Phenotypic manipulation provides insights into the function of a sexually selected trait in a freshwater crustacean species complex

Although sexual selection is a fundamental driver of evolution, our understanding of its causes and evolutionary dynamics is often constrained by poor knowledge of mechanisms of trait function. A powerful method for assessing mechanisms of sexual selection is to combine phenotypic manipulation of male sexual traits with manipulation of behavioural interactions associated with sexual selection. We performed such experiments to assess the function of posterior gnathopods (large, sexually dimorphic claws) in mating and reproduction of Hyalella amphipods. Removal of male gnathopods reduced pairing success and shortened pairing duration. Patterns of pairing success were similar in both the absence and presence of competing males, and results were consistent across three species that live in ecologically distinct habitats. Collectively, our findings demonstrate that gnathopods in Hyalella function during the interactions between males and females that lead to pair formation, and such functions are relatively conserved within the Hyalella species complex. Finally, our findings for gnathopod function in Hyalella contrast markedly with a recent study of gnathopod function in another amphipod, Gammarus pulex, indicating that enlarged gnathopods may have diverse functions among mate-guarding amphipod species.