A Tail of Two Sexes: Sexual Disparity in Brachyuran Crustaceans and Its Impact on Arthropod Trunk Evolution

Abstract

Arthropods are well known for being rampantly speciose. Variation of form and nicheoccupation in arthropods relies on specialized appendages and regionalization of a common bauplan. The specialization of particular body regions leads to displays of sexual dimorphism. Cases of intraspecies morphotypes and sexual dimorphic characters reflect larger trends of morphological variation in all major arthropod clades. Here, I report a morphospace of sexually dimorphic brachyuran crustaceans from the San Juan Islands in the Pacific Northwest and discuss inter- and intraspecies variation as it pertains to marine arthropods as whole. Abdominal shape changes in sexes of Cancer and Hemigrapsus were evaluated using geometric morphometrics and Principal Components Analysis (PCA). Traditional morphometrics, measuring the dimensions of the carapace were also incorporated. The abdominal and body shape disparity between genders of both genera is found to be statistically significant through an analysis of variance (ANOVA) statistical comparison test. Sexual dimorphism is not conserved in interspecies or in intraspecies relationships. Hemigrapsus specimens exhibit a higher degree of sexual dimorphism than Cancer specimens. The morphological results reveal ecological effects and macroevolutionary trends resulting from sexual dimorphism of the arthropod bauplan.