Monogamy and sexual dimorphism: Evaluation of the morphometric relationship of body and secondary sexual structures in heterosexual pairs of Alpheus estuariensis Christoffersen, 1984

Abstract

Sexual monomorphism or reduced sexual dimorphism in the body or cheliped size is expected in caridean shrimps that exhibit a monogamous mating system. In contrast to the mating system models proposed in crustaceans, monogamous heterosexual pairs of same Alpheus species exhibit sexual dimorphism. This raises questions about how sexual selection acts in these shrimps. We hypothesized that secondary sexual structures, but not the body, exhibit a tendency for size and directional allometry in each sex. We evaluate sexual size dimorphism in heterosexual pairs of Alpheus estuariensis, and the morphometric relationship of the body, and secondary sexual structures (cheliped and second pleonal segment), in heterosexual pairs of three populations. Carapace length did not differ between males and females, exhibiting a positive size correlation among monogamous pairs, indicating size-assortative mating across the three locations. Males bear larger cheliped propodus, while females had larger second pleura, indicating sexual size dimorphism. The propodus relationship between the sexes displayed positive allometry in the Paraíba do Norte River, whereas heterosexual pairs from the Paripe River and Massangana River showed isometry in cheliped propodus size between sexes. Furthermore, heterosexual pairs demonstrated negative allometry for pleura width. These findings support the hypothesis that secondary sexual structures, but not the body, exhibit size and directional allometry. We believe that these results are linked to the reproductive success of each sex in A. estuariensis. This implies that even within a monogamous mating system, selective sexual pressures play a role in the sexual dimorphism in these shrimps.