Pseudomystus stenomus

Pseudomystus stenomus View in CoL (Valenciennes in Cuvier & Valenciennes, 1840)

( Fig. 1 View Fig )

Bagrus stenomus Valenciennes View in CoL in Cuvier & Valenciennes, 1840: 415 (type locality: Java); Bleeker, 1858: 165; Roberts, 1993: 27, Fig. 61.

Leiocassis ellenriederii Bleeker, 1860:10 (type locality: Lahat , Sumatra).

Leiocassis stenomus View in CoL : Bleeker, 1862 –63: 54, Pl. 67 Fig. 3 View Fig ; Weber & de Beaufort, 1913: 355 (in part); Jayaram, 1968: 364 (in part).

Liocassis breviceps Regan, 1913: 551 View in CoL (type locality: Deli [= Medan ], Sumatra; in part)

Leiocassis breviceps View in CoL (non Regan, 1913): Weber & de Beaufort, 1913: 361 (in part); Jayaram, 1968: 363 (in part).

Pseudomystus breviceps View in CoL (non Regan, 1913): Ng & Rachmatika, 1999: 178 (in part); Ng & Lim, 2005: 16 (in part); Ferraris, 2007: 103 (in part); Lim & Ng, 2008: 45 (in part); Kottelat, 2013: 265 (in part).

Pseudomystus stenomus View in CoL : Tan & Ng, 2000: 281; Ng & Lim, 2005: 17 (in part); Lim & Ng, 2008: 46 (in part); Tan & Kottelat, 2009: 20; Grant, 2009: 16, Figs. 1 View Fig –5, 8; Kottelat, 2013: 266; Iqbal et al., 2018: 108, Figs. 434–435, 437.

Material examined. RMNH 2986 About RMNH , holotype, 69.1 mm SL; Java . – BMNH 1889.11 .12.65, paralectotype of Liocassis breviceps , 67.2 mm SL; Sumatra : Deli [=Medan]. – ZRC 44163 View Materials , 50 View Materials , 54.4–82.6 mm SL ; ZRC 44166 View Materials , 21 View Materials , 29.0– 42.4 mm SL; Sumatra : Jambi, Sungai Alai. – ZRC 46251 View Materials , 18 View Materials , 25.2–42.8 mm SL; Sumatra : Jambi .

5 mm

A B

Fig. 2. Dorsal views of nuchal plates showing shape in: a, Pseudomystus stenomus ( ZRC 44163, 58.9 mm SL illustrated); b, P. breviceps , P. flavipinnis , and P. nuchalis ( P. breviceps, ZRC 27674, 54.2 mm SL illustrated). Scale bar represents 5 mm.

Diagnosis. Pseudomystus stenomus is distinguished from all congeners except for P. breviceps , P. flavipinnis , P. moeschii , P. myersi , P. nuchalis , P. tuberosus , and P. vaillantii in having a colour pattern of pale yellow patches on the body (vs. uniformly-coloured body, or with either pale transverse bands or pale longitudinal stripes). It differs from P. breviceps , P. flavipinnis , and P. nuchalis in having a narrower nuchal plate with slightly concave lateral margins (vs. broader nuchal plate with straight or slightly convex margins; Fig. 2), and further differs from P. breviceps in having a slenderer caudal peduncle (7.8–9.7% SL vs. 9.2–11.5), from P. flavipinnis in having a broader snout (width 56–59% 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 coalescing to form bands (vs. frequently coalescing to form irregular bands), and from P. nuchalis in lacking (vs. having) a nuchal concavity (compare Figs. 1 View Fig and 4 View Fig ). Pseudomystus stenomus is distinguished from P. moeschii in lacking (vs. having) a supracleithral process, and from P. myersi in having an adipose fin with a shorter base (12.7–17.4% SL vs. 31.5–34.1) whose posterior insertion is separate from (vs. adnate to) the dorsal procurrent caudal-fin rays. It differs from P. tuberosus in having an evenly sloping (vs. strongly humped) predorsal profile, the absence (vs. presence) of rounded tubercles on the head and flanks, an adipose fin with a shorter base (12.7–17.4% SL vs. 23.3–26.1) and a deeper caudal peduncle (7.8–9.7% SL vs. 5.5–6.0), and from P. vaillantii in having a shorter head (21.7–26.2% SL vs. 27.0–29.0) and adipose-fin base (12.7–17.4% SL vs. 20.3–26.7).

