Geometric morphometric analysis of skeletal shape variation across the pleuronectiformes

Pleuronectiformes, commonly called flatfishes, is a large order of highly specialized fishes that display two eyes on one side of the head. Comprised of approximately 716 species, flatfishes share many similar characteristics. However, the complex history of the classification of the group reveals the diversity of shape across the order. This study focused on the diversity of shape across the order by examining skeletal elements. Landmark-based geometric morphometrics was used to visualize shape variation across the order of Pleuronectiformes. A total of 457 specimens were radiographed from collections at the University of Kansas Natural History Museum and the Smithsonian National Museum of Natural History. Sixteen landmarks and one curve were digitized and superimposed using a generalized least squares Procrustes superimposition. A multivariate analysis was performed on all individuals of Pleuronectiformes using a principal component analysis (PCA). PCA analyses were performed on each family individually to examine the shape variation among genera. The PCA of all Pleuronectiformes showed a difference in shape among families. The results of the multivariate analysis revealed tight clustering and clear separation for some families, but showed broad scattering and significant overlap in others. Psettodidae was revealed to have tight clustering and clear separation from the other Pleuronectiformes, suggesting Psettodidae is morphologically distinct from other Pleuronectiformes. Morphological analysis suggests that some families with specialized features had a more conserved shape, whereas some families with generalized characters had greater variation in shape. Paralichthyidae, a non-monophyletic family, showed variation between the three distinct lineages on the PCA of all Pleuronectiformes, suggesting variation in shape across the three lineages. Tephrinectes, a genus of Paralichthyidae which has been suggested to be removed and elevated to the family level, showed no distinct variation in shape from other genera in Paralichthyidae. This result suggests that Tephrinectes did not vary in shape from other genera within Paralichthyidae. Multivariate analysis showed little variation across most genera of Bothidae; however, four genera showed distinct shape within the morphospace, suggesting Bothidae had a large variation in shape across the genera. Furthermore, genera within Achiridae showed distinct shape variation grouped by habitat type (i.e. freshwater, brackish water, and saltwater species) suggesting convergence of shape based on life history. This study is novel in applying landmark-based geometric morphometric methods to shape variation in skeletal elements across the order of Pleuronectiformes. By focusing on skeletal elements, this study helps to clarify shape variation in relation to phylogenetic hypotheses and illustrates the large morphological diversity that flatfishes represent