New freshwater mussel taxa discoveries clarify biogeographic division of Southeast Asia

New Unionidae genera and species from Southeast Asia

Our multi-locus phylogenies were constructed using BEAST v2.6.1, MrBayes v3.2.6 and IQ-TREE v1.6.11 based on the mitochondrial cytochrome c oxidase subunit I (COI), small ribosomal RNA (16 S rRNA), and the nuclear large ribosomal RNA (28 S rRNA) gene fragments. These analyses returned well-resolved consensus phylogenies having a similar topology (Fig. 1 and Supplementary Fig. 1). We found that available freshwater mussel taxa from Southeast Asia cluster to at least 25 genera, four of which are new to science and are described here. The novel genera represent distant monotypic lineages (Scabiellus gen. nov. and Nyeinchanconcha gen. nov.) and well-supported clades with several species (Sundadontina gen. nov. and Thaiconcha gen. nov.). An integrative species delimitation analysis indicates that our dataset contains 12 species that do not have available names and can be considered new to science (Figs. 1–5, Tables 1–3, Supplementary Tables 1–2). Each novel species can be distinguished from its sister taxa by conchological and molecular characters. A description of each new species is presented below. Mean shell parameters for the type series of new species are presented in Table 1. Two more unnamed species-level lineages, i.e. Ensidens sp. ‘Mun’ and Ensidens sp. ‘Thai’ (Fig. 1), appear to represent cryptic species and require separate research.

Figure 1

Time-calibrated multi-locus phylogeny of the Unionidae based on the complete data set of mitochondrial and nuclear sequences (five partitions: three codons of COI + 16 S rRNA + 28 S rRNA). Red numbers near nodes are BPP of BEAST v2.6.1. Black numbers near nodes are the node ages. Node bars are 95% HPD of the divergence time. Age reconstructions for weakly supported nodes (BPP < 0.75) are omitted. Pie charts at nodes indicate the probabilities of certain ancestral areas for clades of interest with respect to combined results under two different statistical modeling approaches (S-DIVA and Bayesian MCMC analysis). New generic and species names are colored red. Outgroup and non-target clades are collapsed. Stratigraphic chart according to the International Commission on Stratigraphy, 2019.

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Figure 2

Shells of the Parreysiini, Indochinellini, Contradentini, and Rectidentini from Southeast Asia. (A) Parreysia rakhinensis sp. nov., Kyeintali Stream, Rakhine Coast, Myanmar (holotype RMBH biv652_1). (B) Parreysia rakhinensis sp. nov., Ann River, Rakhine Coast, Myanmar (paratype RMBH biv659_3). (C) Balwantia baniensis sp. nov., Bani River, Ayeyarwady Basin, Myanmar (holotype RMBH 666_2). (D) Balwantia soleniformis (Benson, 1836) comb. rev., Brahmaputra River, India (specimen USNM 127246). (E) Trapezoideus lenya sp. nov., 14th Mile Stream, Lenya Basin, southeastern Myanmar (holotype RMBH biv629_2). (F) Yaukthwa avaensis sp. nov., unnamed small stream, a tributary of the Ayeyarwady River, Myanmar (holotype RMBH biv680_3). (G) Scabiellus songkramensis (Kongim & Panha, 2015) gen. & comb. nov., Songkhram River, Mekong Basin, Thailand (topotype, collection of S. Tumpeesuwan, Mahasarakham University). Scale bars = 1 cm [A-C, E-G] and 3 cm [D]. Photos: Ekaterina S. Konopleva [A-C, E, F], Ellen Strong [D], and Benchawan Nahok [G].

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Figure 3

Shells of the Pseudodontini from Southeast Asia. (A) Monodontina cambodjensis (Petit de la Saussaye, 1865), Pursat River, Mekong Basin, Cambodia (specimen UMMZ 304350). (B) Monodontina vondembuschiana (Lea, 1840), Java (holotype USNM 86348). (C) Monodontina laosica sp. nov., Houai Pin Stream, a tributary of the Vang Ngao River, Mekong Basin, southern Laos (holotype UMMZ 304650). (D) Monodontina lenyanensis sp. nov., 14 Mile Stream, Lenya Basin, Myanmar (holotype RMBH biv628_2). (E) Monodontina mekongi sp. nov., headwater of the Phong River, Mekong Basin, Thailand (holotype RMBH biv122). (F) Nyeinchanconcha nyeinchani gen. & sp. nov., small stream arising at cave near Ban Kouanphavang, Mekong Basin, central Laos (holotype NCSM 84884). (G) Pseudodon kayinensis sp. nov., Winyaw River, Ataran Basin, southeastern Myanmar (holotype RMBH biv618_1). (H) Pseudodon salwenianus (Gould, 1844), unnamed stream, Salween Basin, Myanmar (a topotype specimen RMBH biv639_3). Scale bars = 1 cm. Photos: Taehwan Lee [A, C], Ilya V. Vikhrev [B], Ekaterina S. Konopleva [D, E, G, H], and Jamie M. Smith [F].

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Figure 4

Shells of the Pseudodontini from Southeast Asia. (A) Sundadontina cumingii (Lea, 1850) gen. & comb. nov., Malacca (holotype USNM 86350). (B) Sundadontina brandti sp. nov., headwater of the Mun River, Mekong Basin, Thailand (holotype RMBH biv475_2). (C) Sundadontina tanintharyiensis sp. nov., Chaung Nauk Pyan Stream, Lenya Basin, Myanmar (holotype RMBH biv643_4). (D) Sundadontina taskaevi sp. nov., headwater of the Mun River, Mekong Basin, Thailand (holotype RMBH biv475_1). (E) Sundadontina moreleti (Crosse & Fischer, 1876) comb. nov., Mekong Basin, Cambodia (syntype MNHN-IM-2000–34623). (F) Sundadontina tumida (Morelet, 1866) comb. nov., Cambodia (holotype NHMUK 93-2-4-1734). (G) Thaiconcha callifera (Martens, 1860) gen. & comb. nov., Siam (holotype NHMUK 1859-8-1-20). (H) Thaiconcha callifera (Martens, 1860) gen. & comb. nov., (a topotype specimen RMBH biv120_11). Scale bars = 1 cm. Photos: Ilya V. Vikhrev [A], Ekaterina S. Konopleva [B-D, H], Kevin Webb (NHMUK Photographic Unit) [F, G], and Manuel Caballer (2018 MNHN Project: RECOLNAT No. ANR-11-INBS-0004) [E].

