Opposites attract: Multiple evidence of sexual antagonistic coevolution driving extreme male-biased sexual size dimorphism in Panopeus meridionalis

Abstract

Explanations for the evolution of male-biased size dimorphism (MBSSD) traditionally focus on male competition and female choice, overlooking the alternative that larger males may be better at coercing females into mating. While displaying considerable diversity, 'true crabs' (Eubrachyura) share common traits that theoretically should promote the evolution of coercive mating strategies. Despite this, there is a conspicuous lack of studies investigating this aspect. We investigated several reproductive and life history traits of Panopeus meridionalis (a mud crab that exhibits extreme MBSSD) to assess whether the specific set of characters is consistent with the hypothesis of sexual antagonistic coevolution at place. We found that the high MBSSD is likely driven by sexual conflict, where males use their size to coerce females into mating. Experimental matings involved male aggression towards females. Females first resist male attempts, but are ultimately subdued. Mating is relatively brief and there is no evident pre or post copulatory guarding. The female reproductive tract lacks complex structures for long-term sperm storage or manipulation, and given the small size of seminal receptacles related to male sperm load capacity, it is unlikely for females to store sperm from multiple partners. All considered, the evidence suggests that females have limited control over paternity and support the existence of an intrinsically coercive mating system in P. meridionalis. We propose this species as an interesting model for studying the resolution of sexual conflict through antagonistic coevolution and selection in the highly diverse group of true crabs.