ABSTRACT Siphonaria is a genus of panpulmonate marine limpets often mistaken for ‘true’ or patellogastropod limpets because of their remarkably convergent appearance. Among the c. 130 species of Siphonaria reported worldwide, S. japonica (Donovan, 1824) is the most commonly encountered species in the northwestern Pacific (NWP) region, including China, Japan and Korea. Siphonaria japonica shells are highly variable, leading to a prevalent view of a single species with considerable ecophenotypic variation. This study presents morphological (shell and radula) and molecular sequence data for the mitochondrial gene cytochrome c oxidase subunit I (COI) evidence leading to the discovery of a still undescribed species that has been previously mistaken for S. japonica in Korea and likely in other NWP localities. Our integrated analysis of shell and radula characters for selected sequenced Korean voucher specimens has revealed an overlooked species within the range of specimens considered to be S. japonica. The ‘tall form’ is distinguished from the ‘short form’ by a larger, oval shell with an elevated, posteriorly curved apex and a weakly undulated margin formed by alternating thick primary and thin secondary ribs, as well as a higher number of radula teeth per row with unicuspid and/or bicuspid mesocone in the innermost lateral teeth. In contrast, the ‘short form’ has a smaller, oval shell with a lower, flattened apex; a coarsely undulated margin due to strong primary ribs without secondary ribs; and fewer radula teeth per row, with a consistently bicuspid mesocone. Mitochondrial COI sequence analysis revealed a substantial genetic divergence (uncorrected p-distance 19.2–20.5%) between the two shell forms, exceeding intraspecific variation observed within each form. Phylogenetic analyses consistently recovered them as distinct, well-supported clades, indicating that they represent separate species. An extensive comparative analysis of both species across the NWP is required to investigate the distribution of the two shell forms and understand the ecological and phylogeographic factors influencing these co-occurring species.
Kim et al. (Fri,) studied this question.