Songkhlanaia songkhramensis, Jeratthitikul & Sutcharit & Prasankok, 2025

Songkhlanaia songkhramensis sp. nov.

Figs 4 View Figure 4 , 5 C View Figure 5 Tables 1 View Table 1 , 4 View Table 4

Type material.

Holotype Thailand – Sakon Nakhon Province • Phang Khon District, Hai Yong Subdistrict, Songkhram Basin, Prahang River ; 17.4376 ° N, 103.7569 ° E; 7 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3024 View Materials (shell length 120.5 mm, shell height 78.5 mm, shell width 55.3 mm) GoogleMaps . Paratype • 3 shells; same collection data as for holotype; MUMNH - UNI 3022 View Materials , 3023 View Materials , 3025 View Materials GoogleMaps .

Other material.

Thailand – Nong Khai Province • 5 shells; Si Chiang Mai District, Nong Pla Pak Subdistrict, Mekong Basin, Nam Mong River ; 17.8914 ° N, 102.5341 ° E; 7 Apr. 2015; E. Jeratthitikul, K. Wisittikoson, and P. Prasankok leg.; MUMNH - UNI 2174 View Materials to 2178 View Materials GoogleMaps • 2 shells; Phon Phisai District, Thung Luang Subdistrict, Mekong Basin, Nam Suai Stream ; 17.9640 ° N, 102.9659 ° E; 8 Apr. 2015; E. Jeratthitikul, K. Wisittikoson, and P. Prasankok leg.; MUMNH - UNI 2214 View Materials , 2215 View Materials GoogleMaps . Udon Thani Province • 2 shells; Ban Dung District, Ban Dung Subdistrict, Songkhram Basin, Songkhram River ; 17.8666 ° N, 103.4034 ° E; 5 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 2978 View Materials , 2979 View Materials GoogleMaps • 2 shells; Ban Dung District, Ban Muang Subdistrict, Songkhram Basin, Songkhram River ; 17.7293 ° N, 103.4101 ° E; 5 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 2981 View Materials , 2982 View Materials GoogleMaps • 7 shells; Thung Fon District, Thung Fon Subdistrict, Songkhram Basin, Songkhram River ; 17.4521 ° N, 103.2808 ° E; 8 Apr. 2015; E. Jeratthitikul, K. Wisittikoson, and P. Prasankok leg.; MUMNH - UNI 919 View Materials , 0925 View Materials to 0927 View Materials , 3003 View Materials to 3005 View Materials GoogleMaps • 3 shells; Sang Khom District, Chiang Da Subdistrict, Mekong Basin, Huai Luang River ; 17.8730 ° N, 103.0875 ° E; 8 Apr. 2015; E. Jeratthitikul, K. Wisittikoson, and P. Prasankok leg.; MUMNH - UNI 2179 View Materials to 2181 View Materials GoogleMaps . Bueng Kan Province • 6 shells; Mueang District, Khok Kong Subdistrict, Mekong Basin, Huay Kam Paeng. 