Rhyacoschistura larreci, Kottelat, 2019

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 : 159-161

Rhyacoschistura larreci , new species

( Figs. 4–6 )

Holotype. MHNG 2727.011 , 57.1 mm SL; Laos: Xayaburi Province : Houay Plong at bridge 4 km South of Ban Houy Chouang, itself 33 km from Hongsa on road to Xayaburi; 528 masl; 19°32’20.1”N 101°29’49.5”E; M. Kottelat & T. Phommavong, 12 December 2018.

Paratypes. CMK 28037, 48; ZRC 60828, 8 ; 26.5–68.8 mm SL; same data .

Diagnosis. Rhyacoschistura larreci is distinguished from R. suber by its colour pattern (flank with numerous narrow slanted bars, very irregularly organised and shaped, more or less connected, or sometimes covering whole flank); suborbital flap present in males (vs. absent); on dorsal surface of pectoral fin, small tubercles along branched rays 1–3 and on membrane behind branched ray 1 (vs. tubercles on unculiferous pad behind branched ray 1).

Description. See Figures 4–6 for general appearance and Table 1 for morphometric data of holotype and 6 largest paratypes. A moderately elongate nemacheilid with body depth slowly increasing up to slightly in front of dorsal-fin origin. Behind dorsal fin, body depth almost uniform until caudal-fin base. Dorsal profile with a very slight concavity between head and body. Head from about circular anteriorly to compressed posteriorly. Interorbital area slightly convex. In lateral view, eye flush with dorsal profile of head. Cheeks not swollen. Snout rounded in dorsal and lateral view. Depth of caudal peduncle 1.1–1.3 times in its length, depth uniform. Dorsal ridge on posterior fourth of post-dorsal area. Ventral ridge on posterior half of caudal peduncle. Largest recorded size 68.8 mm SL.

Dorsal fin with 4 unbranched and 7½ branched rays; distal margin convex. Second branched ray longest. Pectoral fin with 1 unbranched and 10* (6) or 11 (1) branched rays, rounded and reaching half of distance to pelvic-fin base in females, slightly falcate and reaching about ⅔ of distance to pelvic-fin base in males. No axillary pectoral lobe. Pelvic fin with 1 unbranched and 7 branched rays; reaching about halfway of distance to anal-fin origin; falcate, posterior margin straight; origin at vertical through dorsal-fin origin; axillary lobe present, entirely free. Anus situated about 1.5–2 eye diameters in front of anal fin, behind posterior extremity of pelvic fin. Anal fin with 3 unbranched and 5½ branched rays; distal margin straight. Caudal fin with 8+8* branched rays (one specimen with 7+8 rays is probably anomalous); dorsal and ventral procurrent rays cannot be counted; emarginate, lobes rounded, upper lobe slightly longer than lower one.

Body entirely covered by scales, including predorsal area and belly between pectoral-fin bases. All scales very distinct. Lateral line complete in large specimens, interrupted in some smaller ones, with 103–122 pores (difficult to count with accuracy). Cephalic lateral line system with 6 supraorbital, 4 + 10–11 infraorbital, 9 preoperculo-mandibular and 3 supratemporal pores.

Anterior naris pierced on front side of a pointed flap-like tube. Posterior naris adjacent to anterior one. Mouth arched, gape about twice as wide as long ( Fig. 2 ). Lips thick, fleshy. Upper lip with a median notch and short, fine wrinkles. Processus dentiformis present. Lower lip with median interruption; median part with 1–2 shallow sulci, lateral part with fine, shallow wrinkles. Frenum of lower lip wide, median part of lower lip not attached to jaw. Tip of lower jaw not exposed. No median notch on lower jaw. Inner rostral barbel reaching beyond corner of mouth; outer one reaching beyond posterior margin of eye. Maxillary barbel reaching middle of postorbital area.

Intestine with a loop behind stomach ( Fig. 7a ). Air bladder without posterior chamber visible in abdominal cavity. Two halves of bony capsule of anterior chamber of air bladder connected by a manubrium ( Fig. 7b ).

Sexual dimorphism. Males with globulous suborbital flap, with small tubercles along posterior edge; females with neither suborbital flap nor suborbital groove. In males, pectoral fin rigid, slightly curled upwards ( Fig. 1a ). Membrane between unbranched and first branched rays narrower distally than at midlength. First branched ray about 4 times wider than adjacent rays. After branching point, anterior branch wider than posterior one, becoming narrower again at level of branching point of posterior branch. Anterior branch unbranched. Posterior branch branched again. No membrane between branches. On dorsal surface of first branched rays, on distal half of rays and adjacent part of membrane behind posterior branch, small tubercles organised in about 6–7 irregular longitudinal rows.

