Sperm allocation in response to a temporal gradient in female reproductive quality in the stone crab, Hapalogaster dentata

Males with limited sperm reserves should allocate sperm efficiently during the reproductive season. As efficiency depends on the temporal distribution of mating opportunities with females of high reproductive quality, we examined whether the sperm allocation strategy of stone crab males with limited sperm reserves changes in response to the temporal distribution of mating opportunities. First, to examine how sperm number per ejaculate depends on female quality and mating opportunities, we artificially controlled female size or timing of mating within a reproductive season and skewed the sex ratio towards females in laboratory experiments. Next, we investigated the temporal distribution of mating opportunities for males in a natural population. Sperm number per ejaculate increased with increasing female size and with decreasing mating opportunities, but did not change with increasing mating opportunities. The number of fertilized eggs per unit number of sperm increased with increasing female size, indicating that larger females are beneficial as mates. Larger males did not mate with small females during mating trials, implying that the males practised mate choice and avoided females of lower quality. Mating opportunities with larger females were limited to the early reproductive season in a natural population. In conclusion, the observed sperm allocation pattern seems to be adapted to maximize reproductive success with a finite number of sperm. The temporal distribution of mating opportunities with females of high quality during a reproductive season should be considered when predicting sperm allocation strategies.