Songkhlanaia tamodienica Konopleva, Lheknim, Sriwoon, Kondakov, Vikhrev & Bolotov 2022

Songkhlanaia tamodienica Konopleva, Lheknim, Sriwoon, Kondakov, Vikhrev & Bolotov, 2023 Figs 2 A, B, 5 A, Table 1 Songkhlanaia tamodienica Konopleva et al., 2023: 14, fig. 2 c – h. Type Locality: “ Southern Thailand: Klong Plug Pom, middle reach of Klong Tamod, Songkhla Lake Basin, Ban Kok Sai, Tambon Mae Kree, Tamod District, Phatthalung Province, 7.3324 ° N, 100.0917 ° E ”. Bolotov et al. 2023: 12. Material examined. Thailand – Phatthalung Province • 7 shells; Tamot District, Mae Khari Subdistrict, Songkhla Lake Basin, Tamot Stream; 7.3302 ° N, 100.0873 ° E; 17 May 2023; E. Jeratthitikul leg.; MUMNH - UNI 2956 to 2960, UNI 2971 to 2972. Diagnosis. Shell medium, rectangular, thin, rather compressed. Anteriorly constricted, dorsal margin straight, slightly elevated posteriorly. Posterior slope with two prominent folds. Umbo tiny, slightly elevated. Shell surface with fine irregular growth lines, roughened on posterior slope. Right valve with one smooth tubercular or triangular pseudocardinal tooth, left valve with somewhat lingula-shaped tooth. V-shaped furrow on posterior end of hinge structure weak, not prominent. Anterior adductor muscle scar shallow, somewhat drop-like, contiguous with anterior protractor muscle scar. Umbo cavity shallow. Differential diagnosis. This species can be distinguished from the other two congeners by its much smaller (about half size) and thin shell, rectangular shape, and rather compressed lateral profile. It can also be distinguished by 39 fixed nucleotide substitutions in the COI gene fragment (Table 3). Distribution. So far, known only from the type locality in Songkhla Lake Basin, southern Thailand (Fig. 3). Comments. Songkhlanaia tamodienica was described based on a single specimen. The holotype has a relatively small (shell length 44.2 mm), thin, lighter colored shell with shallow adductor muscle scar, and slightly elevated posterior wing (Konopleva et al. 2023: fig. 2 c – h). This specimen appeared to be small and young individual. Recently, we revisited the type locality and collected seven more specimens; two of these shells are larger in size and probably from fully grown specimens. They exhibit a thickened and inflated shell, rounded posterior end, dark periostracum, deep adductor muscle scar, and less pronounced posterior wing (Fig. 2 A). The largest specimen measured is 67.3 mm in shell length. In addition, the outer shell surface is sculpted by irregular growth lines, which are pronounced on the posterior slope and border of the shell (Fig. 2 A). This feature is present in younger specimens but is less prominent (Fig. 2 B).