Schistura colossa, Maurice Kottelat, 2017

Maurice Kottelat, 2017, Schistura colossa and S. klydonion, two new species of loaches from Bolaven Plateau, southern Laos (Teleostei: Nemacheilidae), Raffles Bulletin of Zoology 65, pp. 341-356 : 342-347

Schistura colossa , new species

( Figs. 2–6 )

Holotype. MHNG 2767.084 , 98.0 mm SL; Laos: Champasak Province: Bolaven Plateau: Xe Pian at dam site, near Ban Nongphanouan ; 15°03′28″N 106°31′29″E; 757 masl; M. Kottelat & T. Phommavong, 12 January 2013.

Paratypes. CMK 23313, 7, 43.1–83.9 mm SL; ZRC 26222, 2 , 41.9–54.1 mm SL; same data as holotype. — CMK 23378, 7, 33.0– 81.1 mm SL; Laos: Champasak Province: Bolaven Plateau: Xe Pian north of Ban Houaxang ; 15°04′45″N 106°24′14″E; 960 masl; M. Kottelat & T. Phommavong, 17 January 2013. – CMK 23397, 1, 77.7 mm SL; Laos: Champasak Province: Bolaven Plateau: Tad Set on Houay Set ( Xe Set , tributary of Xe Don ), near Ban Nonghinkhao ; 15°18′03″N 106°18′23″E; 1136 masl; M. Kottelat & T. Phommavong, 19 January 2013. — CMK 23390, 1, 51.4 mm SL; Laos: Champasak Province: Bolaven Plateau: Tad Champi (waterfall) on Houay Champi (tributary of Xe Don ), off road from Pakse to Paksong , north at Ban Lak 38 [Km- 38 village]; 15°12′09″N 106°07′51″E; 938 masl; M. Kottelat & T. Phommavong, 18 January 2013.

Other material (non types). CMK 23443, 1, 36.0 mm SL; Laos: Champasak Province: Bolaven Plateau: Houay Makchan-Gnai at bridge on road from Ban Ta-Od to Ban Nongphanouan ; 15°04′15″N 106°32′34″E; 784 masl; M.

Kottelat & T. Phommavong, 23 January 2013. — CMK 15931, 1, 87.2 mm SL; Laos: Salavan Province: Xe Don basin, Xe Set upstream of reservoir; K. Vattahanatham, no date.

Diagnosis. Schistura colossa is distinguished from the other species of the genus in Southeast Asia by its colour pattern: the body has 16–21 bars, wider than interspaces, quite regularly shaped in juveniles; with increasing size, the bars become more irregular, some become interrupted or joined; in largest individuals (above 60 mm SL), on the caudal peduncle, the bars are broken up in blotches of irregular shape and size and irregularly set. The black pattern at the base of the caudal-fn is made of a vertically elongated blotch occupying the middle half of the fn base, sometimes with a constriction at the level of the lateral line; above and below, this blotch is continued by an arched band, along the base of the principal rays, not reaching the dorsal and ventral midlines.

Additional diagnostic characters, not unique to the species are: relatively large size (up to at least 98 mm SL); depth of caudal peduncle 1.3–1.6 times in its length; 7–8½ branched dorsal-fn rays; pelvic axillary lobe rudimentary and free; usually 9+8 branched caudal-fn rays; no suborbital fap; upper lip with a small median notch; processus dentiformis present, pointed; and origin of dorsal fn above or slightly behind origin of pelvic fn.

Description. See Figs. 2–6 for general appearance and Table 1 for morphometric data of holotype and 9 paratypes. An elongate nemacheilid with body depth slowly increasing up to dorsal-fn origin. Behind dorsal fn, body depth almost uniform until caudal-fn base. Dorsal profle continuous between head and body (no hump). Head slightly depressed; body slightly compressed anteriorly to very compressed posteriorly. Interorbital area fat. In lateral view, upper margin of eye fushed with dorsal profle of head. Cheeks not swollen. Snout pointed. Depth of caudal peduncle 1.3–1.6 times in its length. Low dorsal keel on posterior half of post-dorsal area. Low ventral keel on entire length of caudal peduncle. Dorsal keel continuous with upper margin of caudal fn. Largest recorded size 98.0 mm SL.

Dorsal fn with 4 unbranched and 7½ (1) or 8½ (9*) branched rays; distal margin convex; branched ray 2 or 3 longest. Pectoral fn with 1 unbranched and 8 (1) or 10 (9*) branched rays, rounded, reaching about halfway of distance to pelvic-fn base; origin over base of simple or frst branched pelvic-fn rays. No axillary pectoral lobe. Pelvic fn with 1 unbranched and 7 branched rays; reaching about two thirds of distance to anus; rounded; posterior margin convex; axillary pelvic lobe rudimentary, entirely free. Anus situated about 2 eye diameters in front of anal fn. Anal fn with 3 unbranched and 5½ branched rays; distal margin rounded; branched ray 2 longest. Caudal fn with 9+8 (9*) or 8+8 (1) branched rays; dorsal and ventral procurrent rays cannot be counted; emarginate, lobes rounded, subequal.

