Sewellia pudens, Kottelat, 2025

Sewellia pudens , new species

( Figs. 5–7 View Fig View Fig View Fig )

Holotype. MHNG 2799.022 View Materials , 49.2 mm SL; Laos: Sekong Prov.: Houay Phouang, in Dak Bon , 2 km South of Dakchung; 1187 m asl; 15°27′19″N 107°16′14″E; M. Kottelat et al., 22 May 2011. GoogleMaps

Paratypes. All from Laos: Sekong Province: CMK 22322 , 1 , 47.4 mm SL; unnamed stream, 1.5 km on road from Dak Ta Oc Noy to Dakchung (about 10 km from Dakchung); 1090 m asl; 15°30′43″N 107°19′17″E; 20 May 2011; M. Kottelat et al. — CMK 22381 , 9 , 32.8–43.8 mm SL; Houay Oy [Xe Nam-Oy on map], large stream, on road from Dakchung to Dak Noi, 5 km from Dakchung, 5 km from Dak Noi; 1164 m asl; 15°31′04″N 107°13′27″E; 21 May 2011; M. Kottelat et al. — CMK 22348 , 59 , 27.6–43.6 mm SL [+ 8 fixed in ethanol]; CMK 29201 , 16 , 30.6–46.9 mm SL; ZRC 68312 View Materials , 10 View Materials , 30.4–42.8 mm SL; Houay Phouang, in Dak Bon, 2 km South of Dakchung; 1187 m asl; 15°27′19″N 107°16′14″E; 22 May 2011; M. Kottelat et al. — CMK 22379 , 2 , 30.0– 30.1 mm SL; Houay Champong, in Ban Dak, immediately upstream of confluence with Houay Xe Kaman, 15 km from Dakchung, 4 km after Dak Noi and 4.4 km before Ban Sang Mai; 15°34′24″N 107°13′03″E; 1170 m asl; 22 May 2011; M. Kottelat et al. — CMK 22380 , 16 , 20.9 –43.0 mm SL; Houay Oy [Xe Nam-Oy on map], 13 km from Dakchung, on road to Ban Tong Siang, 4 km after Ban Tongxiang; 1226 m asl; 15°29′05″N 107°11′07″E; 21 May 2011; M. Kottelat et al. — CMK 29189 , 1 , 39.9 mm SL; small stream [possibly Nam Voun, near Dak Do on map] about 15 km on road from Dakchung to Sekong; 1130 m asl; 15°21′39″N 107°12′11″E; 23 May 2011; M. Kottelat et al GoogleMaps .

Diagnosis. Sewellia pudens is distinguished from all species of Sewellia by the poorly developed patterning of the body and fins. On the body, black pigments are present in a vague midlateral stripe and 3–8 small saddles on the back, especially on caudal peduncle. Some irregular markings are also present on the body of some specimens, but not forming a clear pattern. The fins of most specimens have no colour marks, except for thin black lining along rays in the dorsal, anal and caudal fins, and, rarely, 1 or 2 faint blotches along lower edge of the caudal fin. The pectoral and pelvic fins have no black patterning. In all the other species of Sewellia ,

the body and paired fins have conspicuous marking (some large specimens may be almost completely dark brown) and the dorsal and caudal fins have several rows of conspicuous black dots. Sewellia pudens is also unique in the genus in having a longitudinal groove on the snout between patches of large tubercles in males. Additional characters useful for its identification, but not unique are: no dorsal extensions or ridges along the dorsal side of the pectoral-fin rays; the pelvic fin not reaching the anal fin; the anal fin not reaching the base of the caudal fin.

Description. See Figure 5 View Fig for general appearance and Table 1 for morphometric data of holotype and 9 paratypes, 38.9–49.2

mm SL. A moderately elongate Gastromyzontidae with very arched dorsal profile, body depth gradually increasing up to slightly in front of dorsal-fin origin, decreasing under dorsal-fin base. Behind dorsal fin, body depth decreasing regularly until caudal-fin base. Dorsal profile with weakly marked concavity between head and body. Ventral profile straight to slightly concave. Prepelvic part of belly and lower surface of head slightly concave. Head depressed; body very depressed. Interorbital area flat to slightly convex. In lateral view, eye flushed with dorsal profile of head. Snout rounded. Depth of caudal peduncle 1.2–1.5 times in its length, tapering posteriorly, and 1.6–2.0 times in body depth. Largest observed size 49.2 mm SL.

Dorsal fin with 3 unbranched and 7½ branched rays; distal margin straight to slightly convex; 1st and 2nd branched rays longest. Pectoral fin with 1 unbranched and 23 (1), 24 (2), 25 (7*) or 26 (1) branched rays, rounded, reaching about ⅓ of length of pelvic fin (to axillary pelvic lobe); small tubercles on dorsal side of branched and unbranched rays 1–5; on ventral side, unculiferous pads under 10–12 anterior rays (and their branches), and a row of small, blunt tubercles along anterior edge of unculiferous pads of anterior rays; no axillary pectoral lobe.

