Environmental Factors Involved in Setting the Phases of Tidal Rhythm of Color Change in the Fiddler Crabs Uca pugilator and Uca minax1

Abstract

Observations were performed to determine the relationship between the time of maximal pigment dispersion in the fiddler crabs, Uca pugilator and Uca minax, and the tidal events on the beach where the crabs were collected. Uca pugilator obtained from burrows above the high tide mark did not have an overt tidal rhythm but did have 24‐hour and 14.8‐day cycles of color change. The tidal rhythm was probably suppressed because the crabs were free to leave their burrows at any time, whereas crabs from the intertidal zone can leave only when the area where their burrows are located is uncovered by the receding water. Crabs living in the intertidal zone displayed a typical tidal rhythm of color change. The tidal maximum of pigment dispersion progressed across the 24‐hour cycle at the usual rate. Analysis of the tidal rhythms of two groups of crabs, Uca pugilator, collected near the high and low tide marks, respectively, revealed that crabs from burrows near the low tide mark behaved as if low tide occurred for them 9.8 hours later each day than for the high tide crabs. Crabs living near the low tide mark appeared on the beach 10.1 hours later than crabs living near the high tide mark. As a result of these observations the hypothesis was presented that the phases of the tidal rhythm are set primarily according to the time that the area where the crabs live is uncovered by the receding water, and secondarily according to the time required for the area to drain so that the sand is firm enough to support holes from the burrows to the surface.