Rhyacoschistura suber ( Kottelat, 2000 )

Kottelat, Maurice, 2019, Rhyacoschistura larreci, a new genus and species of loach from Laos and redescription of R. suber (Teleostei: Nemacheilidae), Zootaxa 4612 (2), pp. 151-170 : 161-167

Rhyacoschistura suber ( Kottelat, 2000)

( Figs. 8–10 )

Schistura suber Kottelat, 2000: 72 , fig. 60 (type locality: Laos: Vientiane Province: unnamed small forest stream along road from Thad Leuk to Nam Leuk dam site; 18°27’05”N 103°04’06”E, Mekong drainage; holotype: ZRC 45383; noun in apposition, indeclinable)

Examined material. All from Laos: Nam Ngum drainage. CMK 22434, 17, 33.9–54.1 mm SL; small unnamed creek entering Nam Ngum from the east immediately downstream of bridge on road to site of Nam Ngum 3 powerhouse; 405 masl; 18°59’27”N 102°50’20”E; M. Kottelat et al., 11 February 2012 . — CMK 22522, 9, 36.3–57.3 mm SL; unnamed stream, km 20 on road from Nam Phay bridge to Na Xai Sawang; 600 masl; 19°02’51”N 102°44’36”E; M. Kottelat et al., 18 February 2012 . — CMK 22643, 16, 27.7–50.0 mm SL; Nam Ngum upstream of confluence with Nam Ting; 890 masl; 19°24’58”N 102°58’05”E; M. Kottelat et al., 22 February 2012 . — CMK 23557, 4, 32.1–58.2 mm SL; small stream entering Nam Ngum (now in Nam Ngum 2 reservoir) from the west; about 390 masl; 18°57’35”N 102°47’19”E; M. Kottelat et al., 22 March 2013 . — CMK 23567, 143, 24.2–67.8 mm SL; forest creek at about 1.5 km on road from bridge on Nam Ngum (near planned site of Nam Ngum 3 power house) to Na Xai Sawang; 495 masl; 19°00’03”N 102°49’27”E; M. Kottelat et al., 23 March 2013 .

On divide between Nam Ngum and Nam Mang drainages: CMK 13308, 2 paratypes, 26.5–28.2 mm SL; small forest stream along road from Ban Pak Leuk to Ban Longxan; 18°27’05”N 103°04’06”E; M. Kottelat et al., 25 February 1997.

Diagnosis. Rhyacoschistura suber is distinguished from R. larreci by its colour pattern (0–16 bars on flank, of variable width, usually narrower than interspaces, very irregularly shaped and set, sometimes restricted to lower half of body; sometimes bars missing, especially in juveniles); no suborbital flap (vs. present); on dorsal surface of pectoral fin, small sharply pointed tubercles on branched rays 1–3 and on unculiferous pad behind branched ray 1 (vs. tubercles on membrane behind branched ray 1, no unculiferous pad).

Description. Based mainly on CMK 23567. See Figures 8–10 for general appearance and Table 2 for morphometric data of 7 specimens. A moderately elongate nemacheilid with body depth almost uniform from behind head to caudal-fin base. Dorsal profile with a small concavity between head and body. Head slightly depressed; body from about circular anteriorly to compressed posteriorly. Interorbital area arched, with small groove and ledge along upper margin of eye. In lateral view, eye below dorsal profile of head. Cheeks not swollen. Snout rounded. Depth of caudal peduncle 1.0–1.2 times in its length, depth uniform. Low dorsal keel on posterior fourth of post-dorsal area. Low ventral keel immediately in front of caudal-fin base. Dorsal keel separated by a marked concavity from upper margin of caudal fin. Largest recorded size 68.0 mm SL.

Dorsal fin with 4 unbranched and 7½ branched rays; distal margin convex; branched ray 2 longest; origin above pelvic-fin origin or slightly behind. Pectoral fin with 1 unbranched and 11 branched rays, rounded, posterior margin straight in males, reaching about halfway to pelvic-fin base. Pelvic fin with 1 unbranched and 8 branched rays, reaching about ¾ of distance to anus; rounded to falcate; origin slightly in front of dorsal-fin origin; axillary lobe present, small, entirely free. Anus situated about 1–1½ eye diameter in front of anal fin. Anal fin with 3 unbranched and 5½ branched rays; distal margin straight to slightly concave. Caudal fin with 8+8 (5) or 7+8 (or 8+7, difficult to count) branched rays; procurrent rays cannot be counted; emarginate, lobes rounded, subequal.