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Figure 5

Updated freshwater biogeographic division of the mainland Southeast Asia based on freshwater mussel phylogenetics. (A) Freshwater biogeographic division of the mainland Southeast Asia. Color circles indicate the localities of taxa having biogeographic affinities to the Indian (yellow), Western Indochina (green), and Sundaland (pink) faunas. Type localities of new freshwater mussel species and occurrences of two cryptic Ensidens lineages are numbered as follows: Parreysia rakhinensis sp. nov. (1), Balwantia baniensis sp. nov. (2), Trapezoideus lenya sp. nov. and Monodontina lenyanensis sp. nov. (3), Yaukthwa avaensis sp. nov. (4), Ensidens sp. ‘Mun’ (5), Ensidens sp. ‘Thai’ (6), Monodontina laosica sp. nov. (7), M. mekongi sp. nov. (8), Nyeinchanconcha nyeinchani gen. & sp. nov. (9), Pseudodon kayinensis sp. nov. (10), Sundadontina brandti sp. nov. and S. taskaevi sp. nov. (11), and S. tanintharyiensis sp. nov. (12). (B) Boundary between the Western Indochina and Sundaland freshwater subregions at the southern margin of the Kra Isthmus (Tanintharyi – Lenya drainage divide). Violet line indicates the boundary between freshwater subregions based on drainage divides of the corresponding river basins. Green circles indicate records of the Western Indochina fauna representatives: Leoparreysia tavoyensis, Trapezidens scutum, T. exolescens^4,24, Indochinella pugio daweiensis⁶. Pink circles indicate records of the Sundaland fauna representatives: Trapezoideus foliaceus⁷, Trapezoideus lenya sp. nov., Monodontina lenyanensis sp. nov., and Sundadontina tanintharyiensis sp. nov. All freshwater mussel taxa in Malaysia are members of the Sundaland fauna^16,22. Arrows indicate putative ancient (pre-Pleistocene) dispersal routes of the Western Indochina (green) and Sundaland (pink) Unionidae species around the Isthmus of Kra and surrounding areas inferred from our statistical biogeographic analyses and distribution data (Supplementary Tables 1 and 3). Blue gradient shading indicates the putative ancient seaways crossing the Thai-Malay Peninsula based on the ArcGIS modeling (hydrologically conditioned DEM with elevation levels < 120 m) and published data³⁶. The map was created using ESRI ArcGIS 10 software (https://www.esri.com/arcgis); the topographic base of the map was created with Natural Earth Free Vector and Raster Map Data (https://www.naturalearthdata.com), Global Self-consistent Hierarchical High-resolution Geography, GSHHG v2.3.7 (https://www.soest.hawaii.edu/wessel/gshhg), HydroSHEDS (https://www.hydrosheds.org)⁸¹, The General Bathymetric Chart of the Oceans, GEBCO (https://www.gebco.net), and Vector Map (VMap) Level 0 (http://gis-lab.info/qa/vmap0-eng.html) (Maps: Mikhail Yu. Gofarov).

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New freshwater mussel species distribution and their biogeographic affinities in Southeast Asia

Here, we examined freshwater mussels from several poorly known, remote basins such as the rivers of the Rakhine Coast (western Myanmar), the Tanintharyi (former Tenasserim) River, and the Lenya River (southeastern Myanmar) (Fig. 5A). We found that the rivers of the Rakhine Coast are inhabited by one species, Parreysia rakhinensis sp. nov. This is the first record of a Parreysiini member in Myanmar. The fauna of the Tanintharyi Basin includes several species belonging to endemic genera of the Western Indochina Subregion, i.e. Trapezidens scutum and Leoparreysia tavoyensis. In contrast, Trapezoideus lenya sp. nov., Monodontina lenyanensis sp. nov., and Sundadontina tanintharyiensis sp. nov. were discovered from the Lenya River basin, which is located just south of the Tanintharyi Basin (Fig. 5B). These species are members of the genera widely distributed throughout the Sundaland Subregion.

Evolutionary biogeography and time-calibrated phylogeny

Our statistical biogeographic analyses (the combined results of S-DIVA and Bayesian MCMC runs) and time-calibrated Bayesian phylogeny reveal the high levels of endemism of freshwater mussel fauna within each subregion (Fig. 1 and Supplementary Table 3). The fauna of the Sundaland Subregion contains several endemic radiations of freshwater mussels belonging to two subfamilies, i.e. the subtribe Pilsbryoconchina, the tribe Rectidentini, the genera Trapezoideus, Physunio, Contradens (Gonideinae), and the so-called Mekong’s Indochinellini group (Harmandia, Scabies, Scabiellus gen. nov., and Unionetta) (Parreysiinae). The subtribe Pseudodontina, the genus Yaukthwa (Gonideinae), tribe Leoparreysiini, the genera Radiatula, Indochinella, and Trapezidens (Parreysiinae) are endemic clades to the Western Indochina Subregion. The fauna of the Indian Subregion contains the endemic clade Parreysiini, and shares some genus-level subclades with the Western Indochina, i.e. Indonaia, Lamellidens, and, most probably, Balwantia.

