Integrative Taxonomic Reappraisal and Evolutionary Biogeography of the Most Diverse Freshwater Mussel Clade from Southeast Asia (Pseudodontini)

Abstract

Freshwater mussels belonging to the tribe Pseudodontini (Bivalvia: Unionidae: Gonideinae) play a keystone role in riverine and lacustrine environments throughout Southeast Asia. Many of them are narrowly endemic and habitat specialists, which need special conservation efforts. Unfortunately, the systematics of this group is rather poorly understood. Here, we show that Pseudodon inoscularis, the type species of the genus Pseudodon, belongs to the eastern clade of the tribe (Southeast Asia, east of the Salween Basin), and, more certainly, clusters with members of the genus Monodontina. Hence, the latter genus becomes a synonym of Pseudodon, while the subtribe Pseudodontina (=Pilsbryoconchina syn. nov.) should be used as a name for the eastern clade. In turn, the genus Indopseudodon stat. rev. and the subtribe Indopseudodontina subtr. nov. must be applied as valid names for the western clade of the Pseudodontini (Ayeyarwady to Salween). We present a revision of the genus Indopseudodon with an updated synonymy and a description of a new species, I. indawgyiensis sp. nov. We also link four species-level genetic lineages from the Mekong to available taxonomic names in the genera Pseudodon and Sundadontina (Pseudodon cambodjensis, P. vagulus, Sundadontina harmandi, and S. sulcata). Based on morphological features, we revise the taxonomic position of several recent and fossil species that were traditionally placed in the Pseudodontini and transfer them to the genera Parvasolenaia, Pseudodontopsis, Ptychorhynchus, Simpsonasus nom. nov. (a new name for Nasus Simpson, 1900) (Gonideinae: Gonideini), Pseudobaphia (Unioninae: Unionini), Balwantia and Lamellidens (Parreysiinae: Lamellidentini).

1. Introduction

The freshwater mussel tribe Pseudodontini (Bivalvia: Unionidae) represents the most species-rich group of large bivalves in freshwater systems of the mainland part of the Sundaland Subregion (i.e., the Mekong, Chao Phraya, and Mae Klong rivers, as well as the drainages of the Thai–Malay Peninsula) [1,2,3,4,5,6]. Furthermore, a small but remarkable radiation of these mussels does occur in the Western Indochina Subregion, with several species recorded from the Salween (officially Thanlwin), Sittaung, Bago (historically Pegu), and Ayeyarwady (also known as Irrawaddy) river basins in Myanmar [5,7].

Many Pseudodontini species have restricted ranges and specific environmental requirements, being habitat specialists that prefer smaller rivers and streams in primary tropical forests [5,7,8,9,10,11]. Special conservation and collaborative research efforts are urgently needed for such taxa [6,12]. Several narrowly endemic species are probably extinct from the Thai–Malay Peninsula and Borneo, whereas the tropical invasive lineage of Sinanodonta woodiana (Lea, 1834) is now common and widespread in these localities [10,13,14]. There are some concerns of a possible invasive potential of generalist species such as Pilsbryoconcha spp., which could spread through ornamental trade [15]. Conversely, notes on the establishment of a non-native population of Monodontina vondembuschiana (Lea, 1840) in Singapore [16,17] were not confirmed, as it is in fact the tropical lineage of Sinanodonta woodiana [18], which is already widespread throughout Malaysia and Indonesia [19,20].

Despite the fact of high environmental and conservational importance of the Pseudodontini throughout mainland and insular Southeast Asia [14,21,22,23], this tribe deserves little attention from researchers and its systematics are rather poorly understood [5]. Traditionally, only two genera, i.e., Pseudodon Gould, 1844 (with several subgenera) and Pilsbryoconcha Simpson, 1900, were accepted as valid [2,3,6,24,25]. However, a growing body of modern phylogenetic research revealed that the Pseudodontini is a taxonomically rich tribe with several well-defined genera as follows: Bineurus Simpson, 1900; Monodontina Conrad, 1853; Namkongnaia Jeratthitikul et al., 2021; Nyeinchanconcha Bolotov et al., 2020; Pilsbryoconcha Simpson, 1900; Pseudodon Gould, 1844; Songkhlanaia Konopleva et al., 2023; Sundadontina Bolotov et al., 2020; and Thaiconcha Bolotov et al., 2020 [5,8,9,11,26].

From a phylogenetic point of view, all of the Pseudodontini taxa could be delineated into two large clades, the ranges of which are separated by the Salween–Mekong drainage divide [5,7]. Taxonomically, these clades correspond to the subtribes Pseudodontina s. str. (west of the drainage divide in Myanmar—one genus, Pseudodon) and Pilsbryoconchina, Bolotov, Vikhrev & Tumpeesuwan, 2017 (east and southeast of the drainage divide in Laos, Thailand, Cambodia, southern Vietnam, Malaysia, and Indonesia—other genera) [1,5,7]. The identity of the nominal taxon Pseudodon inoscularis (Gould, 1844), the type species of this genus [27], is the most enigmatic taxonomic puzzle, related to the systematics of the entire tribe Pseudodontini. Recently, it was considered a species belonging to the western clade (i.e., the Pseudodontina) based largely upon biogeographic evidence [1,5,7], as it was described from southeastern Myanmar [27]. However, its identity and phylogenetic affinities have never been tested by means of a modern molecular approach.

This study (1) assess the phylogenetic position of Pseudodon inoscularis based on DNA analyses of newly collected topotypes; (2) presents a taxonomic reappraisal of the Pseudodontini systematics with a description of a new subtribe; (3) revises the taxonomy of the western clade of the Pseudodontini from Myanmar using new topotypes of all of the nominal taxa described from the country; and (4) proposes an updated biogeographic reconstruction on the origin and diversification of the tribe. Here, we describe a new species in the genus Indopseudodon Prashad, 1922 stat. rev., as well as assign four genetic lineages to already available specific names in the genera Pseudodon stat. rev., and Sundadontina. Most of these species were discovered during our recent sampling efforts in Myanmar and Laos. We reconsider the taxonomic position of several genera and species from China and northern Vietnam, which were traditionally placed in the genus Pseudodon [1,25], and tentatively transfer these taxa to other tribes based on morphological and biogeographic evidence. Finally, we present a brief overview of available fossil members that were thought to belong to the tribe Pseudodontini with several new combinations.

2. Materials and Methods

2.1. Data Collection and Morphological Studies

New samples of freshwater mussels, including specimens of the new Indopseudodon species and topotypes of other nominal taxa, were collected from Myanmar and Laos in 2020 (Figure 1, Datasets S1–S4). Additional information such as shell images and distribution data were obtained from the MUSSELp Database (http://mussel-project.uwsp.edu, accessed on 5 June 2023) [1], as well as from several museum collections as follows: MCZ—The Louis Agassiz Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA; MNHN—Muséum National d’Histoire Naturelle, Paris, France; NHMUK—Natural History Museum, London, United Kingdom; RMBH—Russian Museum of Biodiversity Hotspots, Federal Center for Integrated Arctic Research of the Ural Branch of the Russian Academy of Sciences, Arkhangelsk, Russia; UMMZ—University of Michigan Museum of Zoology, Ann Arbor, MI, USA.; NCSM—North Carolina Museum of Natural Sciences, Raleigh, NC, USA.

Comparative morphological analyses were carried out, estimating the differences in shell shape, umbo position, sculpture, hinge structure, and development of muscle attachment scars [28,29]. The shell proportions in two lineages of the new species Indopseudodon indawgyiensis sp. nov. were studied based on the shell convexity index (SCI = width/length ratio × 100) and the shell elongation index (SEI = height/length ratio × 100) [30]. Differences in shell proportions between two lineages were assessed using a nonparametric Mann–Whitney test due to the rather small size of samples (<50), as shown in Bolotov et al. [28].

2.2. Molecular Analyses

New sequences of the mitochondrial cytochrome c oxidase subunit I (COI) and 16S ribosomal RNA (16S rRNA), and the nuclear 28S ribosomal RNA (28S rRNA) genes were obtained for specimens collected from different localities of Laos and Myanmar (Datasets S1–S3). PCR amplification and sequencing were carried out using primers and the corresponding protocols for Unionidae, as described in our previous works [4,7]. Available sequences from GenBank, published in a series of earlier studies, were also added [4,5,7,8,9,14,26,31,32,33] (Dataset S1).

Molecular diagnosis for the new species was designed based on available Indopseudodon sequences through a Toggle Conserved Sites tool of MEGA11 at a 50% level [34], as described in our previous works [5,7]. The uncorrected COI p-distances between species of Indopseudodon were also calculated using MEGA11 [34].

2.3. Phylogenetic Analyses and Divergence Time Estimation

A multi-locus phylogeny (3 codons of COI + 16S rRNA + 28S rRNA) was reconstructed based on 44 haplotypes of the Pseudodontini, with Yaukthwa elongatula Bolotov et al., 2019; Y. nesemanni Konopleva, Vikhrev & Bolotov, 2017; and Trapezoideus mitanensis Bolotov et al., 2022 (Unionidae: Gonideinae: Contradentini) as an outgroup (Dataset S1). Gene fragments were aligned separately in MEGA11 using the MUSCLE algorithm [34]. Aligned sequences of COI, 16S rRNA, and 28S rRNA gene fragments were subsequently combined into one combined alignment using an online FASTA sequence toolbox, FaBox v1.61, Aarhus, Denmark (https://birc.au.dk/~palle/php/fabox/ [35], accessed on 1 May 2023).

Figure 1. Distribution of freshwater mussels from the tribe Pseudodontini in mainland Southeast Asia. The color filling indicates the general ranges of the subtribes: Indopseudodontina subtr. nov. (light green) and Pseudodontina (light red). The color stars indicate the type localities and color circles indicate occurrences of Indopseudodon species and Pseudodon inoscularis: I. bogani comb. nov. (pink); I. crebristriatus comb. nov. (violet); I. indawgyiensis sp. nov. (dark yellow); I. kayinensis (dark blue); I. salwenianus comb. rev. (dark green); †I. rostratus (black); and P. inoscularis (red) (Dataset S4). The red line indicates the approximate boundary of the Burma Terrane. The map was created using ESRI ArcGIS 10 software (https://www.esri.com/arcgis, accessed on 5 May 2023).

Figure 1. Distribution of freshwater mussels from the tribe Pseudodontini in mainland Southeast Asia. The color filling indicates the general ranges of the subtribes: Indopseudodontina subtr. nov. (light green) and Pseudodontina (light red). The color stars indicate the type localities and color circles indicate occurrences of Indopseudodon species and Pseudodon inoscularis: I. bogani comb. nov. (pink); I. crebristriatus comb. nov. (violet); I. indawgyiensis sp. nov. (dark yellow); I. kayinensis (dark blue); I. salwenianus comb. rev. (dark green); †I. rostratus (black); and P. inoscularis (red) (Dataset S4). The red line indicates the approximate boundary of the Burma Terrane. The map was created using ESRI ArcGIS 10 software (https://www.esri.com/arcgis, accessed on 5 May 2023).

Maximum Likelihood (ML) phylogenetic analysis was performed using the online server for IQ-TREE v1.6.12 (W-IQ-TREE) with automatic identification of the evolutionary models [36], ultrafast bootstrapping algorithm (UFBoot) and 5000 replicates [37]. The best-fit models of sequence evolution for each partition were as follows: F81 (1st codon of COI); GTR + G (2nd codon of COI); TN + G + I (3rd codon of COI); TIM2 + G + I (16S rRNA); and TIM3 + G (28S rRNA). The mitochondrial COI tree of Pseudodontini was reconstructed using the ML method as stated above.

A time-calibrated phylogeny (COI + 16S rRNA + 28S rRNA) was based on the same dataset as that for the multi-locus ML phylogeny (see above) and was reconstructed in BEAST v2.6.6 [38]. Instead of using the estimated best-fit models, we used the less complex HKY model with corresponding distributions for each partition to avoid overparameterization [4]. We used an external COI evolutionary rate estimated for the Unionidae (1.5 × 10⁻⁹ substitutions per site per million years) [39], a strict molecular clock, and the Yule speciation process as model priors [40]. Three independent runs of 100,000,000 generations were processed for the analyses, with sampling every 1000th generation. The resulting tree sets were combined using LogCombiner v2.6.6 with 10% burn-in and additional resampling every 10,000 generations [38]. The ESS values were checked using Tracer v1.7 [41]. A maximum clade credibility tree was computed with TreeAnnotator v2.6.6 [38].

2.4. Biogeographic Modeling

The ancestral area reconstruction was processed through BioGeoBEARS packages [42,43] implemented in RASP v4.2 [44]. We used a set of primary trees and a maximum clade credibility tree that were generated in the BEAST v2.6.6 analysis (see above). The outgroup was removed from the initial dataset using the appropriate option in RASP v. 4.2. We assigned two possible ancestral areas: the Sunda Plate (A) and Burma Terrane (B) [32]. To select the most appropriate biogeographic models, we carried out a comparative analysis of the relative probability of BioGeoBEARS models (DEC, DIVALIKE, and BAYAREALIKE) using the log-likelihood (LnL), AICc, and model weight according to ΔAICc (AICc wt). The appropriate model was implemented in Statistical BioGeoBEARS analysis (S-BGB).