Body entirely covered by scales, including predorsal area and belly between pectoral-fin bases. All scales very distinct. Lateral line complete, with 115–130 pores. Cephalic lateral line system with 6 supraorbital, 4 + 10–11 infraorbital, 9–10 preoperculo-mandibular and 3 supratemporal pores.

Anterior nare pierced in front side of a pointed flap-like tube, tip reaching at least anterior margin of eye. Posterior nare adjacent to anterior one. Mouth arched, gape about 2–2.5 times wider than long ( Fig. 11 ). Lips fleshy. Upper lip with median notch, with a few shallow wrinkles. Processus dentiformis present. Lower lip with wide median interruption; median part smooth, with 1–3 sulci, lateral part smooth or with small wrinkles. Tip of lower jaw not exposed.A median concavity in lower jaw. Inner rostral barbel reaching corner of mouth; outer one reaching vertical of middle of eye. Maxillary barbel reaching beyond middle of postorbital area.

Intestine with a loop behind stomach ( Fig. 12a ). Air bladder without posterior chamber in abdominal cavity. Two halves of bony capsule of anterior chamber of air bladder connected by a manubrium ( Fig. 12b ).

Sexual dimorphism. Males without suborbital flap, groove or slit. In males, pectoral fin rigid, slightly curled upwards ( Fig. 13 ). Membrane between unbranched and first branched rays somewhat narrower distally than at midlength. First branched ray about 4 times wider than adjacent rays. After branching point, anterior branch somewhat wider than posterior one, becoming narrower again at level of branching point of posterior branch. Anterior branch unbranched. Posterior branch branched again near tip. No membrane between branches. On dorsal surface of first branched ray, on whole length, small sharply pointed tubercles organised in irregular longitudinal rows, where ray is widest. Unculiferous pad along posterior margin of all rays and also along anterior margin of first branched ray. On posterior pad of first branched rays, numerous small tubercles, smaller than on rays.

Second branched ray: anterior branch unbranched, posterior branch branched again near tip. No membrane between branches. Unculiferous pad posterior to branched rays 2–3 with small tubercles, in largest specimens. Branched rays 3–8 branched twice; membrane narrow or absent between branching points 1 and 2.

Tubercles observed only in specimens collected in late March, absent in specimens collected in February.

In female, first branched ray only slightly wider than adjacent rays; branching of rays as in males, but all rays and branches connected by membranes. No tubercles on rays and membranes.

Coloration. After 5 weeks in formalin ( Figs. 8–10 ). Body background colour dark yellowish brown, throat, belly, lower part of caudal peduncle whitish; except otherwise stated, markings dark brown to blackish brown. In adults, dorsal surface of head mottled, very variable but leaving typically some of the following elements: a pale band between eyes, one behind and a longitudinal one on nape, enclosing a rounded dark blotch; a pale line in front of and between nostrils; and a median pale spot near tip of snout. Body with a very irregular bar pattern, with up to 16 slanted bars (usually 9–14), very irregularly set, sometimes partly fused ( Fig. 10a ), reaching dorsal midline or leaving a pale middorsal area ( Fig. 9b ), sometimes reaching to midventral area, sometimes extending upwards from midventral area, especially on caudal peduncle ( Figs. 10 b–c).

Pattern at caudal-fin base: broad, black, arched bar, reaching or almost reaching dorsal and ventral midlines. Posterior margin irregular; bar wider on base of simple rays and of median branched rays, leaving notches in between and at midheight.

Dorsal fin hyaline, with a black spot at base of unbranched rays and branched rays 1–2, and a rows of spots on rays, made of pigments near first branching point, and at midlength of last unbranched ray. Pectoral fin hyaline in female; males with dark pigments along posterior edge of unbranched and branched rays 1–2, forming elongate marks. Other fins hyaline.

Smallest specimens (about 25 mm SL; Fig. 8 ) plain beige to grey, with black basi-caudal bar and spot at base of anterior rays of dorsal fin. A dusky midlateral stripe appearing around 32–35 mm SL. Faint bars start developing, first along midlateral stripe, then extending to whole body depth.