Our time-calibrated Bayesian phylogenetic model reveals that there were several splits between the Western Indochinese and Sundaland clades (Fig. 1). The oldest split between such groups occurred between the subtribes Pseudodontina and Pilsbryoconchina in the Late Cretaceous (mean age = 79.3 Myr, 95% HPD = 63.2–96.8 Myr). The Pseudodontina most likely diversified in the Western Indochinese Region and the Pilsbryoconchina evolved in the Sundaland Subregion (probability = 97.8% and 98.7%, respectively) (Supplementary Table 3). The Rectidentini + Contradentini clade (the former subfamily Rectidentinae²¹) appears to have evolved within the Sundaland Subregion since the Late Cretaceous (probability = 98.4%; mean age = 95.6 Myr, 95% HPD = 76.8–115.0 Myr). A colonization event of the Yaukthwa + Balwantia clade to Western Indochina occurred in the Early Eocene (mean age = 45.6 Myr, 95% HPD = 35.8–55.1 Myr) followed by an intra-area radiation (probability = 95.8%). Conversely, the Indochinellini seems to be a primary Western Indochinese clade evolving in situ since the Late Cretaceous (probability = 75.0%; mean age = 70.4 Myr, 95% HPD = 56.1–87.0 Myr). Divergence of Mekong’s Indochinellini clade from Western Indochinese Radiatula placed in the Late Eocene (probability = 50.9%; mean age = 38.6 Myr, 95% HPD = 30.1–47.5 Myr). After the vicariance event separating these taxa, each group diversified in isolation within a corresponding subregion (probability = 95–100%). Our analyses indicate that the Parreysiini clade is a group evolved within the Indian Subregion (probability = 95.2%), whereas the Leoparreysiini diversified within the Western Indochina (probability = 99.6%). The Indian and Indochinese Indonaia species groups most likely diverged in the mid-Miocene (mean age = 14.6 Myr, 95% HPD = 9.4–21.3 Myr) via a dispersal event from Western Indochina to India but with a rather low probability (47.0%).

Three novel species from the Lenya Basin have sister taxa in the Mekong River and smaller basins emptying into the Gulf of Thailand. These species likely separated by a series of splits occurred from the Oligocene – Miocene boundary to the Late Miocene as follows: (1) Monodontina lenyanensis sp. nov. vs M. mekongi sp. nov. + M. vondembuschiana + M. laosica sp. nov. + M. cambodjensis (mean age = 24.5 Myr, 95% HPD = 16.7–32.6 Myr); (2) Trapezoideus lenya sp. nov. vs T. foliaceus (mean age = 12.1 Myr, 95% HPD = 5.3–19.7 Myr); and (3) Sundadontina tanintharyiensis sp. nov. vs S. cumingii (mean age = 6.2 Myr, 95% HPD = 2.3–10.8 Myr). The split between Parreysia rakhinensis sp. nov. from western Myanmar and several species from India occurred in the Late Miocene (mean age = 5.5 Myr, 95% HPD = 2.9–8.1 Myr).

Taxonomic account. Family Unionidae Rafinesque, 1820

Subfamily Parreysiinae Henderson, 1935

Tribe Parreysiini Henderson, 1935

Type genus: Parreysia Conrad, 1853 (by original designation)

Comments: A monotypic Indian tribe⁴.

Distribution: Indian Subregion from the Indus Basin²⁹ to the coastal basins of the Rakhine State of Myanmar.

Genus Parreysia Conrad, 1853

Type species: Unio multidentatus Philippi, 1847 (by original designation)

Comments: A diverse Indian genus, in which the modern systematics and number of species are still uncertain, due to the lack of available molecular data. Several species from Western Indochina originally assigned to Parreysia were recently transferred to another genus, Leoparreysia Vikhrev, Bolotov & Aksenova, 2017, belonging to the tribe Leoparreysiini Vikhrev, Bolotov & Kondakov, 2017⁴. Parreysia rakhinensis sp. nov. is the first member of the true Parreysiini discovered in Myanmar (Table 3).

Distribution: As for the tribe.

Parreysia rakhinensis sp. nov

Figures 2A,B, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0652_1, MYANMAR: Kyeintali Stream upstream of Ohtein village, 17.9193°N, 94.5946°E, 04.xii.2018, Bogan, Bolotov, Vikhrev, Lopes-Lima, Nyein Chan and local villagers leg. Reference sequence numbers of the holotype are as follows: MN275091 (COI), MN307275 (16 S rRNA), and MN307218 (28 S rRNA). Shell measurements of the holotype are as follows: shell length (SL) 58.7 mm, shell height (SH) 33.7 mm, and shell width (SW) 19.6 mm.

Paratypes: MYANMAR: type locality, same collecting date, and collectors, 4 specimens (RMBH biv0652_2, biv0652_3, biv0652_4, biv0652_6), Sa Lu Stream, 18.1631°N, 94.4997°E, 04.xii.2018, 5 specimens (RMBH biv0653_1, biv0653_2, biv0653_3, biv0654_1, biv0654_2) and 24 specimens (NCSM 113365), Thandwe River near Ywar Shai village, 18.3741°N, 94.4952°E, 04.xii.2018, 4 specimens (RMBH biv0655_1, biv0655_2, biv0655_3, biv0656), Shwehle Stream, 18.6174°N, 94.3508°E, 05.xii.2018, 3 specimens (RMBH biv0657_1, biv0657_2, biv0657_3) and 22 specimens (NCSM 113366), Toungup River, 18.8439°N, 94.3447°E, 06.xii.2018, 3 specimens (RMBH biv0658_1, biv0658_2, biv0658_3) and 13 specimens (NCSM 113367), Ann River near Ann town, 19.8026°N, 94.0449°E, 07.xii.2018, 3 specimens (RMBH biv0659_1, biv0659_2, biv0659_3), tributary of the Ann River, 19.8035°N, 94.0460°E, 07.xii.2018, 3 specimens (RMBH biv0660_1, biv0660_2, biv0660_3) and 10 specimens (NCSM 113360), Bogan, Bolotov, Vikhrev, Lopes-Lima, Nyein Chan and local villagers leg.

Etymology: The new species name is derived from the Rakhine State of Myanmar, in which it is widely distributed.

