The Role of Endogenous Rhythms in Vertical Migration

Photographic recording at hourly intervals of a group of animals contained in a small tank kept in continuous darkness has revealed a 24 h cycle of locomotor activity in Daphnia magna and in Calanus helgolandic A change in periodicity of the rhythm in Daphnia kept under continuous illumination suggests that the cycle is not determined by external stimuli The phasing of the cycle differs in the two animals, but in both there is an increase in activity during the hours of darkness. In Calanus and possibly also in Daphnia, the cycle of activity is absent in the winter months.