Trapezidens exolescens ( Gould, 1843 )

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

Trapezidens exolescens ( Gould, 1843)

Fig. 3c

= Unio exolescens Gould (1843): 141 . Type locality: Tavoy, Burmah [Dawei River]. Type: Lectotype USNM_85473 [selected by R.I. Johnson, 1964]; National Museum of Natural History , Washington, USA (examined by us).

= Trapezoideus exolescens ( Gould, 1843) . – Simpson (1900): 859.

= Lamellidens exolescens ( Gould, 1843) . – Konopleva et al. (2017): 204.

= Trapezidens exolescens ( Gould, 1843) . – Bolotov et al. (2017b): 7.

Topotypes examined. MYANMAR: Dawei (Tavoy) River , 14.5013°N, 98.1558°E, 26.iv.2015, 51 specimens [lots RMBH biv 145 and biv 146, including specimens RMBH biv 145_3, biv 145_6, biv 145_11, biv 145_12, biv 145_21, biv 146_2, biv 146_7, biv 146_11, biv 146_14, biv 146_20, and biv 146_23 being sequenced], local fishermen leg .

Differential diagnosis. Shell rather small, trapezoidal, compressed and thin, with a slightly variable dorso-anterior margin that is higher and more oblique in adult mussels. Periostracum olive-brown to greybrown. The species differs from its congeners by well-developed anterior and posterior muscle attachment scars even in young specimens. Pseudocardinal teeth small, oblique; lateral teeth parallel to the dorsal margin.

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.

Comments. This species was thought to be widespread across Southeast and South Asia ( Zieritz et al. 2018) but later considered a single-drainage endemic species with a restricted range ( Bolotov et al. 2017b; Konopleva et al. 2017, 2019).

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).