Diagnosis: The new species is conchologically and genetically close to a group of Parreysia species from India with affinity to several nominal taxa such as P. corrugata and P. favidens. Its shell varies from ovate-rounded to ovate-elongated, rather thick, umbo slightly elevated, pseudocardinal teeth massive and usually indented, lateral teeth curved and strong, muscle attachment scars deep. The new species differs from all the congeners by fixed nucleotide substitutions in the COI gene fragment, while other genes from Indian Parreysia taxa are not available (Table 2).

Description: Medium-sized mussel: SL 29.3–62.3 mm, SH 18.5–41.7 mm, SW 10.3–24.2 mm. Shell shape variable, from ovate and strongly inflated to ovate-elongated and sub-compressed; inequilateral, rather solid. Umbo usually slightly elevated but may be much more developed at some specimens, with v-shaped sculpture visible only in small mussels due to strong erosion in old mussels. Periostracum from olive-green to brown. Nacre whitish, with bright peach or orange spot near the umbo cavity area, shining. Right valve with one curved lateral tooth and two pseudocardinal teeth, anterior tooth small and somewhat lamellar, posterior tooth massive, very indented. Left valve with two curved lateral teeth and two strongly indented pseudocardinal teeth, the anterior tooth usually higher than the posterior tooth. Anterior adductor scar ovate and deep, posterior adductor scar rounded and well-visible.

Distribution: Rivers and streams of the Rakhine Coast of Myanmar emptying into the Bay of Bengal.

Tribe Indochinellini Bolotov, Pfeiffer, Vikhrev & Konopleva, 2018

Type genus: Indochinella Bolotov, Pfeiffer, Vikhrev & Konopleva, 2018 (by original designation)

Comments: A large Oriental tribe, which contains seven genera: Harmandia Rochebrune, 1882, Indochinella Bolotov, Pfeiffer, Vikhrev & Konopleva, 2018, Indonaia Prashad, 1918, Radiatula Simpson, 1900, Scabies Haas, 1911, Scabiellus gen. nov., and Unionetta Haas, 1955^5,26,27.

Distribution: This group is widespread throughout the Indian, Western Indochina and Sundaland subregions^5,26,27.

Genus Scabiellus gen. nov

Figure 2G Type species: Scabies songkramensis Kongim & Panha, 2015.

Comments: Monotypic genus (Table 3) representing a distinct phylogenetic lineage that is not sister to the other members of Scabies, including its type species, S. scobinatus (Lea, 1856) (Fig. 1). Although Scabiellus gen. nov. is conchologically similar to Scabies and several taxa of Indochinella by having a v-shaped shell sculpture³⁰, this external similarity appears to be only a result of convergence.

Diagnosis: Small mussels, shell length up to 33 mm. Shell thick, rather short, сuneiform, with a broad and elevated umbo, broad anterior margin and narrower posterior margin. Periostracum brown. Dark brown v-shaped sculpture covers the entire shell disc.

Etymology: The name of this genus derived from the genus Scabies, in which its type species was described initially.

Distribution: Scabiellus songkramensis gen. & comb. nov. is recorded from rivers of the Khorat Plateau in Thailand, i.e. the Songkhram and Kam river basins, and the corresponding section of the Mekong River^26,27,30.

Subfamily Gonideinae Ortmann, 1916

Comments: Here we use this subfamily in a broader sense, with the former subfamilies Pseudodontinae²² and Rectidentinae^22,23 being tribes within the monophyletic Gonideinae, as suggested based on the mitogenomic²⁰ and multi-locus nuclear²¹ phylogenies.

Tribe Contradentini Modell, 1942

Type genus: Contradens Haas, 1911 (by original designation)

Comments: This tribe includes six valid genera: Balwantia Prashad, 1919, Contradens Haas, 1911, Trapezoideus Simpson 1900, Physunio Simpson, 1900, Solenaia Conrad, 1869, and Yaukthwa Konopleva et al., 2019^7,21.

Distribution: Northeastern India (Brahmaputra Basin) and Southeast Asia, including the Greater Sunda Islands^2,3,7,29,31.

Genus Balwantia Prashad, 1919 stat. rev

Type species: Anodonta soleniformis Benson, 1836 (by original designation)

Comments: This genus contains ultra-elongated mussels externally resembling members of Solenaia³² (Fig. 2D) but is distantly related to Yaukthwa phylogenetically (Fig. 1). Previously, it was considered a synonym of Solenaia²⁹ and was not used as a valid genus name since the last monograph of Haas³³. Two species were recorded from Myanmar, one of which is new to science and described here (Table 3). They were collected from deep burrows which they dig in hard clay and soft sandstone substrate as does Balwantia soleniformis, their putative Indian relative from the upper Brahmaputra River^34,35.

Distribution: Ayeyarwady and upper Brahmaputra basins^29,32,34.

Balwantia baniensis sp. nov

Figure 2C, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0666_2, MYANMAR: Bani River near Bangong village, 19.3247°N, 94.9839°E, Ayeyarwady Basin, 09.xii.2018, Bogan, Bolotov, Vikhrev, Lopes-Lima, Nyein Chan and local villagers leg. Reference sequence numbers of the holotype are as follows: MN275077 (COI), MN307264 (16 S rRNA), and MN307206 (28 S rRNA). Shell measurements of the holotype are as follows: SL 57.0 mm, SH 23.2 mm, and SW 15.3 mm.

Paratypes: MYANMAR: type locality, same collecting date, and collectors, 4 specimens (RMBH biv0666_1, biv0666_3, biv0666_4, and biv0666_5), 9 specimens (NCSM 113369).

Etymology: The new species name is dedicated to Bani River, a tributary of the Ayeyarwady River, in which it was collected.

Diagnosis: The new species can be distinguished from Balwantia elongatula comb. nov. by having a rostrate anterior margin (vs. rounded), a more inflated shell (vs. flattened), and by the presence of bars from umbo along the dorsal margin. The new species also differs from Balwantia elongatula comb. nov. by fixed nucleotide substitutions in the COI, 16 S rRNA and 28 S rRNA gene fragments (Table 2).

