LIGTH SYNCHRONIZATION OF THE DAILY SPAWNING RHYTHMS OF GILTHEAD SEA BREAM (SPARUS AURATA L) KEPT UNDER DIFFERENT PHOTOPERIOD AND AFTER SHIFTING THE LD CYCLE

Reproduction in most fish is typically a seasonal proces, as spawning takes place usually ay a given time of the year, depending on the reproduction strategy of the species, to ensure maximal survival of offspring. Nevertheless, fish reproduction cannot be considered and exclusively annual phenomenon, because spawning may also show daily rhythmicity. In this study, we investigated the existence of a the daily spawning rhythm in gilthead sea bream exposed to different ligth-dark (LD) cycles and at different times of the year using an automatic and programmable egg collector. Flotability and fertilization rates were analyzed at different times throughout the 24 h. The results showed a daily spawning rhythm with spawning occuring from 14:30 to 21:30 h, with the acrophase (peak time) being 18:29 and 18:08 h in fish exposed to an artificial photoperiod of 9L:15D in winter and in spring. respectively. Nevertheless, in fish exposed to a natural photoperiod of 12 L:12D in spring, the acrophase of the rhythm was recorded later at 21;28 h. The average fertilization rate was 87 % and no significant diffrences were found between the diffrent hours of spawning. Moreover, when the LD cycle (9L:15D) was shifted by 12 h, the daily spawning rhythm gradually resynchronized,resuming a stable phase-relationship after 4-5 transient days, which is characteristic of a endogenous circadian rhythm. Our results clarly demonstrated the existence of a 24 h period of spawning in gilthead sea bream, with a peak anticipating the forthcoming nigth, and its capacity to gradually re-synchronize after a 12 h shift in the LD cycle, pointing to the endogenous nature of the rhythm.The findings will the valuable for better understanding the reproductive physiology of this species and for optimizing the protocols of egg collection and larvae production in aquaculture.