Schistura titan, Maurice Kottelat, 2017

Maurice Kottelat, 2017, Schistura titan, a new species of loach from Dakchung Plateau, southern Laos (Teleostei: Nemacheilidae), Ichthyol. Explor. Freshwaters 28 (1), pp. 65-83 : 67-71

Schistura titan , new species

( Fig. 1 )

Holotype. MHNG 2767.057, 74.8 mm SL; Laos: Xekong Province: 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; M. Kottelat et al., 22 May 2011.

Paratype. CMK 22344, 1, 95.0 mm SL; collected with holotype.

Diagnosis. Schistura titan is distinguished from all the species of the genus in the Mekong drainage by the presence of a posterior chamber of the air bladder in the abdominal cavity. Besides, the following combination of characters is unique in the Mekong drainage: 8 + 7 branched caudal-fin rays; large size (up to at least 95 mm SL); 17 or 18 bars on flank, meeting contralaterals on the back behind dorsal fin, not meeting, irregular or dissociated into small blotches in predorsal area; nostril flap pointed, reaching anterior margin of eye; 8 1 / 2 branched dorsal-fin rays; pelvic-fin origin behind dorsal-fin origin; black pattern at caudalfin base: faint black bar on lower 2 / 3, not extending on ventral crest and a faint blotch at base of upper simple and posterior three procurrent rays.

Description. See Figure 1 for general appearance and Table 1 for morphometric data of holotype and paratype. An elongate nemacheilid with body depth regularly increasing up to slightly in front of dorsal-fin origin. Behind dorsal fin, body depth almost uniform until caudal-fin base, where it is slightly increasing (in paratype). Dorsal profile with a small concavity between head and body. Head slightly depressed; body compressed anteriorly to very compressed posteriorly. Interorbital area convex. In lateral view, eye below dorsal profile of head. Cheeks not swollen. Snout rounded. Caudal peduncle 1.3-1.5 times longer than deep, uniform depth in holotype or increasing until caudal-fin base in paratype. In paratype, high dorsal crest on posterior 1 / 4 of post-dorsal area and high ventral crest on posterior half of caudal peduncle; crests present in holotype but less developed. Dorsal crest continuous with upper margin of caudal fin, supported by procurrent rays. Largest recorded size 95.0 mm SL.

Dorsal fin with 4 unbranched and 8 1 / 2 branched rays; distal margin slightly convex, second branched ray longest. Pectoral fin with 1 unbranched and 11 branched rays, rounded, reaching about halfway of distance to pelvicfin base. Pelvic fin with 1 unbranched and 7 branched rays, reaching to or almost to anus, rounded; posterior margin convex; origin below base of first branched dorsal-fin ray; axillary lobe present, small, entirely free. Anus situated about 1-1 1 / 2 eye diameter in front of anal fin, distance between anus and anal-fin origin about 2 times in distance between anus and pelvic-fin base. Anal fin with 3 unbranched and 5 1 / 2 branched rays; distal margin convex. Caudal fin with 8 + 7 branched rays in holotype (6 + 7 in paratype, but the upper part of the fin was damaged and rays are regrown and deformed); about 9 dorsal and about 6 ventral procurrent rays (difficult to count with transmitted light); emarginate, lobes rounded, subequal.

Body entirely scaled, except between bases of pectoral fins in large specimen; scales deeply embedded. Lateral line complete, with 89-98 pores. Cephalic lateral line system with 6 or 7 supraorbital, 4 +11-13 infraorbital, 9 preoperculo-mandibular and 3 supratemporal pores.

Anterior naris pierced in front side of a long pointed flap-like tube reaching to anterior margin of eye. Posterior naris adjacent to anterior one. Mouth arched, gape about 2.1-2.5 times wider than long ( Fig. 2 ). Lips thick, fleshy. Upper lip without median notch, with several shallow furrows. Processus dentiformis present. Lower lip with median interruption; median part with a few deep furrows, lateral part smooth. Tip of lower jaw not exposed. A very shallow median concavity at tip of lower jaw. Inner rostral barbel reaching at most corner of mouth; outer one reaching anterior margin of eye. Maxillary barbel reaching at most about middle of postorbital area. Intestine with a bend immediately behind stomach ( Fig. 3 ). Posterior chamber of air bladder present, large, extending to above pelvic-fin base.

