Nemacheilus pezidion, a new species of loach from southern Laos (Teleostei: Nemacheilidae) Author Kottelat, Maurice 0000-0003-3638-2824 Rue des Rauraques 6, 2800 Delémont, Switzerland (permanent address); and Lee Kong Chian Natural History Museum, National University of Singapore, 2 Conservatory Drive, Singapore 117377. mkottelat @ dplanet. ch; https: // orcid. org / 0000 - 0003 - 3638 - 2824 mkottelat@dplanet.ch text Zootaxa 2022 2022-04-26 5129 1 92 104 journal article 55813 10.11646/zootaxa.5129.1.5 8f10cdb3-1027-44b0-80da-f61e0fe7ef28 1175-5326 6488205 1CB53F3A-5231-495C-98C3-02B0BA602758 Nemacheilus pezidion , new species ( Fig. 1 ) Holotype . MHNG 2787.088 , 57.8 mm SL; Laos : Attapeu Prov. : Xe Namnoy drainage: Xe Pian at Ban Mai , 14°42’22”N 106°29’46”E ; M. Kottelat et al ., 22 May 1999 . Paratypes . All from Laos : Attapeu Prov. : Xe Namnoy drainage: Xe Pian : CMK 15687, 2 , 48.4–53.8 mm SL; same data as holotype .— CMK 21246, 15 , 46.7–56.9 mm SL ; ZRC 62559, 6 , 49.1–53.9 mm SL; Xe Pian , about 2 km upstream of Ban Mai , 14°43’31”N 106°29’42”E ; M. Kottelat et al ., 20 May 2009 .— CMK 21280, 3 , 43.7– 47.1 mm SL; Xe Pian , about 3 km downstream of Ban Mai , 14°41’33”N 106°28’41”E ; M. Kottelat et al ., 20 May 2009 .— CMK 21330, 14 , 40.1–54.8 mm SL; Houai Pin , a tributary of Xe Pian , entering it about 5 km downstream of Ban Mai ; about 300 m upstream of confluence, 14°40’46”N 106°29’03”E ; M. Kottelat et al ., 21 May 2009 .— CMK 21315, 1 , 39.2 mm SL; Xe Pian , rapids about 6 km downstream of Ban Mai ; 14°39’26”N 106°28’27”E ; M. Kottelat et al ., 21 May 2009 . Diagnosis. Nemacheilus pezidion is distinguished from most species of Nemacheilidae in Southeast Asia by having an obvious dark brown to black midlateral stripe. A somewhat similar pattern is present only in N. binotatus , N. longistriatus , ‘N.’ argyrogaster , N. nandingensis , Schistura rubrimaculata and S. pawensis . It is distinguished from them in having a forked caudal fin with the upper lobe distinctly longer than the lower one (vs. subequal), the pectoral-fin rays with short filamentous extensions in both sexes (vs. without extensions), the presence of an elongate spot on lower ¼ of first branched dorsal-fin ray and adjacent membranes (vs. absence), and the presence of 5 supraorbital pores (vs. 6). In addition, males of N. pezidion have a modified pectoral fin with conspicuous tubercles on anterior rays and wide unculiferous pads (vs. minute tubercles, narrow unculiferous pads), and numerous small tubercles on the middle and posterior part of the flank of large males (vs. without tubercles). Description. See Figure 1 for general appearance and Table 1 for morphometric data of holotype and 10 paratypes . An elongate nemacheilid with body depth gradually increasing up to slightly in front of dorsal-fin origin. Behind dorsal-fin origin, body depth decreasing slowly to caudal-fin base. Dorsal profile continuous between head and body. Head very slightly depressed; body slightly compressed anteriorly to compressed posteriorly. Interorbital area convex. In lateral view, eye usually below dorsal profile of head (flushed in a few specimens). Cheeks not swollen. Snout rounded. Depth of caudal peduncle 1.7–2.0 times in its length, tapering posteriorly. Low dorsal keel on posterior fourth of post-dorsal area. No ventral keel on caudal peduncle. Depth of caudal peduncle 1.5–1.9 times in body depth at dorsal-fin origin. Largest recorded size 57.8 mm SL ( holotype ). Dorsal fin with 4 unbranched and 9½ branched rays; distal margin straight to slightly concave. First branched ray longest. Pectoral fin with 1 unbranched and 10 (1), 11 (7*) or 12 (3) branched rays (including small last ray, usually unbranched), falcate, reaching about ¾ of distance to pelvic-fin base; all rays with short filamentous extensions ( Fig. 2 ). No axillary pectoral lobe. Pelvic fin with 1 