New mitochondrial genomes of Saccostrea (Mollusca, Ostreidae) specimens from Hainan Island

The taxonomic classification of oysters, particularly within the genus <i>Saccostrea</i>, remains poorly resolved due to limitations in traditional morphological and single-gene approaches. To address this, we sequenced the mitochondrial genomes of four <i>Saccostrea</i> specimens (<i>Saccostrea</i> MBSR 211-214) from Hainan Island. The complete mitochondrial genomes (16,280-16,289 bp) revealed a conserved gene arrangement, including 12 protein-coding genes (PCGs), 23 transfer RNA (tRNA), and two ribosomal RNA (rRNA) genes on a single strand, with a fragmented 16S rRNA gene as a distinctive feature. Different OTU delimitation methods consistently identified these four lineages as two novel operational taxonomic units (OTUs). Within one of the OTUs, there are high levels of intraspecific divergence, exhibiting the potential for independent evolution. Our analysis reclassified the publicly available mitochondrial genomes of <i>Saccostrea</i> into nine OTUs, further revealing significant mislabeling on GenBank as indicated by previous studies. The study adds new mitogenomic resources for Saccostrea and provides data useful for future integrative taxonomic work.