3. Results

3.1. Macroevolution and Biogeography of the Tribe Pseudodontini

Our expanded multi-locus phylogeny (COI + 16S rRNA + 28S rRNA gene fragments; see Dataset S1 for details) of this tribe strongly supports two large clades; that is, the eastern clade and the western clade (Figure 1, Figure 2 and Figure S1). Newly collected topotypes of Pseudodon inoscularis from the Dawei (historically Tavoy) River in Myanmar (Dataset S2) are conchologically identical to the lectotype of this nominal species (Figure 3). Phylogenetically, Pseudodon inoscularis belongs to the eastern clade of the Pseudodontini and clusters with members of the genus Monodontina (Figure 2). Based on this evidence, the name Monodontina becomes a junior synonym of Pseudodon, and the name Pilsbryoconchina must be considered a junior synonym of the subtribe Pseudodontina. The latter (eastern) subtribe contains eight valid genera: Bineurus, Namkongnaia, Nyeinchanconcha, Pilsbryoconcha, Pseudodon (=Monodontina sensu Bolotov et al., 2020 [5]), Songkhlanaia, Sundadontina, and Thaiconcha. Finally, the genus Indopseudodon stat. rev. (=Pseudodon sensu Bolotov et al., 2017 [5,7]) and the subtribe Indopseudodontina subtr. nov. are valid names for the western clade of the tribe.

Comparative analysis of biogeographic models calculated for the tribe Pseudodontini (DEC, DIVALIKE, and BAYAREALIKE) reveals that the DIVALIKE model has the highest model weight according to ΔAICc (Table S1). Our time-calibrated biogeographic reconstruction based on the most probable model (Figure 2 and Table S2) indicates that the tribe Pseudodontini most likely originated on the Sunda Plate (with Indochina Block and Sibumasu Terrane) in the Late Cretaceous (probability = 66.7%; mean age = 91.0 Ma, 95% HPD 82.6–99.4 Ma). The subtribe Pseudodontina represents a monophyletic intra-area radiation of freshwater mussels on the Sunda Plate since the Late Cretaceous (probability = 100%; mean age = 74.5 Ma, 95% HPD 68.4–81.0 Ma). The subtribe Indopseudodontina originated via a vicariance event between the Sunda Plate and Burma Terrane in the Eocene (probability = 99.9%; mean age = 52.4 Ma, 95% HPD 40.3–56.3 Ma). A subclade of this subtribe containing most Indopseudodon species, except for I. kayinensis, diversified on the Burma Terrane through an intra-area radiation since the Eocene–Oligocene boundary (probability = 99.9%; mean age = 33.4 Ma, 95% HPD 28.0–38.8 Ma). The two lineages of I. indawgyiensis sp. nov. in the Chindwin and Ayeyarwady rivers were isolated since the Late Miocene (mean age = 7.7 Ma, 95% HPD 4.6–11.2 Ma).

Figure 3. Morphological comparison of the lectotype and topotypes of Pseudodon inoscularis (Gould, 1844) from Dawei (historically Tavoy) River, Myanmar. (A) Lectotype USNM 86352, River Salwen, Tavoy, Brit. Burmah; (B,C) Topotype RMBH biv 1027/1, Yae Pone Stream, Dawei River: inner view of left and outer view of right valves (B), outer view of left and inner view of right (C) valves; (D,E) Topotype RMBH biv 1027/2, the same locality: inner view of left and outer view of right valves (D), outer view of left and inner view of right (E) valves; (F,G) Topotype RMBH biv 1027/3, the same locality: inner view of left and outer view of right valves (F), outer view of left and inner view of right (G) valves; (H,I) pseudocardinal tooth and muscle attachment scars of the right (H) and left (I) valves of a topotype RMBH biv 1027/3. Scale bars = 20 mm (A–G) and 10 mm (H,I). Photos: (A) MUSSELp Database (http://mussel-project.uwsp.edu, accessed on 19 June 2023); (B–I) Ekaterina S. Konopleva.

Figure 3. Morphological comparison of the lectotype and topotypes of Pseudodon inoscularis (Gould, 1844) from Dawei (historically Tavoy) River, Myanmar. (A) Lectotype USNM 86352, River Salwen, Tavoy, Brit. Burmah; (B,C) Topotype RMBH biv 1027/1, Yae Pone Stream, Dawei River: inner view of left and outer view of right valves (B), outer view of left and inner view of right (C) valves; (D,E) Topotype RMBH biv 1027/2, the same locality: inner view of left and outer view of right valves (D), outer view of left and inner view of right (E) valves; (F,G) Topotype RMBH biv 1027/3, the same locality: inner view of left and outer view of right valves (F), outer view of left and inner view of right (G) valves; (H,I) pseudocardinal tooth and muscle attachment scars of the right (H) and left (I) valves of a topotype RMBH biv 1027/3. Scale bars = 20 mm (A–G) and 10 mm (H,I). Photos: (A) MUSSELp Database (http://mussel-project.uwsp.edu, accessed on 19 June 2023); (B–I) Ekaterina S. Konopleva.

3.2. Integrative Taxonomic Revision of the Genus Indopseudodon

Our expanded sample from Myanmar reveals that the genus comprises five recent species: Indopseudodon bogani comb. nov., I. crebristriatus comb. nov., I. indawgyiensis sp. nov., I. kayinensis comb. nov., and I. salwenianus comb. rev. (Figure S2,

Table 1. Taxonomic overview of the freshwater mussel tribe Pseudodontini and related recent and fossil taxa (Bivalvia: Unionidae).