unbranched and 7 branched rays (including small last ray, usually unbranched); reaching ³⁄ to ¾ of distance to anus; triangular; posterior margin straight; origin below base of branched dorsal-fin rays 1–2; axillary lobe present, free, small (equal to base of fin). Anus situated about 1–1½ eye diameter in front of anal fin, behind posterior extremity of pelvic fin. Anal fin with 3 unbranched and 5½ branched rays; distal margin almost straight. Caudal fin with 9+8 branched rays; about 6–7 dorsal and 6–7 ventral procurrent rays (cannot be counted with accuracy, possibly more), forked, lobes pointed, upper lobe 1.1–1.3 times length of lower lobe, 1.9–2.2 times length of median rays. Body entirely covered by scales. Scales embedded. Lateral line complete, with about 93–102 pores (due to shrinkage, lateral line sunk in gutter and impossible to count with accuracy). Scales on flank in large males with tubercles (see details under Sexual dimorphism below). Cephalic lateral line system with 5 supraorbital, 4 + 10–11 infraorbital, 8–10 preoperculo-mandibular and 3 supratemporal pores. Anterior naris pierced in front side of a pointed flap-like tube. Posterior naris adjacent to anterior one. Mouth U-shaped, gape about 2 times wider than long ( Fig. 3 ). Lips thin but fleshy. Upper lip without median notch or with a small, shallow median notch, numerous wrinkles; edge crenulated. Processus dentiformis present. Lower lip without median interruption or small notch in outer part of lip; median part with 4–5 sulci, lateral parts smooth; posterior margin of lip thin, smooth. Tip of lower jaw not exposed. No median concavity in lower jaw. Inner rostral barbel reaching vertical of middle of eye. Outer rostral barbel and maxillary barbel reaching middle of postorbital area of head. FIGURE 1. Nemacheilus pezidion , Laos: Xe Nam Noi: Xe Pian; a, MHNG 2787.088, holotype, 57.8 mm SL, male; b, CMK 15687, paratype, 53.8 mm SL, female; c, CMK 21330, paratype, 54.2 mm SL; female. TABLE 1. Morphometric data of type material of Nemacheilus pezidion (n=11). Range and mean include holotype data.

holotype	range	mean
Standard length (mm)	57.8	46.6–57.8
Total length (mm)	76.7	59.6–76.7
In percent of standard length
Total length	132.8	126.7–133.4	130.1
Head length (dorsal)	18.7	18.0–20.9	19.4
Head length (lateral)	21.8	21.2–22.9	22.0
Predorsal length	47.9	45.2–48.2	46.9
Prepelvic length	48.6	47.6–49.4	48.7
Pre-anus length	69.4	64.2–69.4	67.7
Pre-anal length	76.9	74.6–77.6	76.3
Head depth	11.4	10.7–12.2	11.5
Body depth at dorsal-fin origin	15.4	15.0–19.0	16.9
Depth of caudal peduncle	10.0	9.3–10.5	9.9
Length of caudal peduncle	17.5	17.0–20.3	18.4
Head width	13.0	11.9–13.7	12.6
Body width at dorsal-fin origin	11.0	9.8–13.5	12.1
Snout length	8.6	8.1–8.6	8.3
Eye diameter	5.1	4.3–5.7	5.0
Interorbital width	5.6	5.6–6.9	6.3
Length of dorsal fin	20.4	19.5–22.4	20.5
Length of upper caudal-fin lobe	32.6	27.1–32.6	30.4
Length of median caudal-fin rays	15.3	13.6–16.6	15.1
Length of lower caudal-fin lobe	26.5	22.3–29.6	25.6
Length of anal fin	16.1	15.5–17.8	16.5
Length of pelvic fin	16.3	13.9–16.8	15.6
Length of pectoral fin	22.8	18.0–23.9	20.1
In percent of dorsal head length
Snout length	46	40–46	43
Eye diameter	27	22–29	26
Interorbital width	30	28–36	33
In percent of lateral head length
Snout length	40	36–40	38
Eye diameter	23	20–27	23
Interorbital width	26	24–32	29

