This field study determined which cycle of a lunar day or tide coincides with the time of day of larval release in the terrestrial crab, Sesarma haematocheir. Observations of larval release were made at a riverside 100 m upriver from Kasaoka Bay in the Inland Sea of Japan where the tidal phase differs by several hours from that of the Pacific Ocean. The findings demonstrated that the timing of larval release coincided not with a lunar day cycle but with a local tidal cycle. The larval release pattern of the Kasaoka population showed a relatively strong correlation with tides when compared with the Izu population. This suggests that the Izu population pattern was transitional, going from a combined solar day and tidal pattern to a complete daily rhythm. The timing of incubation and larval release may be based on the following mechanisms: semilunar timing of incubation entrained by lunar cycle, and the time of day of larval release controlled by a combination of solar day and local tidal cycle. This study presents further evidence that the semilunar rhythm of incubation and larval release plays an important role in the survival of larvae.

Larval release activities of the terrestrial crab Sesarma were observed 1.5 km upriver from the sea. The number of Sesarma haematocheir and Sesarma intermedium females releasing larvae peaked twice monthly, during the full and new moon periods. Larval release, at about dusk, coincided with high water at the nearby seacoast. Larvae of S. haematocheir and S. intermedium died quickly in fresh water. The semilunar rhythm of larval release gives the larvae, released just after high water of spring tides and around high water on the days following the full and new moons, a better chance of reaching the sea than otherwise. Sesarma dehaani did not reveal a clear semilunar rhythm and the time of day of larval release did not coincide well with high water. S. dehaani inhabits riverbanks and rice paddies near the sea, and its larvae have the highest tolerance to fresh water. These factors may account for its lack of synchronization with tides.

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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.

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