18.3381 ° N, 103.7625 ° E; 5 Apr. 2015; E. Jeratthitikul, K. Wisittikoson, and P. Prasankok leg.; MUMNH - UNI 0586 View Materials to 0591 View Materials GoogleMaps • 3 shells; Seka District, Nong Thum Subdistrict, Songkhram Basin, Songkhram River ; 17.8822 ° N, 103.8609 ° E; 4 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3051 View Materials to 3053 View Materials GoogleMaps • 1 shell; Seka District, Pong Hai Subdistrict, Songkhram Basin, Nam Hee Stream , Unnamed Check Dam ; 18.0117 ° N, 103.8585 ° E; 4 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3069 View Materials GoogleMaps • 1 shell; Phon Charoen District, Wang Chomphu Subdistrict, Songkhram Basin, Songkhram River ; 17.9557 ° N, 103.6802 ° E; 4 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3107 View Materials GoogleMaps • 2 shells; Mueang District, Na Sawan Subdistrict, Songkhram Basin, Huay Pak Kong Stream , Ban Na Waeng Cheek Dam ; 18.2603 ° N, 103.5425 ° E; 4 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3088 View Materials , 3089 View Materials GoogleMaps • 3 shells; Phon Charoen District, Nong Hua Chang Subdistrict, Songkhram Basin, Huay Pak Kong Stream , Unnamed Cheek Dam ; 18.0863 ° N, 103.5462 ° E; 5 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3048 View Materials to 3050 View Materials GoogleMaps • 9 shells; Seka District, Seka Subdistrict, Market (collected from Nam Hee Stream ); 17.9265 ° N, 103.9455 ° E; 19 Jan 2023; K. Macharoenboon leg.; MUMNH - UNI 2855 View Materials to 2863 View Materials GoogleMaps • 9 shells; Seka District, Tha Sa-at Subdistrict, Songkhram Basin , Songkhram River ; 17.9318 ° N, 103.7600 ° E; 5 Apr. 2015; E. Jeratthitikul, K. Wisittikoson, and P. Prasankok leg.; MUMNH - UNI 0667 View Materials , 0668 View Materials GoogleMaps . Nakhon Phanom Province • 2 shells; Tha Uthen District, Non Tan Subdistrict, Mekong Basin , Thuai River ; 17.5621 ° N, 104.6096 ° E; 2 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3101 View Materials , 3102 View Materials GoogleMaps . Sakon Nakhon Province • 2 shells; Sawang Daen Din District, Khok Si Subdistrict, Songkhram Basin, Songkhram River ; 17.6205 ° N, 103.4020 ° E; 6 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3077 View Materials , 3078 View Materials GoogleMaps • 4 shells; Wanon Niwat District, Khon Sawan Subdistrict, Songkhram Basin, Yam Stream , Huai Kho Check Dam ; 17.5637 ° N, 103.7203 ° E; 7 Apr. 2023; E. Jeratthitikul and W. Siriwut leg.; MUMNH - UNI 3013 View Materials to 3016 View Materials GoogleMaps • 7 shells; Charoen Sin District, Khok Sila Subdistrict, Songkhram Basin, Yam Stream ; 17.5322 ° N, 103.5608 ° E; 3 May 2015; E. Jeratthitikul, K. Wisittikoson, and P. Prasankok leg.; MUMNH - UNI 0344 View Materials to 0350 View Materials GoogleMaps • 1 shell; Kham Ta Kla District, Kham Ta Kla Subdistrict, Songkhram Basin, Songkhram River ; 17.9307 ° N, 103.7572 ° E; 18 Jan. 2023; K. Macharoenboon leg.; MUMNH - UNI 2853 View Materials GoogleMaps • 1 shell; Akat Amnuai District, Tha Kon Subdistrict, Songkhram Basin, Songkhram River ; 17.7786 ° N, 103.9528 ° E; 9 Jan. 2023; local people leg.; MUMNH - UNI 2198 View Materials GoogleMaps .

