Ontogenetic expression rhythms of visual opsins in senegalese sole are modulated by photoperiod and light spectrum.

In the fish retina, rods and cones are responsible for nocturnal vision and colour perception, respectively, and exhibit a repertoire of light-sensitive opsin photopigments that permits the adaptation to different photic environment. The metamorphosis of Senegalese sole determines a migration from pelagic to benthic environments, which is accompanied by essential changes in light intensity and spectrum. In this paper, we analysed the daily expression rhythms of rod opsin and five cone opsins during sole ontogeny in animals maintained under light-dark cycles of white (LDW), blue (LDB), red (LDR) and continuous white (LL) lights. We showed that the expression of visual opsins at early stages of development was enhanced under LDB in relation to LDW, LDR and LL. Moreover, daily rhythms of opsins were more robust under LDW and LDB conditions, in particular, before and after metamorphosis. A shift in the phase of opsin rhythms was observed between hatching and pre-metamorphosis. Metamorphosis was accompanied by a transient loss in the expression rhythms for most of the opsins, which were significantly influenced by light photoperiod and spectrum. In LDR, transcript levels and rhythms were markedly affected for the majority of the opsins analysed. Under LL, most of the opsins examined exhibited endogenous rhythms, although amplitudes and acrophases changed considerably. To the best of our knowledge, this is the first study on the daily expression rhythms of visual opsins during the ontogeny of a metamorphic flatfish and further emphasises the importance of using natural lighting conditions for proper development of Senegalese sole.