Pelvic fin with 1 unbranched and 18 (8) or 19 (3*) branched rays, rounded, reaching about half of distance to anal-fin origin (slightly beyond anus); no extensions or ridge along dorsal surface of rays (or hidden in thick, opaque tissue); last 3 rays adjacent, folded above preceeding rays to form ‘pelvic valve’; axillary lobe present, free (extremity missing in Fig. 5 View Fig ), extending beyond base of last branched rays; on ventral side, unculiferous pads under 10–11 anterior rays (and their branches), and a row of small unsharp tubercles along anterior edge of unculiferous pads. Anus situated under posterior extremity of pelvic fin, at about half of distance between pelvic-fin base and anal-fin origin.

Anal fin with 2 unbranched and 4½ branched rays; not reaching caudal fin; distal margin straight; last unbranched ray with a row of tubercles on each side, along whole length. In largest specimens, a row of tubercles on each side of branched rays 1 to 3.

Caudal fin with 7+8 branched rays; slightly emarginate, tip of lobes rounded, lower lobe longer than upper lobe. Upper 2 and lower 4 principal caudal-fin rays adnate (without membranes) until about one third of their length. Lower principal ray with a row of tubercles on each side; also present along upper principal ray in largest specimens.

Body entirely covered by scales, except on belly from throat to base of last pelvic-fin rays. Scales embedded in about anterior half of body. All scales with rounded posterior edge. Lateral line complete, extending on caudal-fin base, with 61–69 + 1 pored scales. One tubercle on each scale in lower half of body above anal fin, in some specimens tuberculate scales extending forward until tip of pelvic fin and backwards to caudal-fin base.

Anterior nare pierced in front side of a flap. Posterior nare adjacent to anterior one, almost entirely covered by flap. Mouth arched, gape about 1.5–2 times wider than long ( Fig. 6 View Fig ). Two pairs of rostral barbels and one mandibular barbel at each corner of mouth. Rostral barbels fan-shaped, with 3–6 lobes at tip. Lower lip with low papillae. Oro-pectoral fold present. Mouth structure in smallest available juveniles (about 20 mm SL) as in adult.

Tubercles: Large males distinguished from females by the presence of conspicuous tubercles on head, body, and fins (based on Roberts, 1998). See above for tubercles on body and fin rays. Conspicuous tubercles on ventral and lateral sides of snout ( Fig. 7 View Fig ), missing in median area of dorsal surface of snout. A row of tubercles between snout and nostril. Tubercle patch on side of head divided by a groove extending from below eye, along infraorbital canal of cephalic sensory system, then extending forward halfway between under nostril to tip of snout. Above groove, a patch of large tubercles immediately below eye, smaller tubercles under nostril and around posterior edge of eye. Below groove, patch of large tubercles from snout to below eye. Groove already distinct in specimens about 28 mm SL. A broad patch of tubercles along edge of opercle and between opercle and origin of pectoral fin. On ventral surface, small tubercles near base of 3 anterior rays of pectoral fin, connected with a row of small tubercles along anterior edge of unculiferous pads on rays. Patch of small, low tubercles between side of mouth and origin of pectoral fin may be present. In females, tubercles present but less developed, sometimes minute.

Colouration. After about 6 weeks in formalin: head and body background colour yellowish brown; ventral surface of head and belly whitish. In most specimens, colour pattern (dark pigments) restricted to an irregular midlateral stripe ending in a vague blotch at caudal-fin base and 3–5 small saddles behind dorsal-fin base, sometimes with 1–2 along base of dorsal fin and 2 in front of fin. In some specimens, some irregular dark brown marks on body. Pelvic and pectoral fins brown, with hyaline margin. Anal fin hyaline. Dorsal fin hyaline, rays light greyish, with spare pigments and thin black outline of rays and segments. Caudal fin hyaline, rays light greyish, with spare pigments and thin black outline of rays and segments; in a few specimens, some black pigments on rays, possibly forming one or two vague blotches near base of 3 lowermost principal rays, but never forming bars or blotches. No apparent sexual dimorphism. Juveniles identical to adults.

Distribution and habitat. The species has been observed only in tributaries of the Xe Kaman on the Dakchung plateau in Laos ( Fig. 8 View Fig ), at altitudes between 1090 and 1226 m asl.

At the type locality, the Houay Phouang is close to Dakchung village and had a moderate flow of turbid water, with large riffles over sediment covered stones. Few species were present and S. pudens was the most common species, but this may have been a sampling bias because of the turbid water. At the other sites where S. pudens was observed, it was in clear water, flowing over stone or between rocks ( Fig. 9 View Fig ). Eight of the other 10 species observed on the plateau were observed ( Devario gibber , Neolissochilus blanci , Onychostoma meridionale , Poropuntius sp. , Scaphiodonichthys sp. , Annamia normani , Sewellia diardi , Schistura clatrata ) ( Kottelat, 2017). At four of six localities where it was present, S. pudens was obtained with S. diardi , and at each of these localities S. diardi was most numerous (ratios S. pudens : S. diardi : 9:157, 17:196, 2:5, 1:46). No microhabitat difference between the two species was noted in the field.

Etymology. The Latin adjective ‘pudens’ means modest, humble, reserved, discreet. It refers to the absence of conspicuous pattern on the body and fins (compared to most other species of the genus) and the absence of ostentatious modification on pectoral- and pelvic-fin rays. Pudens is indeclinable.

RAFFLES BULLETIN OF ZOOLOGY 2025