Reproductive Biology of Three Land Hermit Crabs (Decapoda: Anomura: Coenobitidae) in Okinawa, Japan

Reproductive ecological research on three land hermit crabs, Coenobita rugosus, C. purpureus, and C. cavipes, was conducted in the southern part of Okinawa-jima island in 1985, 1986, 1987, and for a short period in 1999. Size (carapace length) of the smallest ovigerous female was 3.93 mm for C. rugosus, 3.83 mm for C. purpureus, and 9.49 mm for C. cavipes. Breeding season is late May to November for C. rugosus, late May to mid-September for C. purpureus, and mid-May to late August for C. cavipes. Some females of all three species probably produced at least two broods during the breeding season. The smallest males in which spermatophores were present in dissected vas deferens were 4.24 mm for C. rugosus and 4.94 mm for C. purpureus. Coenobita cavipes females produced more, smaller eggs in comparison with C. purpureus. My observations suggest that coenobitid crabs living in areas with a low supply of shells or with poor shells reproduce at smaller sizes, as is the case in marine hermit crabs. Time of onset of larval release by C. rugosus, with its protracted breeding season, varied according to the seasonal shift in time of sunset. The period during which females of C. rugosus released larvae was about 2 hr in spring tides but was much longer (3 to 5 hr) during neap tides. Larger females of C. purpureus occupied shells derived from the land snail Achatina fulica; smaller ones used shells from the marine snail Lunella granulata. Use of mutually exclusive larval release sites by the larger and smaller females of C. purpureus remained unchanged over 13 yr, from 1986 to 1999. This behavioral difference may be related to the differences in their habitats (i.e., inland versus shore) and to the route traveled by the larger crabs in reaching the sea from inland sites.