Taxa	Type Locality	Distribution	Habitat Requirements and Larval Hosts
Subfamily GONIDEINAE Ortmann, 1916
Tribe Pseudodontini Frierson, 1927
Subtribe Indopseudodontina Bolotov, Konopleva, Kondakov & Vikhrev subtr. nov.
Genus Indopseudodon Prashad, 1922 stat. rev.
I. bogani (Bolotov, Kondakov & Konopleva, 2017) comb. nov. =Pseudodon bogani Bolotov, Kondakov & Konopleva, 2017	Kanni River, 19.0545° N, 96.5131° E, Sittaung Basin, Myanmar [7]	Kanni River, Sittaung Basin, Myanmar	River specialist; host fish unknown
I. crebristriatus (Anthony, 1865) comb. nov. =Monocondyloea crebristriata Anthony, 1865; =Pseudodon (Trigonodon) crebristriatus var. curvata Preston, 1912; TL: Pegu [45]	Pegu, British Burmah (Bago River, Myanmar) [46]	Bago River basin, Myanmar	River/stream specialist; host fish unknown
I. indawgyiensis sp. nov. =Pseudodon cf. avae Bolotov et al., 2017 (identification error) [7]; =Pseudodon cf. inoscularis Bolotov et al., 2020 (identification error) [5]	Nanuinhka Chaung River near Lonton village, 25.1209° N, 96.2812° E, Indawgyi Lake basin, Ayeyarwady River drainage, Myanmar	Upper Chindwin and Upper Ayeyarwady basins, including tributaries and outflow of Lake Indawgyi, Myanmar	River/stream specialist; host fish unknown
I. kayinensis (Bolotov et al., 2020) comb. nov. =Pseudodon kayinensis Bolotov et al., 2020	Winyaw River, 15.6685° N, 97.9496° E, Ataran River basin, Myanmar [5]	Salween, Ataran, and Haungthayaw basins, Myanmar	River/stream specialist; host fish unknown
†I. rostratus Annandale, 1924	A limestone rock in a stream bed about 2.5 miles east of Tichara village, Dawna Hills, Tenasserim (a limestone rock in a stream bed about 2.5 miles east of Hteechara village, 16.7783° N, 98.4581° E, Dawna Range, southern Myanmar) [47,48]	A fossil lacustrine or fluviatile limestone deposit of pre-Pliocene [47] or Pliocene [49,50,51] age on the eastern slope of the Dawna Range, the Cenozoic Mae Sot Series (within the recent Moei River drainage, a tributary of Salween), southern Myanmar	Fossil species
I. salwenianus (Gould, 1844) comb. rev. =Anodon salweniana Gould, 1844; =Monocondyloea peguensis Anthony, 1865 syn. nov.; TL: Pegu, British Burmah [46]; =Monocondylaea avae Theobald, 1873 syn. nov.; TL: Mandalay, Burmah [52]; =Pseudodon manueli Konopleva, Kondakov & Vikrev, 2017 syn. nov.; TL: Pyowne Stream, 18.9694° N, 96.5309° E, Sittaung Basin, Myanmar [7]	Salwen River, British Burmah [27]	Salween, Ataran, Haungthayaw, Sittaung, Bago, and Ayeyarwady basins, Myanmar	River/stream specialist; host fish unknown
Subtribe Pseudodontina Frierson, 1927 =Pilsbryoconchina Bolotov, Vikhrev & Tumpeesuwan, 2017 syn. nov.
Genus Bineurus Simpson, 1900
B. anodontinum (Rochebrune, 1882) =Pseudodon anodontinum Rochebrune, 1882	Sombor-Sombor, Mekong, Cochinchine (Mekong River at Sambour, approx. 12.7726° N, 105.9629° E, Cambodia) [8]	Lower Mekong in Cambodia [8] and Laos	River/stream specialist; host fish unknown
B. exilis (Morelet, 1866) =Monocondylaea exilis Morelet, 1866	In torrentibus montanis Cambodia; the lectotype label reads as follows: ‘lac Tonli-Sap, Cambodia’ (Tonlé Sap Lake, Cambodia); it cannot be found in the lake but was probably collected from a stream or river belonging to the Tonlé Sap Drainage [8]	Mekong Basin in Thailand, Cambodia, southern Vietnam [8], and Laos	River/stream specialist; host fish unknown
B. loeiensis Konopleva et al., 2021	Loei River, 17.0982° N, 101.4814° E, Mekong Basin, Thailand [8]	Loei River, Mekong Basin, northeastern Thailand [8]	River specialist; host fish unknown
B. mouhotii (Lea, 1863) =Monocondyloea mouhotii Lea, 1863	Laos Mts., Cambodia, Siam (most likely Laos Mountains; see [8] for explanation)	Mekong Basin in western and northern Laos, including Nam Ou River; northern Thailand [8]; and southern China: Puwen River, Yunnan [53]	River/stream specialist; host fish unknown
B. panhai Jeratthitikul & Sutcharit, 2023	Phra Sathueng Stream, 13.3346° N, 102.1329° E, Bang Pakong Basin, Wang Thong Subdistrict, Wang Sombun District, Sa Kaeo Province, Thailand [11]	Bang Pakong Basin, eastern Thailand and headwater tributaries of Tonle Sap Basin in Thailand and Cambodia [11]	Stream specialist; host fish unknown [11]
Genus Namkongnaia Jeratthitikul et al., 2021
N. inkhavilayi Jeratthitikul et al., 2021	Bunghona Market, 7 km N of Xe Bangfai River, 17.13674° N, 104.98591° E, Kammoune Province, Laos [26]	Lower Mekong Basin in Laos and northeastern Thailand [26]	Probably generalist; host fish unknown [26]
N. lemeslei (Morelet, 1875) =Anodonta lemeslei Morelet, 1875	Cambodge, Marecages de Battambang (Battambang, approx. 13.0929° N, 103.2001° E, Mekong Basin, Cambodia) [26]	Tonle Sap Lake and its tributaries in Cambodia [26]	Generalist; host fish unknown [26]
Genus Nyeinchanconcha Bolotov et al., 2020
N. nyeinchani Bolotov et al., 2020 = Pseudodon artbogani Thach, 2020 syn. nov.; TL: Las Freci River, near Thakhek, approx. 17.4352° N, 104.8359° E, Mekong River basin, Laos [54]	Small stream arising at a cave near Ban Kouanphavang, 17.4578° N, 104.9263° E, Nam Done River drainage, Mekong Basin, Laos [5]	Mountain tributaries of the Mekong River in Laos	River/stream specialist; host fish unknown
Genus Pilsbryoconcha Simpson, 1900
P. acuta Jeratthitikul & Prasankok, 2022	Dom Yai River at Pho Sai, 15.2311° N, 105.1571° E, Phibun Mangsahan District, Ubon Ratchathani Province, Thailand [55]	Thailand, Laos, and Vietnam [55]	Generalist; host fish unknown [55]
P. carinifera (Conrad, 1837) =Anodonta carinifera Conrad, 1837; =Anodonta sempervivens Deshayes & Jullien, 1876; TL: Arroyo de Peam-Chelang, Cambodge; =Anodonta laminata Rochebrune, 1882; TL: Rapides du Mekong, Cochinchine; =Pilsbryoconcha exilis sensu Pfeiffer & Graf, 2015 (identification error) [55]	Huai Yang Stream at Nong Muang, 13.8695° N, 102.5899° E, Khok Sung District, Sa Kaeo Province, Thailand (based on the neotype) [55]	Headwater of Tonle Sap Basin in Thailand and Cambodia, and probably the Mekong River in Cambodia [55]	Generalist; host fish unknown [55]
P. exilis (Lea, 1838) =Anodonta exilis Lea, 1838; =Anodonta polita Mousson, 1849; TL: Java; =Spatha compressa Martens, 1860; TL: Khao-kho, north-east of Pakpriau, Siam; =Monocondyloea compressa Lea, 1863; unavailable: homonym of Spatha compressa Martens, 1860; TL: Siam; =Anodonta javona Sowerby, 1867; TL: Java (original label), Japan (protologue, erroneous); =Anodon kelletii Sowerby, 1867; TL: unknown [55]	Java? (from the label of a syntype; in the protologue, Lea noted that its locality is unknown) [55]	Indochina, the Malay Peninsula, Singapore, Sumatra, Java, and Borneo [2,3,55]	Generalist [55]; an unspecified cyprinid fish [56]
*P. expressa (Martens, 1900) =Anodonta expressa Martens, 1900	Lake Danau-Baru, Indragiri, Sumatra [57]	Sumatra, Indonesia [55]	No data
P. hoikaab Jeratthitikul & Prasankok, 2022	Kam River at Na Khu, 16.9581° N, 104.5045° E, Na Kae District, Nakhon Phanom Province, Thailand [55]	Songkhram River and tributaries of the Middle Mekong in Thailand and Laos [55]	Probably river specialist; host fish unknown [55]
P. kittitati Jeratthitikul & Prasankok, 2022	Unnamed pond near Nong Ya Sai, 16.9798° N, 103.3371° E, Wang Sam Mo District, Udon Thani Province, Thailand [55]	Unknown beyond the type locality, Chi River drainage, Mekong Basin, Thailand [55]	No data (probably generalist as it was described from a pond) [55]
P. linguaeformis (Morelet, 1875) =Anodonta linguaeformis Morelet, 1875	Cambodia [55]	Mekong Basin in Cambodia, Thailand, and southern Vietnam [55]	Generalist; host fish unknown [55]
P. mekongiana Jeratthitikul & Prasankok, 2022	Tributary of Mekong River at Khok Kong, 18.3382° N, 103.7624° E, Mueang District, Bueng Kan Province, Thailand [55]	Tributaries of Mekong River in Sakon Nakhon Basin, Thailand [55]; and Nam Kadan River in Laos	Generalist; host fish unknown [55]
P. schomburgki (Martens, 1860) =Anodonta (Lamproscapha) schomburgki Martens, 1860	Siam [58]	Chi River drainage, Mae Klong River basin, the headwaters of Mun River and Tonle Sap basin, Khlong Phraphut stream in Thailand; the Tha Taphao and Tapi River basins in southern Thailand [9,55]	Generalist; host fish unknown [55]
Genus Pseudodon Gould, 1844 stat. rev. =Monodontina Conrad, 1853
*P. aeneolus Drouet & Chaper, 1892 comb. rev. =Pseudodon aeneolus Drouet & Chaper, 1892	Sebruang River (approx. 0.4937° N, 111.8931° E), Kapuas Basin, western Borneo [59]	Kapuas Basin, western Borneo	No data
P. cambodjensis (Petit de la Saussaye, 1865) comb. rev. =Monocondylea cambodjensis Petit de la Saussaye, 1865; =Monocondylus orbicularis Morelet, 1866; TL: Battambang, Siam [60]	Battambang (approx. 13.0929° N, 103.2001° E), Mekong Basin, Cambodia [61]	Mekong Basin in Thailand and Cambodia	River specialist; host fish unknown
P. inoscularis (Gould, 1844) =Anodon inoscularis Gould, 1844	River Salwen, Tavoy, Brit. Burmah [27] (here, we recommend to restrict its type locality to Tavoy (now Dawei) River, Myanmar: see Taxonomic Account for explanation)	Dawei River, Myanmar	River/stream specialist; host fish unknown
P. lenyanensis (Bolotov et al., 2020) comb. nov. =Monodontina lenyanensis Bolotov et al., 2020	14 Mile Stream, 11.3508° N, 99.1093° E, Lenya River basin, Myanmar [5]	Lenya Basin, Myanmar [5]	Stream specialist; host fish unknown
P. mekongi (Bolotov et al., 2020) comb. nov. =Monodontina mekongi Bolotov et al., 2020	Headwater of the Phong River, 16.8616° N, 101.9105° E, Mekong Basin, Thailand [5]	Phong River, Mekong Basin, Thailand [5]	River specialist; host fish unknown
*P. nicobaricus (Mörch, 1872) comb. nov. =Alasmodonta nicobarica Mörch, 1872	Nicobar Islands [32]	Nicobar Islands, India	No data
*P. thomsoni Morlet, 1884	Cambodge [8]	Mekong Basin in Cambodia [8]	No data
P. vagulus (Fischer, 1891) comb. rev. =Unio subtrigonus Sowerby, 1867; unavailable name: homonym of †U. subtrigonus Noulet, 1864 [62]; =Unio vagulus Fischer, 1891 (new name for U. subtrigonus Sowerby, 1867) [63]; =Pseudodon cambodjensis tenerrimus Brandt, 1974; TL: Songkram River at Sri Songkram, Thailand [3]; =Pseudodon cambodjensis sensu Pfeiffer & Graf, 2015 (identification error) [33]; =Pseudodon cambodjensis sensu Zieritz et al., 2016 (identification error) [14]	Siam [62]	Mekong Basin in Thailand, Laos, and Cambodia; Malaysia [3,14,33]	River specialist; host fish unknown [3]
P. vondembuschianus vondembuschianus (Lea, 1840) comb. rev. =Margaritana vondembuschiana Lea, 1840; =Alasmodonta crispata Mousson, 1849; TL: Java; =Alasmodonta zollingeri Mousson, 1849; TL: Java; =Monodontina buschiana Conrad, 1853 (new name for Margaritana vondembuschiana); =Monocondyloea planulata Lea, 1859; TL: Java; =Monocondyloea hageni Strubell, 1897; TL: S. Sumatra [5]	Java [64]	Malaysia, Sumatra, and Java [5,9]	River/stream specialist; host fish unknown [3]
P. vondembuschianus laosica (Bolotov et al., 2020) comb. nov. =Monodontina laosica Bolotov et al., 2020	Ca. 300 m upstream of the mouth of Houai Pin Stream, 14.7944° N, 106.4842° E, a tributary of the Vang Ngao River, Mekong Basin, Laos [5]	Mekong River basin in Laos [5,9]	River/stream specialist; host fish unknown
P. vondembuschianus tapienica (Konopleva et al., 2023) comb. nov. =Monodontina vondembuschiana tapienica Konopleva et al., 2023	Main Klong Min, Tapi River basin, Tambon Kaew San, Nabon District, Nakhon Si Thammarat Province, 8.2997° N, 99.5580° E, southern Thailand [9]	Southern Thailand [9]	River/stream specialist; host fish unknown
P. vondembuschianus thasaenica (Konopleva et al., 2023) comb. nov. =Monodontina vondembuschiana thasaenica Konopleva et al., 2023	Main Klong Thasae, Tha Taphao River Basin, nearby Wat Na Srang, Thasae District, Chumphon Province, 10.6753° N, 99.1737° E, southern Thailand [9]	Southern Thailand [9]	River/stream specialist; host fish unknown
†P. vondembuschianus trinilensis (Dubois, 1908) =†Unio trinilensis Dubois, 1908	‘Hauptknochenschicht’ deposits at Trinil site, 7.3667° S, 111.3500° E, Solo River valley, Ngawi Regency, east Java, Indonesia [65,66]	Middle Pleistocene (40Ar/39Ar maximum and minimum age of 0.54 and 0.43 Ma, respectively), Ngawi Regency, east Java, Indonesia [66]	Fossil insular subspecies
†P. vondembuschianus vandervlerki Oostingh, 1935 =†Pseudodon (Trigonodon) vandervlerki Oostingh, 1935	Kali Glagah beds at Boemiajoe, Central Java (Bumiayu District, approx. 7.2620° S, 108.9864° E, Brebes Regency, Central Java, Indonesia) [65]	Early Pleistocene (age ca. 1.8 Ma), Kaliglagah Formation, Bumiayu District, Brebes Regency, central Java, Indonesia [67]	Fossil insular subspecies
*P. walpolei (Hanley, 1871) comb. rev. =Monocondylaea walpolei Hanley, 1871; =Pseudodon crassus Drouet & Chaper, 1892; TL: le Sarawak [59]	Sarawak, Borneo (by the lectotype designation) [5]	Northern Borneo [5]	No data
Genus Songkhlanaia Konopleva et al., 2023
S. tamodienica Konopleva et al., 2023	Klong Plug Pom, 7.3324° N, 100.0917° E, middle reach of Klong Tamod, SLB, Ban Kok Sai, Tambon Mae Kree, Tamod District, Phatthalung Province, southern Thailand [9]	Songkhla Lake, southern Thailand [9]	Probably stream specialist; host fish unknown
Genus Sundadontina Bolotov et al., 2020
S. brandti Bolotov et al., 2020	Headwater of the Mun River, 14.4138° N, 102.0821° E, Mekong Basin, Thailand [5]	Mun River, Mekong Basin, Thailand [5]	River specialist; host fish unknown
S. cumingii (Lea, 1850) =Anodonta cumingii Lea, 1850; =Pseudodus chaperi Morgan, 1885; TL: tous affluents de la rivière Kinta, de Pèrak, Malacca [5]	Malacca [5]	Malaysia [5]	River/stream specialist; host fish unknown [3]
S. harmandi (Crosse & Fischer, 1876) =Pseudodon harmandi Crosse & Fischer, 1876	Cambodia [5]	Lower Mekong Basin in Laos and Cambodia [5]	River/stream specialist; host fish unknown [3]
*S. mabilli (Rochebrune, 1881) =Pseudodon mabilli Rochebrune, 1881	Mekong, Shigloni Breithon, Cochinchina [5]	Lower Mekong Basin in southern Vietnam [5]	No data
*S. moreleti (Crosse & Fischer, 1876) =Pseudodon moreleti Crosse & Fischer, 1876	Mekong, Kompang Cham Province, Cambodia [5]	Lower Mekong Basin in Cambodia [5]	No data
*S. ovalis (Morlet, 1889) comb. nov. =Pseudodon ovalis Morlet, 1889	Srakeo River, Siam (Bang Pakong River, Thailand) [8]	Bang Pakong Basin, Thailand [8]	No data
S. plugpomenica Konopleva et al., 2023	Klong Pa-Payom, 7.8456° N, 99.9933° E, SLB, Ban Teng, Tambon Laem Tanod, Khuan Kanun District, Phatthalung Province, southern Thailand [9]	Songkhla Lake basin, southern Thailand [9]	Stream specialist; host fish unknown
*S. ponderosa (Preston, 1909) =Pseudodon ponderosa Preston, 1909	Nan-ko, Siam (Nan River, Chao Phraya Basin, Thailand) [5]	Chao Phraya Basin, Thailand [5]	No data
S. sulcata (Rochebrune, 1881) =Pseudodon sulcatum Rochebrune, 1881	Mouth of the Mekong River, Cochinchina [5]	Mekong Basin in southern Vietnam [5] and Laos	Probably river specialist; host fish unknown
S. tanintharyiensis Bolotov et al., 2020	Chaung Nauk Pyan Stream, 11.7620° N, 99.1124° E, Lenya River basin, Myanmar [5]	Lenya Basin, Myanmar [5]	Stream specialist; host fish unknown
S. taskaevi Bolotov et al., 2020	Headwater of the Mun River, 14.4138° N, 102.0821° E, Mekong Basin, Thailand [5]	Mun River, Mekong Basin, Thailand [5]	River specialist; host fish unknown
S. tumida (Morelet, 1866) =Monocondylus tumidus Morelet, 1866	Cambodia [5]	Lower Mekong Basin in Cambodia and southern Vietnam [5]	Probably river specialist; host fish unknown
Genus Thaiconcha Bolotov et al., 2020
T. callifera (Martens, 1860) =Anodonta callifera Martens, 1860; =Pseudodon ellipticum Conrad, 1865; TL: Cambodia [8]	Siam (Thailand) [8]	Mekong Basin in Cambodia and Thailand [8]	River specialist; host fish unknown
T. munelliptica Konopleva et al., 2021	Pool site with clay bottom, Mun River upstream of Tha Tum village, 15.3575° N, 103.6637° E, Mekong Basin, Surin Province, Thailand [8]	Mun and Chi rivers, Mekong Basin, northeastern Thailand [8]	River specialist; host fish unknown
T. thaiensis Konopleva et al., 2021	Kham Nong Bua River, trib. Of Mekong River, at Rt. 1016 bridge 0.3 mi west of Rt. 1290, 20.2681° N, 100.0721° E, Chiang Rai, Thailand [8]	Mekong and Upper Chao Phraya basins, Thailand [8]	River specialist; host fish unknown
Tribe Gonideini Ortmann, 1916
Genus Parvasolenaia Huang & Wu, 2019
†Parvasolenaia sublinguaeformis (Tolstikova, 1975) comb. nov. =†Pilsbryoconcha sublinguaeformis Tolstikova, 1975	Dyusyumbay Strata, Chakelmes Mountain, Zaysan Depression, Kazakhstan [68]	Paleocene, Zaysan Depression, Kazakhstan [68]	Fossil species
†Parvasolenaia praeexilis (Tolstikova, 1976) comb. nov. =†Pilsbryoconcha praeexilis Tolstikova, 1976	Kiin-Kerish Mountain, Zaysan Depression, Kazakhstan [69]	Late Eocene, Zaysan Depression, Kazakhstan [69]	Fossil species
Genus Pseudodontopsis Kobelt, 1913
†Pseudodontopsis oettingenae (Modell, 1969) comb. nov. =†Pseudodon oettingenae Modell, 1969; =†Monodontina mogul Modell, 1969 syn. nov.; TL: Charigambir, 6.5 km WSW of Chinji Village, Punjab, Pakistan [70]; our first reviser’s action on the precedence of simultaneous synonyms	Pirawalaban, 9 km ENE of Chinji Village, Punjab, Pakistan [70]	Upper Miocene deposits near Chinji, Punjab, Pakistan [70]	Fossil species
Genus Ptychorhynchus Simpson, 1900 =Cosmopseudodon Haas, 1920 syn. nov.; =Heudeana Frierson, 1922
*Ptychorhynchus liuovatus (He & Zhuang, 2013) comb. nov. =Unio ovatus Liu, Duan & Wang, 1994; unavailable name: homonym of Unio ovatus Say, 1817; =Lamellidens liuovatus He & Zhuang, 2013 (new name for Unio ovatus); =Pseudobaphia liuovata (He & Zhuang, 2013) [1,53]	Jiangkou, Guizhou Province, China [53]	Pearl Basin, China [53]	No data
*P. murinum (Heude, 1883) =Unio murinus Heude, 1883; =Unio pinchonianus Heude, 1883 syn. nov.; TL: Les canaux de la plaine élevée de Tch’eng-tou fou, province du Setchouan (canals of the Chengdu High Plain, Yangtze Basin, Sichuan, China) [71]; =Pseudodon solidus Haas, 1911 syn. nov.; TL: Hunan, Mittelchina [72]; our first reviser action on the precedence of simultaneous synonyms: Unio murinus over U. pinchonianus	Le grand torrent du Kien-té sud, vers ses sources (the main channel of Yangtze River towards its sources, Anhui Province, China) [71]	Yangtze Basin, China	No data
*P. resupinatus (Martens, 1902) comb. nov. =Pseudodon resupinatus Martens, 1902	Than Moi, Tonkin, French Indo China (approx. 21.6313° N, 106.5532° E, Thuong Basin, flowing into Thái Bình River, northern Vietnam) [57]	Thuong Basin, northern Vietnam [57]	No data
Genus Simpsonasus Bolotov & Konopleva nom. nov. (replacement name for Nasus Simpson, 1900)
*Simpsonasus nankingensis (Heude, 1874) comb. nov. =Monocondylea nankingensis Heude, 1874; =Pseudodon secundus Heude, 1877 syn. nov.; TL: Ngan-houe, China [73]	Rivière de Nanking [74]	Lower Yangtze Basin, China	No data
†Simpsonasus politus (Tolstikova, 1974) comb. nov. =†Nasus politus Tolstikova, 1974; =†Pseudodon politus (Tolstikova, 1974) [75,76]	Obaylin Formation, Zaysan Depression, Ulkun-Ulasty River, Kazakhstan [75]	Middle Eocene (ca. 40–50 Ma), Zaysan Depression, Ulkun-Ulasty River, Kazakhstan [77]	Fossil species
Subfamily UNIONINAE Rafinesque, 1820
Tribe Unionini Rafinesque, 1820
Genus Pseudobaphia Simpson, 1900 =Chrysopseudodon Haas, 1920 syn. nov.
*Pseudobaphia aurea (Heude, 1885) comb. nov. =Psudodon (sic!) aureus Heude, 1885	Un torrent du district de Kien-té, Nanking (a watercourse within a former district in Anhui, China) [78]	Lower Yangtze Basin, China	No data
†Pseudobaphia pingi (Otuka, 1942) comb. nov. =†Pseudodon pingi Otuka, 1942	Loc. Kwodo No 30, grey mud of the Lower part of Nihowan beds on a valley floor, SW of Liuchiashaopu, San-Kien-Ho basin, Tsanan, Menchiang, Inner Mongolia, north China [79]	Middle Neogene or pre-Upper Pliocene, Nihowan beds, San-Kien-Ho basin, Inner Mongolia, China [79]	Fossil species
Subfamily PARREYSIINAE Henderson, 1935
Tribe Lamellidentini Modell, 1942
Genus Balwantia Prashad, 1919
†Balwantia longiformis (Takayasu, Gurung & Matsuoka, 1995) comb. nov. =†Lamellidens longiformis Takayasu, Gurung & Matsuoka, 1995; =†Pilsbryoconcha longiformis (Takayasu, Gurung & Matsuoka, 1995) [80,81]	The right bank of the Narayani River, 500 m south of the confluence with the Binai Khola; middle member of the Binai Khola Formation, west-central Nepal [80]	Mio-Pliocene Churia Group, west-central Nepal [80]	Fossil species
Genus Lamellidens Simpson, 1900
†Lamellidens indicus (Modell, 1969) comb. nov. =†Cosmopseudodon indicus Modell, 1969	Parlewali, 4–5 km WSW of Dhok Pathan Village, Punjab, Pakistan [70]	Pliocene deposits near Dhok Pathan, Punjab, Pakistan [70]	Fossil species

