Mitogenomic and phylogenomic analyses identify a cohesive Western Atlantic lineage within the Narcine complex...

Accurate species delimitation within electric rays of the genus Narcine has been hindered by overlapping morphological characters and limited molecular resolution in previous single-locus studies. This study aims to evaluate phylogenetic relationships and species boundaries within the Narcine species complex across the Western Atlantic using complete mitochondrial genomes.

Seven complete mitogenomes were newly assembled from individuals representing distinct morphotypes sampled across geographically widespread Western Atlantic localities and analyzed together with publicly available reference sequences. Mitochondrial protein-coding genes (PCGs) were examined using concatenated nucleotide and amino acid datasets under partitioned maximum-likelihood frameworks. Both approaches recovered highly congruent topologies, consistently supporting a single, well-defined western Atlantic mitochondrial lineage with low internal divergence (0.04-2.13%). Species delimitation analyses based on multiple methods yielded partially congruent results but consistently identified a dominant lineage encompassing all Atlantic samples. In contrast, two Colombian reference mitogenomes formed a separate and highly divergent lineage relative to the Atlantic group, despite showing moderate divergence between them. Comparative mitogenomic analyses revealed conserved genome organization, nucleotide composition bias, codon usage, and transfer RNA (tRNA) structures. All PCGs evolved under strong purifying selection, with Ka/Ks ratios well below unity.

These results support mitochondrial genetic continuity across the Western Atlantic Narcine populations and do not provide mitochondrial evidence for multiple evolutionary lineages within the Western Atlantic. The marked mitochondrial divergence of Colombian reference mitogenomes highlights potential issues in sequence attribution and underscores the importance of data curation. Overall, complete mitochondrial genomes provide a robust framework for species delimitation and future integrative taxonomic assessments within Narcine.