Description: Medium-sized mussel: SL 46.1–61.4 mm, SH 19.5–26.3 mm, SW 12.7–16.7 mm. Shell somewhat trapezoidal, elongated, not very inequilateral, thin, and rather inflated, narrow and rostrate anteriorly, broad and truncated posteriorly, ventral margin slightly curved. Posterior slope covered by elongated, slightly curved bars. Umbo eroded, slightly elevated, without clear sculpture. Periostracum olive-yellow or brownish, the central part of the shell usually lighter than the posterior side. Nacre blue-whitish, sometimes with yellow spots, shining. Lateral teeth very thin, almost straight, by one on each valve. Pseudocardinal teeth reduced. Anterior adductor scars somewhat ovate or drop-like, not deep. Posterior adductor scars ovate or rounded shape, shallow.

Distribution: Bani River, Ayeyarwady Basin, central Myanmar.

Genus Trapezoideus Simpson, 1900

Type species: Unio foliaceus Gould, 1843 (by original designation)

Comments: Small genus with a restricted range that was previously considered a monotypic entity⁷. However, we found one additional congeneric species (Table 3), which is described here.

Distribution: Southern Myanmar (Lenya and Dawei basins) and southwestern Thailand (Mae Klong Basin)⁷.

Trapezoideus lenya sp. nov

Figures 2E, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0629_2, MYANMAR: 14 Mile Stream, 11.3508°N, 99.1093°E, Lenya River basin, 24.xi.2018, Bogan, Bolotov, Vikhrev, Lopes-Lima, Nyein Chan and local villagers leg. Reference sequence numbers of the holotype are as follows: MN275068 (COI), MN307257 (16 S rRNA), and MN307198 (28 S rRNA). Shell measurements of the holotype are as follows: SL 36.3 mm, SH 21.4 mm, and SW 11.3 mm.

Paratypes: MYANMAR: type locality, same collecting date, and collectors, 3 specimens (RMBH biv0629_1, biv0629_3, and biv0629_5), 3 specimens (NCSM 113368).

Etymology: The name of new species is derived from the Lenya River, a coastal freshwater basin in southern Myanmar, from which this species was collected.

Diagnosis: The new species is morphologically similar to Trapezoideus foliaceus but differs in shell shape, being higher posteriorly and narrower anteriorly, with a straight ventral margin. The new species also differs from its congener by fixed nucleotide substitutions in the COI and 16 S rRNA gene fragments (Table 2).

Description: Small mussel: SL 33.1–36.3 mm, SH 19.1–21.4 mm, SW 10.2–11.3 mm. Shell rounded, somewhat trapezoidal, inequilateral, rather thin and compressed, anterior margin rounded and narrow, posterior margin broad and somewhat truncated, dorsal margin high with minute bars extended from the umbo, ventral margin straight. Umbo small, slightly projected, eroded. Periostracum olive-brown. Nacre bluish gray. Pseudocardinal teeth thin and lamellar, two teeth in right valve and one tooth in left valve. Lateral teeth slender, elongated, slightly curved, one in right valve and two in left valve. Anterior adductor scar somewhat drop-like, shallow, posterior adductor scar almost reduced.

Distribution: Lenya River basin, southern Myanmar.

Genus Yaukthwa Konopleva et al . , 2019

Type species: Trapezoideus nesemanni Konopleva, Vikhrev & Bolotov, 2017 (by original designation)

Comments: A large genus with at least eight species⁷, including a species newly described here (Table 3).

Distribution: Endemic to the Western Indochina Subregion⁷.

Yaukthwa avaensis sp. nov

Figure 2F, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0680_3, MYANMAR: Tarkat Stream, 25.2758°N, 97.2722°E, tributary of the Ayeyarwady River, 23.iii.2018, Nyein Chan leg. Reference sequence numbers of the holotype are as follows: MN275071 (COI), MN307259 (16 S rRNA), and MN307200 (28 S rRNA). Shell measurements of the holotype are as follows: SL 38.3 mm, SH 20.8 mm, and SW 14.1 mm.

Paratypes: MYANMAR: type locality, same collecting date, and collector, 9 specimens (RMBH biv0680_1, biv0680_5, biv0680_2, biv0680_4, biv0680_6, biv0680_7, biv0680_8, biv0680_9, and biv0680_10).

Etymology: The name of the new species is derived from the ancient Ava Kingdom in central Myanmar.

Diagnosis: The new species can be distinguished from its sister species Yaukthwa paiensis by having a more curved ventral margin and stronger inflation of the shell. The new species also differs from its congeners by fixed nucleotide substitutions in the COI gene fragment (Table 2).

Description: Small mussel: SL 24.7–46.6 mm, SH 14.1–25.1 mm, SW 8.7–17.0 mm. Shell subtrapezoidal, inequilateral, moderately thick and rather inflated. Anterior margin rounded, dorsal margin elevated posteriorly, ventral outline slightly curved, posterior slope truncated and usually covered by small striae. Umbo slightly elevated and strongly eroded at some specimens. Periostracum from light to dark brown. Nacre bluish, with yellow spots. Pseudocardinal teeth thin, lamellar, one tooth in the left valve and two teeth in the right valve. Lateral teeth elongated, slightly curved, one in the right valve and two in the left valve. Adductor muscle scars shallow.

Distribution: Middle section of the Ayeyarwady River, central Myanmar.

Tribe Pseudodontini Frierson, 1927

Type genus: Pseudodon Gould, 1844 (by original designation)

Comments: This tribe includes seven valid genera: Bineurus Simpson, 1900, Monodontina Conrad, 1853, Nyeinchanconcha gen. nov., Pilsbryoconcha Simpson, 1900, Sundadontina gen. nov., Thaiconcha gen. nov. (subtribe Pilsbryoconchina Bolotov, Vikhrev & Tumpeesuwan, 2017), and Pseudodon Gould, 1844 (subtribe Pseudodontina s. str.).

