Abstract
Sexual dimorphism is prevalent in animal taxa and can result from a variety of factors including natural selection, sexual selection, and the sexes occupying dimorphic niches. Sexual selection typically acts on males via female choice or intra-sexual competition, while the dimorphic niche hypothesis usually applies to females due to reproductive constraints. Fishes exhibit sexual dimorphism as variation in size, body proportions, fins, and/or color. The tule perch, Hysterocarpus traski, is a viviparous (live-bearing) and externally monomorphic, internally fertilizing fish. Despite appearing monomorphic, males and females are expected to differ in body shape because the different reproductive roles occupied by the sexes should influence patterns of selection and, ultimately, lead to differences in morphology. It is also expected that the body shape of a tule perch changes as the fish matures, especially the abdomen shape, as reproductive structures develop. Dimorphic variation among tule perch was investigated using morphometric approaches. The objectives of this study were to determine if tule perch exhibit (1) sexual dimorphism in body shape, (2) size-related changes in body shape, and (3) sexual size dimorphism. Objectives 1 and 2 were accomplished by describing and testing mean body shape differences due to sex and size using a geometric morphometric approach. The third objective was accomplished using a traditional morphometric approach. Results of the Multivariate Analysis of Variance indicated that tule perch exhibited significant sexual dimorphism of body shape. Results of the Canonical Variates Analysis demonstrated that the most effective discriminators between the sexes were in the mid-body and caudal peduncle regions. Specifically, females were narrower through the caudal peduncle and mid-body and had anterior anal fin insertion points that were more posteriorly-located than males. Additionally, results of the multivariate regression indicated that tule perch exhibited size-related changes in body shape. The deformation grid obtained from the regression of shape to size illustrated that larger fish were deeper through the mid-body and blunter through the snout than smaller fish. The caudal peduncle was wider and more rounded and the pectoral fin was shifted ventrally in larger fish, and the eyes of larger fish were relatively smaller and located higher on the body than those of smaller fish. Finally, results of the two-tailed t-test indicated that tule perch did not exhibit sexual size dimorphism with regard to standard length, but did exhibit sexual size dimorphism with regard to weight.