Studies of the Daily Rhythmicity of the Fiddler Crab, Uca. Modifications by Light

Abstract

The diurnal rhythm of the chromatophores of Uca is inhibited by constant illumination. A basic physiol. rhythm persists in the animals even when the chromatophoric expression of the rhythm has been thus inhibited. It is possible to reverse the phases of the diurnal rhythm of the chromatophores by illuminating the animals by night and keeping them in the dark by day. Under conditions of reversed illumination there is no evidence that the basic physiol. rhythm does not come completely into phase with the imposed rhythm. Exposure of the animals to 6-hr. periods of illumination alternating with 6 hrs. of darkness results in the rhythm of the chromatophores becoming 6 hrs. out of phase with a previously observed rhythm. Such a rhythm, 6 hrs. out of phase, may persist for several days in constant darkness and then gradually change to a previously established rhythm if the last period of illumination occurred when the animals were entering the night phase; but it shows no persistence whatsoever when the last period of illumination occurred when the animals were entering the day phase. All those cases of alteration in rhythmicity in which a clear answer is possible suggest that, when a rhythm is altered, it is altered abruptly by a single light change or by a single period of illumination. Analysis of all the data has led to the adoption of a hypothesis of 2 centers of Ixhythmicity within the animal, each one capable of having its rhythm altered independently of the other, and with one of the centers[long dash]that one having a more persistent rhythmicity[long dash]influencing the other.