Description. Morphometric data as in Table 1. Head depressed; dorsal profile evenly sloping or very gently convex. Ventral profile almost straight. Snout broadly rounded when viewed dorsally, width 56–59% HL. Bony elements of dorsal surface of head covered with thin skin. Midline of cranium with fontanelle extending from behind snout to just anterior to level of posterior orbital margin. Supraoccipital process of moderate width, with gently converging sides and notched tip; in contact with 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 3+7 (3), 3+8 (1) or 4+8 (2) 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 at symphysis than premaxillary tooth band, 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 at midline narrower than premaxillary band, widening laterally and then tapering to sharp point postero-laterally.

Barbels in four pairs. Nasal barbel slender, extending to middle of pectoral-fin base. Maxillary barbel slender, extending to level of midpoint between base of last pectoral-fin ray and pelvic-fin origin. Inner mandibular-barbel origin close to midline, thicker and longer than nasal barbel, extending to base of last pectoral-fin ray. Outer mandibular barbel originating postero-lateral of inner mandibular barbel, extending beyond base of last pectoral-fin ray.

Body terete, becoming more compressed toward caudal peduncle. Dorsal profile gently convex from tip of snout to origin of dorsal fin, 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, mid-lateral in position. Vertebrae 18+20=38 (2), 19+20=39 (5), 20+19=39 (2), 19+21=40 (3), 20+20=40 (8), 20+21=40 (5), 19+22=41 (2), 21+20=41 (3), or 21+21=42 (1).

Dorsal fin with spinelet, spine, and 5,i (4), 6 (1), 6,i (15) or 7 (10) rays. Origin of dorsal fin anterior to mid-body, at about one-third of body length. Dorsal fin distal margin convex, usually with anterior branch of fin rays longer than other branches. Dorsal fin spine of moderate length, gently curved, slender, its posterior edge with 4–7 small, retrorse serrations. Nuchal plate narrowly triangular, with slightly concave lateral margins. Pectoral fin with stout spine, sharply pointed at tip, and 6 (1), 6,i (11) or 7 (18) rays. Anterior spine margin smooth; posterior spine margin with 9–14 retrorse serrations along entire length. Pectoral fin margin straight anteriorly, convex posteriorly. Humeral process narrowly triangular, with slightly concave dorsal margin, extending for half of pectoral-spine length. Pelvic fin origin at vertical posterior to posterior end of dorsal-fin base, with i,5 (30) rays, its margin slightly convex; tip of adpressed fin not reaching anal-fin origin. Anal and urogenital opening located at vertical through posterior third of adpressed pelvic fin. Males with elongate genital papilla reaching to anal-fin base. Adipose fin with convex margin for entire length, with deeply-incised posterior portion, 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 v,9 (1), v,10,i (2), vi,9,i (8), v,10,ii (1), v,11,i (1), vii,9,i (1), vi,10,i (13), vi,11,i (2) or vii, 10,i (1) rays, its posterior margin curved. Caudal peduncle of moderate depth. Caudal fin deeply forked, with i,7,8,i (30) principal rays; upper and lower lobes slender, pointed. Procurrent rays extending 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. 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 submarginal brown band. 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 stenomus is known from an unspecified river drainage (very likely either the Ciliwung or

Cisadane drainage) in western Java and the Deli and Batang Hari drainages in Sumatra (Fig. 5). It is likely to be found in river drainages between these two systems in Sumatra and Java (e.g., the Indragiri, Musi, and Tulang Bawang river drainages in Sumatra). The species, however, is likely extirpated in Java ( Ng, 2020) .