Prediction of timing of mating and egg extrusion in the coconut crab Birgus latro judged from female pleonal expansion

A temporal pattern of reproductive activities of biological resources is essential information for the implementation of specific seasonal closures. First we investigated the relationship between the female pleonal expansion and female reproductive condition (i.e., non-ovigerous, ovigerous, or with spermatophores) to clarify the morphological characteristics of female coconut crabs that were able to mate. Second, we mated females with males in captivity to examine the relationship between the degree of female pleonal expansion and mating success. The index of pleonal expansion (ratio of inter-tergite distances to thoracic length) was significantly different among female reproductive conditions; females with spermatophores just before egg extrusion had the most expanded pleon. In the mating trials, all females showing index values higher than 0.35 mated with males. These results indicate that the female pleonal expansion has a strong relation to reproductive activities of females and can be applied as a morphological criterion for selecting females being able to mate. Next, we investigated whether reproductive activities of female coconut crabs are related to the lunar cycle. The reproductive activities were synchronized with the lunar cycle, and most females mated and extruded eggs around the new moon phase. These results have considerable implications for resource management and aquaculture for this species.