A taxonomic review of Trapezidens (Bivalvia: Unionidae: Lamellidentini), a freshwater mussel genus endemic to Myanmar, with a description of a new species

Trapezidens Bolotov, Vikhrev & Konopleva, 2017 (Bivalvia: Unionidae: Lamellidentini) is a peculiar freshwater mussel genus, the range of which is confined to the Western Indochina Subregion (Myanmar). Here we show that this genus contains five allopatric species: Trapezidens angustior (Hanley & Theobald, 1876) from the Bago, Sittaung, and Bilin rivers; T. dolichorhynchus (Tapparone-Canefri, 1889) from the Ayeyarwady River; T. exolescens (Gould, 1843) from the Dawei River; T. scutum (Sowerby, 1868) from the Tanintharyi (Great Tenasserim) River; and T. yeti sp. nov. from the Ye River. Phylogenetically, Trapezidens angustior, T. dolichorhynchus, and T. yeti sp. nov. are closely related to each other, forming a species complex, while T. exolescens and T. scutum represent the most distant lineages within the genus. An updated synonymy for each species is compiled.


Introduction
Southeast Asia is one of the most species-rich hotspots of freshwater mussel diversity worldwide (Bolotov et al. 2017a(Bolotov et al. , b, 2018(Bolotov et al. , 2019aLopes-Lima et al. 2017;Pfeiffer et al. 2018;Zieritz et al. 2018;Konopleva et al. 2019). The genus Trapezidens Bolotov, Vikhrev & Konopleva, 2017 (Bivalvia: Unionidae: Lamellidentini) was established for a small group of freshwater mussels endemic to Myanmar (Bolotov et al. 2017b). Representatives of this genus were recorded from the Ayeyarwady, Bago, Bilin, Dawei, Sittaung, Tanintharyi (Great Tenasserim), and Ye rivers (Konopleva et al. 2017;Bolotov et al. 2017b; this study). Although recent research revealed that this genus may contain at least three valid species (Bolotov et al. 2017b), its taxonomy has not yet been studied in detail. It was found that several mussel-associated leech species (Hirudinea: Glossiphoniidae) use Trapezidens species as a secondary host and shelter (Bolotov et al. 2019b), whereas other ecological features of these freshwater mussels such as habitats and life cycle are almost unknown.
This study aims to revise the genus Trapezidens by an integrative approach and to describe a new species recently collected from the Ye River in southeastern Myanmar.

