The Role of Rhythmic Systems in the Adaptation of Fiddler Crabs to the Intertidal Zone

Abstract

Fiddler crabs inhabiting the intertidal zone must adapt their activity to both the day-night cycle and the cycle of the tides. The tidal cycle imposes on the animals a rhythmic alternation between terrestrial and marine periods of existence. At the same time the crabs are influenced by the day-night cycle, since they exhibit specific diurnal and nocturnal habits. Moreover, the interaction of daily and tidal rhythms may give rise to semi-monthly variations in activity. It is now well established that persistent daily and tidal rhythms in physiological processes underlie rhythmic variations in the behavior of crabs in the field. The present paper reviews more recent studies, which have compared the persistent rhythms in crabs from different tidal and non-tidal regions. Rhythmic patterns recorded in the laboratory are found to be closely correlated with tidal conditions in the native habitat of the crabs. It has also been shown that the persistent pattern can be modified by transplanting crabs to the intertidal zone of another coast where they are exposed to tidal cycles which differ in form from those in the original habitat.