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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.