Sexual dimorphism. None of the characters usually associated with sexual dimorphism in Nemacheilidae were observed (no suborbital flap, modified pectoral fins, pads of unculi or tubercles on cheek and fins). This is probably uninformative because both available specimens are possibly females (see below).

Coloration. After about two months in formalin. Head and body background colour pale olive brown, throat, belly, lower part of caudal peduncle pale yellowish; except otherwise stated, markings dark brown. Body with 17 or 18 bars (6 predorsal, 4 subdorsal, 7 or 8 postdorsal); bars of quite regular width and shape, slightly wider than interspaces, reaching downwards slightly beyond lateral line, continuous over back with contralaterals on caudal peduncle, irregular or broken down into small blotches on back in front of dorsal.

A faint black bar at caudal-fin base, on about lower 2 / 3 of fin base, not extending on ventral crest. A faint blotch at base of upper simple and 3 posterior procurrent rays.

Dorsal fin hyaline; a small black spot at base of unbranched rays and branched rays 1-2, separated by a small hyaline area from a blackish brown elongated blotch along base of remaining rays. In large specimen, rays darker near first branching; a small elongated patch of black pigments on lowermost third of all membranes. Other fins hyaline.

Notes on biology. Both specimens were dissected. The holotype is a female with only a few yellow eggs about 0.8 mm diameter. The paratype has a distended belly as usual for females but its sex could not be determined. They were collected on 22 May 2011 and this suggests that they had already spawned earlier in the dry season. The stomach of the paratype was large, filled by three unidentified insect larvae. Both specimens were infected by small worms free in the abdominal cavity as well as attached to the air bladder. Schistura titan was obtained in a stream stretch under tree cover, with moderate flow over stone bottom but with important sand and mud deposit coming from activity further upstream (possibly gold mining judging from December 2012 pictures on Google Earth, which would explain the very low number of fish present).

Distribution. Schistura titan was observed only at the type locality on Dakchung Plateau at Ban Dak ( Fig. 4 ). The toponymy of the area is quite confused. The villagers identified their village as Ban Dak and the streams as Houay Champong and Houay Xe Kaman. Ban Dak appears to be Ban Lak Tai on the 1: 100 0 0 0 topographic map (Service Géographique d’ État, 1987, sheet D-48- 23) and Houay Xe Kaman as Houay Poyo. The map shows Houay Poyo as a tributary of Nam Poay-O, in turn a tributary of Xe Kaman. Houay Xe Kaman is not the same as Xe Kaman, the main river on Dakchung Plateau. To further confuse the issue, Nam Poay-O was called Xe Kaman by the Ban Dak inhabitants that were interviewed. Ban Dak appears as Dak Long on Google Earth, but on the map this is the name of a village about 2 km west of Ban Dak.

Etymology. From the Greek τιτάν (one of the Titans, giant deities), a reference to the large size of the specimens. A noun in apposition.

Remarks. There is no clue to the affinities of Schistura titan . It is peculiar in having 8 + 7 branched caudal-fin rays. The vast majority of species of Schistura s. l. have 9 +8 branched rays (as do the majority of Cypriniformes ). A smaller number of caudal-fin rays is usually found in small-size species, like S. crabro , S. punctifasciata , S. fusinotata , S. diminuta , S. daubentoni , and S. rikiki . Very few species of the size of S. titan have a reduced number of branched caudal-fin rays; S. antennata (from the Song Lam drainage in Central Vietnam) and S. namboensis (from Srepok and Sesan [Mekong drainage] and coastal drainages in southern Central Vietnam [between Quy Nohon and Phan Rang]) are known to reach 74 mm SL and to have 8 + 8 branched rays ( Freyhof & Serov, 2001). In these two species, the air bladder does not have a posterior chamber in the abdominal cavity. Besides S. antennata is immediately distinguished by its more slender appearance, its unique colour pattern in which the black bars are broader with age and are joined at their upper and lower extremities and only leave narrow yellowish interspaces. Schistura titan is further distinguished from S. namboensis by having a complete lateral line (vs. reaching at most to anal-fin origin), and a very different appearance with 17 or 18 narrow and regular bars on the body (vs. 7-10, irregular, often dissociated into a saddle and a blotch), and no axial band of deep pigmentation (vs. presence of such a band) (data on S. antennata and S. namboensis from specimens and from Freyhof & Serov, 2001).

