MORPHOLOGICAL CHARACTERISTICS, TAXONOMIC STATUS REVISION, AND PHYLOGENETIC ANALYSIS OF YLISTRUM JAPONICUM

<italic>Amusium japonicum</italic> is an important economic scallop species in China, and there has been a long-term phenomenon of homonym. In this study, the morphology of <italic>A. japonicum</italic> was studied using traditional morphological, molecular biological, and geometric morphological methods. The study aimed to identify the differences between <italic>A. japonicum</italic> and similar species and re-describe and revise its taxonomic classification in the South China Sea, thereby contributing to scallop taxonomy and phylogenetics research. The results indicated that <italic>A</italic>. <italic>japonicum</italic> occupied a distinct phylogenetic position relative to <italic>A. pleuronectes</italic>. and the taxonomic status of Chinese <italic>A</italic>. <italic>japonicum</italic> was corrected for the first time, from the genus <italic>Amusium</italic> into a newly established genus <italic>Ylistrum</italic>, with the updated scientific name <italic>Ylistrum japonicum</italic>. A maximum likelihood phylogenetic tree clearly separated <italic>Y.</italic><italic>japonicum</italic> from <italic>A. pleuronectes</italic>, highlighting their divergent evolutionary lineages. There are 34—52 inner ribs in the left shell of <italic>Y.</italic><italic>japonicum</italic>, with an average of 40, and 43—55 in the right shell, with an average of 47, and <italic>Y</italic>. <italic>japonicum</italic> is gonochorism. There are 22—30 internal ribs in the left shell of the <italic>A. pleuronectes</italic>, with an average of 25, and 22—30 in the right shell, with an average of 26, androgynous. According to divergence time estimation, the divergence time of the <italic>Y.</italic><italic>japonicum</italic> is approximately 108.47 million years ago (Mya), corresponding to the early Cretaceous period of the Mesozoic era.