Trapezidens scutum ( Sowerby, 1868 )

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 : 53-54

Trapezidens scutum ( Sowerby, 1868)

Fig. 3d

= Unio scutum Sowerby (1868) : Pl. 94, sp. 510. Type locality: Tenasserim. Type: Holotype BMNH_ 1907-10-28 -239 [by monotypy]; Natural History Museum , London, UK (examined by us).

= Lamellidens scutum ( Sowerby, 1868) . – Simpson (1900): 857; Konopleva et al. (2017): 210.

= Trapezidens scutum ( Sowerby, 1868) . – Bolotov et al. (2017b): 10.

Topotypes examined. MYANMAR: Nga Wun River, 11.8567°N, 99.1181°E, Tanintharyi River basin, 25.xi.2018, 4 specimens [ RMBH biv 630_1, biv 632_1, biv 630_2 (ethanol-preserved tissue snip only), and biv 632_2 (ethanol-preserved tissue snip only); all sequenced], Bolotov, Nyein Chan and Vikhrev leg. ; Nga Wun Stream, 11.8233°N, 99.14934°E, Tanintharyi River basin, 2 specimens [ RMBH biv 644 and biv 645_1; all sequenced], Nyein Chan leg.

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.

Distribution. Endemic to the Tanintharyi (Great Tenasserim) River, Myanmar.

Comments. Previously, we assumed that Trapezidens scutum may be conspecific with T. exolescens based on the proximity of their ranges and conchological similarity ( Konopleva et al. 2017). However, this hypothesis was not supported by the novel molecular data, because these taxa represent two divergent phylogenetic lineages.

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).