Melatonin is involved in the transduction of light information and the photoperiodic control of many important physiological functions in fish. Although artificial photoperiods have been used to improve fish growth and manipulate reproduction, there is little information about the characteristics of light 'quality'. In this paper we describe the effects of a light pulse in the middle of the dark phase on plasma and ocular melatonin in European sea bass. We first determined the light intensity necessary to elicit a melatonin response using white light of varying intensities (0.6-600 mu W/cm(2), experiment 1). Secondly, we tested the effect of the light spectrum on melatonin production using three differently coloured lights (half-peak bandwidth=434-477, 498-575 and 610-687 nm for the blue, green and red lamp, respectively, experiment 2) and, finally, we determined the effect of light orientation (downwards directed versus upwards directed, experiment 3). The results show that the minimum light intensity needed to inhibit or stimulate melatonin levels in both plasma and the eye was 6.0 mu W/cm(2). A linear correlation was found between the logarithm of light intensity and the relative inhibition. In addition, the blue wavelength was more effective in decreasing melatonin levels in the former and increasing the levels in the latter. Nevertheless, red light at sufficient intensity proved effective at significantly suppressing circulating melatonin. Downwards light had a greater effect than upward-directed illumination in suppressing plasma melatonin. In conclusion, the results point to the importance of giving proper consideration to the characteristics of light, to adequately control melatonin production and its related physiological processes.

23This paper investigates the impact of different thermo-and photo-cycles of 24 distinct wavelengths on Solea senegalensis larvae from day 1 to 30 post-hatching. In 25 experiment 1, larvae were exposed to 12 h light:12 h dark (12L:12D) cycle and (A) 26 constant temperature (20.7ºC), (B) thermocycle of 12h thermophase: 12h cryophase, 27 22.1ºC day: 19.0ºC night (referred to as TC) or (C) 12h cryophase: 12h thermophase, 28 19.2ºC day: 22.0ºC night (referred to as CT). In experiment 2, larvae were kept under 29 constant temperature (20.8ºC) and exposed to (A) continuous light (LL), (B) continuous 30 darkness (DD), and LD 12L:12D cycles of (C) white light (LD W ), (D) blue light (LD B ) 31 or (E) red light (LD R ). The sole larvae achieved the best performance, and showed 32 fastest development and lowest degree of deformity under natural thermocycle 33 conditions (TC) with a deformity percentage of 31.1% and LD B cycles with 27.7% of 34 malformation, conditions which were nearest their natural aquatic environment. Larvae 35 reared under TC started eye migration at 9 day post-hatching (DPH), while larvae 36 exposed to CT started eye migration at 11 DPH. In larvae under the LD B treatment the 37 migration of the eye started earlier than in the other treatments (9 DPH), while larvae 38 reared under LL and DD photoperiods died before metamorphosis. These findings 39 highlight the importance of light and temperature cycles during the early development 40 of Solea senegalensis larvae, which should be taken into consideration in experimental 41 or rearing protocols.42

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A specific chronology for puberty and changes at the brain-pituitary-gonad axis for sea bass are reviewed. Recent findings demonstrate that the Kisspeptin system, gonadotropin releasing hormones, follicle stimulating hormone, 11-ketotestosterone, and leptin are potential candidates for the onset of puberty of this fish species, stressing the importance of the daily and annual rhythms of some of these hormones. Environmental control of puberty is also reviewed, specifically the manipulations of constant photoperiods for altering or even suppressing the onset of puberty in sea bass. Recently, a possible narrow sensitive period for suppressing gonadogenesis in sea bass has been identified.