Morphology and mitochondrial genome-based analysis of the systematics and evolution of Acanthochitona species (Polyplacophora: Acanthochitonidae)

Chitons, known as marine living fossils, have retained their ancestral traits for approximately 300 million years. The genus Acanthochitona (Polyplacophora: Acanthochitonidae), characterized by the presence of 9 pairs of sutural tufts on a well-expanded girdle, is distributed across the intertidal zones of South Korea, Japan, China, and the Indo-Pacific. This study examined five Acanthochitona species from South Korea: A. achates, A. circellata, A. defilippii, A. rubrolineata, and A. feroxa sp. nov. Their mitochondrial genome sequences ranged from 14,986 to 15,006 bp in length and with a gene content typical for Polyplacophora. Genetic (including a transitive consistency score [TCS] genetic network), principal coordinate, phylogenetic network, and CO1-based barcoding gap analyses confirmed a new species, A. feroxa sp. nov., which exhibited morphologically distinct dorsal spicules and radulae. Maximum likelihood (ML) and Bayesian inference (BI) trees were constructed based on the CO1 sequences of 28 polyplacophoran species belonging to 9 families, which placed these five Acanthochitona species within a monophyletic family, Acanthochitonidae. The analyses also indicated the polyphyletic nature of Mopaliidae, recommending a reclassification. Divergence time estimation revealed that Acanthochitona deviated during the Late Cretaceous (ca. 83.94 mya), with continued speciation occurring in the Paleogene and Neogene periods. Additionally, we constructed a pictorial key based on the ML tree for morphologically identifying the five Acanthochitona species. This study contributes to the understanding of speciation and phylogenetic relationships within the Acanthochitonidae, offering valuable insights into the classification scheme and mitochondrial genome evolution of chitons in the western Pacific.