Entrainment of a semilunar rhythm by a simulated moonlight cycle in the terrestrial crab, Sesarma haematocheir.

Sesarma haematocheir is a species of terrestrial crabs inhabiting hillsides and paddy fields near the sea. Females show a semilunar rhythm of zoea-release coinciding with days of spring tides and in addition with the time of high water occurring at the nearby seacoast about dusk. Nature of environmental stimuli or zeitgebers that induce the semilunar rhythm of zoea-release was examined experimentally. In case that a population of males and females was kept under the condition of a 24-h light-dark cycle (LD 14:10) only, females showed a free-running semilunar rhythm of egg-laying and zoea-release synchronized from field conditions. On the other hand, a semilunar rhythm of egg-laying and zoea-release was entrained by the combination of the 24-h LD and a simulated 24.8-hr moonlight cycle of which the phase was shifted in relation to the natural lunar cycle. This result suggests that the 24.8-h moonlight cycle acts as a zeitgeber of a semilunar rhythm. The 12.4-h tidal cycle parallels with the 24.8-h moonlight cycle in the field. On the basis of the perception of a distinct phase relationship between the 24-h LD and the 24.8-h moonlight cycle, it is considered that crabs substitute the 24.8-h moonlight cycle for the 12.4-h cycle of tides as a zeitgeber to synchronize the phase of the semilunar rhythm with a tidal situation.