Assessing the intraspecific osteological variation of the spottail shiner (Hudsonius hudsonius) (Cypriniformes: Leuciscidae).

Hudsonius hudsonius (Cypriniformes: Leuciscidae), commonly known as the spottail shiner, is a small cyprinoid fish species found across much of North America. H. hudsonius has traditionally been regarded as one of the most basal and plesiomorphic species among the notropin fishes, that is, fishes traditionally placed in or associated with the large, polyphyletic genus Notropis, including but not limited to Alburnops, Cyprinella, Ericymba, Lythrurus, Miniellus and Paranonotropis. The basal nature of H. hudsonius has been further corroborated by recent molecular studies. The osteology of notropin fishes has either focused on a specific region of the skeleton, including the cranium, pharyngeal teeth and dental formulae, or the caudal skeleton. Others have aimed to determine evolutionary relationships using osteological characteristics. However, no research has focused on the osteology of H. hudsonius specifically nor emphasized the intraspecific osteology of a single notropin species. Thus, this study aimed to describe and analyse key osteological characters of H. hudsonius, highlighting key osteological variants within and among 15 populations from Alberta, the Northwest Territories, Manitoba and Ontario. Particular attention is given to the elements of the oral jaws, suspensorium, opercular series, branchial apparatus, hyoid region, circumorbital series, pectoral girdle, pelvic girdle, caudal skeleton and skull. My analysis of the osteology of H. hudsonius reveals variation in all examined regions except the pelvic girdle. Regional differentiation between eastern and western populations is especially evident in the degree of fusion of the pectoral girdle, fusion of the parhypural and first hypural of the caudal skeleton and shape of the urohyal. The results of this analysis provide an overview of the osteology of a basal notropin species and highlight which regions are subject to variation. It also raises further questions about the current taxonomic classification of H. hudsonius based on the regional differences observed here.