Sperm limitation: possible impacts of large male-selective harvesting on reproduction of the coconut crab Birgus latro

For many biological resources, larger males are more likely to be harvested than smaller ones. In such harvested populations, the remaining males are relatively small and participate in more matings than in pristine populations. Because the number of sperm passed to females generally decreases with decreasing male body size and increasing male mating frequency, the reproductive success of females in populations subjected to such harvesting patterns can be restricted due to sperm limitation. The Japanese population of the coconut crab Birgus latro is typically subject to large male-selective harvesting, which may be one of the causes of the presently observed B. latro population decline. To examine the impact of harvesting on coconut crab populations, the effects of male size and mating frequency on male reproductive potential (number of ejaculated sperm and fertilization rate), sperm recovery rate, and the number of sperm received by females in a harvested population were investigated. Larger males provided females with larger numbers of sperm and achieved higher fertilization rates. Although the number of ejaculated sperm decreased with increasing mating frequency regardless of male size, larger males always passed more sperm to successive mates than smaller ones. The number of used sperm did not increase even after 30 d, suggesting a low sperm recovery rate. More than half of the females collected after the beginning of the reproductive season had very low numbers of sperm. These results suggest that the reproductive rate of the coconut crab has declined due to females suffering from sperm limitation.