The geographic distribution of the imperiled Barrens Darter, Etheostoma forbesi, and threats of hybridization with the closely related Fringed Darter, Etheostoma crossopterum.

The Barrens Darter, Etheostoma forbesi, is one of the most geographically restricted freshwater fish species in North America, with a distribution limited to headwater portions of nine streams in the western part of the upper Caney Fork, a tributary of the Cumberland River in Tennessee. This limited geographic distribution makes E. forbesi especially vulnerable to potential threats posed by human alterations to rivers and streams, and the risk of ecological competition and introgressive hybridization with the closely related Fringed Darter, E. crossopterum. Museum collection records and targeted surveys conducted since its description suggest that E. forbesi's present-day range does not include several streams it previously inhabited– some as recently as 15 years ago. We investigate the geographic distribution and variation in meristic traits of both E. forbesi and E. crossopterum in the upper Caney Fork system through the examination of all available museum vouchers, and assess phylogeographic patterns among populations using mitochondrial DNA sequence data. We report a newly discovered population of E. forbesi from the upper Collins River, and present evidence that E. crossopterumhas displaced E. forbesi in the Hickory Creek system and Mud Creek in the upper Barren Fork River system. Meristic differences between the species are slight, but and fin ray counts can provide a visual means of identifying species and potential hybrid individuals. Analysis of mtDNA variation indicates gene flow between E. forbesi and E. crossopterum. Our results suggest that sympatry of the two species in the western upper Caney Fork system is a potential threat to the persistence of E. forbesi.