Note: * Species whose DNA sequences are not available. All of the other species were studied by means of a molecular approach. † Fossil taxa.

and Dataset S3). Most of these species are transferred from the genus Pseudodon sensu Bolotov et al., 2017 [5,7].

Newly collected topotypes of Indopseudodon crebristriatus and Pseudodon (Trigonodon) crebristriatus var. curvata from the Bago (Pegu) River reveal that these nominal taxa are synonymous (Table 1). In particular, Pseudodon crebristriatus var. curvata differs from the typical form by having a concave ventral margin (Figure 4). Indopseudodon crebristriatus seems to be a species endemic to the Bago River basin.

Our phylogenetic analyses reveal that the nominal species Monocondylaea avae is a close relative of Indopseudodon salwenianus based on available DNA sequences of newly collected topotypes (mean uncorrected COI p-distance of 1.7%) and that these species are synonymous (Table 1 and Dataset S3). Conchologically, M. avae represents an elongated form of I. salwenianus without a projected wing, which is known to occur in the Ayeyarwady Basin (Figure 5).

In turn, the nominal species Pseudodon manueli, which was thought to be endemic to the Sittaung Basin, differs from Indopseudodon salwenianus by a few nucleotide substitutions in the COI gene fragment (mean uncorrected COI p-distance of 0.5%) and also represents a synonym of the latter species (Table 1 and Dataset S3). From a conchological point of view, P. manueli differs from the typical form of I. salwenianus by having an ovate shell with broader anterior margin and less-developed wing (Figure 5). At the same time, the nominal species Pseudodon manueli seems to be a synonym of Monocondylaea peguensis Anthony, 1965 based on conchological similarities, such as general outlines, teeth structure and the shape of muscle attachment scars. Our new results indicate that the range of Indopseudodon salwenianus covers not only rivers emptying to the Salween Estuary but also the Sittaung, Ayeyarwady, and Bago basins.

Indopseudodon indawgyiensis sp. nov. contains two divergent allopatric lineages in the Upper Ayeyarwady and Upper Chindwin basins (Figure 2, Figure 6 and Figure S2). Here, we consider these lineages as belonging to a single biological species due to the lack of clear morphological differences, especially for younger specimens, and a relatively low level of genetic divergence (mean uncorrected COI p-distance of 2.0 ± 0.5%).

The mean values of the shell elongation index (SEI ± s.e.m.) for Chindwin and Ayeyarwady lineages are 63.95 ± 0.56 (N = 24) and 63.15 ± 0.45 (N = 35), respectively (Mann–Whitney test: U = 371, p = 0.45). The mean values of the shell convexity index (SCI ± s.e.m.) for the same lineages are 32.14 ± 0.44 and 32.64 ± 0.36, respectively (Mann–Whitney test: U = 372, p = 0.46). Thus, the lineages do not share significant differences in the shell shape. The shell elongation index vs shell convexity index scatterplot demonstrates that 95% confidence ellipses are transversed (Figure S3). Two specimens, i.e., RMBH biv 0925/2 and 0110/6, fall out of the 95% confidence ellipse of the Ayeyarwady lineage. A complete description of Indopseudodon indawgyiensis sp. nov. is presented below in the taxonomic account.

3.3. Newly Sequenced Samples of Little-Known Pseudodontini Species

Our expanded sampling in Laos revealed two Sundadontina species, the DNA sequences of which differ from all already-sequenced taxa in this genus (Figure 2, Figures S1 and S2). Based on thorough morphological investigation of the newly collected specimens and historical museum lots, we conclude that these samples belong to Sundadontina harmandi and S. sulcata (Table 1 and Dataset S2). The type localities of these species are situated in the Lower Mekong Basin. Our new sample of Sundadontina harmandi has a rounded shell with a broader posterior margin and conchologically corresponds well to the syntypes of this nominal taxon (Figure 7). Newly collected specimens of Sundadontina sulcata have a characteristic radial plication over the posterior part of the shell and broad umbo (Figure 7).

Figure 5. Shells of Indopseudodon salwenianus (Gould, 1844) comb. rev. and nominal species synonymized with it (A–G), as well as I. kayinensis (Bolotov et al., 2020) comb. nov. (H,I). (A,B) Topotype of I. salwenianus, RMBH biv 639/3, unnamed stream, Salween Basin, Myanmar: inner view of left and outer view of right valves (A), outer view of left and inner view of right valves (B); (C) Holotype of Monocondyloea peguensis Anthony, 1865 syn. nov., MCZ 161877, Pegu, British Burmah: outer view of left and inner view of right valves; (D,E) Holotype of Pseudodon manueli Konopleva, Kondakov & Vikhrev, 2017 syn. nov., NCSM 103022 (former RMBH biv 246/3) Pyowne Stream, Sittaung Basin, Myanmar: inner view of left and outer view of right valves (D), outer view of left and inner view of right valves (E); (F,G) Topotype of Monocondylaea avae Theobald, 1873 syn. nov., RMBH biv 670, Nga Wun River, Ayeyarwady Basin, Myanmar: inner view of left and outer view of right valves (F), outer view of left and inner view of right valves (G); (H,I) Holotype of I. kayinensis, RMBH biv 618/1, Winyaw River, Ataran basin, Myanmar: outer view of left and inner view of right valves (H), inner view of left and outer view of right valves (I). Scale bar = 20 mm. Photos: Ekaterina S. Konopleva.

Figure 5. Shells of Indopseudodon salwenianus (Gould, 1844) comb. rev. and nominal species synonymized with it (A–G), as well as I. kayinensis (Bolotov et al., 2020) comb. nov. (H,I). (A,B) Topotype of I. salwenianus, RMBH biv 639/3, unnamed stream, Salween Basin, Myanmar: inner view of left and outer view of right valves (A), outer view of left and inner view of right valves (B); (C) Holotype of Monocondyloea peguensis Anthony, 1865 syn. nov., MCZ 161877, Pegu, British Burmah: outer view of left and inner view of right valves; (D,E) Holotype of Pseudodon manueli Konopleva, Kondakov & Vikhrev, 2017 syn. nov., NCSM 103022 (former RMBH biv 246/3) Pyowne Stream, Sittaung Basin, Myanmar: inner view of left and outer view of right valves (D), outer view of left and inner view of right valves (E); (F,G) Topotype of Monocondylaea avae Theobald, 1873 syn. nov., RMBH biv 670, Nga Wun River, Ayeyarwady Basin, Myanmar: inner view of left and outer view of right valves (F), outer view of left and inner view of right valves (G); (H,I) Holotype of I. kayinensis, RMBH biv 618/1, Winyaw River, Ataran basin, Myanmar: outer view of left and inner view of right valves (H), inner view of left and outer view of right valves (I). Scale bar = 20 mm. Photos: Ekaterina S. Konopleva.

Figure 6. Shells of Indopseudodon indawgyiensis sp. nov. (A,B) Holotype RMBH biv 0110/10, Ayeyarwady Basin, Myanmar: inner view of left and outer view of right valves (A), outer view of left and inner view of right valves (B); (C) Paratype RMBH biv 0925/3, Washawng Stream, Ayeyarwady Basin, Myanmar: outer view of left and inner view of right valves; (D) Paratype RMBH biv 0936/1, unnamed stream, Ayeyarwady Basin, Myanmar: outer view of left and inner view of right valves; (E,F) Paratypes RMBH biv 0906/1 (E) and biv 0906/2 (F), Shin Lon Ga Stream, Chindwin Basin, Myanmar: outer view of left and inner view of right valves; (G,H) Paratype RMBH biv 0896/1, Lamon Chaung Stream, Chindwin Basin, Myanmar: inner view of left and outer view of right valves (G), outer view of left and inner view of right valves (H). Scale bar = 20 mm. Photos: Ekaterina S. Konopleva.

