Trapezidens angustior ( Hanley & Theobald, 1876 )

Konopleva, Ekaterina S., Bolotov, Ivan N., Kondakov, Alexander V., Kononov, Oleg D., Gofarov, Mikhail Yu., Tomilova, Alena A., Lunn, Zau, Chan, Nyein, Win, Than & Vikhrev, Ilya V., 2020, A taxonomic review of Trapezidens (Bivalvia: Unionidae: Lamellidentini), a freshwater mussel genus endemic to Myanmar, with a description of a new species, Ecologica Montenegrina 27, pp. 45-57 : 50-52

Trapezidens angustior ( Hanley & Theobald, 1876)

Fig. 3a

= Unio generosus var. angustior Hanley & Theobald (1876): 22 , Pl. 46, Fig. 7. Type locality: Pegu [Bago River, Myanmar]. Type : Holotype BMNH_1968655 [by monotypy]; Natural History Museum, London, UK (examined by us).

= Lamellidens marginalis var. sublamellatus Preston (1912): 305 ; Preston (1915): 181, Fig. 19. Type locality: Burma. Type: Holotype M4711 [by original designation]; Zoological Survey of India, Kolkata, India (fide Ramakrishna et al. 2004) (not examined by us). The holotype is listed in the Catalogue of the type specimens of the Mollusca section of the Zoological Survey of India ( 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) . – Bolotov et al. (2017b): 10.

= Trapezidens obesa feae Kondakov, Konopleva & Vikhrev in Bolotov et al. (2017b): 8, Fig. 3d . Type locality: Myanmar: Sittaung, Myit Kyi Pauk Stream. Types: Holotype NCSM 103035 [by original designation; transferred from RMBH biv 250_4]; North Carolina Museum of Natural Sciences , Raleigh, USA (examined by us). Paratypes NCSM 103029 (transferred from RMBH biv 250_3 and biv 250_7), RMBH biv 250_8. This taxon is considered a synonym of T. angustior based on molecular sequence data.

Topotypes examined. MYANMAR: Bago River , 17.5334°N, 96.3315°E, 18.ii.2018, 11 specimens [ RMBH biv 382, including specimens biv 382_1, biv 382_2, and biv 382_3 being sequenced], Bolotov, Nyein Chan and Vikhrev leg.

Other material examined. MYANMAR: Myit Kyi Pauk Stream, 18.9613°N, 96.4455°E, Sittaung River basin, 26.11.2016, 4 specimens [ NCSM 103029 (transferred from RMBH biv 250_3 and biv 250_7), NCSM 103035 (transferred from biv 250_4), and RMBH biv 250_8; the three latter specimens are sequenced], Vikhrev leg. ; Thauk Ye Kupt River, 18.9840°N, 96.4361°E, Sittaung River basin, 26.xi.2016, 1 specimen [ RMBH biv 255_1, sequenced], Vikhrev leg ; Bilin River , 17.3306°N, 97.2418°E, 13.ii.2018, 5 specimens [ RMBH biv 370, including specimens RMBH biv 370_1, biv 370_2, and biv 370_3 being sequenced], Bolotov, Nyein Chan and Vikhrev leg. ; Chain Stream, 17.9769°N, 96.7659°E, Sittaung River basin, 20.ii.2018, 5 specimens [ RMBH biv 394_2, biv 394_5, biv 394_6, biv 394_1, and biv 394_3; the two latter specimens are sequenced], Bolotov, Nyein Chan and Vikhrev leg. ; Moload Stream, 17.6011°N, 96.2861°E, Bago River basin, 18.ii.2018, 5 specimens [ RMBH biv 385_1, biv 385_5, biv 385_6, biv 385_8, biv 385_9; the first specimen is sequenced], Bolotov, Nyein Chan and Vikhrev leg. ; Kyak Phar Stream, 17.6660°N, 96.2465°E, Bago River basin, 19.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.

Distribution. Endemic to the Bago, Sittaung, and Bilin rivers, Myanmar.

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).

Figures

Figure 3. Shells of five Trapezidens species: a) T. angustior [topotype RMBH biv 382_3]; b) T. dolichorhynchus [topotype RMBH biv 417_2]; c) T. exolescens [topotype RMBH biv 145_12]; d) T. scutum [topotype RMBH biv 632_1]; e) T. yeti sp. nov. [holotype RMBH biv 616_1]. Scale bar = 20 mm. (Photos: Ekaterina S. Konopleva).