Body entirely scaled, except belly in front of pelvic fins and predorsal area (anterior extremity in smaller specimens, entirely in specimens over about 60 mm SL). Scales embedded, deeply embedded in anterior predorsal area. Lateral line complete, with 92–107 pores (diffcult to count with accuracy). Cephalic lateral line system with 6 supraorbital, 4 + 12–13 infraorbital, 9–11 preoperculomandibular and 3 supratemporal pores.

Anterior nostril pierced in front side of a pointed fap-like tube. Posterior nostril adjacent to anterior one. Mouth strongly arched, gape about 2–2½ times wider than long ( Fig. 7 ). Lips thick. Upper lip with small median notch, with a few shallow furrows in median area and near corner of mouth, edge not crenulated. Processus dentiformis present. Lower lip with narrow median interruption; median part with 1–3 shallow sulcus, lateral part smooth. Tip of lower jaw not exposed. A deep median concavity in lower jaw (in adults). Inner rostral barbel reaching corner of mouth; outer one almost reaching vertical of anterior margin of eye. Maxillary barbel reaching vertical of posterior margin of eye. Intestine with a bend behind stomach ( Fig. 8 ). Air bladder without posterior chamber in abdominal cavity.

Sexual dimorphism. None observed. Ripe females deeper bodied.

Colouration. About 3 weeks after fxation. Head and body background colour pale brown; throat, belly and lower part of caudal peduncle pale greyish. Except otherwise stated, all markings dark brown. Body with 16–21 bars (6–7 predorsal, 3–4 subdorsal, 7–10 postdorsal), extending from dorsal midline to level of pectoral fn, wider than interspaces, some continuous across back with contralaterals, others dissociated into blotches in predorsal area. Bars of quite regular shape in juveniles. With increasing size, bars becoming more irregular in shape, some becoming interrupted or joined, more obvious posteriorly. In specimens above about 60 mm SL, on caudal peduncle, bars broken up in blotches of irregular shape and size and irregularly set. Axial inner stripe faint or indistinct.

Black mark at caudal-fn base made of a vertically elongated blotch occupying middle half of fn base, sometimes with constriction at level of lateral line (best seen in Fig. 3 c). Above and below, blotch continued by an arched band, along base of principal rays, not reaching dorsal and ventral midlines; these bands less intensely black than central blotch ( Fig. 3 a). Pattern sometimes blurred by other dark brown marks overimposed, but central black blotch always distinct.

Dorsal-fn membranes hyaline, with pigments on whole length of rays. With increasing size, membranes becoming covered by tiny pigments and pigments on rays becoming denser at level of frst branching but not forming a row of spots. In largest specimen (holotype), most membranes covered by densely set small pigments. A black blotch at base of simple rays.

Caudal fn dark orange to red. Caudal, anal, pelvic and pectoral fns with membranes hyaline, with pigments on whole length of rays. With increasing size, membranes becoming covered by tiny pigments, and pigments on rays becoming denser at level of frst branching but not forming a row of spots.

Notes on biology. A dissected female (CMK 23378, 76.6 mm SL) had ripe ovaries with ova about 2.1 mm diameter. Its stomach was empty, which is probably related with low activity because of the very low temperature at time of collection (night air temperature was about 7°C; air and water temperatures were estimated around 15°C at time of collecting, in the morning). Schistura colossa was observed in streams 3 to about 30 m wide, over a pebble to boulder bottom, in clear, moderate to fast current, in riffes. Other species of Schistura collected together with S. colossa are: in the Xe Pian: S. tizardi , S. clatrata ; in the Houay Makchan-Gnai: S. bolavenensis , S. clatrata , S. tizardi ; in the Houay Champi: S. clatrata ; and in the Xe Set: S. clatrata , S. systomos .

Distribution. Schistura colossa has been collected on Bolaven Plateau in the Xe Pian drainage, a tributary of the Xe Kong, in southern Laos ( Fig. 9 ). A single specimen caught in the Champi and one caught in the Xe Set, both on Bolaven Plateau, cannot be distinguished from the samples from the Xe Pian.

Etymology. The Latin adjective colossus (-a, -um) means giant. Allusion to the size of the species, a giant among Southeast Asian nemacheilids.

Remarks. With a maximum size of at least 98 mm SL, S. colossa is a giant among Southeast Asian nemacheilines. Its general appearance and the colour pattern (see description and diagnosis) are observed in no other species of the genus.

