Comparative studies of tidal rhythms. VII. The circalunidian locomotor rhythm of the brackish‐water fiddler crab,Uca minax

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Abstract
The spontaneous locomotor activity of 31 Uca minax fiddler crabs was studied in constant conditions. All of the crabs displayed persistent, circalunidian rhythms, i.e., cycles which were approximately the length of the lunar day. Individual crabs remained rhythmic in the laboratory for 18 to 70 days, with 37 days being the average. It was typical for a crab's rhythm to change spontaneously in length several times. It was concluded that the twice/lunar day peaks in this species—like those in the 4 other decapods previously studied in this series—are under the control of separate clocks. This conclusion derives from the demonstrated independent manifestations of the peaks. For example: they can differ in form and amplitude; one may disappear either temporarily or permanently; they can have period lengths that differ significantly from one another; and one can split while the other does not. All these displays could result if each peak was governed by its own timepiece. It should be emphasized that in nature, and usually in the laboratory, the two clocks are coupled, but constant conditions can sometimes cause them to disengage.