THE RHYTHMIC UNIVERSE 369 



furnished by studies made on the color change in the skin of 

 the fiddler crab, Uca pugnax.^ Near dawn on the beaches, the 

 skin of this crab is observed to begin to darken, becoming 

 darkest at noon, while near sunset it begins to blanch, becoming 

 lightest at midnight. In its natural habitat the fiddler crab 

 begins feeding at dawn, and it is believed that the darkening 

 of the skin protects it from the radiant energy of the sun and 

 makes it less conspicuous to its predators. When collected and 

 taken to a photographic darkroom where light, temperature 

 and other environmental factors are maintained constant, these 

 crabs continue to change color as if they were still on their 

 native beaches, although this color change has no longer any 

 survival value. In the course of studying these changes, the 

 observers detected not only a diurnal color change produced by 

 a diurnal rhythm of melanin dispersion (causing darkening) , 

 but also a supplemental tidal color change accompanying a tidal 

 rhythm of dispersion. This latter tidal rhythm of darkening 

 and blanching was closely related to the feeding periodicity 

 and was in phase with the times of high and low tide of the 

 crab's natural habitat. So true was this, that crabs collected 

 from beaches that had tide times different from those of the 

 location of the laboratory where they were observed, main- 

 tained their rhythm of color dispersion in step with their former 

 home. 



Although these diurnal and tidal rhythms held constant in 

 the laboratory, they could, nevertheless, be " re-set " out of 

 phase with the external solar and tidal times by exposing the 

 animals to very low temperatures or to continuous illumination 

 over a period of several days. The crabs would then keep the 

 regular twenty-four hour cycle and the twelve and one-quarter 

 hour cycle, but with a six-hour lag. Thus, instead of beginning 

 to darken at six o'clock in the morning, the crabs would begin 

 to darken at noon, blanching not at six o'clock in the evening, 



^ Frank A. Brown, Jr., Milton Fingerman, Muriel I. Sandeen and H. M. Webb, 

 " Persistent Dirunal and Tidal Rhythms of Color Change in the Fiddler Crab, Uca 

 pugnax," Jour. Exp. ZooL, CXXIII (1953) , 29-60. 



