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

¹ Department of Biological Sciences, Northwestern University, Evanston, Illinois, and the Museu Paraense "Emilio Goeldi," Belém-Pará, Brazil

1. Experimental observations were made on daily and tidal rhythms in four species of fiddler crabs, Uca, collected at Belém and Salinópolis near the equator in Brazil.

2. In naturally changing illumination, overt daily rhythms of melanophore activity were present in U. mordax, U. rapax, and U. maracoani. The form of the rhythms was related to the equatorial cycle of daylight and darkness.

3. There was a melanophore rhythm in U. thayeri, but its amplitude was less than one-third of the amplitude of the other species. In U. thayeri, as in U. pugilator, the melanophores failed to coalesce during the diurnal phase of dispersion.

4. The red chromatophores of U. rapax and U. maracoani displayed daily rhythms of concentration and dispersion.

5. In constant illumination the amplitude of the melanophore rhythms was reduced. In at least two of the species, U. mordax and U. rapax, the melanophore rhythm deviated in period from 24 hours.

6. In its spontaneous motor activity U. maracoani possessed a tidal rhythm which persisted in constant illumination and temperature. U. mordax had a more complex and variable pattern which could be resolved into persistent solar and lunar components.

7. Differences in the activity patterns of the two species appeared to be related adaptively to differences in their habitats.

8. The frequency of the persistent tidal rhythms seemed to follow the actual systematic variations in frequency of the tidal cycle. Such deviations about a true lunar frequency would be expected if the tidal rhythms were timed by two components, one of solar and the other of lunar frequency.

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