Figure 6. Shells of Indopseudodon indawgyiensis sp. nov. (A,B) Holotype RMBH biv 0110/10, Ayeyarwady Basin, Myanmar: inner view of left and outer view of right valves (A), outer view of left and inner view of right valves (B); (C) Paratype RMBH biv 0925/3, Washawng Stream, Ayeyarwady Basin, Myanmar: outer view of left and inner view of right valves; (D) Paratype RMBH biv 0936/1, unnamed stream, Ayeyarwady Basin, Myanmar: outer view of left and inner view of right valves; (E,F) Paratypes RMBH biv 0906/1 (E) and biv 0906/2 (F), Shin Lon Ga Stream, Chindwin Basin, Myanmar: outer view of left and inner view of right valves; (G,H) Paratype RMBH biv 0896/1, Lamon Chaung Stream, Chindwin Basin, Myanmar: inner view of left and outer view of right valves (G), outer view of left and inner view of right valves (H). Scale bar = 20 mm. Photos: Ekaterina S. Konopleva.

Figure 7. Shells of little-known Pseudodontini species discussed in this study, i.e., Sundadontina harmandi (Crosse & Fischer, 1876) (A–D) and S. sulcata (Rochebrune, 1881) (E–G). (A,B) Holotype of S. harmandi, MNHN-IM-2000-1717, Cambodia: outer view of right and inner view of left valves (A), inner view of right and outer view of left valves (B); (C,D) Newly collected specimen of S. harmandi, RMBH biv 0881, Huay Tuay River, Mekong Basin, Laos: outer view of right and inner view of left valves (C), inner view of right and outer view of left valves (D); (E) Syntype of S. sulcata, MNHN-IM-2000-1795, southern Vietnam; (F,G) Newly collected specimen of S. sulcata, RMBH biv 869/3, Phaphou Stream, Mekong Basin, Laos: outer view of right and inner view of left valves (F), inner view of right and outer view of left valves (G). Scale bar = 20 mm. Photos: (A,B) Manuel Caballer (2018 MNHN Project: RECOLNAT No. ANR-11-INBS-0004); (C,D,F,G) Ekaterina S. Konopleva; (E) MUSSELp Database (http://mussel-project.uwsp.edu, accessed on 19 June 2023).

Figure 7. Shells of little-known Pseudodontini species discussed in this study, i.e., Sundadontina harmandi (Crosse & Fischer, 1876) (A–D) and S. sulcata (Rochebrune, 1881) (E–G). (A,B) Holotype of S. harmandi, MNHN-IM-2000-1717, Cambodia: outer view of right and inner view of left valves (A), inner view of right and outer view of left valves (B); (C,D) Newly collected specimen of S. harmandi, RMBH biv 0881, Huay Tuay River, Mekong Basin, Laos: outer view of right and inner view of left valves (C), inner view of right and outer view of left valves (D); (E) Syntype of S. sulcata, MNHN-IM-2000-1795, southern Vietnam; (F,G) Newly collected specimen of S. sulcata, RMBH biv 869/3, Phaphou Stream, Mekong Basin, Laos: outer view of right and inner view of left valves (F), inner view of right and outer view of left valves (G). Scale bar = 20 mm. Photos: (A,B) Manuel Caballer (2018 MNHN Project: RECOLNAT No. ANR-11-INBS-0004); (C,D,F,G) Ekaterina S. Konopleva; (E) MUSSELp Database (http://mussel-project.uwsp.edu, accessed on 19 June 2023).

Furthermore, we clarify the taxonomic status and phylogenetic position of Pseudodon cambodjensis and P. vagulus using a set of new DNA sequences (Figure S2, Table 1 and Dataset S1). In particular, our new sample from Laos conchologically corresponds to a syntype of the nominal species Pseudodon cambodjensis (Figure 8) but phylogenetically differs from available sequences attributed to this species in NCBI GenBank, e.g., P. cambodjensis sensu Pfeiffer & Graf, 2015 (Figure S2 and Dataset S1). The sequenced specimen of the latter species (UMMZ 304350; Pursat River, just downstream (north) of Highway 5 Bridge, 12.5331° N, 103.9195° E, Pursat City, Cambodia) [33] has a triangular shell with a well-developed, angulate wing. Conchologically, this specimen is nearly identical to the holotype of Unio subtrigonus Sowerby, 1867 (NHMUK 1874-12-11-3; Siam). The latter name is a junior homonym of †Unio subtrigonus Noulet, 1864 and it was renamed as Unio vagulus [63]. Here, we consider Pseudodon cambodjensis sensu Pfeiffer & Graf, 2015 [14,33], as P. vagulus and our new sample from Laos as P. cambodjensis in its original sense (Table 1 and Dataset S1).

We also present a series of first and new records of several Pseudodontina species from Laos (Lower Mekong Basin) based on an expanded set of DNA sequences (Dataset S2). In particular, Bineurus anodontinum and B. exilis were recorded from this country for the first time. New occurrences are presented for Nyeinchanconcha nyeinchani, Pilsbryoconcha exilis, P. hoikaab, P. mekongiana, and P. acuta. Finally, Pseudodon vondembuschianus laosica that was described on the basis of a single specimen [5] was discovered in four additional streams belonging to the Lower Mekong Basin.

3.4. Taxonomic Account of the Tribe Pseudodontini with a New Species Description

• Family Unionidae Rafinesque, 1820
• Subfamily Gonideinae Ortmann, 1916
• Tribe Pseudodontini Frierson, 1927

Subtribe Indopseudodontina Bolotov, Konopleva, Kondakov & Vikhrev subtr. nov. (type genus: Indopseudodon Prashad, 1922).

LSID. https://zoobank.org/urn:lsid:zoobank.org:act:1A367762-D8E3-487A-942D-741277651797 (accessed on 19 June 2023)

Comments. A monotypic subtribe; diagnosis and distribution as for the genus.

Genus Indopseudodon Prashad, 1922 stat. rev. (type species: Anodon salweniana Gould, 1844; by subsequent designation by Thiele, 1934 [82]).

=Trigonodon Conrad, 1865 (type species: Monocondyloea crebristriata Anthony, 1865; by monotypy; unavailable name: junior homonym of Trigonodon Sismonda, 1847, Pisces).

Diagnosis. Shell ovate or rounded, somewhat winged, dorsal and posterior margins usually covered by fine wrinkles; pseudocardinal teeth well developed, knob-like.

Distribution. Endemic to the Western Indochina Subregion (Ayeyarwady to Salween) [5].

Comments. This genus contains five recent species (Table 1), including one species new to science, which is described here. Additionally, we re-describe one little-known species, I. crebristriatus, based on a series of newly collected topotypes from the Pegu (currently Bago) River. One fossil species, †I. rostratus Annandale, 1924, was described from a limestone deposit of the pre-Pliocene [47] or Pliocene [49,50,51] age on the eastern slope of the Dawna Range, southern Myanmar. This fossil site is situated within the recent Moei River drainage, a tributary of Salween. Conchologically, this fossil species resembles the recent Indopseudodon salwenianus and I. kayinensis but does not have a characteristic sculpture over the posterior region of the shell [48].

• Indopseudodon crebristriatus (Anthony, 1865)

=Monocondyloea crebristriata Anthony (1865): 205, pl. 18, Figure 1 [46];

=Pseudodon (Trigonodon) crebristriatus var. curvata Preston (1912): 295 [45].

Figure 4

Type and type locality. Holotype MCZ 161877; Pegu, British Burmah (Bago River, Myanmar) [46].

Topotypes examined. MYANMAR: Bago River, 17.6726° N, 96.2357° E, 21.iii.2020, 20 specimens (RMBH biv 0986, including biv 0986/1, biv 0986/2 and biv 0986/3 sequenced), Bolotov, Vikhrev, Nyein Chan, Kondakov, and Gofarov leg.; Bago River, 17.6797° N, 96.2318° E, 22.iii.2020, 1 specimen (RMBH biv 0992, sequenced), Bolotov, Vikhrev, Nyein Chan, Kondakov, and Gofarov leg.; Bago River, 17.6795° N, 96.2347° E, 19.ii.2018, 1 specimen (RMBH biv 391A, dead shell), Bolotov, Vikhrev, and Nyein Chan leg.; Ba Mawe Khong Stream, Bago Basin, 17.9461° N, 95.7687° E, 23.iii.2020, 5 specimens (RMBH biv 0995, including biv 0995/1 sequenced), Bolotov, Vikhrev, Nyein Chan, Kondakov, and Gofarov leg.

Diagnosis. The shell is rhombic, older specimens are more elongated, rather thick, moderately inflated, anterior margin rounded, dorsal margin curved, ventral margin straight or concave, posterior margin truncated or biangular. The umbo is not projected, with a zigzag sculpture, and eroded. The posterior slope with two prominent folds, and for younger specimens they are strongly striated, compared to older individuals. Pseudocardinal teeth are developed, erected, there is usually one thick tooth in the right valve and it is bilobed in the left valve. Anterior adductor muscle attachment scars are oval-shaped or bean-like, usually contiguous with retractor and protractor scars, posterior adductor muscle attachment scars are rounded or drop-liked, and less projected.

Habitat and ecology. Rivers and streams; larval hosts are unknown.

Distribution. Endemic to the Bago River basin, Myanmar.

Comments. Haas [2] synonymized Monocondylaea peguensis with Indopseudodon crebristriatus, although clear differences in the structure of pseudocardinal tooth of these taxa, as well as their shell shape and inflation, indicate that this solution was erroneous. Here, we consider Monocondylaea peguensis as a synonym of Indopseudodon salwenianus (see Figure 5).

• Indopseudodon indawgyiensis Bolotov, Konopleva, Kondakov & Vikhrev sp. nov.

=Pseudodon avae Bolotov et al. (2017): 6, Figure 1 (identification error) [7];

=Pseudodon cf. avae Bolotov et al. (2017): Figure 4, Supplementary Table S1 (identification error) [4];

=Pseudodon cf. inoscularis Bolotov et al. (2020): 3, Supplementary Table S1 (identification error) [5]; Jeratthitikul et al. (2021): 124 [26]; Jeratthitikul et al. (2022): 9, Supplementary Table S1 [55]; Jeratthitikul et al. (2023): 174 [11]; Konopleva et al. (2021): 3, Supplementary Table S1 [8]; Kongim et al. (2023): 244 [83].

Figure 6

LSID. https://zoobank.org/urn:lsid:zoobank.org:act:FC40FA9A-85CE-4B65-B501-0FDA53EFE045 (accessed on 19 June 2023)

Type and type locality. Holotype RMBH biv 110/10 (shell length 69.1 mm; shell height 43.0 mm; shell width 21.5 mm); MYANMAR: Nanuinhka Chaung River near Lonton village, 25.1209° N, 96.2812° E, Indawgyi Lake basin, Ayeyarwady River drainage, 25.iii.2014, Bolotov, Vikhrev, Gofarov, and Spitsyn leg.

Paratypes. The type locality, the same date and collectors, 10 specimens (RMBH biv 0110, including biv 0110/5 and biv 0110/11 sequenced); Washawng Stream, 25.3771° N, 97.6095° E, Ayeyarwady River basin, 12.iii.2020, 6 specimens (RMBH biv 0925, including biv 0925/1, biv 0925/2, and biv 925/3 sequenced), Bolotov, Vikhrev, Nyein Chan, Kondakov, and Gofarov leg.; unnamed stream, 25.4670° N, 96.8567° E, Ayeyarwady River basin, 13.iii.2020, 14 specimens (RMBH biv 0936, including biv 0936/1, biv 0936/2, and biv 0936/3 sequenced), Bolotov, Vikhrev, Nyein Chan, Kondakov, and Gofarov leg.; Bonelee Stream, 25.4999951° N, 97.5941° E, Ayeyarwady River basin, 13.iii.2020, 6 specimens (RMBH biv 0947, including biv 0947/1 and biv 0947/2 sequenced; 4 dead shells), Bolotov, Vikhrev, Nyein Chan, Kondakov, and Gofarov leg.; Nan Ma Tee River, 25.3890° N, 97.0235° E, Ayeyarwady River basin, 13.iii.2020, 2 specimens (RMBH biv 0955, including biv 0955/1, and biv 0955/2 sequenced), Bolotov, Vikhrev, Nyein Chan, Kondakov, and Gofarov leg.; Lamon Chaung Stream, 26.3463° N, 96.6685° E, Chindwin River basin, Ayeyarwady River drainage, ii.2020, 13 specimens (RMBH biv 0896, including biv 0896/1, biv 0896/2, and biv 0896/3 sequenced); Shin Lon Ga Stream, 26.5236° N, 96.6168° E, Chindwin River basin, Ayeyarwady River drainage, ii.2020, 11 specimens (RMBH biv 0906, including biv 0906/1, biv 0906/2 and biv 0906/3 sequenced), Nyein Chan and locals leg. Shell measurements of the type series are presented in Table S3.

Etymology. The name of this species is derived from Lake Indawgyi; its type locality is situated in an inflow stream of the lake.

Differential diagnosis. The shell of the new species is ovoid or oval-elongated, and the dorsal margin is curved or just slightly elevated. Pseudocardinal teeth represent projected tubercles, usually elevated, rounded or slightly pointed, anterior muscle scars are pronounced, adductor scars are mainly oval-shaped; posterior muscle scars are shallower. The new species is morphologically similar to Pseudodon inoscularis, but has a wider anterior margin, more flattened dorsal margin (not winged), and more elevated pseudocardinal teeth.

