Pseudomystus nuchalis, Ng, 2025

Pseudomystus nuchalis , new species

( Fig. 4 View Fig )

Holotype. MZB 17249 View Materials , 86.2 mm SL; Borneo: Kalimantan Tengah, Barito River drainage, a small tributary of Sungai Laung , 0°20′57″S 114°43′00″E; D. Siebert et al., 16 July 1992. GoogleMaps

Paratypes. BMNH 2001.1.15.2248–2261, 14, 36.3–74.3 mm SL; Borneo: Kalimantan Tengah, Barito River drainage, Sungai Laung at Desa Maruwei , 0°21′59″S 114°44′06″E; D. Siebert et al., 15 July 1992 GoogleMaps . – BMNH 2001.1.15.2323–2325, 3, 81.8–86.9 mm SL; Borneo: Kalimantan Tengah, Barito River at Muara Laung , 0°35′34″S 114°44′12″E; D. Siebert et al., 20 February 1991 GoogleMaps . – BMNH 2001.1.15.2327–2346, 20, 50.7–91.3 mm SL; Borneo: Kalimantan Tengah, Barito River at Muara Laung , 0°35′34″S 114°44′12″E; D. Siebert et al., 8 July 1992 GoogleMaps . – BMNH 2001.1 .15.2104, 1, 78.8 mm SL; Borneo: Kalimantan Tengah, Barito River drainage, Sungai Pangaren, a small tributary of Sungai Laung, approximately 1 km from Muara Laung; D. Siebert et al., 7 July 1992 . – BMNH 2001.1.15.2347–2352, 6, 70.1–94.0 mm SL; Borneo: Kalimantan Tengah, Barito River drainage, Barito River at Purukcahu , 0°36′56″S 114°44′22″E; D. Siebert et al., 12 July 1992 GoogleMaps . – ZRC 38875 View Materials , 10 View Materials , 29.8–65.4 mm SL; Borneo: Kalimantan Tengah, Barito River drainage, market at Purukcahu; M. Kottelat, 22 June 1995 .

Diagnosis. Pseudomystus nuchalis is distinguished from all congeners except P. breviceps , P. flavipinnis , P. moeschii , P. myersi , P. stenomus , P. tuberosus , and P. vaillanti in having a colour pattern of pale yellow patches on the body (vs. a uniformly-coloured body, or with either pale yellow transverse bands or longitudinal stripes). It differs from P. breviceps , P. flavipinnis , P. moeschii , P. myersi , and P. stenomus in having (vs. lacking a nuchal concavity; compare Figs. 1 View Fig , 4 View Fig , 6 View Fig and 8 View Fig ), further differing from P. flavipinnis in having a broader snout (55–63% HL vs. 48–53; Fig. 3 View Fig ), adipose fin more anteriorly positioned relative to the anal fin (adipose-fin origin at vertical through anal-fin origin vs. posterior to vertical) and the pale yellow patches on the body never (vs. frequently) coalescing to form irregular bands, from P. moeschii in lacking (vs. having) a supracleithral process, from P. myersi in having an adipose fin with a shorter base (13.2–19.0% SL vs. 31.5–34.1) whose posterior insertion is separate from (vs. adnate to) the dorsal procurrent caudal-fin rays, and from P. stenomus in having a broader nuchal plate with straight or slightly convex margins (vs. narrower nuchal plate with slightly concave lateral margins; Fig. 2). Pseudomystus nuchalis is distinguished from P. vaillantii in having a shorter head (22.1–24.5% SL vs. 27.0–29.0) and adipose-fin base (13.2–19.0% SL vs. 20.3–26.7).

Description. Morphometric data as in Table 2. Head depressed; dorsal profile with distinct nuchal concavity. Ventral profile almost straight. Snout broadly rounded when viewed dorsally, width 55–63% HL. Bony elements of dorsal surface of head covered with thin skin. Midline of cranium with fontanelle extending from behind snout to level of posterior orbital margin. Supraoccipital process of moderate width, with gently converging sides and emarginate tip; not reaching nuchal plate. Eye ovoid, horizontal axis longest, subcutaneous; located entirely in dorsal half of head. Gill openings wide, extending from post-temporal to beyond isthmus. Gill membranes free from isthmus. First branchial arch with 4+8 (1), 4+9 (1) or 4+10* (1) gill rakers.