Intestine with a loop shortly behind stomach. Air bladder without posterior chamber in abdominal cavity. Sexual dimorphism. Males distinguishable by presence (vs. absence) of a suborbital flap (modified lateral ethmoid; Fig. 4a ), with a few tiny tubercles at extremity and on inner surface. In males, dorsal surface of branched pectoral-fin rays 1–6 with densely-set tubercles on ray itself and on fleshy pad extending along its posterior margin (only behind rays 1–3); on last rays, tubercles restricted to a single row along proximal part of ray ( Fig. 2a ). In females, fin somewhat shorter, no tubercles ( Fig. 2b ). In males, a line of black pigments along anterior and posterior edge of branched ray 1 and behind branched rays 2 and 3; in large females, black pigments on branched ray 1 and on membranes between it and adjacent rays. Tubercles on first branched ray already visible in smallest available male (CMK 21315, 39.2 mm SL). In males, a patch of scales with minute tubercles on flank, longitudinally from about tip of pectoral fin to above anal-fin base, on lateral line scales, and on about 3 rows of scales above and about ⅔ of flank below lateral line ( Fig. 4b ); tubercles missing in females. Ripe females deeper bodied. FIGURE 2. Nemacheilus pezidion , pectoral fin in dorsal view of: a, male, MHNG 2787.088, holotype, 57.8 mm SL (note small tubercles on branched rays 1–6); and b, female, CMK 21246, 52.9 mm SL. FIGURE 3. Nemacheilus pezidion , CMK 21246, 52.9 mm SL; mouth. FIGURE 4. Nemacheilus pezidion , MHNG 2787.088, holotype, 57.8 mm SL, male: a, nostril and suborbital flap; and b, flank with small tubercles on scales. Coloration. Preserved: Upper half of head and body background colour pale olive brown, lower half yellowish; unless otherwise stated, markings dark brown to black. Two transverse lines in interorbital area and on posterior part of head. One spot between tip of snout and nostril. Two spots between nostrils. A midlateral stripe from base of outer rostral barbel to eye and to upper part of opercle. Body with a row of 11–15 narrow saddles along back (4–5 predorsal, 2–3 subdorsal, 5–7 postdorsal); saddles of regular width and shape, sometimes paired or with paler central area. A stripe from upper extremity of gill opening to end of caudal peduncle; anteriorly as a very regular band of pigments of same intensity, width about equal to eye diameter; posteriorly becoming narrower, less regular. Pattern at caudal-fin base: A conspicuous black spot at base of fin, slightly lower than middle, extending on lower 2 rays of upper lobe and upper 6 rays of lower lobe. Spot aligned with midlateral stripe, separated from it by narrow unpigmented patch, black, darker than rest of patterning on body. Dorsal fin hyaline, with: an elongate spot on lower ¼ of first branched ray and adjacent membranes; upper half of last unbranched ray black; 2 or 3 irregular rows of spots, usually at branching points of rays; membranes hyaline. Caudal fin hyaline with dusky membranes near base, or with a few spots on median rays, or 1 or 2 vague vertical rows of spots corresponding to pigments on rays approximately at branching points. Anal and pelvic fins hyaline. Pectoral fin hyaline, dark band on first and second branched rays (see more details under sexual dimorphism, above). Colour pattern of juvenile unknown. In life: yellowish, patterning dark brown; spot at caudal-fin base black, contrasting with pale area around it. Notes on biology. A dissected female (CMK 21330, 53.6 mm SL) had unripe ovaries with eggs about 0.6–0.7 mm diameter. Its stomach contained unidentifiable material (possibly mud). Distribution. Nemacheilus pezidion has been observed only in the Xe Pian, a tributary of Xe Kong, flowing on the southern slope of the Bolaven plateau ( Fig. 5 ). All localities are in the floodplain; at the time of collecting (end of dry season–beginning of wet season), the water of the Xe Pian was turbid at the sampling sites, with the bottom varying from gravel to mud ( Fig. 6 ). FIGURE 5. Southern Laos, showing localities of examined material of Nemacheilus pezidion (circles) and ‘N’. argyrogaster (triangles). Each symbol may represent several localities. White circle is type locality of N. pezidion . Dams, reservoirs and other alterations of original hydrographic network are not shown. Etymology. From the ancient Greek πεζίΔΙΟΝ (pezidion), a ribbon, reference