ADAPTIVE SIGNIFICANCE OF A SEMILUNAR RHYTHM IN THE TERRESTRIAL CRAB SESARMA

¹ Okayama University, College of Liberal Arts, Department of Biology, Tsushima-Naka 2-1-1, Okayama, 700 Japan

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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