Trapezidens yeti, Konopleva & Bolotov & Kondakov & Kononov & Gofarov & Tomilova & Lunn & Chan & Win & Vikhrev, 2020

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 : 54

Trapezidens yeti sp. nov.

Figs. 3e , 4

https://zoobank.org/ urn:lsid:zoobank.org:act:CE1C5610-AB1F-4E46-BEA4-4C466B109963

Holotype RMBH biv 616_1: MYANMAR: Ye River near Kyaung Ywar village, 15.2957°N, 97.9984°E, 20.xi.2018, Nyein Chan and Vikhrev leg.

Paratypes. The same locality, date and collectors, 14 specimens [ RMBH biv 616_2, biv 617_1, biv 617_2, biv 616_3, biv 616_4, biv 616_5, biv 617_3, biv 617_4, biv 617_5, biv 617_6, biv 617_7, biv 617_8, biv 617_9, biv 617_10; the three first specimens are sequenced]; Ye River, 15.2957°N, 97.9984°E, 2018, 3 specimens [ RMBH biv 675, including specimens RMBH biv 675_1 and biv 675_2 being sequenced], Than Win leg.

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 well-developed, 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.

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).

Figure 4. Type locality of Trapezidens yeti sp. nov.: Ye River near Kyaung Ywar village. (Photo: Ilya V. Vikhrev).