Molecular phylogeny reveals a new genus of freshwater mussels from the Mekong River Basin (Bivalvia: Unionidae) Author Jeratthitikul, Ekgachai 088AC693-052D-4EBF-AB49-E57D7A922FC8 Animal Systematics and Molecular Ecology Laboratory, Department of Biology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand. ekgachai.jer@mahidol.ac.th Author Sutcharit, Chirasak C2E2FA6B-A3F9-4F33-B447-B59B1BD322D4 Animal Systematics Research Unit, Department of Biology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. chirasak.s@chula.ac.th Author Ngor, Peng Bun 77BC565D-7067-46C6-8744-A0C50A6E2B3F Faculty of Fisheries, Royal University of Agriculture, Khan Dongkor, P. O. Box 2696, Phnom Penh, 12401, Cambodia, Fisheries Administration, no. 86, Norodom Boulevard., PO Box 582, Phnom Penh, Cambodia and Wonders of the Mekong Project, c / o IFReDI, no. 86, Norodom Boulevard., PO Box 582, Phnom Penh, Cambodia. pengbun.ngor@gmail.com Author Prasankok, Pongpun 3F701F9E-05D4-4597-B712-426000E72C0C School of Biology, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand. prasankok@sut.ac.th text European Journal of Taxonomy 2021 2021-10-22 775 119 142 journal article 3902 10.5852/ejt.2021.775.1553 bec1ec23-b368-4653-bd22-700cf359ccc4 2118-9773 5593519 370D0699-44B3-4C39-BF60-F282552AB04F Genus Namkongnaia gen. nov. urn:lsid:zoobank.org:act: 77AD43CF-83EC-42B8-B021-A950D4E701A0 Type species Namkongnaia inkhavilayi gen. et sp. nov. , by present designation. Differential diagnosis The new genus is distinguished from other genera in Pseudodontini by having a narrow, elongated, and less inflated shell. It is also represented as a distinct clade in multi-locus phylogenetic analyses. Description Shell Medium-sized, thin, narrow and elongated, rather compressed, very inequilateral; anteriorly round; posteriorly round or somewhat pointed; umbonal area not elevated and usually eroded. Ligament very narrow. Hinge without dentition, posterior end of the hinge structure with V-shaped fossette. Anterior adductor muscle scar shallow, ovate, and fused with pedal retractor muscle scars; posterior adductor muscle scar very shallow. Excurrent aperture smooth, shorter than incurrent; incurrent with 1-2 rows of conical papillae. Gills elongated and slightly ribbed; anterior margin of inner gills slightly longer and wider than outer gills. Etymology The generic name “ Namkongnaia ” is from the word “Namkong” , a name for the Mekong River used by Thai and Lao peoples, and Greek word “naiad” meaning freshwater mussels. The name of this genus thus means “freshwater mussels from Mekong River”. Distribution Mekong River and its tributaries in Laos , Cambodia , and Thailand ( Fig. 3 ; Brandt 1974 ; Ng et al. 2020 ). The distribution range may include the Mae Klong River Basin in Thailand ( Brandt 1974 ), although the taxonomic status of the specimens recorded in Brandt (1974) needs to be confirmed. Remarks The new genus is currently composed of two species, which are confirmed by our multi-locus molecular data. Other potential species are probably among those previously recognized under the name ‘ Pilsbryoconcha lemeslei ’ by Brandt (1974) . This includes the specimens from ‘Bang Pae’, Mae Klong River Basin, Ratchaburi Province (USNM-786217, SMF-BR2663, and ANSP-H19041). These specimens show elongate and narrow shell, which are diagnostic characters of the genus, but differ from other species by having a minute concave ventral margin, and truncated posterior margin with a pointed posterior end. Further examination of fresh materials and DNA investigation would confirm the taxonomic status of these populations.