Photoperiod effects on circadian rhythms and puberty onset in African catfish Clarias gariepinus

Photoperiod manipulation is routinely used in the aquaculture industry with the aim to enhance growth by manipulating the timing of reproduction in several commercially important temperate fish species. However, there are clear gaps in our understanding of how photoperiod is perceived by the circadian axis and transmitted to the brain to alter reproduction. Furthermore, due to the wide range of environments inhabited by fish, it is unlikely that one single organization exists. It is therefore believed that comparative studies of temperate species “models” with tropical species such as the African catfish (Clarias gariepinus) that adapted to different environments characterized by weaker light signals can help in such an aim. A number of studies were therefore performed in this PhD project to expand our knowledge on circadian biology and environmental physiological effects in African catfish. The first aim was to characterize the circadian melatonin system in this species (chapter 3). Results clearly showed that the control of melatonin production by the pineal gland was very different in the African catfish as compared to temperate species such as salmon and trout. Indeed, melatonin production appeared to mainly depend on light stimuli perceived by the eyes as opposed to salmonids where light directly perceived by the pineal gland regulates its own melatonin production within photoreceptors. The main evidence was obtained in ophthalmectomised fish that were unable to synthesize and release melatonin into the blood circulation during the dark period. This was the first time that such a decentralized organisation, similar in a way to the mammalian system, was found in any teleost species. In vitro results also supported such findings as African catfish pineal glands in isolation were not able to normally produce melatonin at night as usually seen in all other fish species studied so far. This indirectly suggested that pineal gland photo-sensitivity might be different in this tropical species. Further studies were performed to better determine the amount of light that can be perceived by the African catfish pineal gland depending on light transmittance though the skull (where the pineal gland is located). Surprisingly, it appeared that catfish cranium act as a stronger light filter than in other species resulting in lower light irradiance of the pineal gland. This could explain, although it still needs to be further confirmed, why African catfish photic control of melatonin produced by the pineal would have evolved differently than in temperate species. The work then focused on better characterizing diel melatonin production and endogenous entrainment through exposure to continuous photic regimes (continuous light, LL or darkness, DD) (chapter 4). Daily melatonin profiles of fish exposed to 12L:12D photoperiod (routinely used in indoor systems) confirmed low melatonin production at day (