Circadian rhythms of feeding activity in sea bass, Dicentrarchus labrax L.: dual phasing capacity of diel demand-feeding pattern.

The nocturnal versus diurnal feeding patterns of sea bass under controlled experimental conditions were studied in order to investigate the existence of such a dualistic feeding behavior. The animals (six groups of 4 animals and 8 single fish) were held in tanks filled with recirculating salt water and installed in a "chronolab" under constant conditions (23.5 degrees C and 2.4% salinity). The fish were given access to self-demand feeders and first exposed to a photoperiod regime of 12:12 (12 h light, 70 lx, and 12 h dark, complete darkness) and then to light:dark (LD) pulses (40 min light, 40 min dark). The LD 12:12 cycle was reversed by doubling the light period in day 16, and reversed again in day 27 by doubling the dark period. The circadian rhythm of food demand was strongly synchronized with the LD cycle, and fish exhibited both diurnal and nocturnal patterns. In most fish, the shift of the feeding rhythm to the new LD cycle was very fast for each reversal (bringing forward or delaying their feeds 12 h), indicating a weak participation of an endogenous circadian rhythm. However, when submitted to LD pulses, fish began to free-run with a periodicity of about 23 h and kept feeding in the light or dark phase according to their prior behavior. The existence of a dualism in the diel feeding pattern in sea bass was thus clearly demonstrated and it appeared that the diurnal and nocturnal behavior did not depend exclusively on a circadian phase inversion of the feeding rhythms as this pattern of behavior was enhanced under ultradian LD pulses. The ecological implications of such dual capacity and the switch from one type of phasing to another are hitherto unknown and need further field and laboratory investigation.