Repeatability of Habitat-Associated Divergence in Shell Shape of Turtles

Repeated patterns of phenotypic divergence between environments across disparate taxa provide strong evidence for the generation of adaptive phenotypes. Flow velocity is an important selective force in aquatic habitats; however, among vertebrates, the study of its effects on morphology has been limited almost exclusively to fully-aquatic bony fishes. We tested whether three confamilial species of semi-aquatic freshwater turtle (family Emydidae: Graptemys pseudogeographica, Graptemys nigrinoda, and Pseudemys concinna) displayed similar patterns of phenotypic divergence in carapace shape between fast- and slow-flowing aquatic environments. We used (1) geometric morphometrics to quantify shell shape, (2) multivariate analysis of variance to test the effects of species, sex, and flow, and (3) phenotypic trajectory analysis to compare patterns of divergence for six species-sex groups. We found significant effects on shell shape for all factors. In general, ecomorphs from fast-flowing habitats had flatter shells than those from slow-flowing habitats. Furthermore, results of trajectory analysis indicate that the degree to which, as well as the way in which, ecomorphs differed were concordant across all species. Our findings demonstrate that the effects of flow are not limited to fully-aquatic vertebrates, and provide evidence of the ability of flow to drive repeatable phenotypic divergence in tetrapods.