The circatidal clock of an estuarine semi-terrestrial crab, Sesarma erythrodactylum

Abstract

The influence of the tidal cycle decreases with the distance from the sea, and this may affect the behavioral timing systems in estuarine animals. In addition, the circatidal rhythm of these animals may be controlled by light-sensitive systems. To investigate the timing systems in estuarine animals, the larval release activity of a semi-terrestrial crab, Sesarma erythrodactylum, was monitored in the laboratory without any tidal influence. The larval release rhythm free-ran under constant dim light conditions, which suggests that the timing of release is under the control of an endogenous clock. The free-running period was somewhat different for each individual. Under an artificial 24-h light-dark (LD) cycle in phase with that in the field, the timing of release coincided with high tides at night. In contrast, the rhythm changed to match a phase-shifted 24-h cyclic light regimen. These results demonstrate that a light-sensitive mechanism is certainly involved in the circatidal timing systems of S. erythrodactylum. The role of the 24-h LD cycle is not only to shift the synchrony of the timing of release onto the other high tide when necessary for maintaining a nocturnal schedule, but also to drive the phase of the circatidal rhythm. While the tidal behavioral rhythms of intertidal animals reflect two parameters of the tidal cycle, i.e., the 12.4-h period and the tidal amplitude, those of estuarine crabs lose synchrony with the tidal amplitude and show a nocturnal pattern instead. These properties of the circatidal rhythm of larval release in S. erythrodactylum and other intertidal and estuarine crabs can be explained by a coupled oscillator hypothesis.