Interactions of diurnal and tidal rhythms in the fiddler crab, Uca pugnax.

Abstract

1. Locomotor activity of the fiddler crab, Uca pugnax, was recorded in constant illumination following a variety of light regimes and in natural illumination.2. When pre-treatment caused a shift in the phase relations of the diurnal rhythm, a shift of activity peaks with respect to time of low tide was also observed; the shift in phase of the tidal rhythm was about the same number of hours as that of the diurnal rhythm.3. Under recording conditions of constant illumination the rate of progression of activity peaks showed systematic variations such that periods calculated for a few days would range from 24.0 hours to 25.8 hours. The apparent period was a function of the phase of the diurnal rhythm in which the peak occurred.4. When recording was carried out under conditions of natural illumination, the tidal rhythm showed a consistent 24.8-hour period.5. The imposition of a 24.75-hour cycle by four-hour supplemental light periods yielded an apparent period of about 25.5 hours for 11 successive days. This is interpreted as indicating successive daily phase shifts of both diurnal and tidal rhythms.6. The imposition of 24.75-hour cycles by means of two-hour supplemental light periods failed to produce any consistent effects on rate of peak progression.