Relationships between life history traits and sexual dimorphisms in two varunid crabs, Hemigrapsus takanoi Asakura & Watanabe, 2005 and H. sinensis Rathbun, 1931 (Brachyura: Varunidae)

Differences in life histories and mating systems can influence the strength of sexual selection, leading to different degrees of sexual dimorphism among allied species. To test this premise, we compared the life history traits (including operational sex ratios) between two brachyuran crabs (Hemigrapsus takanoi Asakura & Watanabe, 2005 and H. sinensis Rathbun, 1931) that have different degrees of sexual dimorphism. In H. sinensis, both sexes have setal patches on their chelae, but patches are found only on the males of H. takanoi. Our field surveys showed that sexual dimorphism about body size (carapace width) differed between the two species, males being larger than females in H. takanoi, whereas females were slightly larger than males in H. sinensis. Ovigerous females of H. takanoi occurred from May to October, with two peaks in the abundance trend; ovigerous females of H. sinensis were found from March to June, with one peak of abundance when almost all females were ovigerous. The population density of H. takanoi in the field far exceeded that of H. sinensis. Although the operational sex ratio was highly biased toward males in both species due to the rarity of receptive females, the bias was more pronounced in H. takanoi than in H. sinensis, partly due to the longer receptive duration of H. sinensis females than that of H. takanoi females. The larger male biased operational sex ratio and higher population densities of H. takanoi (compared to H. sinensis) may explain interspecific differences in sexual dimorphism of morphological features, including the occurrence of the setal patch.