Isannaia occultata Jeratthitikul, Sutcharit & Prasankok, 2024, sp. nov.

Isannaia occultata sp. nov. Figs 1, 3, 4 C, D Type material. Holotype. Thailand • Udon Thani Province, Wang Sam Mo District, Nong Ya Sai Subdistrict, Yang Stream (a tributary of Lam Pao River); 16.9886 ° N, 103.3638 ° E; K. Wisittikoson leg.; MUMNH -UNI 2895; shell length 53.84 mm, shell height 31.68 mm, and shell width 20.41 mm. Paratypes. Thailand • 2 shells; same collection data as for holotype; MUMNH -UNI 2896, UNI 2897. Diagnosis. This species resembles I. fortunata sp. nov., but it can be distinguished by the overall shell shape being more rounded (vs rhomboidal), slightly higher shell (H / L ratio 0.58–0.60 vs 0.55–0.57), wider anterior margin (vs rather constricted), more truncated and rounded posterior margin (vs slightly ventrally elongate), less curved ventral margin (vs curved ventral margin). It also differs in characteristics inside the shell by having higher and more triangulate pseudocardinal tooth on the right valve (vs very shallow and hill-like pseudocardinal tooth), well-developed muscle scars (vs very shallow and sometimes almost invisible), 3 to 5 well-developed muscle scars in the umbo cavity (vs 1 to 2 tiny muscle scars in the umbo cavity), and well-marked pallial line (vs faint pallial line). It can also be distinguished from its congeners by fixed nucleotide substitution positions in the COI gene fragment of base A on the 54 th, 114 th, 363 rd, and 414 th; base T on the 90 th, 141 st, 591 st, and 609 th; base C on the 57 th, and 216 th; and base G on 150 th, 117 th, 192 nd, 558 th, 480 th, 552 nd, 627 th, 639 th, 645 th, and 657 th. Description. Shell medium-sized (shell length 43.8–53.8 mm, shell height 26.7–31.7 mm, shell width 13.8–20.4 mm; Table 2), ovate to rectangular, elongate (H / L ratio = 0.58–0.60), rather thin, very inequilateral, moderately inflated. Anterior margin rounded, shouldered; posterior margin truncated to rounded; ventral margin straight to slightly curved. Dorsal margin slightly curved; anterior low, posterior end slightly higher. Umbo slightly elevated, heavily eroded even in young specimens. Posterior ridge wide and obtuse, not prominent; posterior slope with two faint lines running from umbo to posterior margin, invisible in old specimens. Periostracum thin, brown to brownish-black, eroded part white to coppery-brown. Shell surface with visible growth lines. Ligament long, very narrow, dark brown. One hill-like or triangulate pseudocardinal tooth on right valve; pseudocardinal tooth on left valve less developed, hill-like. Posterior end of hinge structure with small V-shaped fossette. Anterior adductor muscle scar well-developed, ovate, contiguous with anterior pedal retractor and anterior protractor muscle scars; posterior adductor muscle scar round, shallow, contiguous with posterior retractor muscle scars; posterior retractor muscle scars triangular. Pallial line well-marked, continuous. Umbo cavity moderately deep, with 3 to 5 muscle scars in cavity. Nacre blue-whitish to yellowish, more yellowish toward the umbo cavity. Exhalant aperture almost smooth, with a row of tiny tubercle-like papillae on the edge of aperture, similar length with inhalant. Inhalant aperture with a row of very short conical papillae. Small epithelial fold divides exhalant and inhalant apertures. Gills elongated and slightly ribbed; inner gills slightly wider, and anterior margin of inner gills slightly longer than that of outer gills. Labial palps elongate, slightly curved, pointed at tip. Glochidia unknown. Etymology. The species name “ occultata ” is derived from the Latin adjective “ occultatus ” meaning concealed or hidden, a reference to this new freshwater mussel being hidden in a small stream far from the main river. Distribution. This new species is known only from the type locality, Yang Stream in Udon Thani Province. Yang Stream is a headwater tributary of the Lam Pao River in the Chi River Basin. The type locality is above the Lam Pao Dam. Remarks. This new species is rare and difficult to find despite repeated surveys. They were collected from a network of dense tree roots on a clay wall of a small stream. Nearby, there was a community of unionid mussels living in muddy or sandy substrate of the stream bottom, including Scabies phaselus (Lea, 1856), Physunio modelli Brandt, 1974, Namkongnaia inkhavilayi Jeratthitikul et al., 2021, P. exilis, S. brandti, and Thaiconcha callifera (Martens, 1860).