Data from: A new species of spottail darter endemic to the Clarks River in Kentucky and Tennessee (Percidae: Etheostomatinae: Etheostoma)

<p><em>Etheostoma xanthovum</em>, the Clarks Darter, is described as a new species endemic to the Clarks River in Kentucky and Tennessee, USA. <em>Etheostoma xanthovum</em> was previously delimited as <em>Etheostoma oophylax</em>, the Guardian Darter, based on morphological trait data. Subsequent to the description of <em>E. oophylax</em>, molecular phylogenetic analyses consistently resolved specimens from the Clarks River system and <em>E. chienense</em>, the Relict Darter, as sister species, which together formed a sister clade to all other sampled populations of <em>E. oophylax</em>. Our analyses of morphological traits, mitochondrial DNA (mtDNA), and genomic sampling using double digest restriction-site associated DNA (ddRAD) sequencing support the distinctiveness of <em>Etheostoma xanthovum</em>. Morphologically, <em>Etheostoma xanthovum</em>, differs from <em>Etheostoma oophylax</em> in the number of dorsal fin rays (12 versus 11), anal fin rays (8 versus 7), and in the number of scale rows around the caudal peduncle (21.8 versus 20.37). <em>Etheostoma xanthovum</em> does not share mtDNA haplotypes with <em>Etheostoma oophylax</em> or <em>Etheostoma chienense</em>. Phylogenomic analysis of an average of 28,448 ddRAD loci per sampled specimen resolves <em>Etheostoma xanthovum</em> and <em>Etheostoma chienense</em> as sister species, and assessment of genomic divergence supports the hypothesis that each of these two species represents a distinct and independently evolving lineage. In addition, we report a range extension of <em>Etheostoma oophylax</em> in the Obion River system, a direct tributary of the Mississippi River. </p>