Systematics, Gill Raker Morphology, and Pharyngeal Arch Development of Suckers (Cypriniformes: Catostomidae)

Abstract

Morphological diversity is shaped by past evolutionary history, function, and ontogeny. Evolutionary history plays an important role in shaping morphological diversity and morphology itself can affect the future evolutionary trajectory of taxa. Morphology is shaped by function through selection and function can be constrained by morphology. Differences among taxa in their morphology arise due to changes in the pattern of development in those taxa. Therefore, a full understanding of morphological diversity requires knowledge about evolutionary history, morphological function, and ontogeny. Suckers, family Catostomidae, are a group of freshwater fish with interesting trophic morphology and feeding habits. I used molecular sequence data and fossil calibrations to reconstruct the phylogeny and divergence times of the families of Cypriniformes, including Catostomidae. I found evidence that gene choice, base compositional heterogeneity, and rate heterogeneity provide challenges to reconstructing the evolutionary history of the order and I found that body size is correlated with the rate of molecular evolution not only in Cypriniformes but in many groups of fishes. Suckers, like most fish, have gill rakers which are complex, repetitive, finger-like projections in the interior of the branchial arches that are important in feeding and vary among species. I quantified and compared gill raker morphology among sucker species and found that multiple aspects of gill raker morphology interact to predict function. Gill rakers are just one component of the complex trophic apparatus of suckers. I described and compared the sequence of chondrification and ossification of the paired elements of the pharyngeal arches and the histological development of the palatal organ, chewing pad, and pharyngeal teeth in five species of suckers and found several examples where heterochrony has shaped the trophic morphology of suckers.