Entrainment of the circadian rhythm of food demand by infradian cycles of light-dark alternation in Hoplosternum littorale (Teleostei)

Hoplosterum littorale (Hancock, 1828) shows a marked circadian rhythm of food demand. Feeding activity is mainly nocturnal, with two peaks, and is synchronized with the diel light cycle. We tested the effect of infradian (period > 28 h) light/dark cycles on this rhythm in fish with demand-feeders: fish subjected to a 13.5 L/22.5 D light/dark cycle for 8 days and a 25.5 L/10.5 D cycle for a further 8 days showed a strictly nocturnal feeding activity, a response that only passively reflected the cyclic alternation of light and dark.