An integrative approach to a revision of the freshwater mussel genus Songkhlanaia (Bivalvia, Unionidae), with the description of a new species Author Jeratthitikul, Ekgachai 0000-0002-3477-9548 Animal Systematics and Molecular Ecology Laboratory, Department of Biology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand Author Sutcharit, Chirasak 0000-0001-7670-9540 Animal Systematics Research Unit, Department of Biology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Author Prasankok, Pongpun 0000-0001-5967-223X School of Biology, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand text ZooKeys 2025 2025-01-28 1224 187 209 journal article 10.3897/zookeys.1224.140549 8F8AAC17-7284-46F8-A2B5-1BA03FDB54E1 Songkhlanaia moreleti ( Crosse & Fischer, 1876 ) comb. nov. Figs 2 C – F , 5 B , Table 1 Monocondylea [sic] tumida Deshayes & Jullien, 1876: 117–120 , pl. 5, figs 1–3 [in part; non Monocondylus tumidus Morelet, 1866 ]. Pseudodon moreleti Crosse & Fischer, 1876: 330 . Type Locality: “ les marécages qui avoisinent les rives du Mékong; dans un lac, à Préai-Bac, arroyo de Peam-Chelang; ďeau de la province de Compong-Soai ” [= Peam Chileang and Kampong Svay, Cambodia ]. Fischer 1891: 221 . Fischer and Dautzenberg 1904: 436 . Dautzenberg and Fischer 1905: 452 , 453. Pseudodon ( Pseudodon ) moreleti — Simpson 1900: 838 . Simpson 1914: 1094 , 1095. Haas 1924: 137 , 138. Haas 1969: 130 . Sundadontina moreleti — Bolotov et al. 2020: 10 , fig. 4 e. Graf and Cummings 2021: 22 . Bolotov et al. 2023: 13 . Pseudodon thomsoni Morlet, 1884: 401 , 402, pl. 13, figs 2, 2 a. Type Locality: “ Cambodge ” [= Cambodia ]. Morlet 1889: 166 . Fischer 1891: 221 . Fischer-Piette 1950: 154 . Zieritz et al. 2018 : supplementary material 1. Bolotov et al. 2023: 11 . Syn. nov. Margaritana thomsoni — Paetel 1890: 174 . Pseudodon ( Pseudodon ) thomsoni — Simpson 1900: 838 . Simpson 1914: 1092 , 1093. Pseudodon ( Bineurus ) thomsoni — Haas 1920: 336–337 . Haas 1924: 144 . Graf and Cummings 2007: 311 . ? Pseudodon thomsoni — Konopleva et al. 2021: 11 , fig. 3 h. Type material. Syntypes MNHN - IM- 2000–34623 ( 2 shells ). Syntype MNHN - IM- 2000-1800 ( 1 shell ) of Pseudodon thomsoni Morlet, 1884 , (inadvertently stated as holotype by Fischer-Piette, 1950 ). Other material examined. Thailand – Sa Kaeo Province • 1 shell ; Aranyaprathet District , Aranyaprathet Subdistrict , Tonle Sap Basin , Phrom Hot Stream ; 13.6694 ° N , 102.5210 ° E ; 31 Jan. 2015 ; E. Jeratthitikul leg.; MUMNH - UNI 0544 • 1 shell ; Aranyaprathet District , Aranyaprathet Subdistrict , Tonle Sap Basin , Phrom Hot Stream ; 13.6718 ° N , 102.5166 ° E ; MUMNH - UNI 3428 • 2 shells ; Watthana Nakhon District , Phak Kha Subdistrict , Tonle Sap Basin , Phrom Hot Stream ; 13.7490 ° N , 102.4271 ° E ; 5 May 2015 ; P. Prasankok leg.; MUMNH - UNI 0300 to 0301 . Diagnosis. Shell large, subrhomboidal to ovate, moderately thickened, rather inflated. Anteriorly constricted, dorsal slightly curved, slightly elevated posteriorly. Posterior slope with one or two faint folds. Umbo rounded, moderately elevated. Shell surface with irregular growth lines, roughened on posterior slope and border of shell. Right valve with one smooth tubercular pseudocardinal tooth, left valve with hill-like or triangulate pseudocardinal tooth, subcompressed. V-shaped furrow on posterior end of hinge structure prominent, wide. Anterior adductor muscle scar impressed, ovate, separated from anterior protractor muscle scar. Umbo cavity moderately deep. Differential diagnosis. This species is much larger and more inflated than the type species. Irregular growth lines