The presence of a posterior chamber of the air bladder in the abdominal cavity had been used to diagnose some genera in Nemacheilidae (e.g., Physoschistura , Triplophysa ) but even in these it is variously developed or present. Within Schistura s. l. a free posterior chamber has been reported only in S. similis from the Salween drainage ( Kottelat, 1990: 210). Schistura titan is distinguished from S. similis by having a very different appearance, 17 or 18 narrow and regular bars on the body (vs. 7 or 8 broad blackish brown regular and very contrasted bars), 8 +7 branched caudal rays (vs. 9 + 8), the intestine with a bend immediately behind the stomach (vs. at some distance behind the stomach), and a more slender caudal peduncle (depth 1.3-1.5 times in its length, vs. 0.8-1.3).

Peculiar patterns of black markings at the base of the caudal fin are known to be diagnostic for some species or some lineages within Schistura s. l. That of S. titan is faint and of a very generalised type and, therefore, uninformative.

With a maximum known size of 95 mm SL, S. titan is a giant within Schistura s. l. Most other species have a maximum length below 70 mm SL. Other large species in the Mekong drainage are: S. sokolooi which also reaches 95 mm SL ( Freyhof & Serov, 2001: 172) from headwaters of the Sesan and Srepok rivers in Vietnam and S. colossa (98 mm SL) on Bolaven Plateau. Schistura titan is distinguished from them by the presence of the posterior chamber of the air bladder (vs. absence), by having 8 +7 branched caudal rays (vs. 9 + 8), and by having the pelvic-fin origin behind the dorsal-fin origin (vs. below or slightly in front of it). Schistura titan is further distinguished from S. sokolooi by having more bars on the body (17 or 18, vs. 11-15, in both cases quite regularly set, continuous with contralaterals at least behind the dorsal fin), the more backward pelvic-fin origin (prepelvic length 53.8-55.3 % SL, vs. 50.4-53.2), a wider head (maximum width 16.5-18.0 % SL, vs.

13.4-16.9), and a shorter snout (41-44 % of lateral head length, vs. 44-54) (morphometric data from Freyhof & Serov, 2001). Schistura titan is further distinguished from S. colossa by its colour pattern (17 or 18 regular bars on the flank, vs. 16-21 bars quite regularly shaped in juveniles and with increasing size becoming more irregular and, on the caudal peduncle, broken up in irregular blotches), a rounded snout (vs. pointed), wider head (16.5- 18.0 % SL, vs. 11.8-14.7) and body (12.0-13.6 % SL, vs. 9.9-13.5), a greater interorbital distance (34-36 % of lateral head length, vs. 25-31), a longer head (lateral head length 24.7-25.2 % SL, vs. 21.1-23.8), a greater prepelvic length (53.8- 55.3 % SL, vs. 48.8-52.2), and a greater preanal length (76.2-77.9 % SL, vs. 73.9-76.4).

By its general appearance and large size, S. titan is somewhat reminiscent of S. klydonion , also from Bolaven Plateau, of which the largest known individual is 76 mm SL. However, S. titan is distinguished from S. klydonion by the presence of the posterior chamber of the air bladder (vs. absence), 8+ 7 branched caudal-fin rays (vs. 9 + 8), the colour pattern (17 or 18 regular bars on the flank, continuous with contralaterals at least behind the dorsal fin, vs. a midlateral row of 12-21 bars that do not reach the dorsal midline and that alternate with a middorsal row of saddles or small blotches, leaving a pale zigzag line between the two rows), the lips without sparsely-set pointed papillae (vs. presence), a more backward pelvic-fin origin (prepelvic length 53.8-55.3 % SL, vs. 50.3-52.2), a wider head (maximum width 16.5-18.0 % SL, vs. 13.3-16.1), and a greater interorbital distance (34-36 % of lateral head length, vs. 26-33). Besides, the black pattern at the base of the caudal fin in S. klydonion is made of a vertically elongated blotch occupying the middle half of the base of the fin, sometimes with constriction at the level of the lateral line; above and below, the blotch is continued by an arched band, along the base of the principal rays, not reaching dorsal and ventral midlines.

