Complete mitochondrial genomes defining two distinct lancelet species in the West Pacific Ocean

For many years, the lancelet Branchiostoma belcheri had been considered a widely distributed species throughout the West Pacific Ocean. Several recent studies divided it into two separate species based on significant morphological and genetic divergence. However, this taxonomic subdivision has not yet been fully accepted by the scientific community. In order to further clarify the taxonomic status of the West Pacific lancelets, we present here complete mitochondrial genomes (mitogenomes) of two species and compare them at both genomic and phylogenetic levels. These comparisons reveal a significant genetic differentiation and show a deep phylogenetic division. Moreover, to get more insights into the phylogeography of this species complex, we sampled lancelets from Qingdao and Maoming (China) and sequenced their 12S rRNA gene, then combined the data with those from Xiamen and Japan lancelets for phylogenetic analysis. The results were consistent with our previous identification and proved that the lancelets distributed in Qingdao and Japan as well as B. japonicum in Xiamen waters belong to the same species, and those in Maoming and B. belcheri in Xiamen waters are different. Finally, we discuss the implications of these findings on the distribution of lancelets in the West Pacific Ocean.