on the shell surface are rougher, especially on the posterior slope and along the shell border. This species is also distinct from congenerics due to 14 fixed nucleotide substitutions in the COI gene fragment (Table 2 ). Distribution. Tonle Sap Basin in Thailand and Cambodia ( Crosse and Fischer 1876 ; Deshayes and Jullien 1876 ; this study), and the Lower Mekong Basin in Cambodia ( Morlet 1884 ) (Fig. 3 ). Comments. Originally, Songkhlanaia moreleti was described based on a partially misidentified specimen from Cambodia as Monocondylus tumidus Morelet, 1866 by Deshayes and Jullien (1876) . Crosse and Fischer (1876) reexamined the specimens and provided it with a new name, Pseudodon moreleti . This nominal species had been recognized as valid by subsequent studies for more than a hundred years (e. g., Fischer 1891 ; Fischer and Dautzenberg 1904 ; Dautzenberg and Fischer 1905 ; Simpson 1900 , 1914 ; Haas 1924 , 1969 ). Later, Brandt (1974: 271) and Graf and Cummings (2007: 311) listed this taxon as a junior synonym of either species or subspecies of ‘ tumidus Morelet, 1866 ’. Recently, Bolotov et al. (2020) raised this species as valid and placed it in their new genus, Sundadontina . However, this resurrection and generic placement seemed provisionally based on conchological characters alone. Fortunately, specimens collected from Sa Kaeo Province in Thailand (Fig. 2 E ), the location of the headwaters of the Tonle Sap Basin and the type locality for this species, have been found to match well with the syntypes (Fig. 2 C ). Furthermore, these specimens cluster within the phylogenetic position of the Songkhlanaia (Fig. 1 ). Therefore, we propose transferring this species to the more appropriate genus Songkhlanaia . Pseudodon thomsoni Morlet, 1884 was described based on specimens collected from Cambodia by Auguste Jean-Marie Pavie. Later, Morlet (1889: 166) detailed and specified the type locality as " Etang de Pnom-Penh ( Cambodge ) " [= pond in Phnom Penh, Cambodia ]. It was recognized as a distinct species for more than a century, until it was recently treated as a junior synonym of Thaiconcha callifera ( von Martens, 1860 ) by some authors ( Bolotov et al. 2020: 10 ; Graf and Cummings 2021: 22 ). The following year, it was resurrected as a valid species by Konopleva et al. (2021) and again by Bolotov et al. (2023) . However, the syntype of Pseudodon thomsoni Morlet, 1884 is relatively small (Fig. 2 D ; shell length 53 mm ), and its shell characteristics generally resemble those of young specimens of S. moreleti (Fig. 2 F ), such as the long obovate shell that is constricted anteriorly, moderately elevated umbo, and a bean-shaped anterior protractor scar. Furthermore, the type locality in ‘ Phnom Penh, Cambodia’ is in the lower Mekong Basin (Fig. 3 ), the same basin as the type locality of S. moreleti . Based on this conchological and biogeographic evidence, we thus synonymise this species with S. moreleti . The molecular data examined in this study included individuals of S. moreleti solely collected from the headwater areas of the Tonle Sap Basin in Thailand . In fact, previous freshwater mollusk surveys of areas surrounding the Tonle Sap Lake in Cambodia by Ng et al. (2020) did not recover any specimens identified as S. moreleti from over 40 sampling localities. This possibly suggests a low abundance or local disappearance from the area. Further intensive surveys throughout the basin, including the headwater area and its tributaries, may encounter more specimens, which would be beneficial for assessing the genetic viability and conservation status of this species.