Comparative studies of tidal rhythms XI. Ultradian and circalunidian rhythmicity in four species of semiterrestrial, intertidal crabs

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Abstract
Activity‐rhythm data from four species of semiterrestrial, intertidal crabs collected in Woods Hole, Massachusetts, USA, and Dunedin, New Zealand were investigated using a battery of sensitive digital signal analysis techniques. The animals were tested in a variety of constant lighting regimens, constant darkness, and at a variety of temperatures. In all species a small number of animals showed ultradian rhythms not attributable to either circatidal or circalunidian oscillations. Several animals were monitored for long intervals. In animals with adequately lengthy records, It was possible to demonstrate that ultradian rhythms appear when tidal periodicities weaken, and vice versa, thus strengthening the hypothesis that long‐period rhythms are produced by an ensemble of ultradian fundamental oscillators. Up until our present examination, ultradian rhythms were known only from species that typically displayed circadian frequencies. That these high‐frequency cycles are now known to exist in organisms that display tidal rhythms implies a commonality between circadian and tidal horological systems lending further weight to the hypothesis that circalunidian and circadian rhythms may well be the product of oscillators of similar period and mechanism.