Diel feeding rhythms of greenback flounder Rhombosolea tapirina (Gunther 1862): the role of light-dark cycles and food deprivation

The role of light-dark (LD) cycles and food deprivation in the regulation of diel feeding rhythms in greenback flounder Rhombosolea tapirina (Gunther 1862) was examined. A computerized monitoring system was developed to record uneaten pellets, and food intake was estimated as the difference between pellets presented and those uneaten. Three groups of five fish each (75-281 g) were exposed sequentially to the LD cycles: LD 12:12. LD 12:12 (scotophase advanced by 9h), LD 6:18, DD, LD 6:6:6:6, LL and LD 12:12 (return to the initial cycle). Illumination was 120 lux at the water surface (complete darkness at scotophase), and water temperature was 15 ± 1°C. Fish fed mainly during the light hours under LD 12:12 and resynchronized to the scotophase-advanced LD 12:12 regime after about 12h. Feeding started at the onset of light and extended to the early scotophase under LD 6:18. A circadian feeding rhythm was detected in fish under DD and LD 6:6:6:6 regimes. Under the LL regime, two groups of fish displayed arrhythmic feeding patterns and did not resynchronize to LD 12:12 for at least 13 days. In contrast, the third group of fish exhibited a circadian feeding rhythm under the LL regime and immediately re-entrained to LD 12:12. Fish were deprived of food for a single period of 26-57 h once feeding patterns had been established under LD 12:12, LD 12: 12 (scotophase advanced by 9 h) and DD regimes, and feed delivery was resumed during the scotophase or subjective night; the timing of feeding rhythms was shifted by the reintroduction of feed, but progressively resynchronized to the LD cycles. The results suggest that the greenback flounder is a diurnal feeder, that a LD 12:12 cycle is a potent environmental cue to entrain circadian feeding rhythms, and that a biological clock is involved in the timing of feeding.