A functional approach to ecomorphological patterns of feeding in cottid fishes

Synopsis

One of the major goals of an ecomorphological analysis is to correlate patterns of interspecific differences in morphology with patterns of interspecific differences in ecology. Information derived from functional morphological studies may provide a mechanistic framework supporting the correlation, but the move from a correlational relationship to a causal relationship requires experimental evidence that the interspecific morphological differences create performance differences and therefore ecological differences. The goal of this study was to examine ecomorphological relationships in the diets of cottid fishes (Scorpaeniformes: Cottidae) from the northeastern Pacific by using functional classifications of predators (based on their mode of attack) and of prey (based on their anti-predator defenses). The proposed ecomorphological relationships were tested by examining capture success among the cottids in attacks on prey with different escape abilities. As predicted on functional morphological grounds, both multivariate and univariate analyses indicated that the gravimetric importance of ‘elusive prey’ (i.e. fishes, shrimp, mysids, and octopods) was greater in cottid species with a larger relative mouth size. Supporting a causal link between morphology and ecology, performance tests indicated that larger-mouthed cottids had significantly higher capture success on Heptacarpus shrimp than did smaller-mouthed species. As predicted, there were no differences in capture success among predators regardless of their attack style or ecomorphological patterns in attack on crabs (Cancer and Petrolisthes spp.) when the crabs were presented on a sand surface (i.e. ‘easy prey’). Several difficulties still exist when trying to apply a functional group approach to ecomorphology. These include the behavioral plasticity of the predators, the confounding factor of evolutionary history in identifying correlated ecomorphological features, multiple morphological solutions to common functional problems, the limitations of traditional dietary studies during extremes of prey abundance, and an inadequate understanding of the anti- predator defenses of most prey, including modifications that occur during ontogeny or in different habitats.