Molecular diagnosis. The nearest neighbor is Indopseudodon bogani with an uncorrected COI p-distance of 8.1 ± 1.0% (Table S4). The new species also differs from its congeners by fixed nucleotide substitutions in the COI (71 A, 89 G, 236 C, 290 A, 305 G, 314 A, 407 C, 500 C, 509 A, 563 A, 575 C, and 638 A) and 16S rRNA (19 T and 320 T) gene fragments.

Description. The shell is ovoid to more or less oval-elongated, rather thick and moderately inflated, anteriorly rounded, posteriorly truncated, slightly rounded or somewhat rostrated, the dorsal margin is usually curved, and the ventral margin is straight or curved. There are two allopatric lineages of this species. The first lineage from the Indawgyi Lake and its tributaries differs by a more elongated shell as opposed to the second lineage from the Chindwin Basin, with a more compressed shell on the anterior and posterior sides. The umbo is not projected, but more or less elevated under the hinge line, and usually eroded, especially for the lineage from the Chindwin Basin. The periostracum is light to dark brown, sometimes with rusty tint on the posterior side of the shell around the ventral margin. The growth lines are well projected. Thin, more or less visible lines may cross the growth lines of the shell directing from the umbo to ventral margin. The surface behind the umbo and around the posterior slope can be covered by small wrinkles. The nacre is whitish with yellow spots. There is one strong tubercle-like pseudocardinal tooth on the right valve, usually rather thick and projected, elevated, and smooth and rounded on both sides. The pseudocardinal tooth on the left valve is often smaller and more pointed. Lateral teeth look like rudimentary lamellae, with one on each valve. The umbo cavity is moderately deep, with a yellow tint. Anterior adductor muscle scars are drop-like or oval-shaped, well-developed, and contiguous with the anterior retractor, and sometimes with protractor scars; posterior adductor muscle scars are somewhat rounded, less projected, contiguous with the posterior retractor scar. The pallial line not very deep and clean, continuous

Habitat and ecology. The new species inhabits rivers and streams; larval hosts are unknown.

Distribution. The Ayeyarwady River basin, including Lake Indawgyi with its tributaries (Ayeyarwady lineage) and the Chindwin River basin (Chindwin lineage). The two allopatric lineages are phylogenetically divergent (mean uncorrected COI p-distance ± standard error of 2.0 ± 0.5%) and may represent subspecies-level taxa.

• Subtribe Pseudodontina Frierson, 1927 (type genus: Pseudodon Gould, 1844; by original designation).

=Pilsbryoconchina, Bolotov, Vikhrev & Tumpeesuwan, 2017 syn. nov. (type genus: Pilsbryoconcha Simpson, 1900; by original designation).

Comments. Representatives of this clade are characterized by a broad range of conchological variability; from ultra-elongated, thin shells with weakly developed teeth (e.g., Namkongnaia and Pilsbryoconcha) to rounded, massive shells with large, knob-like pseudocardinal teeth (Sundadontina). This subtribe ranges east and southeast of the Salween–Mekong drainage divide in southern Myanmar, Laos, Thailand, Cambodia, southern Vietnam, Malaysia, and Indonesia (Sumatra, Java, and Borneo) (Table 1). The maximum taxonomic richness occurs within the Mekong River basin (7 genera and 31 species).

• Genus Pseudodon Gould, 1844 stat. rev. (type species: Anodon inoscularis Gould, 1844; by original designation).

=Monodontina Conrad, 1853 (type species: Margaritana vondembuschiana Lea, 1840; by original designation).

=Suborbiculus Simpson, 1900 (type species: Monocondylus orbicularis Morelet, 1866; by original designation).

=Diplopseudodon Haas, 1920 (type species: Pseudodon crassus Drouët & Chaper, 1892; by original designation).

Diagnosis. The shell is ovate or rounded, winged, and truncated posteriorly, and rather thin; pseudocardinal teeth are weakly developed.

Distribution. Lenya and Dawei river basins in southern Myanmar, the entire Mekong Basin, Thai–Malay Peninsula, the Greater Sunda Islands [5], and the Nicobar Islands [32].

Comments. This genus contains 10 recent species (Table 1). In our earlier revisions [5,8], almost all of these species were placed in the genus Monodontina, which is found to be a synonym of Pseudodon based on the phylogenetic position of P. inoscularis topotypes. The type locality of the latter nominal taxon was a matter of doubt. Dr. A.A. Gould described two new Pseudodontini species, the shells of which were said to be collected in the Salween River by Rev. Francis Mason, a Baptist missionary and enthusiastic naturalist in Tavoy [27,84]. The original descriptions do not contain any reference to the type localities of these taxa, with the exception of the Salween River, mentioned in the title. The lectotype of Indopseudodon salwenianus is labelled as “Salwen River, British Burmah”. In contrast, the lectotype label of Pseudodon inoscularis reads as follows: “River Salwen, Tavoy, Brit. Burmah”. The latter locality, however, is nonsense, because the Tavoy Province (District), which has belonged to the Tenasserim Division of British Burma, did not cover the Salween River basin [85]. It is indeed strange, as Rev. Mason [86] has prepared a brief description of the Tavoy Province and its detailed map for the first time. A possible explanation of this inconsistency is that Rev. Mason sent to Dr. Gould the type series of the two taxa from two different localities (Salween River and Tavoy Province) with a single label such as “River Salwen, Tavoy, British Burmah”. It is worth to note that Rev. F. Mason was a missionary in Tavoy for many years (since 1831) [87], and he collected numerous samples of freshwater shells throughout this province [84,88]. Recently, Indopseudodon salwenianus was rediscovered from the Lower Salween Basin, and, hence, its type locality was stated correctly. Rev. Mason had probably collected the type series of this species when he visited the Amherst Province (District) of the Tenasserim Division [87], covering the Lower Salween basin. Conversely, a sample, which is nearly identical conchologically to the lectotype of Pseudodon inoscularis, was collected by one of the authors (T.W.) in the upstream section of the Tavoy (now Dawei) River. Based on the line of reasoning, presented above, we argue that the lectotype of Pseudodon inoscularis has originated from the latter river and that the reference to Salween River should be omitted from its type locality (Table 1).

The widespread species P. vondembuschianus (Lea, 1840) was separated to four recent and two fossil subspecies (Table 1). The two fossil subspecies were described from the Pleistocene deposits of Java [65]. Furthermore, several fossil nominal species were placed in the genera Pseudodon and Monodontina. These species are as follows: †Pseudodon oettingenae Modell, 1969 and †Monodontina mogul Modell, 1969 from the Upper Miocene of Pakistan, †Pseudodon politus (Tolstikova, 1974) from the Middle Eocene of Kazakhstan, †Pseudodon pingi Otuka, 1942 from the Middle Neogene or pre-Upper Pliocene of China, and †Pseudodon indicus (Modell, 1969) from the Pliocene of Pakistan (Table 1). Modell [70] placed †P. oettingenae within Pseudodon with caution and noted that it may also belong to the genus Pseudodontopsis Kobelt, 1913 (Gonideini). We agree with the latter opinion, because †P. oettingenae has a rounded shell shape that better fits the diagnosis of the genus Pseudodontopsis Kobelt, 1913. †M. mogul could represent a conchological variety of †P. oettingenae, because it was described from the same (Upper Miocene) deposits and its type locality is close to that of the latter species [70]. †P. politus was described in the genus Nasus Simpson, 1900 that was considered to be a synonym of Pseudodon. However, we show that Nasus is a separate genus, the name of which is unavailable due to homonymy, and propose a replacement name, Simpsonasus nom. nov. (Gonideini). Here, the nominal taxon †P. politus is transferred to the latter genus, because it closely resembles the recent species Simpsonasus nankingensis (Heude, 1874) comb. nov. based on conchological features. †Pseudodon pingi represents a fossil relative of the recent species Pseudobaphia aurea (Heude, 1885) comb. nov. and conchologically corresponds to the genus Pseudobaphia Simpson, 1900 (Unionini). Finally, †Pseudodon indicus was briefly described based on a single specimen, the shell shape of which corresponds well to that of Lamellidens Simpson, 1900 (Lamellidentini). The new combinations and synonyms for fossil taxa are presented in Table 1. In summary, pre-Pleistocene fossil Pseudodon species are still unknown.

• Pseudodon cambodjensis (Petit de la Saussaye, 1865) comb. rev.

=Monocondylea cambodjensis Petit de la Saussaye (1865): 16, pl. 4, Figure 4 [61].

=Monocondylus orbicularis Morelet (1866): 167 [60].

Figure 8

Type and type locality. Holotype NHMUK_1874-12-11-3; Siam (Thailand).

Material examined. LAOS: Nam Ngum River, 18.1796° N, 103.0585° E, Mekong River drainage, 26.ii.2020, 1 specimen (RMBH biv 0811, sequenced), Bolotov, Vikhrev, Inkhavilay, Konopleva, Chapurina, and local villagers leg.

Diagnosis. The shell is ovate subquadrate, rather large, moderately thick and compressed, with rounded anterior, curved dorsal and ventral margins, and a slightly pointed posterior margin. The posterior slope is covered with two long folds, spanning from the umbo. The umbo is not projected. The periostracum is brownish, the nacre is whitish with yellow spots. Pseudocardinal teeth on both valves are tubercle-like, moderately developed, with a broad base, and smooth. Lateral teeth are reduced. The anterior adductor muscle attachment scars are projected, contiguous with strong anterior retractor and protractor scars; the posterior muscle attachment scars are less developed.

Habitat and ecology. Unidentified nonbiting midge larvae (Diptera: Chironomidae) were found in the mantle cavity of the mussel from Nam Ngum River.

Distribution. Lower Mekong Basin.

Comments. A specimen previously identified as P. cambodjensis (GenBank acc. No KP795028) [33] belongs to P. vagulus (Fischer, 1891) based on morphological traits such as a rounded triangular shell shape and narrow hinge plate (Table 1). Additionally, several sequenced specimens from Malaysia and Thailand that were considered P. cambodjensis (see Figure S2 for detail) also belong to P. vagulus.

• Pseudodon inoscularis (Gould, 1844)

=Anodon inoscularis Gould (1844): 160 [27].

Figure 3

Type and type locality. Lectotype USNM 86352; River Salwen, Tavoy, Brit. Burmah [27]. Here, we recommend to restrict its type locality to Tavoy (currently Dawei) River, Myanmar (see above for explanation).

Topotypes examined. MYANMAR: Yae Pone Stream near Yae Pone village, 14.5481° N, 98.1873° E, Dawei River basin, 26.i.2020, 11 specimens (RMBH biv 1027, including biv 1027/1, biv 1027/2, and biv 1027/3 sequenced), Than Win leg.

Diagnosis. The shell is elongately ovate, moderately thick and inflated, posteriorly slightly winged, the posterior margin is wider than the anterior margin, the anterior margin is rounded, the dorsal margin is elevated, the ventral margin is curved, and the posterior margin is truncated or slightly rounded. The umbo is not projected, and eroded. Pseudocardinal teeth are tubercle-like, with one on each valve. The pseudocardinal tooth on the right valve is usually more pronounced, with wide base, hill-like or somewhat trapezoidal, and right-directed; the pseudocardinal tooth on the left valve may be smaller and pointed. The lateral teeth are reduced. Anterior adductor muscle attachment scars are oval-shaped, pronounced, and contiguous, with anterior retractor and often protractor muscle scars; the protractor muscle scar may be elongated and narrow. Posterior adductor muscle attachment scars are rounded and less visible. The pallial line is continuous and rather clear. Morphologically, the species is rather similar to Indopseudodon indawgyiensis sp. nov. but winged posteriorly and differs by having a narrower anterior margin and less elevated pseudocardinal teeth.

Habitat and ecology. The species inhabits rivers and streams with sand and clay substrate.

Distribution. Dawei River basin.

Comments. Previously, specimens of Indopseudodon indawgyiensis (RMBH biv 0110) were erroneously linked to P. inoscularis [9].

• Genus Bineurus Simpson, 1900 (type species: Monocondyloea mouhotii Lea, 1863; by original designation).

Diagnosis. The shell is elongate-rhomboid or kidney-shaped, inequilateral, usually with a concave ventral margin, compressed, and rather thin; the umbo is not prominent; the shell sculpture is not traced; the hinge plate is narrow, with a small, tubercle-like pseudocardinal tooth in each valve; muscle scars are usually more or less well-marked.

Distribution. Mekong Basin in Laos, Thailand, Cambodia, and southern Vietnam [8].

Comments. This genus contains five recent species (Table 1), most of which are habitat specialists and prefer fast-flowing mountain streams and smaller rivers [8]. It was revised in our earlier paper, delineating four valid species [8], while the fifth species was recently discovered and described by another team [11]. Fossil records of Bineurus are not available.

• Genus Namkongnaia Jeratthitikul et al., 2021 (type species: Namkongnaia inkhavilayi Jeratthitikul et al., 2021; by original designation).

