INTERACTIONS OF DIURNAL AND TIDAL RHYTHMS IN THE FIDDLER CRAB, UCA PUGNAX

Abstract

Locomotor activity was monitored under conditions of natural illumination, constant illumination following a series of LD regimes, and constant illumination plus supplemental light scheduled on a 24.75 hour cycle. Under recording conditions of constant illumination the time of maximal activity relative to time of low tide in the area of collection was found to vary systematically as a function of time of day and thus as a function of phase relations between biurnal and tidal rhythms. Such systematic variations are absent under recording conditions of natural illumination. LD schedules which shifted the diurnal rhythm shifted the tidal rhythm in the same direction and to approximately the same extent. Four-hour supplemental light periods, initiated 45 minutes later each day (24.75 hour cycles) yielded an activity rhythm with a 25.5 hour period. The apparent inconsistency is explained by assuming that the light periods induced successive daily shifts in the diurnal rhythm with accompanying shifts in the tidal rhythm.