Twilight and day length affects the seasonality of entrainment and endogenous circadian rhythms in a fish, Couesius plumbeus

Abstract

Twilight and day-length portions of the light-dark cycle determine the seasonal course of (i) the phase relationship (ψ) between activity and the daily environmental light–dark cycle, (ii) the duration of activity (α), and (iii) the circadian period (τ) under constant darkness of the lake chub (Couesius plumbeus). With fish held under seasonally appropriate light–dark cycles with twilight (LD + t), the onset of diurnal activity occurred during dawn and its timing (ψonset) followed a bimodal annual pattern that was correlated with seasonal changes in the duration and physical characteristics of twilight. The end of activity occurred during dusk (ψoffset) and followed a unimodal annual pattern that was determined by day length. α followed a sigmoidal annual pattern under LD + t. The circadian period underwent significant seasonal changes with maximum and minimum τ values occurring during summer and winter, respectively. Fish that were entrained to rectangular light–dark cycles that excluded twilights (LD) failed to show any seasonal changes in ψ and τ and the sigmoidal relation between α and day length was absent.