STUDIES OF THE DAILY RHYTHMICITY OF THE FIDDLER CRAB, UCA. MODIFICATIONS BY PHOTOPERIOD

Abstract

The persistent daily rhythmicity of the black chromatophores of Uca pugnax resulting in a darkening by day and lightening by night was shown to be modifiable by length of the daily light period in such a manner that the magnitude of the dark phase was increased by longer light periods and decreased by shorter ones. The influence of daylength was seen to consist of two components: a gradual alteration of a transient character persisting only so long as the crabs were subjected to the daily light changes and a more basic alteration which persisted when the animals were later placed in continuous darkness. The relations of the phases of the rhythm to the day-night phases of the solar cycle were shown also to be readily modifiable in a persistent manner by the time of day the daily light periods were given. The data indicated clearly that two centers of rhythmicity within the animal were involved. The results of the experiments were readily interpretable if one of these centers was postulated to be altered in a persistent manner by the length of the photoperiod and the second to be altered in a transient manner by light and by darkness and, when other factors were equal, by the first center.