Four species of Schistura have been collected together with S. colossa : S. bolavenensis , S. clatrata , S. systomos and S. tizardi . Schistura tizardi ( Fig. 10 ) has a distinctive appearance, with a fat head, depressed snout, eyes protruding over the dorsal profle, and humped back; besides, S. colossa has a larger size (up to 98 mm SL, vs. 59), and 16–21 bars, contrasted, of quite regular shape in juveniles, becoming interrupted or joined and irregularly set in adults, especially in posterior part of body (vs. 7–10 bars, not very contrasted and regularly shaped at all sizes). Schistura systomos reaches a maximum known size of 36 mm SL (ovigerous females), has a very small mouth (gape about 3 times in maximum head width, occupying only median half of head width), a blunt snout and the eyes are visible in ventral view ( Kottelat, 2017c).

Further to the differences mentioned above, S. colossa is distinguished from S. bolavenensis ( Fig. 11 ) by its stouter body and caudal peduncle, 16–21 bars, often dissociated into small blotches in predorsal area, wider than interspaces, irregularly set and shaped in adults, especially in posterior part of body (vs. 15–24, meeting contralaterals on back, about as wide as interspaces, regularly shaped and set), a different black pattern at caudal-fn base (bar made of a vertically elongated blotch occupying middle half of fn base; above and below, the blotch is continued by an arched band, along the base of the principal rays, not reaching the dorsal and ventral midlines, vs. a bold black bar usually reaching the dorsal and ventral midlines).

Schistura colossa is further distinguished from S. clatrata ( Fig. 12 ), among others, by having 16–21 bars on the body, often dissociated into small blotches in predorsal area, irregularly set and shaped in adults, especially in posterior part of body (vs. 7–13 bars, meeting contralaterals on back, in most individuals regularly shaped and set), no distinct pattern along dorsal-fn base (a black spot at base of unbranched and frst branched rays, followed by a bright orange spot at base of second branched ray and then a black band along base of remaining rays); a different black pattern at caudal-fn base (bar made of a vertically elongated blotch occupying middle half of fn base; above and below, the blotch is continued by an arched band, along the base of the principal rays, not reaching the dorsal and ventral midlines, vs. a bold black bar between bases of upper and lower unbranched principal rays, sometimes reaching the dorsal and ventral midlines); and the presence of a median notch in the upper lip (vs. absence).

Schistura klydonion (described below) is the only other species of the genus found on the Bolaven Plateau but not in the same drainages as S. colossa . Schistura colossa differs from S. klydonion in having the pattern of bars and saddles becoming irregular in posterior part of body in largest specimens (vs. quite regular on whole body at all sizes), the head is shorter (lateral head length 21.1–23.8% SL, vs. 23.2–25.1), presence of a median notch in the upper lip (vs. no notch), and the spots at the base of the caudal fn form a continuous band reaching close to dorsal and ventral midlines (vs. the spots are separated).

Figures

Fig. 2. Schistura colossa, MHNG 2767.084, holotype, 98.0 mm SL; Laos: Xe Pian River on Bolaven Plateau.

Fig. 3. Schistura colossa, paratypes; Laos: Xe Pian on Bolaven Plateau; a, CMK 23378, 33.0 mm SL; b, CMK 23313, 43.1 mm SL; c, CMK 23313, 51.5 mm SL; and d, CMK 23313, 71.9 mm SL.

Fig. 4. Schistura colossa, CMK 23390, paratype, 51.4 mm SL; Laos: Houay Champi on Bolaven Plateau.

Fig. 5. Schistura colossa, CMK 23397, paratype, 77.7 mm SL; Laos: Xe Set on Bolaven Plateau.

Fig. 6. Schistura colossa, CMK 23443, 36.0 mm SL; Laos: Houay Makchan-Gnai on Bolaven Plateau.

Fig. 7. Schistura colossa, CMK 23378, paratype, 81.1 mm SL; mouth.

Fig. 8. Schistura colossa, CMK 23378, paratype, 76.6 mm SL; digestive tract. Scale bar = 1 mm.

Fig. 9. Type locality of Schistura colossa; Laos: Bolaven Plateau: Xe Pian near Ban Nongphanouan; 12 January 2013.

Fig. 10. Schistura tizardi, CMK 23331, 48.1 mm SL; Laos: Xe Namnoy on Bolaven Plateau.

Fig. 11. Schistura bolavenensis, CMK 23330, 50.5 mm SL; Laos: Xe Namnoy on Bolaven Plateau.

Fig. 12. Schistura clatrata, CMK 23381, 77.9 mm SL; Laos: Xe Pian on Bolaven Plateau.