to the black stripe along the flank. A noun in apposition. Remarks. As mentioned in the diagnosis, N. pezidion is distinguished from most other species of Nemacheilidae in Southeast Asia in having an obvious dark brown to black midlateral stripe. A somewhat similar colour pattern is observed in N. binotatus ( Fig. 7 ) known from the Chao Phraya and Mae Khlong drainages in Thailand . The two species are distinguished by a forked caudal fin with the upper lobe distinctly longer than the lower one in N. pezidion (vs. subequal in N. binotatus ), the pectoral-fin rays with filamentous extensions (vs. without extensions), the mid-dorsal series of saddles (vs. a black stripe from top of head to caudal-fin base), the presence of an elongate spot on lower ¼ of first branched dorsal-fin ray and adjacent membranes (vs. absence) and the absence of small vertically elongated spots above and in contact with the midlateral stripe (vs. presence of one, rarely two or three, spots at about level of the tip of the pectoral fin). Further, male N. pezidion have a modified pectoral fin with conspicuous tubercles on anterior rays and wide unculiferous pads (vs. minute tubercles, narrow unculiferous pads), and numerous small tubercles on the middle and posterior part of the flank of large males (vs. without tubercles). Data on N. binotatus are from Kottelat (1990) and re-examination of specimens cited therein. FIGURE 6. Laos: Xe Kong watershed: Houai Pin, tributary of Xe Pian, entering it about 5 km downstream of Ban Mai, 21 May 2009, locality of Nemacheilus pezidion , CMK 21330. FIGURE 7. Nemacheilus binotatus , CMK 16044, 34.6 mm SL; Thailand: Chao Phraya drainage: Nan River. Nemacheilus pezidion had at first been misidentified as N. longistriatus ( Fig. 8 ), a species known from the Mekong drainage upstream of the Khone falls, because of the presence of the midlateral stripe ( Kottelat, 2009 ). The two species are distinguished by a forked caudal fin with the upper lobe distinctly longer than the lower one in N. pezidion (vs. subequal in N. binotatus ), the pectoral-fin rays with filamentous extensions (vs. without extensions), the presence of an elongate spot on lower ¼ of first branched dorsal-fin ray and adjacent membranes (vs. absence). Further, male N. pezidion have a modified pectoral fin with conspicuous tubercles on anterior rays and wide unculiferous pads (vs. pectoral fins not modified, tubercles minute and few, on a single row along posterior margin of branched rays 1–3), and numerous small tubercles on the middle and posterior part of the flank of large males (vs. without tubercles), and the suborbital flap well developed (vs. rudimentary). The smallest observed specimen of N. pezidion is 39.2 mm SL and all specimens have the colour pattern described above of a quite regular stripe. In specimens of N. longistriatus the stripe is less regular and small specimens have small bars on the flank, connected with the saddles on the back; the bars then become fainter, and only remain as spots on the course of the lateral line, becoming connected by a midlateral stripe; vestiges of the spots persist, more or less distinct, until adult, when the stripe is irregular ( Fig. 8a ). Nemacheilus pezidion has up to now been recorded only from the Mekong drainage downstream of Khone falls. Data on N. longistriatus are from Kottelat (1990) and additional examined material. FIGURE 8. Nemacheilus longistriatus , Mekong drainage: a, CAS 62547, holotype, 53.2 mm SL; Thailand: Mekong at Chiang Khan; b, CMK 23085, 37.5 mm SL; Laos: Xe Bangfai. ‘Nemacheilus’ argyrogaster ( Fig. 9 ) is a second species from the Xe Kong watershed that had also been misidentified as N. longistriatus (e.g., Baird et al . 1999: 70 ; Rainboth et al . 