Entrainment of Tidal and Semilunar Rhythms by Artificial Moonlight Cycles

Abstract

A marked feature of the larval release activity of the terrestrial crab Sesarma haematocheir is its synchronization with the time of high water. This activity occurs only at night, so that the pattern of the tidal rhythm recurs at semi-monthly intervals. When adult specimens from Seto (Okayama Prefecture) and Shima (Mie Prefecture) populations were brought from the field into 24-h light-dark conditions in spring, the larval release occurred at night, but the overall activity pattern gave no indication of a tidal component. On the contrary, under simulated moonlight cycles the timing of release was strongly coordinated and exhibited a well-defined tidal component arranged at semi-monthly intervals. The phase difference between the evoked tidal rhythms of the two populations of 4-5 h was about equal to the phase difference of the tidal cycles in their natural habitats. Synchronization of larval release with the artificial moonlight cycle required more than 40 days of exposure. In addition to entraining the tidal rhythm, artificial moonlight induced a semilunar rhythm in both populations. Entrainment could be achieved with exposure to moonlight for just a few days around the time of the full moon. In this paper, underlying mechanisms of the Sesarma larval release rhythm, which involves both tidal and semilunar components, are explained in terms of circadian oscillatory systems.