PERSISTENT TIDAL CYCLES OF SPONTANEOUS MOTOR ACTIVITY IN THE FIDDLER CRAB, UCA PUGNAX

¹ Sweet Briar College, Sweet Briar, Virginia, Northwestern University, Evanston, Illinois, and the Marine Biological Laboratory, Woods Hole, Massachusetts

1. The spontaneous locomotor activity of groups of fiddler crabs, Uca pugnax, was recorded during the summers of 1955 and 1956 under constant laboratory conditions.

2. This species shows an overt rhythm of activity of primary lunar or tidal frequency. Within solar days, there are two peaks of activity which are 12 to 13 hours apart. These maxima move across succeeding solar days at an average tidal rate. The cycles are precise for at least a week under constant conditions, but after this time, some warping and displacement of maxima occur.

3. A low amplitude solar rhythm of activity is apparent upon analysis of 29 days of continuous data. This rhythm is characterized by high activity between hours 6 and 12 of the solar day.

4. There was observed a difference in phase relationships of the tidal rhythms between 1955 and 1956. The state of the problem of the setting of phases of persistent tidal cycles is discussed.

5. Since this rhythm is precise for at least a week under constant conditions, and since the cycles of two groups of crabs recorded independently correlate to a high degree, this cycle appears to be an excellent one with which to study experimental modifications of persistent tidal rhythms.

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