Materials and methods
The specimens of Trapezidens species, including topotypes, and the type specimens of Trapezidens yeti sp. nov. were studied in the malacological collection of the Russian Museum of Biodiversity Hotspots (RMBH thereafter), Federal Center for Integrated Arctic Research of the Russian Academy of Sciences, Arkhangelsk, Russia. These samples were collected throughout Myanmar during 2015-2018, and the topotypes of all nominal taxa belonging to the genus Trapezidens were located (Fig. 1). Comparative analyses of the shell morphology were carried on the shell shape, umbo position, the pseudo-cardinal, and lateral teeth structures, and muscle attachment scars (Konopleva et al. 2019).
The length, height, and width of the shell (all at the maximum diameter) of each specimen from the new species were measured using dial calipers (±0.1 mm).
Morphological studies demonstrate that Trapezidens species can be distinguished from each other by shell shape and general outlines (Fig. 3). The hinge and muscle attachment scars structure in all Trapezidens species are typical for the genus, and the development of these features primarily depends on the shell size. Young specimens of Trapezidens angustior resemble those of Lamellidens species based on the shell shape, while older specimens share a smoother shell outline without a marked wing. Trapezidens dolichorhynchus is the largest representative of the genus with an elongated and thick shell, strong pseudocardinal teeth and deep muscle attachment scars. Trapezidens scutum has a rather large shell with well-developed conchological traits. Its specific feature among other Trapezidens taxa is the presence of small granules on the umbo cavity surface. Trapezidens exolescens and T. yeti sp. nov. are the smallest representatives of the genus and morphologically are very similar to each other, whereas phylogenetically they represent distant lineages. All five species appear to be taxa endemic to the Western Indochina Subregion.   (Ramakrishna et al. 2004). This nominal taxon has been assigned to T. angustior based on conchological features of the holotype, i.e. the elongate-rectangular shell shape, specific structure of the pseudocardinal teeth, and clear radial lines on the outer side of the valve (Preston 1915 : Fig. 19). =Trapezidens obesa angustior (Hanley & Theobald, 1876   ii.2018, 17 specimens [RMBH biv 387_4, biv 387_6, biv 387_7, biv 387_8, biv 387_9, biv 387_10, biv 388 (nos. 4-10), biv 387_3, biv 388_1, biv 388_2, biv 388_3; the four latter specimens are sequenced], Bolotov, Nyein Chan and Vikhrev leg.
Differential diagnosis. Shell rhomboid and somewhat winged in young specimens (similar to that in Lamellidens species) to elliptical and elongated in adult mussels, inequilateral, slightly inflated and moderately thick. In the process of shell growth, the dorsal margin becomes smooth and almost parallel to the ventral margin, posterior slope maybe biangular or truncated. Adult specimens are very similar to Trapezidens dolichorhynchus, but differ from it by smaller pseudocardinal teeth. Lateral teeth usually straight. The outer side of the shell is usually covered by clear radial lines. Umbo rather small slightly elevated above hinge line. Anterior muscle scar somewhat deep, bean-like; posterior muscle scar shallow. Umbo cavity is not deep.
Molecular diagnosis. Trapezidens angustior is closely related to T. dolichorhynchus and T. yeti sp. nov. (mean uncorrected COI p-distances: 2.14% and 1.87% respectively), but it can be distinguished from the sister species by six fixed substitutions in the COI gene fragment as follows: 8 A, 65 C, 128 A, 293 T, 314 T and 429 T; one fixed substitution in 16S rRNA gene fragment as follows: 49 G.
Comments. Previously, we described Trapezidens obesa feae as a subspecies-level lineage endemic to the Sittaung River (Bolotov et al. 2017b). However, an expanded dataset reveals that this taxon has a broader range and is conspecific with Trapezidens angustior that was described from the Bago River (Hanley and Theobald 1876). Fig. 3b =Unio marginalis var. obesa Hanley & Theobald (1876): 20, Pl. 43, Fig. 3 Simpson, 1900. Types: Whereabouts unknown, but probably in ZSI (Prashad 1922 Differential diagnosis. Shell large, very elongated, inequilateral, irregularly elliptic with slightly curved or straight ventral margin and slightly raised dorsal margin, rather thick and moderately inflated. Periostracum dark-brown with well visible growth lines. Pseudocardinal teeth massive, lateral teeth long and sharp, mussel scars deep.

Trapezidens dolichorhynchus (Tapparone-Canefri, 1889)
Molecular diagnosis. Trapezidens dolichorhynchus is closely related to T. angustior and T. yeti sp. nov. (mean uncorrected COI p-distances: 2.14% and 1.96% respectively), but it can be distinguished from the sister species by two fixed substitutions in the COI gene fragment as follows: 479 T and 569 G; two fixed substitutions in 16S rRNA gene fragment as follows: 186 T and 196 C.
Distribution. Endemic to the Ayeyarwady River, Myanmar.

