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Abstract
The orientational response to light of the intertidal cranny crab, Cyclograpsis, was tested in three separate experiments and found to describe a circadian rhythm. In constant ambient illumination, crabs cycled between the lighted and darkened ends of a “phototactorium”; with a period of 24.9 hours or slightly longer. When in the natural habitat such a clock‐driven response would help keep the crab hidden in dark nooks under rocks during the daytime. The spontaneous locomotor activity of the crab was studied in both constant darkness and constant illumination. In the former, 90% of the animals displayed persistent circalunidian rhythms with periods somewhat longer than 24.8 hours. The animal's response to constant light was blurred by a great deal of noise: Twenty five percent of the subjects were arrhythmic, while most of the rest expressed greatly enfeebled circalunidian rhythms. Several characteristics of this animal's locomotor rhythm are predicted by the circalunidian hypothesis.