuclei reported in the juvenile are present, except for the population observed in the lobus inferior hypothalami. We observed that the nucleus recessus lateralis stained on day 58, the raphe caudal nucleus on day 60, the nuclei raphe pallidus and obscurus on day 65, the area praetectalis on day 80, and the nucleus recessus posterioris and the anterior group of the nucleus recessus lateralis on day 92. At the complete resorption of the yolk sac (days 175–190), the nucleus reticularis paragigantocellularis, the nuclei raphe pallidus and obscurus and the caudal raphe of the brainstem are very weakly immunoreactive, and in juveniles, only one caudal immunoreactive nucleus is still present. A comparison of our result with those previously reported in the three-spined stickleback, Gasterosteus aculeatus, suggest a similar distribution of 5-HTir neurons in the brain of both species but also reveal species differences in the sequence of appearance of 5HT immunoreactivity in the different neuronal populations.