One becomes three: An integrative morphological and molecular analysis of the windowpane oyster <i>Placuna</i> (Bivalvia: Pectinida) reveals new species

For decades, many marine animals have been considered to exhibit cosmopolitan or transoceanic distribution. This situation is prevalent in Asia, where many species were collected and named by American or European experts in the 1700s to early 1900s. Using the windowpane oysters <i>Placuna</i>-a small genus of bivalves with five recognized species-we show that careful analysis is required to reassess the validity of these species. Currently, only two species of <i>Placuna</i> (<i>P. placenta</i> and <i>P. ephippium</i>) widely reported in the Indo-Pacific region have been recorded from Chinese coastal waters. Here, we described two new species of <i>Placuna</i> from China. <i>Placuna vitream</i> sp. nov. can be distinguished from <i>P. placenta</i> by its larger ridge angle. Phylogenetic analysis using five gene fragments fully supported that <i>P. vitream</i> sp. nov. is a sister to the specimen from Singapore identified as <i>P. placenta</i> and more distant from other <i>Placuna</i> species with available molecular data. Besides, based on subfossil shells, we describe <i>Placuna aestuaria</i> sp. nov. that differs from its congeneric species by its broad hinge, medium ridge angle, and nearly straight ridges. Finally, we suggest a combination of hinge structure and ridge angle that can be used for identifying <i>Placuna</i> species and preparing a key to this genus. Our findings of two new species expand the diversity of <i>Placuna</i> and prompt reassessment of the many presumably widely distributed marine species in Asia.