Diagnosis. The shell is ovate, elongated, and not winged, with a smoothly rounded posterior part of the shell, which is compressed and very thin; the hinge plate is without dentition, and its posterior end with a V-shaped furrow.

Distribution. Mekong Basin in Laos, Cambodia, and Thailand [26].

Comments. This recently described genus contains two valid species (Table 1). Previously, freshwater mussels belonging to this peculiar genus and having an elongated shell were thought to be representatives of the genus Pilsbryoconcha [26]. Fossil records of Namkongnaia are lacking.

• Genus Nyeinchanconcha Bolotov et al., 2020 (type species: Nyeinchanconcha nyeinchani Bolotov et al., 2020; by original designation).

Diagnosis. The shell is ovate, winged, rather thin, and usually with a light brown periostracum and green radial lines posteriorly; the posterior area has weakly developed corrugate plication; pseudocardinal teeth are rather well developed.

Distribution. Tributaries of the Mekong River in Laos [5].

Comments. This genus is monotypic (Table 1); its single species is a strict habitat specialist and prefers fast-flowing mountain streams and smaller rivers. Fossil records of Nyeinchanconcha are unknown.

• Genus Pilsbryoconcha Simpson, 1900 (type species: Anodonta exilis Lea, 1838; by original designation).

Diagnosis. The shell is elongated, rhomboidal, and winged, with an angulate, triangular posterior part of the shell which is compressed and very thin; teeth are absent.

Distribution. Mekong, Chao Phraya, and Mae Klong basins, Thai–Malay Peninsula, and the Greater Sunda Islands [2,26,55].

Comments. Based on a modern comprehensive revision [55], this genus contains not less than nine recent species (Table 1). The nominal taxon Anodonta carinifera Conrad, 1837 (original TL: Ohio or one of its tributaries below Louisville, Kentucky, USA) [89] is somewhat enigmatic. Simpson [90] transferred it to Pilsbryoconcha and stated that it could have originated from Southeast Asia (with a note “probably”). Subsequent researchers accepted this proposal, and it was listed as a valid Pilsbryoconcha species from the 1900s up to now [1,2,6,25]. However, any evidence on an Asian origin of the type of this nominal taxon is lacking. Based on the description and type locality, this taxon in its original understanding may represent a junior synonym of Hemistena lata (Rafinesque, 1820) (TL: Kentucky River, Ohio Basin, USA), which is known to occur in the Ohio Basin of eastern North America, including the Tennessee and Cumberland rivers [2,91]. However, the type specimen of Anodonta carinifera seems to be lost, and Jeratthitikul et al. [55] recently designated its neotype, choosing it from a Pilsbryoconcha sample. This taxonomic action finally clarified the status of Pilsbryoconcha carinifera as a valid species, belonging to a Southeast Asian freshwater mussel assemblage.

Three fossil species were placed in this genus; that is, †Pilsbryoconcha sublinguaeformis Tolstikova, 1975, †P. praeexilis Tolstikova, 1976, and †P. longiformis (Takayasu, Gurung & Matsuoka, 1995). The first two species were described from the Paleogene deposits of Kazakhstan. Their diagnostic features are as follows: the shell is small, elongate-triangular, and tapering anteriorly, with a narrow anterior and broad posterior margin; pseudocardinal teeth are lamellar and reduced; anterior muscle scars are large [68,69]. These features indicate that the two fossil species from Kazakhstan do not correspond to Pilsbryoconcha even by a specific (triangular) shell shape and should be transferred to the genus Parvasolenaia Huang & Wu, 2019 (Gonideini). The two new combinations are presented here (Table 1). †P. longiformis from the Middle Miocene-Pliocene Siwalik Group of Nepal was initially described in the genus Lamellidens [80] and was subsequently transferred to Pilsbryoconcha based on an elongated shell shape [81]. However, this taxonomic action is rather doubtful, because the structure of adductor scars, hinge, and teeth of this fossil species are unknown [80]. From a conchological point of view, it better fits the genus Balwantia Prashad, 1919 (Lamellidentini) due to very similar shell outline with narrower anterior margin. Here, we propose †Balwantia longiformis comb. nov. (Table 1). Hence, fossil members of Pilsbryoconcha are yet to be discovered.

• Genus Sundadontina Bolotov et al., 2020 (type species: Anodonta cumingii Lea, 1851; by original designation).

Diagnosis. The shell is ovate or rounded, without a prominent wing, rather thick, usually with dark brown or black periostracum; the umbo is not prominent; the shell sculpture is not traced; pseudocardinal teeth are stout and knob-like; the anterior muscle scar is ovate and well-marked, usually contiguous with the pedal retractor scar; the posterior muscle scar is shallow.

Distribution. Lenya Basin in southern Myanmar, Mekong Basin in Thailand, Laos, Cambodia, and southern Vietnam, the Chao Phraya Basin in Thailand, and rivers of the Thai–Malay Peninsula [5,9].

Comments. This widespread genus contains 12 recent species (Table 1). Additionally, Sundadontina sp. ‘Lenya’ from a tributary of the Lenya Basin in southern Myanmar represents a divergent phylogenetic lineage (Figure 2) but we need more DNA sequences to estimate the status of this putative new species. Here, we clarify the phylogenetic position of two nominal species, that is, Sundadontina harmandi and S. sulcata, based on newly sequenced specimens from the Mekong Basin in Laos (Table 1 and Dataset S2). Fossil Sundadontina species are not known.

 

• Sundadontina harmandi (Crosse & Fischer, 1876)

=Pseudodon harmandi Crosse & P. Fischer (1876): 331, pl. 10, Figure 2 [92].

Figure 7

Type and type locality. Holotype MNHN-IM-2000-1717; Cambodgia (Cambodia).

Material examined. LAOS: Huay Tuay River, 14.8732° N, 106.9339° E, Xe Khong River basin, Mekong River drainage, 02.iii.2020, 2 specimens (RMBH biv 0881, including biv 0881/1 sequenced and one broken dead shell), Bolotov, Vikhrev, Inkhavilay, Konopleva, Chapurina, and local villagers leg.

Diagnosis. The shell is ovate, moderately thick and inflated, the posterior margin is wider than the anterior margin, the dorsal and ventral margins are curved, the posterior slope is rather high with two hardly projected folds. The umbo is small and eroded. Slightly visible, thin radial lines cover the surface of the shell from the umbo to the ventral margin. The pseudocardinal tooth on right valve is elevated, somewhat triangular. The pseudocardinal tooth on the left valve is bell-shaped and ragged. Lateral teeth are reduced. The anterior adductor muscle attachment scar is bean-like, visible, and contiguous with the anterior retractor and close banded with the protractor muscle scars. The posterior adductor muscle attachment scar is somewhat rounded and shallow. The pallial line is not pronounced.

Habitat and ecology. Rivers with sandy and rocky substrate.

Distribution. Mekong River drainage in Laos and Cambodia.

• Sundadontina sulcata (Rochebrune, 1882)

=Pseudodon sulcatum Rochebrune (1882): 41 [93].

Figure 7

Type and type locality. Syntype MNHN-IM-2000-1795; Embouchure du Mékong (mouth of the Mekong) [93].

Material examined. LAOS: Phaphou Stream, 14.7276° N, 106.0933° E, Mekong River drainage, 01.iii.2020, 10 specimens (RMBH biv 0869, including biv 0869/1, biv 0869/2, and biv 0869/3 sequenced), Bolotov, Vikhrev, Inkhavilay, Konopleva, Chapurina, and local villagers leg.

Diagnosis. The shell is oviform, the posterior slope is high, covered by two more or less-developed radial folds stretching from the umbo to posterior margin, undulating ridges cover the dorsal and posterior half of the shell and sometimes the anterior margin, and it is more developed in younger individuals. The umbo is moderately projected, and usually eroded. The pseudocardinal tooth on the left valve is massive, looking like a concave tubercle, the pseudocardinal tooth on the right valve is variable, from a triangular to rectangular shape. Lateral teeth are reduced. The anterior adductor muscle attachment scars are oval-shaped, closely banded with the anterior retractor and protractor muscle scars. The posterior adductor muscle attachment scars are somewhat drop-like and shallow. The pallial line is well-visible.

Habitat and ecology. Representatives of Pseudodon, Bineurus, and Lens were discovered living together with S. sulcata in the Phaphou Stream, Laos.

Distribution. Lower Mekong basin in Laos and Cambodia.

Comments. Haas [2] regarded this species as a synonym of P. cumingii (Lea, 1850) but this opinion is incorrect.

• Genus Thaiconcha Bolotov et al., 2020 (type species: Anodonta callifera Martens, 1860; by original designation).

Diagnosis. The shell is large, thick, elliptical or rounded, elongated posteriorly, moderately inflated, and the ventral margin is not concave; the umbo is not prominent; the shell sculpture is not traced; pseudocardinal teeth are rather well-developed; muscle attachment scars are well-marked.

Distribution. Mekong and Chao Phraya basins, Thailand and Cambodia [8].

Comments. This genus contains three recent species (Table 1). It was revised in our earlier paper [8]. Fossil Thaiconcha species are unknown.

3.5. A Brief Overview of Taxa That Should Be Excluded from the Tribe Pseudodontini

• Family Unionidae Rafinesque, 1820
• Subfamily Gonideinae Ortmann, 1916
• Tribe Gonideini Ortmann, 1916

• Genus Ptychorhynchus Simpson, 1900 (type species: Unio pfisteri Heude, 1874; by original designation).

=Cosmopseudodon Haas, 1920 syn. nov. (type species: Pseudodon resupinatus Martens, 1902; by original designation).

=Heudeana Frierson, 1922 (type species: Unio murinus Heude, 1883; by original designation).

Diagnosis. The shell is elongated, elliptical, and inequilateral; the umbo is not prominent; the posterior slope is usually with a prominent gonial ridge crossed by curved, parallel folds, although this feature is lacking in some species; the right valve is with one blunt, low, more or less elongated pseudocardinal tooth and one lateral tooth, sometimes having a weak auxilliary lamella; the left valve is with two low, short, roughened pseudocardinal teeth and one or two rather weak lateral teeth; muscle attachment scars are well-marked but shallow.

Distribution. Pearl, Yangtze, and Huang He (Yellow) basins and Hainan Island in eastern China, and Thuong Basin in northern Vietnam [53].

Comments. Based on the newest global checklist of freshwater mussel taxa, this genus contains five valid species [1]. Here, we transfer four nominal taxa (two additional valid species and two new synonyms) to this genus based on morphological and biogeographical evidence (Table 1). In particular, Pseudodon resupinatus Martens, 1902 morphologically corresponds to Ptychorhynchus by having a posterior slope with a prominent gonial ridge crossed by curved, parallel folds and similar structure of the hinge plate. Alternatively, it externally resembles Obovalis omiensis (Heimburg, 1884), a single member of the monotypic genus Obovalis Simpson, 1900, but the latter taxon differs from Pseudodon resupinatus by having a small, knob-like pseudocardinal tooth and traces of one lateral tooth in each valve. Hence, we propose here Ptychorhynchus resupinatus comb. nov. Two nominal taxa, i.e., Unio pinchonianus Heude, 1883 and Pseudodon solidus Haas, 1911, are currently placed within the genus Pseudodon, with the second name being a junior synonym of P. pinchonianus [1]. Conchologically, both the taxa are within the range of shell variability of Ptychorhynchus murinum (Heude, 1883), which shares an elliptical to broadly elliptical shell without clear sculpture, a non-prominent umbo, blunt, elongated pseudocardinal teeth, and weakly developed lateral teeth. Here, we propose the synonymy as follows: Ptychorhynchus murinum (=Unio pinchonianus syn. nov.; = Pseudodon solidus syn. nov.). Additionally, we transfer the nominal taxon Lamellidens liuovatus He & Zhuang, 2013 from Pseudobaphia Simpson, 1900 to Ptychorhynchus, based on morphological features such as an elliptical shell shape, a non-prominent umbo, and a characteristic teeth structure [53], and propose Ptychorhynchus liuovatus comb. nov.

• Genus Simpsonasus Bolotov & Konopleva nom. nov. (replacement name for Nasus Simpson, 1900).

=Nasus Simpson, 1900 (type species: Monocondylea nankingensis Heude, 1874; by original designation; unavailable name: homonym of Nasus Basilewski, 1855, Pisces).

LSID. https://zoobank.org/urn:lsid:zoobank.org:act:AD6E2012-213A-47E2-A8DF-0C1166B39BB7 (accessed on 5 June 2023)

Diagnosis. The shell is ovate or narrowed and elongated; the posterior half of the shell tapers posteriorly, with a more-or-less pointed end; the umbo is not prominent; the shell sculpture is not traced; there is a small, lamella-like, triangular or rounded pseudocardinal tooth in each valve; lateral teeth are slightly curved and elongated; muscle attachment scars are well-marked.

Distribution. Lower Yangtze Basin, eastern China.

Comments. Here, we introduce a replacement name for Nasus Simpson, 1900 and transfer this genus to the tribe Gonideini based on morphological and biogeographical evidence. The two nominal taxa belonging to this genus, i.e., Pseudodon nankingensis (Heude, 1874) and P. secundus Heude, 1877 syn. nov., are considered conspecific based on morphological similarity and overlapping type localities (Table 1). Additionally, one fossil species, †Simpsonasus politus comb. nov., is transferred to this genus (Table 1).

