Comparative studies of tidal rhythms. VI. Several clocks govern the activity of two species of fiddler crabs

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Abstract
The tide‐associated persistent activity rhythms of the ocypod crabs, Uca pugnax and U. pugilator were observed in constant conditions, some for as long as three months. In some animals the twice/day peaks of activity were found to scan the day at significantly different rates from one another. Individual peaks in some animals split into two fragments. And at times, one peak would fade and then return, or vanish completely. These examples of the independence between tide‐related activity peaks have become the basis for a hypothesis that each peak is controlled by its own clock(s). The basic period of these clocks is “circalunidian”, with periods varying up to about 4–8% on either side of 24.8 hours.