Distribution: Southeast Asia from the Ayeyarwady River to the Mekong Basin, Malaysia and the Greater Sunda Islands^2,3,4,22,31.

Subtribe Pilsbryoconchina Bolotov, Vikhrev & Tumpeesuwan, 2017

Type genus: Pilsbryoconcha Simpson, 1900 (by original designation)

Genus Monodontina Conrad, 1853

=Suborbiculus Simpson, 1900

Type species: Margaritana vondembuschiana Lea, 1840 (by original designation)

Comments: This genus contains seven species (Table 3), three of which are new to science and described here.

Distribution: Sundaland Subregion (Lenya Basin in Myanmar, Mekong Basin, Malaysia, and the Greater Sunda Islands)⁴.

Monodontina laosica sp. nov

Figure 3C, 5A, Tables 1–2, Supplementary Table 2

Holotype: UMMZ 304650, LAOS: 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, 21.v.2009, Kottelat et al. leg. Reference sequence numbers of the holotype are as follows: KP795029 (COI) and KP795052 (16 S rRNA). Shell measurements of the holotype are as follows: SL 61.4 mm, SH 41.4 mm, and SW 19.0 mm.

Etymology: The name of the new species is derived from the country of Laos, in which it was recorded.

Diagnosis: This species can be distinguished from its sister taxa by having a higher dorsal margin and reduced pseudocardinal teeth. The new species also differs from its congeners by fixed nucleotide substitutions in the COI and 16 S rRNA gene fragments (Table 2).

Description: Medium-sized mussel. Shell ovate, inequilateral, rather inflated, with high dorsal margin, creating a wing, rounded anteriorly, truncated posteriorly, ventral margin curved. Umbo not prominent, eroded. Periostracum brownish with yellow and rusty sites. Nacre blue-whitish with cream tint near the umbo. Pseudocardinal teeth weak. Both muscle scars shallow.

Distribution: Vang Ngao River, Mekong Basin, southern Laos.

Monodontina lenyanensis sp. nov

Figures 3D, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0628_2, MYANMAR: 14 Mile Stream, 11.3508°N, 99.1092°E, Lenya River basin, 24.xi.2018, Bogan, Bolotov, Vikhrev, Lopes-Lima, Nyein Chan and local villagers leg. Reference sequence numbers of the holotype are as follows: MN275055 (COI), MN307246 (16 S rRNA), and MN307187 (28 S rRNA). Shell measurements of the holotype are as follows: SL 63.4 mm, SH 40.1 mm, and SW 23.7 mm.

Paratypes: MYANMAR: type locality, same collecting date, and collectors, 9 specimens (RMBH biv0628_1, biv0628_3, biv0628_4, biv0628_5, biv0628_6, biv0628_7, biv0628_8, biv0628_9, and biv0628_10), 9 specimens (NCSM 104012).

Etymology: This new species is dedicated to the Lenya River, its type locality.

Diagnosis: This species can be distinguished from its sister taxa by presenting an ovate, elongated, rather solid and inflated shell, not elevated umbo, tubercular or pyramidal pseudocardinal teeth, and rather reduced muscle scars. The new species also differs from its congeners by fixed nucleotide substitutions in the COI and 16 S rRNA gene fragments (Table 2).

Description: Medium-sized mussel: SL 31.2–63.4 mm, SH 20.3–40.1 mm, SW 11.0–23.7 mm. Shell ovate, elongated, inequilateral, rather solid and inflated, rounded anteriorly, broad and truncated posteriorly, dorsal margin high, ventral margin slightly curved. Umbo small, slightly elevated, eroded. Periostracum rusty-brown, smooth. Nacre white-yellowish. One pseudocardinal tooth in each valve, which is tubercular-like or more pyramidal and sharper, rather high and strong, smooth or slightly ribbed. Lateral teeth reduced. Anterior adductor scar ovate, rather prominent; posterior adductor scar reduced, weakly developed.

Distribution: Lenya River basin, southern Myanmar.

Monodontina mekongi sp. nov

Figures 3E, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0122, THAILAND: clay bottom, headwaters of the Phong River, 16.8616°N, 101.9105°E, Mekong Basin, Loei Province, 09.iv.2014, Bolotov, Vikhrev, Spitsyn and Gofarov leg. Reference sequence numbers of the holotype are as follows: KX865861 (COI), KX865632 (16 S rRNA), and KX865733 (28 S rRNA). Shell measurements of the holotype are as follows: SL 65.7 mm, SH 42.2 mm, and SW 20.3 mm.

Etymology: The name of this species is derived from the Greater Mekong Basin, its type locality.

Diagnosis: This species is conchologically and genetically related with Monodontina vondembuschiana and M. laosica sp. nov. but it can be distinguished from these species by presenting an uninflated, weak pseudocardinal teeth (vs. stouter), and a curved and lower dorsal margin (vs. strait and higher). The new species also differs from its congeners by fixed nucleotide substitutions in the COI, 16 S rRNA and 28 S rRNA gene fragments (Table 2).

Description: Medium-sized mussel. Shell obovate, slightly higher posteriorly, inequilateral, thin, semitransparent, not inflated; anterior margin rounded, posterior margin angulate, ventral margin curved. Umbo slightly elevated, eroded, without clear sculpture. Periostracum olive yellow. Nacre whitish. Pseudocardinal teeth weak, flatten, more developed in the right valve than in the left one. Both muscle scars shallow, anterior scar irregular; posterior scar somewhat drop-like, almost invisible.

Distribution: Phong River, Mekong Basin, northern Thailand.

Genus Nyeinchanconcha gen. nov

Type species: Nyeinchanconcha nyeinchani gen. & sp. nov.

Comments: Remarkable monotypic genus (Table 3).

Diagnosis: Shell elliptical, resembling that of the genus Lamellidens Simpson, 1900 (Parreysiinae: Lamellidentini), slightly elevated posteriorly, moderately thick and inflated, umbo not elevated, pseudocardinal teeth strong and somewhat pyramidal in each valve; anterior adductor scar drop-like, developed, usually contiguous with pedal retractor scar; the posterior muscle scar shallow.

