LARVAL RELEASE RHYTHM COINCIDING WITH SOLAR DAY AND TIDAL CYCLES IN THE TERRESTRIAL CRABSESARMA— HARMONY WITH THE SEMILUNAR TIMING AND ITS ADAPTIVE SIGNIFICANCE

Abstract

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.