DAILY AND TIDAL PATTERNS OF ACTIVITY IN INDIVIDUAL FIDDLER CRAB (GENUS UCA) FROM THE WOODS HOLE REGION

¹ Department of Biological Sciences, Northwestern University, Evanston, Illinois and the Marine Biological Laboratory, Woods Hole, Massachusetts

1. The motor activity of individual specimens of U. pugnax, U. pugilator, and U. minax was recorded in actographs in both constant illumination and natural cycles of daylight and darkness for periods which exceeded six weeks in some crabs.

2. In the natural light cycle crabs exhibited overt, semi-diurnal tidal rhythms of high accuracy which persisted, in the case of U. minax, for at least 46 days. Superimposed on the tidal rhythm was a 24-hour component which consisted generally of activity between sunset and sunrise.

3. In constant illumination the tidal rhythm in U. minax clearly exceeded the natural tidal period of 12.4 hours. In some crabs of this species the tidal rhythm persisted for well over a month.

4. In both U. minax and U. pugilator transitions from a tide-related pattern to an apparently circadian pattern were observed in constant illumination.

5. Evidence was presented for dissociation of components of the tidal clock system in U. pugnax.

6. Variations in the activity pattern occur at the species level and within species. Within species individual variation may be due to such factors as exposure to tidal influence, reproductive state, phase of social activity, and molting.