Synchronizing effect of photoperiod on the dual phasing of demand-feeding rhythms in sea bass

Under natural environmental conditions, sea bass feeding rhythms are noctur nal in winter and diurnal the rest of the year. In this paper we describe t he effect of contracting and expanding photoperiods and two skeleton photop eriods (SP) on the dual feeding rhythms of sea bass (Dicentrarchus labrax L .). To this end, twelve animals were held individually with access to self- feeders. First, the lights on and lights off were progressively delayed and advanced respectively by one hour in group I (G1), and conversely in group 2 (G2), so that the fish were exposed from a light/dark (LD) 12L:12D cycle to 2:22 LD (G1) and DL (G2) cycles and finally 0.25:23.75 LD (G1) and DL ( G2). In the second experiment two SP's were used involving two light pulses separated by 12 hours, each pulse lasting 0.25 hours during the first two weeks and one hour during the succeeding two weeks. The results showed that diurnal and nocturnal sea bass tended to confine their feeding phase follo wing the contraction of the LD cycle. Both SP's failed to simulate a comple te photoperiod. In conclusion, the LD cycle appeared to drive the daily fee ding rhythms but, the photoperiod length did not itself control the inversi ons of nocturnal and diurnal fish, so that other factors, in addition to ph otoperiod, may he involved in the control of the annual rhythms of phase in versions in sea bass.