Sukanya MuantaEkgachai JeratthitikulSomsak PanhaPongpun PrasankokChulalongkorn UniversitySuranaree University of TechnologyMahidol University2020-01-272020-01-272019-01-01Journal of Molluscan Studies. Vol.85, No.2 (2019), 224-23114643766026012302-s2.0-85071285667https://repository.li.mahidol.ac.th/handle/20.500.14594/49872© 2019 The Author(s). Published by Oxford University Press on behalf of The Malacological Society of London, All rights reserved. The freshwater bivalve genus Ensidens Frierson, 1911 is widely distributed in Thailand and the surrounding parts of mainland Southeast Asia. While the identification of the two currently accepted species, Ensidens ingallsianus (Lea, 1852) and E. sagittarius (Lea, 1856) is based on external morphology, classification using only morphology can lead to underestimation of species diversity and misidentification, due to phenotypic plasticity. Here we present the results of a phylogeographic study of these two nominal species based on 82 individuals and using DNA sequence data from mitochondrial (COI and ND1) and nuclear (ITS1) gene fragments. Phylogenetic trees based on maximum likelihood and Bayesian inference revealed that both species contain deeply divergent clades that may represent previously unrecognized species. The six phylogenetic lineages within Thai Ensidens correspond to the geographic structure of Thailand's river basins. These six consist of two lineages of E. ingallsianus from basins draining to the Gulf of Thailand (Chao Phraya, Mae Klong, Phetchaburi, Bang Prakong, Prasae and Ta Pi) and the Mekong River (Mun and Tonle Sap), and four lineages of E. sagittarius from the Khorat basin (Mekong). These geographically disjunct lineages are hypothesized to have formed during the complex drainage history of Indochina. The presence of multiple lineages of Ensidens endemic to the northern Khorat basin emphasizes the conservation importance of this region.Mahidol UniversityAgricultural and Biological SciencesArticleSCOPUS10.1093/mollus/eyz013