Etymology: This genus is dedicated to our friend Mr. Nyein Chan, an enthusiastic conservation biologist from FFI – Myanmar Program, Yangon, Myanmar, for his valuable contribution to the conservation of freshwater ecosystems in Southeast Asia. This genus name means “Shell of Nyein Chan” (“concha” being shell in Latin).

Distribution: Mekong Basin in Laos.

Nyeinchanconcha nyeinchani gen. & sp. nov

Figure 3F, 5A, Tables 1–2, Supplementary Table 2

Holotype: NCSM 84884, LAOS: small stream arising at a cave near Ban Kouanphavang, 17.4578°N, 104.9263°E, Nam Done River drainage, Mekong Basin, Khammouane Province, 17.v.2012, M. Kottelat et al. leg. Reference sequence numbers of the holotype are as follows: KX822662 (COI) and KX822618 (28 S rRNA). Shell measurements of the holotype are as follows: SL 50.9 mm, SH 27.8 mm, and SW 15.6 mm.

Paratypes: LAOS: type locality, same date, and collectors, 1 specimen (NCSM 113351); Nam Phiat River near Phon Bong village, 18.0839°N, 104.9781°E, ca. 2 km from confluence with the Namkading River, Mekong Basin, Bolikhamsai Province, 12.v.2009, 1 specimen (UMMZ 304648), M. Kottelat et al. leg.

Etymology: This species is dedicated to our friend Mr. Nyein Chan, a conservation biologist from FFI – Myanmar Program, Yangon, Myanmar.

Diagnosis: The species is morphologically and genetically more similar to Sundadontina brandti sp. nov. but differs from it by a more elliptical shell without marked elevation of the dorsal margin, and by pyramidal and weaker pseudocardinal teeth. The new species also differs from other Pseudodontini taxa by fixed nucleotide substitutions in the COI and 16 S rRNA gene fragments (Table 2).

Description: Medium-sized mussel: SL 33.7–59.9 mm, SH 15.1–37.0 mm, SW 7.9–15.6 mm. Shell elliptical, inequilateral, rounded anteriorly, truncated posteriorly, dorsal margin elevated, ventral margin slightly curved. Umbo not prominent, eroded. Periostracum dark brown. Nacre whitish with cream tint near the umbo. Pseudocardinal teeth somewhat pyramidal, stout. Anterior adductor scar pronounced; posterior adductor scar weak.

Distribution: Nam Done and Nam Phiat rivers, Mekong Basin, Laos.

Genus Sundadontina gen. nov

Type species: Anodonta cumingii Lea, 1850.

Comments: This genus contains at least 10 species, three of which are new to science and described here (Table 3).

Diagnosis: Shell ovate or elongate-ovate, rather thick and strong, umbo not projected, pseudocardinal teeth stout and tubercular-like; anterior muscle scar ovate and well-developed, usually contiguous with pedal retractor scar; the posterior muscle scar shallow.

Etymology: The name of this genus is derived from that of the genus Monodontina, but with another prefix highlighting its broad distribution across the ancient Sundaland.

Distribution: Sundaland Subregion: Lenya Basin in Myanmar, Mekong Basin in Thailand, Cambodia, and southern Vietnam, Chao Phraya Basin in Thailand, Malaysia.

Sundadontina brandti sp. nov

Figures 4B, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0475_2, THAILAND: headwater of the Mun River, 14.4138°N, 102.0821°E, Mekong Basin, Khorat Plateau, Nakhon Ratchasima Province, 12.iii.2018, Bolotov, Vikhrev, and local villagers leg. Reference sequence numbers of the holotype are as follows: MN275058 (COI), MN307249 (16 S rRNA), and MN307190 (28 S rRNA). Shell measurements of the holotype are as follows: SL 85.4 mm, SH 53.7 mm, and SW 27.7 mm.

Paratypes: THAILAND: type locality, same collecting date, and collectors, 2 specimens (RMBH biv0475_3, biv0475_4).

Etymology: This species is named in the memory of Dr. Rolf Arthur Max Brandt (1917–1989), one of the most influential freshwater malacologists of the last century. This prominent scientist worked in Southeast Asia on freshwater mollusks and authored the freshwater mollusks of Thailand³¹.

Diagnosis: The species is similar to Sundadontina taskaevi sp. nov. but can be distinguished from it by having a more slender and higher pseudocardinal tooth on the right valve. The new species also differs from its congeners by fixed nucleotide substitutions in the COI and 16 S rRNA gene fragments (Table 2).

Description: Large mussel: SL 71.3–85.4 mm, SH 43.8–53.7 mm, SW 24.1–27.7 mm. Shell ovate, very inequilateral, solid, not very inflated, rounded anteriorly, truncated posteriorly, dorsal margin curved, ventral margin slightly rounded. Periostracum brownish-black. Nacre creamy. Umbo very small, not developed, eroded. Left valve with one tubercle-like pseudocardinal tooth, right valve with one rectangular and high pseudocardinal tooth. Anterior muscle scar rather well-developed, ovate; posterior muscle scar slightly visible.

Distribution: Mun Basin, Thailand.

Sundadontina tanintharyiensis sp. nov

Figure 4C, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0643_4, MYANMAR: Chaung Nauk Pyan Stream, 11.7620°N, 99.1124°E, Lenya River basin, 16.vi.2018, Nyein Chan leg. Reference sequence numbers of the holotype are as follows: MN275057 (COI), MN307248 (16 S rRNA), and MN307189 (28 S rRNA). Shell measurements of the holotype are as follows: SL 57.1 mm, SH 40.8 mm, and SW mm.

Paratypes: MYANMAR: type locality, same collecting date, and collectors, 2 specimens (RMBH biv0643_1, biv0643_6) and 3 specimens (NCSM 113364).

Etymology: The new species name is derived from the Tanintharyi Region of Myanmar, in which its type locality is situated.

