Sexual Shape Dimorphism in the Claws of an Estuarine Snapping Shrimp: When Static Allometry Does Not Amplify Sexual Dimorphism.

Sexual dimorphism in secondary sexual structures used as weapons is mainly attributed to sexual selection, which promotes asymmetry in these structures between sexes. However, when selective pressures also act on female weaponry, sexual dimorphism may decrease or even be absent. We evaluated the effect of static allometry, that is, shape variation associated with increasing size of the structure, on claw shape in three populations of the snapping shrimp Alpheus estuariensis, a socially monogamous species abundant in estuarine mudflats along the northeastern coast of Brazil. We tested the hypothesis that static allometry promotes a similar effect on claw shape in both sexes, thereby promoting sexual shape monomorphism. Our results show that although males bear larger claws, shape variation in response to growth is slight, leading to morphological overlap between sexes. Nonetheless, sexual shape dimorphism, although slight, was statistically observed, suggesting that males and females possess functionally similar weaponry. These results partially support our hypothesis and contrast with the pronounced sexual shape dimorphism previously reported for other Alpheus species. Furthermore, our findings indicate that the selective pressures acting on the weaponry of A. estuariensis may differ from those operating in congeneric species. Biomechanical and comparative analyses among species are needed to assess how selective pressures mediated by social and ecological interactions shape the evolution of weapons in snapping shrimp.