Namkongnaia lemeslei ( Morelet, 1875 ) Jeratthitikul & Sutcharit & Ngor & Prasankok, 2021

Jeratthitikul, Ekgachai, Sutcharit, Chirasak, Ngor, Peng Bun & Prasankok, Pongpun, 2021, Molecular phylogeny reveals a new genus of freshwater mussels from the Mekong River Basin (Bivalvia: Unionidae), European Journal of Taxonomy 775, pp. 119-142 : 132-135

Namkongnaia lemeslei ( Morelet, 1875) gen. et comb. nov.

Figs 3 , 5 ; Table 1

Anodonta lemeslei Morelet, 1875: 328 , pl.14 fig. 1.

Pilsbryoconcha lemsleyi [sic] Simpson, 1900: 558.

Anodonta lemeslei – Crosse & Fischer 1876: 333. — Morlet 1889: 167.

Pilsbryoconcha lemeslei – Simpson 1914: 244. — Haas 1920: 300, pl. 37 fig. 4.; 1969a: 382. — Brandt 1974: 263 (in part, only records from “Aranyapratet (Prov. Prachin Buri)”, Thailand). — Graf & Cummings 2007: 311. — Zieritz et al. 2018: 29–44. — Ng et al. 2020: 122 , fig. 3j.; — Graf & Cummings 2021a: 22.

Type material

Syntype CAMBODIA • 2 shells; “Cambodge, Marecages de Battambang ”; Le Mesle leg.; MNHN-MP-3150 .

Other material

CAMBODIA • 6 shells; Siem Reap, Chi Kraeng, Kampong Kdei, Kampong Kdei River ; 13.13279° N, 104.33989°E; E. Jeratthitikul, C. Sutcharit, W. Siriwut, S. Chhuoy and T.H. Ng leg.; MUMNH-UNI2825

to UNI 2829, UNI 2669 • 5 shells; Grand Lacs; MNHN-MP-3156 • 2 shells; Phnom Penh; MNHN- MP-3158 • 4 shells; Cambodge; MNHN-MP-3163, MNHN-MP-3171.

Differential diagnosis

This species resembles the type species, but it can be distinguished by the relatively smaller size with a shell length of 67.0–81.0 mm (vs 80.9–102.2 mm), anterior adductor muscle scar placed near the dorsal margin (vs relatively far from the dorsal margin), adductor muscle scar relatively deep for the thin shell (vs shallower, although thicker shell), nacre yellowish (vs whitish), anterior margin rounded and little shouldered (vs wider and not shouldered), and the anterior portion of the dorsal margin somewhat lower than the posterior (vs at the same level in both anterior and posterior portions).

Description

Shell medium-sized, rather thin, narrow and elongated (H/L ratio = 0.39), very inequilateral, compressed. Dorsal margin straight; somewhat lower anteriorly than posteriorly. Umbonal area eroded, not elevated. Anterior margin round, little shouldered; posterior margin elongated, somewhat pointed. Ventral margin almost straight, or minutely concave in the middle in old specimens, posteriorly slightly curved upward. Posterior ridges low, wide and obtuse, not prominent. Periostracum thin, yellowish to dark brown, the eroded part coppery-brown. Shell surface with fine growth lines. Ligament very narrow. Hinge without dentition, posterior end of the hinge structure with V-shaped fossette. Anterior adductor muscle scar placed near the dorsal margin, relatively deep for the thin shell, ovate, fused with pedal retractor muscle scars; posterior adductor muscle scars very shallow, almost invisible. Pallial line very faint. Nacre yellowish with creamy tint near the umbo.

Additional description

Examination of the soft body of newly collected specimens in this study yielded additional description of the animal: excurrent aperture smooth, shorter than incurrent; incurrent with 1–2 rows of conical papillae, similar in length; small epithelial folds form a fused bridge separating excurrent and incurrent aperture; gills elongated and slightly ribbed; anterior margin of inner gills slightly longer and wider than that of outer gills. However, no brooding specimens were available to examine glochidia.

Distribution

This species is restricted to Tonle Sap Lake and its tributaries in Cambodia. Its distribution probably reaches the headwater of Tonle Sap basin in eastern Thailand ( Brandt 1974).

Habitat

This species was recorded in mud substrate in still sections of rivers or in lentic habitat, i.e., ponds and lakes ( Morelet 1875; Morlet 1889; Brandt 1974).

Remarks

This species has been noted for its rarity ( Brandt 1974; Ng et al. 2020). Only a few lots are available in museum collections. We obtained additional fresh materials from the Kampong Kdei River, Siem Reap Province. These specimens resemble the syntype MNHN-MP-3150 ( Fig. 5D ). The only detected variations are less erosion of the umbo area and more yellowish-brown colour of the periostracum. Morelet (1875) described Anodonta lemeslei based on specimens collected by M. Le Mesle. The figured specimen is eroded, with a concave ventral margin ( Fig. 5B–C ), while another shell has a relatively parallel shell and a straight ventral margin ( Fig. 5D ). Morelet (1875) identified the ventrally concave specimens as fully-grown individuals, while the ventrally straight specimens were considered to be young individuals. The concavity trait is quite unusual for species that live in stagnant water habitats. Nevertheless, some specimens from Tonle Sap Lake (MNHN-MP-3156) also show the characteristic of a concave ventral margin. However, all of our specimens have a straight ventral margin. This raises some suspicions about the existence of both variations. In this study, we retain both variations as the same species until topotype specimens with concave ventral margin are available for DNA analysis.

Apart from a ventral margin, other morphological variations are also observed. The posterior ends of some specimens are more or less rounded than others. Crosse & Fischer (1876) noted that some specimens from Cambodia are narrower and more elongated than the type figure. The posterior dorsal margin in some specimens is also somewhat higher. Morelet (1875) noted that one of his type specimens is undoubtedly similar to Anodonta schomburgki Martens, 1860 [= Pilsbryoconcha compressa (Martens, 1860) ]. However, it is more or less narrow, and the high posterior cannot be considered a posterior wing as in Anodonta schomburgki Martens, 1860 .

Several records and specimens previously identified as Pilsbryoconcha lemeslei have been examined and listed in this species. However, a specimen from Khanh Hoa, Vietnam, is conchologically different and does not belong to this species ( Thach 2007; Do et al. 2018); rather, it is probably a species of Pilsbryoconcha . This specimen shows more or less curved ventral margin and relatively wider shell.

Similarly, the DNA sequence of specimen assigned to Pilsbryoconcha lemeslei by Bolotov et al. (2020) and Konopleva et al. (2021) is recovered as monophyletic within the Pilsbryoconcha , together with specimens of Pilsbryoconcha exilis from Java, Indonesia (type species) and Pilsbryoconcha linguaeformis , another sympatric species with Namkongnaia lemeslei gen. et comb. nov., from Tonle Sap Lake, in our phylogenetic analyses. However, without examination of the specimen mentioned in Bolotov et al. (2020) and Konopleva et al. (2021), it can only be classified to the genus Pilsbryoconcha .

Figures

Fig. 3. Map showing distribution localities of Namkongnaia gen. nov. Boundaries of river basins follow Abell et al. (2008).

Fig. 5. Namkongnaia lemeslei gen. et comb. nov. A. Labels associated with the syntype lot. B. Original figure (after Morelet 1875: pl. 14 fig. 1). C–D. Syntype MNHN MP 3150 (photographs by V. Heros and M. Caballer) from Battambang Province, Cambodia. E. Specimen MUMNH-UNI2669. F. Specimen MUMNH-UNI2829 from Kampong Kdei River, Siem Reap Province, Cambodia. Scale bars: 10 mm.