The evolution of cranial design, diet, and feeding mechanisms in batoid fishes

The batoid fishes (electric rays, sawfishes, skates, guitarfishes, and stingrays) are a trophically and morphologically diverse clade in which the observed range of diets is a product of a feeding mechanism with few parts and therefore a limited number of functional interactions. This system allows an intriguing comparison to the complex network of associations in the feeding apparatus of bony fishes and an anatomically simple framework for investigations of the mechanisms underlying the evolution of functional and phenotypic diversity. We quantified morphology from reconstructed CT scans of 40 batoid species, representing more than half of the extant genera. We used pairwise comparisons to evaluate the extent of coevolution among components of the feeding apparatus and among morphologies and diets. These relationships were then used to predict diets in poorly studied taxa and in a reconstruction of the batoid ancestor. Although functionally there are fewer examples of convergence in the batoid feeding mechanism than in bony fishes, our data show multiple evolutions of similar dietary compositions underlain by a broad morphological diversity. Elements of the feeding apparatus evolved independently of one another, suggesting that decoupling components of the head skeleton created separate but interacting evolutionary modules that allowed trophic diversification. Our data imply that food habits exhibit strong independent and convergent evolution and that suites of morphologies are associated with certain diets; however, lack of behavioral data for this clade, and one example of divergent diets underlain by convergent morphology, caution against the assumption of simplistic relationships between form and function. We therefore urge future work to ground truth our study by testing the functional, dietary and evolutionary hypotheses suggested by our data.