Predictors of body shape among populations of a stream fish (Cyprinella venusta, Cypriniformes: Cyprinidae)

Abstract

Performance-related variation in fitness can manifest as morphological responses to ecological and evolutionary pressures. Eco-morphological studies often utilize stark binary comparisons, such as lentic to lotic populations of freshwater fishes, to characterize relationships between form and function despite possible complications from confounding factors. In the present study, we compared body shape variation among lotic populations of a stream fish (Cyprinella venustaGirard) to disentangle the influence of ecological and evolutionary drivers of phenotypic change. We assessed the extent to which body shape corresponded to three key environmental factors (mean channel velocity, mean discharge, and mean annual run-off), phylogeny (mitochondrial DNA divergence), and body size (centroid size). We also examined relationships between these parameters and a fineness index, which is a measure of streamlining and morphological optimization for steady swimming performance. All three environmental variables had some explanatory power, although morphological characteristics were predominantly associated with variation in mean annual run-off. Phylogeny was also a strong predictor of morphological variation, whereas body size had little predictive power. Populations experiencing higher mean annual run-off exhibited a shorter base of the dorsal fin, a more slender body and caudal peduncle, a smaller head in both horizontal and vertical dimensions, and a more anterior placement of the eye. With some exceptions, such as variation in jaw length, differences in body shape associated with phylogenetic history were similar to those associated with run-off. Notably, all clades exhibited parallel responses to variation in run-off. Populations experiencing high mean annual run-off approached a hydrodynamic optimum, suggesting a morphology optimized for steady swimming performance. In contrast to previous studies that emphasize the importance of average water velocity, the findings of the present study indicate that morphological variation among populations of stream fishes is tightly linked to more complex aspects of hydrology and evolutionary history.