Seasonal changes in the circadian period of the lake chub, Couesius plumbeus

Abstract

Systematic seasonal and annual changes are present in the length of the circadian period, τ, of the lake chub, Couesius plumbeus. Maximum and minimum period values of 28.1 and 24.8 h were recorded under constant darkness from fish obtained at the times of the winter and summer solstices, respectively. There was an asymmetric annual pattern in the τ values obtained from the increasing and decreasing photoperiods of spring and fall. Annual variations in τ were relatively independent of temperature. They were interpreted as being aftereffects of natural photoperiodic entrainment. Relations of annual changes in the length of the period to models of circadian organization and entrainment are examined.