DNA barcoding and <i>COI</i> -based phylogenetic placement of <i>Ellochelon vaigiensis</i> (Mugiliformes: Mugilidae) from the Vietnamese Mekong Delta

The squaretail mullet (Ellochelon vaigiensis) is widely distributed in Indo-Pacific coastal and estuarine systems, yet molecular data from the Vietnamese Mekong Delta remain limited. In this study, we generated 12 new mitochondrial cytochrome c oxidase I (COI) sequences from specimens collected in the Mekong Delta and integrated them with 36 publicly available sequences to evaluate their phylogenetic placement within the species. All sequences were standardized to a 621 bp overlap and analysed using Maximum Likelihood phylogenetic inference. COI variation within the Vietnamese samples included 56 segregating sites defining five haplotypes (Hd = 0.79), with nucleotide diversity (π) = 0.03 and an average of 15.99 pairwise differences. In the global phylogenetic framework (n = 48), all Vietnamese specimens clustered within the E. vaigiensis clade. However, two individuals formed a well-supported mitochondrial sublineage showing relatively elevated COI divergence compared with other regional samples. These results confirm the taxonomic identity of E. vaigiensis in the Mekong Delta and provide the first standardized COI barcode reference dataset for the region. The observed mitochondrial lineage divergence warrants further investigation using additional nuclear markers and broader geographic sampling.