2012 : pl. 33 fig. 685). When first collected, it was identified as a Nemacheilus , in fact not even distinguished from N. pezidion . Later examination showed them to be distinct, with the molecular study by Šlechtová et al. (2021) showing that, if included in Nemacheilus , the latter genus ceases to be monophyletic. It therefore falls within Schistura sensu lato and a more accurate generic placement will only be possible after relationships within Schistura s.l. are better understood; Schistura has long been known to be a polyphyletic catchall for Southeast and South Asian nemacheilids (e.g., Kottelat, 1990 ; Sember et al ., 2015 ; Freyhof et al ., 2016 ; Chen et al ., 2019 ). The two species are distinguished by a forked caudal fin with the upper lobe distinctly longer than the lower one in N. pezidion (vs. subequal in ‘N’. argyrogaster ), the pectoralfin rays with filamentous extensions (vs. without extensions), the presence of an elongate spot on lower ¼ of first branched dorsal-fin ray and adjacent membranes (vs. absence), and the flank colour about the same above and below the midlateral stripe (vs. the flank above the midlateral stripe being yellowish brown, pale yellow to silvery below the stripe). Further, large male N. pezidion have numerous small tubercles on the middle and posterior part of the flank (vs. without tubercles). FIGURE 9. ‘Nemacheilus’ argyrogaster , MHNG 2785.032, holotype, 63.0 mm SL; Laos: Mekong drainage: Xe Kong watershed. Based on the present data, N. pezidion and ‘N.’ argyrogaster have allopatric distributions, with N. pezidion observed only in the Xe Pian (a tributary of Xe Kong), and ‘N.’ argyrogaster observed only in the Xe Kong proper and in tributaries upriver of Attapeu ( Fig. 5 ); this may, however, be only a sampling artefact, related to different habitats targeted by different components of the surveys. At the time ( 20–22 May 1999 and 2009) and sites of sampling, N. pezidion was in flowing turbid water over mostly gravel to mud bottom, in low gradient streams in the flood plain. Specimens of ‘N’. argyrogaster were collected during the same period ( 20–28 May 1999 and 2009), in streams with faster gradient, in rapids, with substrate comprised of stones to boulders. [CMK 15687, types of N. pezidion , were erroneously listed as ‘N’. argyrogaster in Kottelat, 2021: 277 ; this removes the only record of ‘N.’ argyrogaster from the Xe Pian]. Nemacheilus nandingensis Zhu & Wang , described from the Salween drainage in Yunnan , also has a yellowish to whitish body with a conspicuous black midlateral stripe and a middorsal series of 16–19 squarish saddles ( Zhu & Wang, 1985 ). Nemacheilus nandingensis was treated as a synonym of Pteronemacheilus meridionalis (Zhu) by Yang (in Chu & Chen 1990: 49 ), as a valid species of Schistura by Zhu (1989: 54) , Kottelat (2012: 113) and Chen (2013: 296) , and as a valid species of Nemacheilus by Kottelat (2021: 285) . No topotypical specimen could be examined. A species apparently similar to N. nandingensis , or the same species, is present in the Mekong drainage in Xishuangbanna ( Yunnan ) ( Fig. 10 ); the single available specimen, a male, has a small triangular suborbital flap and a pectoral fin with no tubercles (the first branched ray is thickened and its branches are not secondarily branched). The two species are distinguished by a forked caudal fin with the upper lobe distinctly longer than the lower one in N. pezidion (vs. subequal in N. nandingensis ), the pectoral-fin rays with filamentous extensions (vs. without extensions), the presence of an elongate spot on the lower ¼ of the first branched dorsal-fin ray and adjacent membranes (vs. absence), and a thinner midlateral stripe, wider anteriorly, where clearly less than eye diameter (vs. width uniform from head to caudal-fin base, about equal to eye diameter). Further, male N. pezidion have a modified pectoral fin with conspicuous tubercles on the anterior rays and wide unculiferous pads (vs. without), in addition to numerous small