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

Observations were performed to detcrminc the relationship bctwccn 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 collcctcd. 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 bccausc the crabs wcrc free to leave their burrows at any time, whereas crabs from the intertidal zone can lcavc 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, rcvealcd 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 annearcd on the beach 10.1 hours later than crabs living near Lhe high tide mark. As a &ult 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 arca to drain-so that the saitnd is firm enough to subport holes from the burrows to the surface. A review of the literature concerning tidal and semilunar rhythms of color change has appeared recently (Fingerman 1957a). Brown et al. (1953) published the first report of these rhythms. The fiddler crab Uca pugnax collected in the region of Woods Hole, Massachusetts, where the tides are semidiurnal, would darken by day and blanch by night in accordance with a 24-hour rhythm of color change. Superimposed upon the latter was a tidal cycle that progrcsscd across the 24-hour rhythm at the average rate of 48.8 minutes per day. The tidal rhythm was evidenced by a supplementary dispersion of pigment in the melanophorcs about the time of low tide. A semilunar rhythm was also cvidcnt from analysis of the data. The frequency of this rhythm was 14.8 days, the average interval between days on which the 24-hour and tidal rhythms repeated similar time relations to one another. Marc recently, Brown et al. (1955) showed that Uca pugnax has the endogenous ability to mark off 1 This investigation was supported by Grant No. B-838 from the National Institutes of Ilcalth. periods of solar and lunar day-lengths in the absence of all possible rhythmic external signals. Persistent tidal and semilunar rhythms of color change were observed in the blue crab, Callinectes sapidus, by D’ingerman (1955). These rhythms were similar to those described above for Uca pugnax in spite of the fact that the Cullinectes were collected in the vicinity of New Orleans, Louisiana, a region of diurnal tides. Evidently the “center” of tidal rhythmicity of color change in blue crabs operated on the basis of tides spaced 12.4 hours apart, although the crabs were collected in a region where 24.8 hours was the average interval between successive low tides. In addition to Uca pugnax, tidal rhythms of color change have been observed in Uca pugikator and Uca speciosa by Fingerman (1956). The latter two species were collcctcd at Ocean ‘Springs, Mississippi, where the tides are diurnal. The tidal rhythms of both species wcrc similar to the tidal rhythms of Callinectes sapidus and Uca pugnax. Both the Uca pugilator and