Seaward migration and larval release of the land hermit crab Coenobita brevimanus Dana, 1852 (Anomura: Coenobitidae) on Iriomote Island, Japan

Abstract

The seaward migration and larval release of Coenobita brevimanus Dana, 1852 on a sandy beach of Iriomote Island, Japan, was studied between May and November 2009 and between May and December 2010. Seaward migration and larval release were mainly observed for several days before and after new moons during the period from June to November, and the migration was mainly focused on 0.82 days after the new moon. Around full moons, some crabs were also found, with the highest numbers at 1.34 days after spring tides (semi-lunar rhythms). Almost half of the crabs that appeared on the beach entered seawater, and almost half of the immersed crabs released zoeas or exhibited swinging behavior. Appearance of crabs on the beach and entering seawater occurred only after sunset between 19:00 to 23:10. The peak time of the appearance on the beach was within a certain range each month. Crabs entered seawater at a mean of 2 h after the nighttime high tide. Seaward migration of the crabs showed clear seasonality, and many crabs migrated and entered seawater during the period from July to September, the months with higher temperature (＞25°C). This species shows a phenology and rhythm of spawning common to coenobitids on the Ryukyu Islands.