tubercles on the middle and posterior part of the flank of large males (vs. without tubercles). The mouth of N. nandingensis has the characters of Nemacheilus (see Kottelat, 1990 ) and definitively neither the conspicuous median interruption in the lower lip nor the deep sulci in its median parts typical of Schistura s.l. This is possibly the species identified as N. longistriatus by Lin et al . (2021: 873) from the Mekong River mainstream at Menghan, Yunnan , China . Their pictures definitively do not depict N. longistriatus (compare their figure 2 with Fig. 10 of the present article); the suboptimal fixation of their specimen does not allow a more detailed discussion. FIGURE 10 Nemacheilus aff. nandingensis , CMK 23949, 56.6 mm SL; China: Xishuangbanna: Mekong drainage: Nam Xing. Schistura rubrimaculata Bohlen & Šlechtová (from the Arakan range in Rakhine , Myanmar ) and S. pawensis Bohlen & Šlechtová (from Irrawaddy drainage in Myanmar ) too, have a colour pattern that includes a midlateral stripe and a pale belly, but the back is dark brown, with males lacking modified pectoral-fin rays. Besides, S. rubrimaculata has up to 6 saddles on the back and a maximum known size of 28 mm SL, while S. pawensis has 6–7 weakly marked saddles on the back, no suborbital flap, and 7–8 + 7 branched caudal-fin rays (data from Bohlen & Šlechtová, 2013a ). Nemacheilus pezidion was tentatively identified as N. aff. pallidus by Kottelat (2021: 285) . In addition to the presence of the bold black midlateral stripe (vs. a midlateral row of 17–19 blackish spots and a middorsal row of 14–17 narrow saddles), N. pezidion is distinguished from N. pallidus (s.s., from Chao Phraya drainage; Fig. 11 ) by a more slender caudal peduncle (length 17.0–20.3 % SL vs 14.1–16.3; depth 9.3–10.5 % SL vs. 11.6–12.4, 1.7–2.0 (vs. 1.2–1.4) times in its length, and 1.5–1.9 (vs 1.4–1.6) times in depth at dorsal-fin origin). The fish widespread in the Mekong drainage (from at least the Xe Banghiang watershed northwards to at least the Nam Ngum watershed) identified as N. pallidus by Kottelat (1990: 63 , fig. 36b, 1998: 69, 2001: 100), is an unnamed species whose description is in preparation. FIGURE 11. Nemacheilus pallidus , ZRC 38468, holotype, 54.0 mm SL; Thailand: Mae Nam Yom. Comparison material. List not exhaustive. Nemacheilus binotatus : CMK 1768 , 6, 32.8–39.8 mm SL; Thailand : Chiang Mai Province : Mae Nam Ping in Chiang Dao gorges . N. longistriatus : CMK 5402, 4 paratypes , 36.0– 46.5 mm SL; Thailand : Loei Province : Mekong mainstream from Chiang Khan to 70 km downstream.— CMK 13704, 22 , 30.6–38.8 mm SL ; Laos : Savannakhet Province : Xe Bang Hiang , about 6 km downstream of Xepon. — CMK 13777, 7 , 32.4–37.3 mm SL ; Laos : Savannakhet Province : Xe Bang Hiang at Ban Tat Hai Xe. — CMK 15950, 1 , 67.2 mm SL ; Laos : Bolikhamsai Province : Nam Xao , a tributary of Nam Ngiep. — CMK 19312, 7 , 29.8-36.5 mm SL ; Laos : Khammouan Province : Xe Bangfai at Keng Mou Man. — CMK 23085, 56 , 29.7–46.6 mm SL ; Laos : Savannakhet Province : Xe Bangfai drainage: Xe Noy at Keng Boua . N. nandingensis : CMK 23949, 1, 56.6 mm SL; China : Yunnan : Xishuangbanna : Nam Xing River . N. zonatus : CMK 19373, 13, 25.1–30.2 mm SL; Laos : Khammouan Province : Xe Bangfai at Ban Tha Ponsaoe . N. pallidus : CMK 4835, 7 paratypes , 34.9–50.0 mm SL; Thailand : Lampang Province : Mae Nam Yom drainage: Huai Mae Phlung near Ban Phong. N. masyae : CMK 5180, 7, 48.1–65.8 mm SL; Thailand : Surat Thani Province : Tapi River drainage: Khlong Sok.— CMK 5165 , 1 , 61.5 mm SL ; Thailand : Surat Thani Province : Khlong Phum Duang, upstream of Phunphin . ‘N.’ argyrogaster : MHNG 2785.032 , holotype , 63.0 mm SL ; CMK 21663, 26 , 45.3–76.9 mm SL; ZRC 61628, 6 , 45.6–61.8 mm SL; Laos : Mekong drainage: Xe Kong watershed : Xe Kong Prov. : confluence of Xe Kong and Houai Peung , downstream of Ban Kaleum .