Branched ray 2: posterior branch branched again; no membrane between branches. Branched rays 3–8 branched twice; membrane narrow or absent between branching points 1 and 2. A few small tubercles along posterior edge of branched ray 2 and sometimes behind ray 3.

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 ( Fig. 1b ). No tubercles on rays and membranes.

Coloration. After 2 weeks in formalin ( Figs. 4 , 6 ). Body background colour beige to pale brown, darker posteriorly, paler on belly; except otherwise stated, all markings darker grey to dark brown. Head dark brown to blackish on top and side, with a variable pattern of pale to whitish patches, typically organised in: a large blotch on nape, and one on each side of it anteriorly; a v-shaped mark between eyes and another one between nostrils; a median longitudinal line on snout and one between eye and base of outer rostral barbel; and a white patch behind and posterior to eye. Rostral barbels dark brown; inner ones whitish distally. Maxillary barbels whitish with a narrow black area at base.

In large specimens ( Figs. 4 , 6e ), flank with numerous narrow slanted bars, extending downwards to level of in- sertion of pectoral fin, very irregularly organised and shaped, more or less connected, or sometimes covering whole flank. In smallest specimens, flank covered by dark pigments, denser on dorsal half ( Fig. 6a ); with increasing size, pigments become denser at midheight of flank, forming a midlateral band from head to caudal-fin base ( Fig. 6 b–c), later dissociated into irregular bars ( Fig. 6d ). Back crossed by narrow bars, which are often distinct and merging into a dark background colour.

Black bar at caudal base arched, extending on base of rays, almost complete, except along upper and lower extremities. A narrow pale band in front of black bar.

Dorsal fin hyaline; in smaller specimens with a black spot at base of unbranched ray and first branched ray; in larger specimens black band extending along entire base. A black mark at midlength of membrane covering unbranched rays. A few black pigments along rays and between segments at level of first branching point, forming a faint row of spots.

Caudal, anal and pelvic fins hyaline.

Pectoral fin hyaline; black pigments along middle posterior edge of unbranched and branched rays 1–2, and their branches, forming faint longitudinal marks, more intense in males than in females.

In life ( Fig. 5 ): background colour pale grey, marks dark brown, bar at caudal-fin base and markings on dorsal fin black. Fins yellowish, pectoral fin more intense. (Sampling happened in late afternoon and in rain, therefore vague information).

Notes on biology. Two dissected females (CMK 28037, 50.2, 62.9 mm SL) did not have ripe ovaries.

Distribution. Rhyacoschistura larreci is presently known only from the type locality, Houay Plong, a small mountain creek tributary of Nam Met, itself a tributary of Nam Houng, itself a tributary of Mekong. It was obtained in a very small stream, about 1 m wide and 20 cm deep, among stones and boulders. It is probably present in the uppermost parts of many other tributaries of the Nam Met, and possibly adjacent streams, but time was not available to reach and sample more sites with similar morphology.

Etymology. Named for LARReC, Living Aquatic Resources Research Center, Vientiane, for its 20th anniversary, and appreciation to several of its staff for 20 years of collaboration in the field. Treated as a masculine noun in genitive, indeclinable.

Figures

FIGURE 1. Rhyacoschistura larreci, left pectoral fin, dorsal view, of: a, male, MHNG 2727.011, holotype, 57.1 mm SL; and b, female, CMK 28034, 68.8 mm SL.

FIGURE 2. Rhyacoschistura larreci, CMK 28034, 68.8 mm SL; mouth.

FIGURE 4. Rhyacoschistura larreci, MHNG 2727.011, holotype, 57.1 mm SL; Laos: Xayaburi: Nam Houng watershed.

FIGURE 5. Rhyacoschistura larreci, CMK 28037, paratype, 57.8 mm SL; Laos: Xayaburi: Nam Houng watershed; immediately after fixation.

FIGURE 6. Rhyacoschistura larreci, CMK 28037, paratypes; Laos: Xayaburi: Nam Houng watershed; a, 23.9 mm SL; b, 26.5 mm SL; c, 29.2 mm SL; d, 63.6 mm SL; e, 68.8 mm SL.

FIGURE 7. Rhyacoschistura larreci, CMK 28034; a, 49.8 mm SL; digestive tract; b, 50.2 mm SL; air bladder capsule.