Trapezidens exolescens (Gould, 1843)
Molecular diagnosis. Trapezidens exolescens is closely related to T. scutum (mean uncorrected COI p-distance: 3.56%), but it can be distinguished from the sister species by eight fixed substitutions in the COI gene fragment as follows: 29 G, 47 A, 107 C, 122 G, 155 T, 158 C, 419 C and 554 C; two fixed substitutions in 16S rRNA gene fragment as follows: 318 A and 430 A; one fixed substitution in 28S rRNA gene fragment -701 C, as well as deletion from 582 to 587 positions.
Distribution. Endemic to the Dawei River, Myanmar.
Differential diagnosis. Shell rhomboid, winged posteriorly, inequilateral, moderately thick and inflated. Periostracum dark-brown to black. Pseudocardinal teeth strong, anterior tooth smaller, posterior tooth bigger, somewhat leaf-like, ribbed. Umbo cavity deeper than that in other Trapezidens species, with small granules on its surface. Molecular diagnosis. Trapezidens scutum is closely related to T. exolescens (mean uncorrected COI p-distance: 3.56%), but it can be distinguished from the sister species by nine fixed substitutions in the COI gene fragment as follows: 68 A, 134 C, 179 C, 308 A, 335 G, 443 C, 638 A, 647 C and 653 T; three fixed substitutions in 16S rRNA gene fragment as follows: 14 G, 285 C, and 335 G.
Etymology. The name of the new species refers to the Ye River, its type locality. Differential diagnosis. Morphologically, specimens of the new species, especially young mussels, can hardly be distinguished from Trapezidens exolescens. Adult specimens differ from other congeners by a more elongated shell with wider anterior margin and usually without prominently winged dorsal margin, and by peculiar curved pseudocardinal teeth in the right valve.
Molecular diagnosis. Phylogenetically, Trapezidens yeti sp. nov. is closely related to T. angustior and T. dolichorhynchus (mean uncorrected COI p-distances: 1.87% and 1.96% respectively), forming a species complex, but it can be distinguished from the sister species by five fixed substitutions in the COI gene fragment as follows: 257 A, 314 G, 344 C, 413 A, and 614 T; two fixed substitutions in 16S rRNA gene fragment as follows: 382 C and 488 C. Description. A rather small mussel, with shell length 41.3-64.2 mm, shell height 20.9-31.7 mm, and shell width 10.8-16. 3 mm. Shell somewhat trapezoidal, elongated, inequilateral, thin and not inflated. Anterior margin rounded, ventral margin usually straight, dorsal margin slightly curved, posterior slope diagonally truncated. Umbo small, slightly elevated under hinge line, eroded. Periostracum brownish, sometimes with darker posterior margin. Thin radial lines cross the shell from the umbo area to the ventral margin. Dorsal margin sometimes has small ridges. Nacre whitish with dark-yellow areas, which may almost cover the entire inner side. Two pseudocardinal teeth on the right valve, posterior tooth strong and welldeveloped, the anterior tooth usually smaller and lamellar. On the left valve, one elongated pseudocardinal tooth with a cavity and a triangular edge. Lateral teeth elongated, one on the right valve and two on the left valve. Anterior muscle scars bean-like, pronounced, posterior muscle scars rounded and less developed.
Distribution. Endemic to the Ye River, Myanmar.

Discussion
This small correspondence is the following step towards a complete revision of the Unionidae from Myanmar. Based on previous works (Bolotov et al. 2017a(Bolotov et al. , 2017b(Bolotov et al. , 2018(Bolotov et al. , 2019aKonopleva et al. 2017Konopleva et al. , 2019, the taxonomic concept for this area has shifted from a few widespread species crossing a variety of freshwater basins to multiple species with restricted ranges and endemic to a single drainage basin or even to a section within a freshwater system. The genus Trapezidens was established for a distant Lamellidentini clade containing members of the so-called exolescens-group (Konopleva et al. 2017;Bolotov et al. 2017b). Initially, we assumed that this genus may contain at least three valid species (Bolotov et al. 2017b). However, further broad-scale sampling of these peculiar mussels throughout Myanmar with a special focus on the type localities of nominal taxa indicates that there are five allopatric Trapezidens species. The range of Trapezidens angustior crosses several river drainages, i.e. the Bago, Sittaung, and Bilin rivers. Trapezidens dolichorhynchus is restricted to the massive Ayeyarwady River system, with exception of its upstream tributaries (e.g. north of Putao) having a stony bottom and strong current with multiple rapids and waterfalls. In contrast, the three other species are confined to smaller freshwater basins, i.e. Trapezidens exolescens is endemic to the Dawei River, T. scutum to the Tanintharyi (Great Tenasserim) River, and T. yeti sp. nov. to the Ye River. Interestingly, all Trapezidens species were recorded from flowing water, while taxa in their sister genus Lamellidens strongly prefer lacustrine environments, being typical inhabitants of lakes, reservoirs, fishponds, and stagnant ditches. Previously, we proposed that Trapezidens exolescens and T. scutum may be conspecific based solely on conchological features (Konopleva et al. 2017). However, newly collected topotypes of Trapezidens scutum share distant molecular sequences supporting the status of this taxon as a valid species. The small granules on the umbo cavity surface are considered here as a diagnostic feature for this peculiar species. The new species, Trapezidens yeti sp. nov., was collected from the Ye River, which is directly adjacent to the Dawei River, where T. exolescens was found. Based on the proximity of those river systems and morphological similarity, we initially considered the sample from the Ye River as representatives of Trapezidens exolescens, but later they were found as two distant phylogenetic lineages, sharing another remarkable example of convergence in conchological traits.
Finally, future studies of Trapezidens species must be focused on their biological and ecological patterns, i.e. fish hosts, life cycles, and habitat requirements. This information is urgently needed to estimate the conservation status of these species.