Fighting does not influence the morphological integration of crustacean claws (Decapoda: Aeglidae)

Abstract

Animal weapons are generally complex systems composed of more than one structure. A crab's claw, for instance, is composed of a dactyl (a movable finger) and a propodus (where the muscle resides). Any weapon feature that increases winning probability also increases an individual's fitness, meaning that all moving parts of a weapon will be under the same selective force - increasing their integration. By comparing weapons to homologous structures not used for fighting, we can test whether fighting increases morphological integration. Here, we tested that hypothesis by comparing the right (non-fighting) and left (fighting) claws of males of Aegla. First, we tested if the shape and size of the propodus and dactyl differed between claws. Then, we compared the degree of morphological integration. We showed that claws differed in their shape and size, but both claws showed similarly high levels of integration. This pattern can occur if selection favours an adequate squeeze on both claws, even though squeezing performance might differ between the claws. Nevertheless, the dactyl is important in determining the integration of a claw but it has been greatly ignored in crustacean studies and its study might increase our knowledge of claw evolution.