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| Namkongnaia inkhavilayi gen. et sp. nov. Namkongnaia lemeslei (Morelet, 1875) Jeratthitikul, Sutcharit, Ngor & Prasankok, 2021 |
ABSTRACT
A new genus of freshwater mussels (Bivalvia: Unionidae) is described from the Mekong River Basin as Namkongnaia gen. nov. The validity of the new genus is supported by its unique conchological characteristics, namely the lack of hinge dentition and elongated shells, together with its evolutionary distinctiveness as estimated by multi-locus phylogenetic analyses (mitochondrial COI and 16S, and nuclear 28S genes). The new genus includes two lineages with deep divergence, shown by 5.10% uncorrected COI p-distance. One lineage is a type species described herein as Namkongnaia inkhavilayi gen. et sp. nov. The other is a recognized species under the name ‘Pilsbryoconcha lemeslei (Morelet, 1875)’. Molecular phylogenetic analysis further shows that the new genus belongs to the tribe Pseudodontini, and evolutionarily is closely related to the genus Monodontina Conrad, 1853. However, its conchology is similar to the genus Pilsbryoconcha Simpson, 1900. Time-calibrated phylogeny suggests that the main radiation events of the tribe Pseudodontini occurred during the Late Cretaceous to the Eocene, with the divergence between the new genus and Monodontina placed in the Miocene. The discovery of new freshwater mussel taxa in this study highlights the importance of the Mekong River Basin as one of the world’s biodiversity hotspots for freshwater fauna.
Keywords: Indochina, freshwater mussels, multi-locus phylogeny, new taxa, time-calibrated phylogeny
Family Unionidae Rafinesque, 1820
Subfamily Gonideinae Ortmann, 1916
Tribe Pseudodontini Frierson, 1927
Subtribe Pilsbryoconchina Bolotov et al., 2017
Genus Namkongnaia gen. nov.
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 in equilateral; 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.
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| Map showing distribution localities of Namkongnaia gen. nov. Boundaries of river basins follow Abell et al. (2008). |
Namkongnaia inkhavilayi gen. et sp. nov.
Pilsbryoconcha lemeslei – Brandt 1974: 263, pl. 18 fig. 22 (in part, only records from “Nang Rong (Prov. Burirum)”, Thailand) [non Anodonta lemeslei Morelet, 1875].
Etymology: This new species is dedicated to our colleague and distinguished malacologist, Dr Khamla Inkhavilay, from the National University of Laos
Namkongnaia lemeslei (Morelet, 1875) gen. et comb. nov.
Distribution: This species is restricted to Tonle Sap Lake and its tributaries in Cambodia. Its distribution probably reaches the headwater of Tonle Sap basin in eastern Thailand (Brandt 1974).
Habitat: This species was recorded in mud substrate in still sections of rivers or in lentic habitat, i.e., ponds and lakes (Morelet 1875; Morlet 1889; Brandt 1974).
Distribution: This new species appears to occur in the Lower Mekong watershed. In Laos, it was recorded from several locations near Vientiane, Kammoune, and Champasak Provinces. In Thailand, it is known from the Songkram and Mun river basins in the northeast region (Brandt 1974).
Ekgachai Jeratthitikul, Chirasak Sutcharit, Peng Bun Ngor and Pongpun Prasankok. 2021. Molecular Phylogeny reveals A New Genus of Freshwater Mussels from the Mekong River Basin (Bivalvia: Unionidae). European Journal of Taxonomy. 775(1), 119-142. DOI: 10.5852/ejt.2021.775.1553