OBSERVATIONS ON DAILY AND TIDAL RHYTHMS IN SOME FIDDLER CRABS FROM EQUATORIAL BRAZIL

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Abstract
Three species of fiddler crabs, Uca mordax, U. rapax, and U. maracoani, collected at Belem and Salinopolis in northern Brazil, exhibit persistent daily rhythms of color change. In a fourth species, U. thayeri, the rhythm is of much lower amplitude. In spontaneous running activity U. maracoani possesses a persistent tidal rhythm. U. mordax has a more complex activity pattern which can be resolved into persistent solar and lunar components. The frequency of the persistent activity in rhythms in both species seems to follow the actual systematic variations in the frequency of the ocean tidal cycle in the original habitat of the crabs. Such variations in frequency would be expected if the persistent activity rhythms were timed by the interaction of two components, one of solar and the other of lunar frequency.