Pseudobagarius eustictus, Ng & Kottelat, 2021

Ng, Heok Hee & Kottelat, Maurice, 2021, Pseudobagarius eustictus, a new species of catfish from northern Laos (Siluriformes Akysidae), Zootaxa 4996 (2), pp. 322-330 : 323-327

Pseudobagarius eustictus , new species

( Fig. 1 , 2 , 5a )

(?) Akysis sp. (1)— Taki, 1974: 78, fig. 77; Kottelat, 2001: 140, fig. 394 (from Taki, 1974).

Type material. Holotype: MHNG 2786.062 , 34.4 mm SL; Laos: Xayaburi Province, Nam Heung, Keng Ban Veun rapids at Ban Mouang Mor, 1.5 km downstream of bridge about 7 km S of Kenethao , 242 m a.s.l., 17°40’59”N 101°24’03”E; M. Kottelat et al., 15 December 2018.

Diagnosis. Pseudobagarius eustictus is distinguished from congeners in having a unique combination of the following characters: a weakly-produced snout in which the upper jaw extends only slightly beyond the margin of the lower jaw when viewed ventrally, 3 tubercles on the posterior margin of the pectoral spine, eye diameter 8% HL, head width 24.1% SL, dark yellow dorsal and lateral surfaces of the head, pectoral spine lacking elongate extensions, pectoral fin reaching the pelvic-fin base when adpressed against the body, dorsolateral surfaces of body without longitudinal series of prominent tubercles, body depth at anus 13.7% SL, length of adipose-fin base 17.7% SL, caudal-peduncle depth 7.0% SL, and 33 vertebrae.

Description. Morphometric data as in Table 1 . Body moderately compressed. Dorsal profile gently convex from tip of snout to origin of dorsal fin, sloping gently ventrally to end of caudal peduncle. Ventral profile flat to anal-fin base, sloping gently dorsally to end of caudal peduncle. Anus and urogenital openings located at vertical through middle of adpressed pelvic fin. Skin tuberculate. Vertebrae 16+17=33.

Head depressed and broad, with rounded snout margin when viewed from above.Anterior nostril enclosed by flap of skin, not tubular, separated from posterior nostril by only base of nasal barbel. Gill openings narrow, extending from immediately ventral to posttemporal to point immediately lateral to ventral midline of body. Bony elements of dorsal surface of head covered with thick, tuberculate skin. Eye ovoid, horizontal axis longest; located entirely in dorsal half of head.

Barbels in four pairs. Maxillary barbel long and slender, extending to middle of pectoral-fin base. Nasal barbel slender, extending to base of pectoral spine. Inner mandibular-barbel origin close to midline, extending to base of pectoral spine. Outer mandibular barbel originating posterolateral of inner mandibular barbel, extending to middle of pectoral-fin base.

Mouth subterminal, upper jaw slightly longer than lower, premaxillary tooth band slightly exposed when mouth closed. Oral teeth small and villiform, in irregular rows on all tooth-bearing surfaces. Premaxillary tooth band gently arched, of equal width throughout. Dentary tooth band much narrower than premaxillary tooth band at symphysis, tapering laterally.

Dorsal fin located above anterior third of body, with I,5 rays; fin margin convex; spine short, straight, with smooth anterior and posterior margins.Adipose fin with anterior margin slightly concave, posterior margin angular; origin at vertical through middle of pelvic-fin base. Caudal fin strongly forked, with i,6,6,i principal rays. Procurrent rays symmetrical, extending slightly anterior to fin base.Anal-fin origin at vertical through approximately midpoint of adipose-fin base. Anal fin with convex margin and iii,6,i rays. Pelvic-fin origin at vertical through posterior end of dorsal-fin base. Pelvic fin with slightly convex margin and i,5 rays; tip of adpressed fin not reaching anal-fin origin. Pectoral fin with I,6,i rays; fin margin posteriorly convex; anterior spine margin smooth, posterior margin with 3 pointed tubercles.

Coloration. In ethanol: snout and preorbital region dark yellow. Dorsal and lateral surfaces of head roughly corresponding to area defined by anterior three-quarters of neurocranium medium brown, with irregular darker chocolate-brown markings randomly distributed. Posterior quarter of neurocranium and area around nuchal plate elements dark yellow. Lateral surfaces of body below and posterior to dorsal fin dark chocolate-brown, with numerous irregular pale patches (dark yellow dorsolaterally, grading to cream ventrally). Belly, chest and ventral surfaces of head cream, with moderately dense pattern of melanophores imparting finely-speckled appearance. Body posterior to dorsal fin with two dark yellow saddle-shaped patches: first on body covering posterior half of interdorsal region and anterior third of adipose-fin base, second covering pelvic-fin-base and most of region between it and anal-fin base. Caudal peduncle with irregular dark yellow bar formed by two dorso-ventrally overlapping ovoid patches. Proximal half of dorsal-fin membrane chocolate-brown; distal half hyaline, with irregular chocolate-brown reticulate marking. Pectoral and pelvic fins hyaline with elongate brown patches forming indistinct transverse bands through middle of fins; 3 such bands visible on pectoral fin and 2 such bands visible on pelvic fin. Anal fin hyaline with irregular chocolate-brown reticulate marking. Caudal fin with reticulate chocolate-brown W-shaped band on posterior third of fin. Adipose fin mostly dark yellow; middle third chocolate brown with irregular dark yellow patches forming reticulate pattern. Nasal and maxillary barbels dark yellow, annulated with brown rings. Mandibular barbels cream, with outer mandibular barbels annulated with brown rings. Color in life similar, but much more orange ( Fig. 2 ).

Distribution. Presently known only from the Nam Heung, a tributary of the Mekong River entering it at 17°49’09”N 101°33’10”E ( Fig. 3 ). The holotype was collected at shallow depth (about 30 cm) in rapids, in a short patch of gravel behind large stones ( Fig. 4 ).

Etymology. The specific epithet is the latinization of the Greek adjective ἐὔστικτος, meaning variegated, in reference to the mottled head of this species. This is in contrast with the nearly uniform-colored heads of congeners from the Mekong River drainage.

Figures

FIGURE 1. Pseudobagarius eustictus, MHNG 2786.062, holotype, 34.4 mm SL; Laos: Nam Heung.

FIGURE 2. Pseudobagarius eustictus, MHNG 2786.062, holotype, 34.4 mm SL immediately after capture and fixation, showing live coloration.

FIGURE 3. Map of the middle and lower Mekong showing collection locality of Pseudobagarius eustictus.

FIGURE 4. Laos: Nam Heung: Keng Ban Veun rapids at Ban Mouang Mor. Type locality of Pseudobagarius eustictus, 15

FIGURE 5. Ventral views of heads of Pseudobagarius showing conditions of snout in: a. P. eustictus, MHNG 2786.062, holotype, 34.4 mm SL; b. P. inermis, ZRC 45401, holotype, 34.9 mm SL (condition also observed in P. baramensis, P. fuscus and P. leucorhynchus); c. P. similis, UMMZ 241332, 33.7 mm SL (condition also observed in P. filifer, P. hardmani, P. meridionalis, P. nitidus, P. pseudobagarius and P. subtilis). Images not to scale.