Mouth subterminal. Oral teeth small and villiform, in irregular rows on all tooth-bearing surfaces. Premaxillary tooth band rounded, of equal width throughout. Dentary tooth band much narrower than premaxillary tooth band at symphysis, tapering laterally. Vomerine tooth band unpaired, continuous across midline; smoothly arched along anterior margin, tapering laterally to point extending posteriorly past level of premaxillary band; band width narrower than premaxillary band at midline, widening laterally and then tapering to sharp point postero-laterally.

Barbels in four pairs. Nasal barbel slender, extending to tip of supraoccipital process. Maxillary barbel slender, extending to slightly beyond tip of humeral process. Inner mandibular-barbel origin close to midline, thicker and longer than nasal barbel and extending slightly beyond pectoral-spine base. Outer mandibular barbel originates postero-lateral of inner mandibular barbel, extending slightly beyond base of last pectoral-fin ray.

Body terete, becoming more compressed towards caudal peduncle. Dorsal profile straight from tip of snout to about one and a half eye diameter postorbitally, with distinct concavity in profile in nuchal region, sloping gently ventrally from origin of dorsal fin to end of caudal peduncle. Ventral profile slightly convex to anal-fin base, then sloping slightly dorsally to end of caudal peduncle. Skin smooth. Lateral line complete and mid-lateral in position. Vertebrae 19+20=39 (1), 20+19=39 (1), 19+21=40 (1), 19+22=41 (1) or 20+21=41* (3).

Dorsal fin with spinelet, spine, and 6,i (10) rays. Origin of dorsal fin anterior to mid-body, at about one-third of body length. Dorsal fin margin convex, usually with anterior branch of fin rays longer than other branches. Dorsal fin spine of moderate length, gently curved; posterior edge with 5–10 small, retrorse serrations. Nuchal plate triangular, with straight or slightly convex lateral margins. Pectoral fin with stout spine, sharply pointed at tip, and 6,i (4) or 7* (6) rays. Anterior spine margin smooth; posterior spine margin with 10–16 retrorse serrations along entire length. Pectoral fin margin straight anteriorly, convex posteriorly. Humeral process narrowly triangular, with slightly concave dorsal margin and extending for half to two thirds of pectoral-spine length. Pelvic fin origin at vertical through posterior end of dorsal-fin base, with i,5 (10) rays and slightly convex margin; tip of adpressed fin not reaching anal fin origin. Anal and urogenital openings located at vertical through posterior third of adpressed pelvic fin. Males with elongate, conical genital papilla reaching to anal-fin base. Adipose fin with convex margin for entire length, with deeply-incised posterior portion and origin separate from base of last dorsal-fin ray; fin-base of moderate length, spanning one-third of postdorsal distance. Anal fin origin at vertical through origin of adipose fin, with iv,11 (1), v,9,i (3), v,10 (1), v,10,i (1), vi,9,i* (1) or vi,10 (3) rays and curved posterior margin. Caudal peduncle of moderate depth. Caudal fin deeply forked, with i,7,8,i (10) principal rays; upper and lower lobes slender and pointed. Procurrent rays extend anterior to fin base, with evenly-sloping anterior margins.

Colour. In 70% ethanol: head and body dark brown above, fading to pale yellow on ventral third of head and body. Abdomen pale yellow, lateral margins speckled with brown melanophores. Faint irregularly-shaped yellowish patches on either side of nape, frequently coalescing to form irregular band. Single faint ovoid yellowish patch on sides of body behind tympanic region and below dorsal fin (absent in some individuals). Series of paired faint irregularly-shaped yellowish patches on body: first series above and below lateral line on interdorsal region (between dorsal and adipose fins), second series above and below lateral line below posterior portion of adipose-fin base, third series on dorsal and ventral procurrent caudal-fin rays (absent in some individuals). Dorsal fin with dark brown base and dark brown submarginal bar. Pectoral and pelvic fins with dark brown base and scattered brown melanophores on middle third of fin rays forming diffuse brown band on middle third of fins. Adipose fin dark brown, with narrowly ellipsoid yellowish patch on dorsal surface at anterior origin (absent in some individuals), its posterior margin hyaline. Anal fin hyaline with both a dark brown base and diffuse brown melanophores on proximal three-quarters of fin rays. Caudal fin hyaline with diffuse brown melanophores on fin rays.

Distribution. Pseudomystus nuchalis is known from the Barito River drainage in southeastern Borneo (Fig. 5).

Etymology. The Latin noun ‘nucha’ means ‘the nape of the neck’. The name is used to refer to the nuchal concavity seen in this species.