The migration process of megalopae from the sea to a tidal river is an important subject for catadromous Brachyura. The process of invasion and settlement of the megalopae of the Japanese mitten crab Eriocheir japonica was investigated in a small river in temperate Japan. First, to elucidate the relationship between settlement and lunar rhythm, megalopae were collected using artificial grass mats (900 cm2) in the upper tidal river area every 3–4 days from November to December. An evident semi-lunar rhythm was detected, and the settlement was concentrated in the spring tide around the days of full moon and new moon. All the megalopae completed metamorphosis within 5 days before the next spring tide. Second, the relationship between settlement and tidal cycles within a day was investigated in the upper tidal river by collecting megalopae using artificial grass mats every 2 h during 1 day in November. Megalopae exhibited in their settlement an evident nocturnal behavior and synchronization with the tidal cycles. A mass of megalopae suddenly began settling on the mat during the night when the tide came in and high salinity water (ca. >10‰) reached the sampling site. In contrast, after the sunrise, very few megalopae were collected even during the flood tide. Third, the upstream migration process of megalopae in the tidal river was surveyed by hand-net sweeping at five stations during the night every hour for 7 h in May. No larvae were collected during the ebb tide before sunset, but swimming megalopae suddenly gathered at the rivermouth just after sunset. Aggregated megalopae shifted upstream with the flow of the highly saline water mass until the latter reached a weir in the uppermost tidal area. In the subsequent ebb tide, the megalopae remained in the uppermost area. Therefore, they utilized a selective tidal transport mechanism; megalopae migrated upstream from the sea preferably during the night around the spring tide and reached the uppermost tidal river during a single flood tide. Combined with previous data showing the distribution pattern of parent crabs and the settled megalopae, it was estimated that all megalopae were supplied from the sea’s offshore area, and river population dynamics depend on the supply of megalopae from the sea. This also suggests the larval exchange and gene flow between local river populations of E. japonica within a metapopulation structure.
