Comparative species delimitation of a biological conservation icon.

The United States Endangered Species Act (ESA) of 1973 set a precedent for biodiversity conservation across the globe.¹ A key requirement of protections afforded by the ESA is the accurate delimitation of imperiled species. We present a comparative reference-based taxonomic approach to species delimitation that integrates genomic and morphological data for objectively assessing the distinctiveness of species targeted for protection by governmental agencies. We apply this protocol to the Snail Darter (Percina tanasi), a freshwater fish from the Tennessee River that was discovered in 1973 and declared an endangered species under the ESA in 1975.² Concurrently, the Snail Darter's habitat was slated to be destroyed through the construction of the Tellico Dam by the Tennessee Valley Authority (TVA),^3,4 inspiring nationwide protests advocating for the suspension of the federal project. This David versus Goliath struggle between supporters of the 3-inch fish and the TVA culminated in the first major legal conflict over protections afforded by the ESA, the US Supreme Court case Hill v. TVA, 437 U.S. 153 (1978), with a 6 to 3 ruling in favor of protecting the Snail Darter and interrupting the completion of the Tellico Dam. Here, we integrate multiple lines of evidence in a comparative framework to demonstrate that despite its legacy, the Snail Darter is not a distinct species but is a population of the Stargazing Darter (Percina uranidea) described in 1887. These results illustrate how a reference-based framework for species delimitation dramatically aids the proper direction of efforts toward protecting biodiversity.