In life: background pale brown on back, whitish on belly, with very contrasting bars.

Notes on biology. Rhyacoschistura suber inhabits the uppermost stretches of forest creeks, head waters and torrents, with fast and cool water, over stone to rock bottom ( Fig. 14 ). It has occasionally been observed in larger rivers, near rapids.

Among dissected females, one (CMK 23567, 56.1 mm SL) obtained in late March had ripe ovaries with eggs 0.8 mm diameter. Females collected in February had no ripe ovaries. Stomach of dissected specimens contained larvae of aquatic insects about 1–2 mm long. Males collected in February had the modified pectoral-fin rays, but no tubercles were present. Tubercles are present only in specimens collected in late March. No samples have been obtained during other months.

Distribution. Rhyacoschistura suber has been observed in the Nam Ngum watershed, upstream of Nam Ngum 2 reservoir; it was not observed in other parts of the watershed, or on the Plain of Jars. The type series was obtained in 1997 in a tiny rivulet on the divide between the Nam Ngum and Nam Mang watersheds, apparently flowing to the Nam Mang watershed.

Figures

FIGURE 8. Rhyacoschistura suber, juveniles; a, CMK 23567, 19.3 mm SL; Laos: Nam Ngum watershed; b, CMK 22643, 28.7 mm SL; Laos: Nam Ngum watershed; and c, ZRC 45383, holotype, 29.7 mm SL; Laos: Nam Ngum–Nam Mang divide.

FIGURE 9. Rhyacoschistura suber, Laos: Nam Ngum watershed; a, CMK 22434, 51.5 mm SL; b, CMK 23567, 40.4 mm SL.

FIGURE 10. Rhyacoschistura suber, Laos: Nam Ngum watershed; a, CMK 23567, 41.6 mm SL; b, CMK 22522, 44.8 mm SL; c, CMK 22522, 49.2 mm SL; d, CMK 23567, 56.7 mm SL; e, CMK 23557, 59.0 mm SL.

FIGURE 11. Rhyacoschistura suber, CMK 23567, 62.8 mm SL; mouth.

FIGURE 12. Rhyacoschistura suber, CMK 23567, 58.7 mm SL; a, digestive tract; b, air bladder capsule.

FIGURE 13. Rhyacoschistura suber, CMK 23567, 62.8 mm SL; left pectoral fin.

FIGURE 14. Unnamed creek entering Nam Ngum near planned Nam Ngum 3 powerhouse, Xaisomboun Province, Laos (site of CMK 22434); habitat of Rhyacoschistura suber; 11 February 2012.

Tables

TABLE 2. Morphometric data of Rhyacoschistura suber (CMK 23567, n=7).

	range	mean
Standard lenth (mm)	55.3–60.4
Total lenth (mm)	65.9–74.3
In percent of standard length
Total length	119.1–123.0	120.8
Head length (dorsal)	19.3–20.6	19.9
Head length (lateral)	22.6–24.8	23.5
Predorsal length	55.6–57.9	56.5
Prepelvic length	52.3–54.6	53.9
Pre-anus length	74.5–77.2	76.3
Pre-anal length	78.9–81.5	80.4
Head depth	11.4–12.6	12.0
Body depth at dorsal-fin origin	17.0–18.0	17.6
Depth of caudal peduncle	12.4–13.6	13.1
Length of caudal peduncle	13.3–15.5	14.4
Head width	15.8–17.5	16.5
Body width at dorsal-fin origin	12.9–15.5	14.4
Snout length	9.2–10.5	9.8
Eye diameter	2.6–3.4	3.0
Interorbital width	7.5–8.8	8.3
Length of dorsal fin	16.5–18.7	17.3
Length of upper caudal-fin lobe	19.2–22.8	20.9
Length of median caudal-fin rays	16.8–19.4	18.1
Length of lower caudal-fin lobe	19.1–23.0	21.0
Length of anal fin	16.0–18.9	17.8
Length of pelvic fin	17.2–19.6	18.8
Length of pectoral fin	18.5–21.1	19.9
In percent of dorsal head length
Snout length	45–51	49
Eye diameter	13–17	15
Interorbital width	38–46	42
In percent of lateral head length
Snout length	38–44	42
Eye diameter	11–14	13
Interorbital width	32–38	35