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

Abstract

Hoplosterum littorale (Hancock, 1828) shows a markcd circadian rhythm of food demand. Feeding activity is mainly nocturnal, with two pcaks, and is synchronized with the diel light cycle. We tested the cffect of infradian (period >28 h) lightldark cycles on this rhythm in fish wilh dcmand-fccdcrs: fish subjected to a 13.5 L122.5 D lightldark cycle for 8 days and a 25.5L/10.5D cycle for a further 8 days showed a strictly nocturnal fccding activity, a response that only passively reflected the cyclic altcrnation of light and dark.