A single other species of Schistura s. l. was observed on Dakchung Plateau, S. clatrata . Schistura titan is distinguished from S. clatrata by the presence of the posterior chamber of the air bladder (vs. absence), 8 + 7 branched caudal rays (vs. 9+ 8), and the colour pattern (17 or 18 regular bars on the flank, continuous with contralaterals at least behind the dorsal fin, vs. 9-13, usually continuous with contralaterals also in front of the dorsal fin, with exceptions). Schistura clatrata is widely distributed in the Xe Kong drainage; there is considerable variation in the colour pattern between different samples, and it cannot be excluded that more than one species are involved. Awaiting further analysis, they are herein recognised as a single species. In the samples from the upper Xe Kamam watershed on Dackchung Plateau, the bars are usually not very contrasted, short, not reaching much downwards beyond the lateral line ( Fig. 5 ).

Only 11 fish species have been observed from a total of seven sites on Dakchung Plateau: Deoario gibber, Neolissochilus blanci , Onychostoma meridionale , Poropuntius cf. carinatus , Scaphiodonichthys sp. ( Cyprinidae ), Annamia normani , Sewellia diardi , Sewellia sp. ( Gastromyzontidae ), Schistura clatrata , S. titan (Nemacheilidae) , and Channa gachua (Channidae) . Of them, three are new to science ( Schistura titan , described here, and Scaphiodonichthys sp. and Sewellia sp., to be described soon). The variability in Schistura clatrata has been mentioned above. The status of Poropuntius cf. carinatus is not clear yet. It seems to be unnamed too but a number of nominal species of Poropuntius have been reported or described in recent years from adjacent areas of Vietnam, including in the Xe Kong drainage ( P. aluoiensis , P. kontumensis , P. yalyensis ). Unfortunately these descriptions and the accompanying illustrations are of limited use and it is presently not possible to reach reasonable conclusions.

Figures

Fig. 1. Schistura titan; Laos: Xekong Province: Dakchung Plateau: Houay Champong; aı MHNG 2767.057, holotypeı 74.8 mm SL; bı CMK 22344, paratype, 95.0 mm SL (distal part of dorsal fin and upper part of caudal fin damaged and regrown).

Fig. 2. Schistura titan, CMK 22344, paratype, 95.0 mm SL; ventral view of mouth.

Fig. 3. Schistura titan, CMK 22344, paratype, 95.0 mm SL; digestive tract. Scale bar 5 mm.

Fig. 4. Dakchung Plateau (North East; A) and Bolaven Plateau (West; B) in southern Laos, and Xe Kong River flowing from North to South between them.: type locality of Schistura titan.

Fig. 5. Schistura clatrata, CMK 22329, 70.9 mm SL; Laos: Xekong Province: Dakchung Plateau.

Tables

Table 1. Morphometric data of Schistura titan (MHNG 2767.057, holotype; CMK 22344, paratype). H, holotype; P, paratype.

	H	P
Standard length (mm)	74.8	95.0
Total length (mm)	90.1	111.3
In percent of standard length
Total length	120.4	117.1
Head length (dorsal)	21.2	20.5
Head length (lateral)	24.7	25.2
Predorsal length	55.6	53.9
Prepelvic length	55.3	53.8
Pre-anus length	72.3	70.3
Pre-anal length	77.9	76.2
Head depth	14.0	14.7
Body depth at dorsal-fin origin	16.7	18.4
Depth of caudal peduncle	12.0	11.2
Length of caudal peduncle	16.0	16.2
Head width	16.5	18.0
Body width at dorsal-fin origin	12.0	13.6
Snout length	10.9	10.4
Eye diameter	4.0	3.8
Interorbital width	8.3	9.1
Length of dorsal fin	16.3	14.0
Length of upper caudal-fin lobe	21.5	18.7
Length of median caudal-fin rays	17.8	14.0
Length of lower caudal-fin lobe	19.3	17.3
Length of anal fin	17.3	16.3
Length of pelvic fin	15.6	13.0
Length of pectoral fin	17.9	16.5
In percent of dorsal head length
Snout length	51	50
Eye diameter	19	19
Interorbital width	39	44
In percent of lateral head length
Snout length	44	41
Eye diameter	16	15
Interorbital width	34	36