Three cyprinids often referred to as the Gila robusta complex, G. robusta, G. nigra, and G. intermedia, are morphologically similar and genetically indistinguishable at the currently recognized species level. Current taxonomy is based on purported morphometric and meristic differences that are detailed in a classification key; however, the ability of the key to reliably distinguish the species has recently come into question. Chubs were collected from locations in Arizona, and two analysis methods were used to predict species' identification success using the key: 1) correct assignment to species using cluster analysis and multinomial logistic regression; and 2) observer identification success by species. Cluster analysis and multinomial logistic regression correctly assigned only 62% and 74% of fish, respectively, to the assumed species designation. Identification success using both analysis methods was most successful for G. robusta (82% cluster; 82% regression), followed by G. intermedia (53% cluster; 80% regression), and G. nigra (49% cluster; 58% regression). Overall observer identification success was 54%, led by G. intermedia (68%), followed by G. robusta (63%) and G. nigra (33%). The high level of misidentification appears to be due to overlap in morphometric and meristic characters among assumed species groups. Although the three species are currently considered allopatric, sympatry was found in 88%, 76%, and 100% of locations in the cluster analysis, regression analysis, and observer analysis, respectively. These results indicate that the morphometric and meristic characters in the key do not consistently distinguish the three putative chub species. Because independent genetic analyses also fail to support the delineation of the three species, we consider G. robusta as a single polymorphic species a viable hypothesis. Furthermore, a recent formal taxonomic review of the three species conducted by the American Fisheries Society–American Society of Ichthyologists and Herpetologists Committee on Names of Fishes concluded the available morphological and genetic data (including a pre-publication version of this study) support recognition of only one species, G. robusta.
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Vol. 106 • No. 2