Diagnosis.

Shell large, subrhomboidal to ovate, thick, rather inflated. Anteriorly constricted, dorsal slightly curved, slightly elevated posteriorly. Posterior slope with one or two faint folds. Umbo rounded, wide, moderately elevated. Shell surface with irregular growth lines, roughened on posterior slope and border of shell. Right valve with one smooth triangular or high tubercular pseudocardinal tooth, left valve with well-developed, rectangular, rather broad and high pseudocardinal tooth. V-shaped furrow on posterior end of hinge structure prominent and wide. Anterior adductor muscle scar impressed, somewhat droplet-like, separated from anterior protractor muscle scar. Umbo cavity moderately deep and wide.

Differential diagnosis.

This new species can be distinguished from S. moreleti by having well-developed pseudocardinal teeth (Fig. 5 View Figure 5 ), particularly the one on the left valve, which is characterized as a rectangular, rather broad, and high tooth (vs subcompressed, hill-like, or triangulate in S. moreleti ); a wider V-shaped furrow at the posterior end of the hinge structure; and a deeper umbo cavity. The new species also possesses a set of unique fixed nucleotide substitutions in the COI gene fragment that make it genetically distinct from its congeners (Table 3 View Table 3 ). The new species is genetically closely related to S. moreleti , with a 4.17 % uncorrected p-distance in the COI gene. They also form a sister clade in the phylogenetic analysis (Fig. 1 View Figure 1 ).

Description.

Shell large-sized (shell length 104.7–120.5 mm, shell height 73.8–78.5 mm, shell width 46.2–55.3 mm; Table 4 View Table 4 ), thick, rather high (H / L ratio = 0.65–0.70), inequilateral, subrhomboidal to ovate shape, rather inflated. Anterior margin rounded; posterior margin oblique above, subtruncate below; ventral margin slightly curved to straight. Dorsal margin curved to slightly curved; anterior low, rather constricted, slightly elevated to posterior end; posterior end high, resembling posterior wing in young specimens (Fig. 4 C View Figure 4 ). Umbo rounded, wide, moderately elevated, usually eroded. Posterior ridge wide and obtuse, not prominent; posterior slope with one or two faint folds running as curved line from umbo to posterior margin; lower one more prominent, ending at approximately middle of posterior margin, forming angulate point. Periostracum moderately thick, dark brown to black, eroded part white to coppery-brown. Shell surface with irregular growth lines, roughened on posterior slope and border of shell.

Inner side of shell: ligament long, narrow, dark brown in color. Pseudocadinal teeth one on each valve; smooth, triangular or high tubercular shape on right valve; well-developed, smooth, rectangular shape, rather broad and high on left valve; in shell coupling position, pseudocadinal tooth on right valve situated well anteriorly. Lateral teeth obsolete. Posterior end of hinge structure with wide V-shaped furrow. Anterior muscle scars impressed; anterior adductor muscle scar somewhat droplet-like, contiguous with anterior pedal retractor, but separated from anterior protractor muscle scars; pedal retractor muscle scar rounded, protractor muscle scar bean-shaped. Posterior adductor muscle scar large, drop-like to ovate, shallow. Pallial line well-marked and continuous. Umbo cavity deep, wide, with one row of 5–10 muscle scars. Nacre whitish blue to yellowish.

Siphon apertures with strip of dark pigmentation running along aperture edge. Exhalant aperture smooth, shorter than inhalant. Inhalant aperture with one row of conical papillae, varying in length, with more of the longer papillae near ventral edge. Small epithelial fold divides exhalant and inhalant aperture. Gills elongated and slightly ribbed; outer gills narrower than inner gills; anterior margin of inner gills slightly longer than outer gills. Labial palps elongate, somewhat pointed at tip. Glochidia unknown.

Etymology.

The species name songkhramensis refers to the Songkhram Basin, a sub-river basin of the Middle Mekong Basin in northeastern Thailand, in which this species is highly abundant. The type locality of the species is also situated in the Songkhram Basin.

Distribution.

The new species occurs in the Songkhram Basin and tributaries of the Mekong River in northeastern Thailand. It is a common freshwater mussel in the middle to upper part of Songkhram Basin (Fig. 3 View Figure 3 ) and is usually found in high abundance.

Comments.

Among the mussel species commonly found sympatrically with S. songkhramensis sp. nov., Thaiconcha callifera is the most similar in overall shell features, and thus may confuse the identification. However, the new species can be easily distinguished from T. callifera by its thick shell (vs moderately thick), subrhomboidal to ovate shape (vs elliptical or rounded shape), higher shell (vs somewhat elongate), wider and more elevated umbo (vs narrow and slightly elevated), less shiny shell (vs somewhat shiny shell), shell surface sculptured with irregular growth lines, heavily roughened on the posterior slope (vs shell surface rather smooth, with fine growth lines, slightly roughened on the posterior slope), and rectangular and rather broad pseudocardinal teeth (vs tubercular pseudocardinal teeth) ( Bolotov et al. 2020; Konopleva et al. 2021).