 

• Subfamily Unioninae Rafinesque, 1820
• Tribe Unionini Rafinesque, 1820

• Genus Pseudobaphia Simpson, 1900 (type species: Unio biesianus Heude, 1877; by original designation [90]).

=Chrysopseudodon Haas, 1920 syn. nov. (type species: Psudodon (sic!) aureus Heude, 1885; by original designation [94]).

Diagnosis. The shell is Parreysia-like, oval, massive, and thick; the umbo is enlarged, prominent, and elevated; the shell sculpture is not traced; the hinge plate is strongly curved; there is a massive, irregular or rounded pseudocardinal tooth in the right valve, two large pseudocardinal teeth in the left valve; two short lateral teeth in each valve; anterior muscle attachment scars are very deep, posterior muscle attachment scars are shallow but well-marked. The DNA-based diagnosis for this genus is not available. It should be noted that the newly described genus Postolata Dai et al., 2023 (Gonideini) from the upstream section of the Pearl Basin in Guangxi, southern China [95] conchologically resembles Pseudobaphia. At first glance, Pseudobaphia may also be a Gonideini member but the lack of molecular data precludes any final solution on its tribal placement.

Distribution. Yangtze and Pearl basins in eastern China and northern Vietnam [96].

Comments. This genus contains three recent and one fossil species: Pseudobaphia biesiana (Heude, 1877), P. banggiangensis Bogan & Do, 2018 [1], P. aurea (Heude, 1885) comb. nov. (=Pseudodon aureus), and †P. pingi (Otuka, 1942) comb. nov. (=Pseudodon pingi) (Table 1). The nominal taxon Lamellidens liuovatus He & Zhuang, 2013 was also placed in this genus [1] but, in our opinion, it better fits Ptychorhynchus based on morphological features (see above). The genus Chrysopseudodon becomes a synonym of Pseudobaphia, as its type species is transferred to the latter genus. Based on the protologue with an image of the type specimen [78], P. aurea comb. nov. shares characteristic features of the genus Pseudobaphia such as a specific shell shape with an elevated umbo, a thick shell, a rounded pseudocardinal tooth in the right valve, and short laterals. This type most likely represents a juvenile specimen (shell length 31 mm), which explains the yellowish–green color of its periostracum [78]. Unfortunately, the type specimen of P. aurea comb. nov. is probably lost, whereas other samples of this species are not available [53].

4. Discussion

4.1. Taxonomic Novelties and Their Consequences for the Modern Concept of the Tribe Pseudodontini

Here, we clarify the phylogenetic position of Pseudodon inoscularis, the type species of the genus. Although this species occurs in the Dawei River in southern Myanmar, which was considered a part of the Western Indochina Subregion [5], it phylogenetically belongs to the eastern clade of the Pseudodontini and clusters with members of the genus Monodontina. Hence, the name Monodontina becomes a junior synonym of Pseudodon, while Indopseudodon stat. rev. and the subtribe Indopseudodontina subtr. nov. are valid names for the western clade of the tribe.

One of the challenges in the study of the tribe Pseudodontini, as for other Unionidae, is a high level of intraspecific conchological variability, and age-induced differences between individuals within one population, as well as the presence of cryptic taxa [8,9,28]. Usually, these peculiarities could be driven by environmental and hydrological gradients such as the type of substrate, flow rate, etc. [8,30]. This pattern must be considered in morphological analyses, although, in some cases, a molecular approach is a sole tool for establishing differential diagnoses and solving the taxonomic issues.

Our review of original and published data indicates that the tribe Pseudodontini contains 48 species, belonging to nine genera and two subtribes (see Table 1 for detail). Altogether, 39 species were already sequenced, while 9 species are still delineated on the basis of morphological data alone. None of the DNA sequences is available for nominal species described from Sumatra and Borneo (Pilsbryoconcha expressa, Pseudodon aeneolus, and P. walpolei) and from the Nicobar Islands (Pseudodon nicobaricus). Additionally, five nominal species described from Thailand, Cambodia, and southern Vietnam are still known based on historical shell lots only; that is, Pseudodon thomsoni, Sundadontina mabilli, S. moreleti, S. ovalis, and S. ponderosa. Here, we confirm that none of the Pseudodontini representatives ranges in the East Asian Subregion. Our preliminary analysis indicates that several recent species described from the Yangtze and Pearl River basins in China and placed in the genus Pseudodon [1,53] morphologically correspond to other genera such as Ptychorhynchus, Simpsonasus nom. nov. (Gonideini), and Pseudobaphia (Unionini). In particular, Pseudodon aureus is transferred to Pseudobaphia; P. pinchonianus, P. resupinatus, and P. solidus to Ptychorhynchus; P. nankingensis, and P. secundus to Simpsonasus nom. nov. (see Table 1 and Taxonomic Account for detail). Hence, the Pseudodontini could be considered a tribe endemic to Southeast Asia, the range of which exclusively covers the Western Indochina (Ayeyarwady to Salween) and Sundaland (Mekong, Chao Phraya, Mae Klong, and the drainages of the Thai–Malay Peninsula and Greater Sunda Islands) subregions. The boundaries of these freshwater subregions were defined previously based on a broad-scale phylogenetic and biogeographic research on freshwater mussels [5,97].

4.2. Ancient Radiations of the Pseudodontini in Mainland Southeast Asia

In many cases, large river basins support diverse and species-rich assemblages of freshwater mussels and fishes [1]. Two large monophyletic radiations of freshwater mussels (mean age ca. 50–55 Ma) belonging to the tribes Pseudodontini and Rectidentini were discovered within the former paleo-Mekong catchment (Mekong, Siam, and Malacca Strait paleo-river drainage basins) [4]. This discovery reveals that the Mekong may be considered an example of a long-lived river that has existed throughout the entire Cenozoic epoch [4]. Paleontological data also indicates that the Unionidae faunas of the Mekong and Yangtze rivers must have developed as separate radiations since the early Paleogene [98]. Our new results based on a greatly expanded DNA sequence dataset of Pseudodontini species support these hypotheses but indicate older mean age for the MRCAs of the subclades.

Brandt [3] noted that the fauna of freshwater Mollusca of Myanmar is different from that of Thailand and China but closely related to that of Bengal and Bangladesh. Based on a broad-scale phylogenetic reconstruction, it was shown that the fauna of western Indochina (Myanmar) represents an amalgam of two groups of freshwater mussels, having different origin [32]. The first group (Parreysiinae) originated in East Gondwana and arrived to Asia on two fragments of the former supercontinent serving as ‘biotic ferries’, i.e., the Indian Plate and Burma Terrane. The second group (Gonideinae: Pseudodontini and Contradentini) originated in Laurasia and colonized the Burma Terrane after the collision of this tectonic block with the Sunda Plate. Available paleogeographic data indicate that this collision occurred in the Late Eocene (approximately 40–41 Ma) [99,100], although some authors propose older dating for this event [101]. A collision zone between the Burma Terrane and the western margin of the Sunda Plate (Sibumasu or Shan-Thai Terrane) may be considered an ancestral area for Indopseudodon. In particular, Indopseudodon kayinensis, the basal lineage in the genus, was discovered in rivers emptying into the Salween Estuary. Our time-calibrated phylogeny returns an older mean age for the split between Indopseudodon kayinensis and other species in this genus (52.4 Ma). However, our sampling efforts did not cover several basins at the western margin of the Sunda Plate such as the Tanintharyi (historically Great Tenasserim), Mae Klong, and Moei rivers. These freshwater basins may house some additional Indopseudodon species, whose addition could decrease the mean age of the split between species groups from the Sunda Plate and Burma Terrane. Conversely, our dating largely aligns with the India–Eurasia collision at around 50 Ma [102].

Unfortunately, paleontological records of the Pseudodontini are scarce. Here, we tentatively revised these records and found that †Indopseudodon rostratus is the oldest fossil species ever discovered in the tribe. It was described from a pre-Pliocene or Pliocene deposits belonging to the Cenozoic Mae Sot Series. †Pseudodon vondembuschianus vandervlerki and †P. vondembuschianus trinilensis from Java represent two additional fossil taxa of the Early and Middle Pleistocene age, respectively. All other fossil nominal species currently placed in Pilsbryoconcha and Pseudodon are reconsidered here as belonging to other genera in the tribes Gonideini, Unionini, and Lamellidentini based on morphological and biogeographic evidence. In particular, †Pilsbryoconcha sublinguaeformis and †P. praeexilis are transferred to Parvasolenaia (Gonideini); †Pseudodon oettingenae and †Monodontina mogul to Pseudodontopsis (Gonideini); †Pseudodon politus to Simpsonasus nom. nov. (Gonideini); †Pseudodon pingi to Pseudobaphia (Unionini); †Pilsbryoconcha longiformis to Balwantia (Lamellidentini); and †Pseudodon indicus to Lamellidens (Lamellidentini) (see Table 1 and Taxonomic account for detail). There is a need for a thorough revision of fossil freshwater mussel taxa, because this information is necessary for the future development of time-calibrated phylogenies.

4.3. Ecology and Larval Hosts of the Pseudodontini

Our review indicates that most of the Pseudodontini species are habitat specialists, preferring lotic ecosystems such as rivers and streams (see Table 1). Nyeinchanconcha nyeinchani and Bineurus mouhotii are known to exclusively occur in mountain streams and rivers [5,8]. A few species (e.g., Pseudodon vondembuschianus) were also recorded from irrigation channels with flowing water [56]. In contrast, species from the genera Pilsbryoconcha and Namkongnaia are mostly habitat generalists that were recorded from both lentic and lotic waters [3,26,55].

The body of available data on larval hosts of the Pseudodontini representatives is extremely limited (see Table 1). In particular, certain hosts of all species are unknown, although an unspecified cyprinid fish was mentioned as a host of Pilsbryoconcha exilis glochidia [56]. Moreover, suitable hosts were determined for only a few freshwater mussel species from other tropical Asian tribes, that is, Hyriopsis myersiana (Lea, 1856) (Rectidentini) [103], Chamberlainia duclerci (Rochebrune, 1882) (Chamberlainiini) [104,105], Lens contradens (Lea, 1838), Physunio superbus (Lea, 1843) (Contradentini) [56], Lamellidens ferrugineus (Annandale, 1918), and L. marginalis (Lamarck, 1819) (Lamellidentini) [106]. Panha [56] listed 14 fish species and one unspecified palaemonid shrimp (Decapoda: Palaemonidae) as larval hosts of freshwater mussels from the Chao Phraya Basin in Thailand but, in most cases, it was impossible to perform a taxonomic identification of the observed glochidia. Several genera of stream specialists belonging to the Pseudodontini such as Bineurus, Indopseudodon, Nyeinchanconcha, Songkhlanaia, and Sundadontina may have a narrow host range, being associated with a few local endemic species of fish. Undoubtedly, the almost complete lack of information on hosts is the most striking gap in our knowledge of the tribe Pseudodontini and other tropical freshwater mussels, because this information is urgently needed for conservation purposes.

5. Conclusions

In this study, we revised the taxonomy of the tribe Pseudodontini, using new topotypes of many nominal taxa, and proposed an updated biogeographic reconstruction on the origin and diversification of this large group. We showed that Pseudodon inoscularis, the type species of this genus, belongs to the eastern clade of the tribe and represents a member of the genus Monodontina. Hence, Monodontina is a synonym of Pseudodon, while the subtribe Pseudodontina (=Pilsbryoconchina syn. nov.) should be used as a name for the eastern clade. The genus Indopseudodon stat. rev. and the subtribe Indopseudodontina subtr. nov. were applied as valid names for the western clade of the Pseudodontini (Ayeyarwady to Salween). We also revised the genus Indopseudodon and showed that it contains five recent species, including one species new to science, I. indawgyiensis sp. nov. Additionally, we re-described Indopseudodon crebristriatus, based on a series of newly collected topotypes from the Pegu (currently Bago) River. We clarified the taxonomic status and phylogenetic position of Pseudodon cambodjensis, P. vagulus, Sundadontina harmandi, and S. sulcata.

Based on morphological traits, we concluded that several recent and fossil species from East, South and Central Asia traditionally placed in the Pseudodontini belong to Parvasolenaia, Pseudodontopsis, Ptychorhynchus, Simpsonasus nom. nov. (a new name for Nasus Simpson, 1900) (Gonideinae: Gonideini), Pseudobaphia (Unioninae: Unionini), Balwantia and Lamellidens (Parreysiinae: Lamellidentini). We revealed that the Pseudodontini more likely represents a tribe endemic to Southeast Asia, the range of which exclusively covers the Western Indochina and Sundaland subregions.

We demonstrated that most of the Pseudodontini species, belonging to the genera Bineurus, Indopseudodon, Nyeinchanconcha, Songkhlanaia, and Sundadontina, are habitat specialists, preferring lotic ecosystems such as rivers and streams. Thus, further research on the Pseudodontini, a diverse group of species with restricted ranges and specific environmental requirements, is urgent to supply special conservation and monitoring efforts for these taxa.