Is the Setal Patch on the Chelae of Hemigrapsus takanoi and Hemigrapsus sinensis (Crustacea, Brachyura, Varunidae) Advantageous in Fighting and Mating?

Abstract

Many animals have unique morphological characters that function in social behavior. Sexual selection can affect the expression of such traits in males and females, leading to sexual dimorphism. We investigated the social function of setal patches on the chelae of two species of varunid crabs, one in which males, but not females, have setal patches (Hemigrapsus takanoi), and one in which both sexes have setal patches (Hemigrapsus sinensis). We experimentally removed setal patches and compared fighting and mating behavior of individuals with and without setal patches. In H. sinensis, males with setal patches removed were inferior fighters compared to intact males. In male H. takanoi and female H. sinensis, setal removal did not influence the outcome of fights. In mating, males lacking setal patches had a similar ability to copulate with females as intact males in both species. However, male H. takanoi with their setae removed tended to take more time to initiate copulation than did intact males. When females were given the opportunity to choose intact males or males without setal patches, females of H. takanoi did not discriminate between the two. Female H. sinensis, however, copulated with intact males more frequently compared to males lacking setal patches. Male H. sinensis showed no preferences for the presence of setal patches or the body size of females. Thus, our results indicate that setal patches have a social function in male H. takanoi and male H. sinensis, but not in female H. sinensis, suggesting that the setal patches of male crabs are a sexually selected trait in both species. However, the social function of male setal patches was more prominent in the species in which both sexes possess setal patches than in the species in which only males bear setal patches.