Diagnosis: The new species is similar to Sundadontina cumingii gen. & comb. nov. but can be distinguished from it by having a more rounded and inflated shell. The new species also differs from its congeners by fixed nucleotide substitutions in the COI and 16 S rRNA gene fragments (Table 2).

Description: Medium-sized mussel: SL 39.2–61.2 mm, SH 27.2–45.1 mm, SW 14.7–23.2 mm. Shell ovate or slightly elongated, inequilateral, moderately solid and rather inflated. Anterior margin rounded, dorsal and ventral margin curved, posterior margin subangular. Umbo not elevated, eroded. Periostracum rusty-brown, smooth. Nacre yellowish white. Each valve with one tubercle-like, smooth pseudocardinal tooth. Lateral teeth reduced. Anterior muscle scar ovate or drop-like, rather prominent; posterior muscle scar drop-like and very shallow.

Distribution: Lenya River basin, southern Myanmar.

Sundadontina taskaevi sp. nov

Figures 4D, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0475_1, THAILAND: headwater of the Mun River, 14.4138°N, 102.0821°E, Mekong Basin, Khorat Plateau, Nakhon Ratchasima Province, 12.iii.2018, Bolotov and Vikhrev leg. Reference sequence numbers of the holotype are as follows: MN275061 (COI), MN307251 (16 S rRNA), and MN307192 (28 S rRNA). Shell measurements of the holotype are as follows: SL 82.7 mm, SH 52.2 mm, and SW 29.3 mm.

Paratypes: THAILAND: type locality, same collecting date, and collectors, 1 specimen (RMBH biv0475_5).

Etymology: This species is named in memory of the late Dr. Anatoly Ivanovich Taskaev (1944–2010), a well-known Russian biologist.

Diagnosis: The new species is similar to Sundadontina brandti sp. nov. but can be distinguished from it by having broader, stronger, tubercle-like pseudocardinal teeth. The new species also differs from its congeners by fixed nucleotide substitutions in the COI and 16 S rRNA gene fragments (Table 2).

Description: Large mussel: SL 60.0–82.7 mm, SH 42.8–52.2 mm, SW 22.6–29.3 mm. Shell elongate-ovate, inequilateral, rather solid and inflated, rounded anteriorly, dorsal margin convex, ventral margin slightly curved. Periostracum blackish, with brown lines. Nacre creamy. Umbo slightly elevated, eroded. Each valve with one tubercle-like, smooth pseudocardinal tooth. The tooth in right valve more developed, rather trapezoidal, with broader base. Anterior muscle scar ovate, rather well-developed; posterior muscle scar shallow.

Distribution: Mun River, Thailand.

Genus Thaiconcha gen. nov

Type species: Anodonta callifera Martens, 1860.

Comments: This genus contains at least two valid species (Table 3).

Diagnosis: Shell large, thick, elliptical or rounded, moderately inflated. Pseudocardinal teeth rather well developed, muscle attachment scars deep.

Etymology: The name of this genus means “a shell from Thailand”.

Distribution: Mekong Basin in Thailand and Cambodia.

Subtribe Pseudodontina Frierson, 1927

Type genus: Pseudodon Gould, 1844 (by original designation)

Genus Pseudodon Gould, 1844

Type species: Anodon inoscularis Gould, 1844 (by original designation)

Comments: This genus contains eight species, one of which is new to science and described here (Table 3).

Distribution: Endemic clade to the Western Indochina Subregion⁴.

Pseudodon kayinensis sp. nov

Figure 3G, 5A, Tables 1–2, Supplementary Table 2

Holotype: RMBH biv0618_1, MYANMAR: Winyaw River, 15.6685°N, 97.9496°E, Ataran River basin, 20.xi.2018, Vikhrev, Bogan, Lopes-Lima, and local villagers leg. Reference sequence numbers of the holotype are as follows: MN275043 (COI). Shell measurements of the holotype are as follows: SL 59.6 mm, SH 34.9 mm, and SW 17.7 mm.

Paratypes: MYANMAR: type locality, same collecting date, and collectors, 4 specimens (RMBH biv0618_2, biv0618_3, biv0618_4, biv0618_5) and 4 specimens (NCSM 104014); Ko Du Kwe Stream, 15.6132°N, 98.2363°E, Zami River, Ataran River basin, 26.ii.2018, 5 specimens (RMBH biv0637_1, biv0637_2, biv0637_3, biv0637_4, biv0637_5) and 5 specimens (NCSM 113362), Than Win leg.; unnamed stream, 17.0292°N, 97.8100°E, Hlaingbwe River basin, 17.xi.2018, 3 specimens (RMBH biv0638_1, biv0638_2, biv0638_3) and 5 specimens (NCSM 104015), Than Win leg.

Etymology: The name of new species is derived from its distribution range, i.e. the Kayin State in Myanmar.

Diagnosis: The new species is conchologically more similar to Pseudodon bogani but can be distinguished from it by having a more curved dorsal margin and clearly ornamented posterior side. The new species also differs from its congeners by fixed nucleotide substitutions in the COI, 16 S rRNA and 28 S rRNA gene fragments (Table 2).

Description: Rather large mussel: SL 37.8–71.0 mm, SH 22.4–42.5 mm, SW 10.3–21.2 mm. Shell from ovate to elliptical, elongated, inequilateral, moderately inflated and thick. Anterior margin rounded, posterior margin somewhat truncated, dorsal margin curved and rather high, ventral margin straight or slightly curved. Umbo not elevated, eroded. Periostracum olive-brown to dark brown, the surface from umbo to posterior margin clearly ribbed, having curved bars covering the entire dorsal margin and then radiate along the posterior slope. Nacre whitish, sometimes with yellow sites. Pseudocardinal teeth high, tubercular-like, in each valve. Anterior adductor scar rather well developed, ovate; posterior adductor scar drop-like, more or less visible.

Distribution: Hlaingbwe and Ataran River basins in southern Myanmar.

Source: Ecology - nature.com