Nemacheilus argyrogaster, Kottelat, 2021

Kottelat, Maurice, 2021, ‘ Nemacheilus’ argyrogaster, a new species of loach from southern Laos (Teleostei: Nemacheilidae), Zootaxa 4933 (2), pp. 277-288 : 278-287

‘Nemacheilus’ argyrogaster , new species

( Figs. 1–2 )

Holotype. MHNG 2785.032 , 63.0 mm SL; Laos: Mekong drainage: Xe Kong watershed : Xe Kong Prov. : confluence of Xe Kong and Houai Peung, downstream of Ban Kaleum, 15°42’54’’N 106°48’9”E; M. Kottelat et al., 28 May 2009.

Paratypes. CMK 21663, 26 (7 males 45.3–55.0 mm SL, 18 females 49.8–76.9 mm SL, 1 fixed in 95% ethanol) ; ZRC 61628, 6 , 45.6–61.8 mm SL; same data as holotype .— CMK 21684, 3 , 52.8–66.5 mm SL; Xe Kong Prov. : Xe Kong drainage: Houai Peung, about 100 m upstream of confluence with Xe Kong, downstream of Ban Kaleum, 15°42’56’’N 106°48’16”E; M. Kottelat et al., 28 May 2009 .

Additional materal (non-types). All from Laos: Mekong drainage: Xe Kong watershed. CMK 15576, 7 , 37.0– 45.6 mm SL; Attapeu Prov.: Xe Kong between Attapeu and downstream to Ban Ouk , 14°44’51’’N 106°43’59”E; M. Kottelat et al., 20 May 1999 .— CMK 15616, 1 , 57.3 mm SL; Attapeu Prov.: unnamed creek entering Xe Kaman from the north at Xe Kaman dam site, 14°57’40’’N 107°9’16”E; M. Kottelat et al., 21 May 1999 .— CMK 15650, 6 , 40.6 –63.0 mm SL; Attapeu Prov.: rapids on Xe Kaman , 14°53’29’’N 107°7’20”E; M. Kottelat et al., 21 May 1999 .— CMK 15672, 15 , 41.4–59.4 mm SL; Attapeu Prov.: Xe Kaman at Muang Xaisettha , 14°48’27’’N 106°55’52”E; M. Kottelat et al., 22 May 1999 .— CMK 15687, 3 , 48.5–58.1 mm SL; Attapeu Prov.: Xe Pian at Ban Mai , 14°42’22’’N 106°29’46”E; M. Kottelat et al., 22 May 1999 .— CMK 15723, 6 , 47.0– 60.4 mm SL; Xe Kong Prov.: Xe Kong at Keng Luang , 15°26’24’’N 106°43’39”E; M. Kottelat et al., 24 May 1999 .— CMK 18996, 2 , fixed in 95 % ethanol; same data as CMK 15723 .— CMK 15775, 2 , 36.5–42.1 mm SL; Xe Kong Prov.: Xe Namnoy, rapids about 1 km upriver of Tad Feak waterfall; 135 masl, 15°14’9’’N 106°44’55”E; M. Kottelat et al., 25 May 1999 .— CMK 15819, 6 , 44.9–61.7 mm SL; Xe Kong Prov.: Houay Pao, a west side tributary of Xe Kong , entering it about 16 km upstream of Muang Kaleum , 15°50’17’’N 106°45’40”E; M. Kottelat et al., 26 May 1999 .— CMK 19006, 1 , fixed in 95 % ethanol; same data as CMK 15819 .— CMK 21455, 1 , 41.2 mm SL; Attapeu Prov.: Xe Kong, riffles in gravel banks at Ban Khanmaknao , 14°36’46’’N 106°33’13”E; M. Kottelat et al., 22 May 2009 .— CMK 21521, 31 , 39.5–58.5 mm SL; 3, fixed in 95 % ethanol; Xe Kong Prov.: Xe Nam Noy immediately below Taad Fek waterfall, downstream of Ban Dan ; 115 masl, 15°14’42’’N 106°45’7”E; M. Kottelat et al., 25 May 2009 .— CMK 21570, 1 , 48.0 mm SL; Xe Kong Prov.: Xe Kong at Keng Chang rapids, near Ban Song Khone , 15°30’17’’N 106°46’2”E; M. Kottelat et al., 26 May 2009 .

Diagnosis. ‘Nemacheilus’ argyrogaster is distinguished from most other species of Nemacheilidae in Southeast Asia by having an obvious bold black midlateral stripe separating the yellowish brown dorsal part of the body from the silvery whitish ventral part. A somewhat similar pattern is present only in N. binotatus , N. longistriatus , Schistura nandingensis , S. rubrimaculata and S. pawensis , from which it is distinguished by having male with a globulous suborbital flap with tubercles along its free, posterior edge; male with pectoral fin with thickened anterior rays, and branched rays 1–4 and the unculiferous pads behind them covered with small tubercles; lips thin, lower lip with narrow median notch; and 9+8 branched caudal-fin rays.

Description. See Figures 1–2 for general appearance and Table 1 for morphometric data of holotype and 11 paratypes. A moderately elongate nemacheilid with body depth gradually increasing up to slightly in front of dorsalfin origin. Behind dorsal fin, body depth decreasing gradually to end of anal-fin base, then almost uniform until caudal-fin base. Dorsal profile with slight concavity between head and body. Head slightly depressed; body slightly compressed anteriorly, to compressed posteriorly. Interorbital area convex. In lateral view, eye flush with dorsal profile of head. Cheeks not swollen. Snout rounded. Depth of caudal peduncle 1.3–1.6 times in its length, depth almost uniform. Very low dorsal and ventral keels on posterior half of caudal peduncle. Dorsal keel continuous with upper margin of caudal fin. Largest recorded size 76.9 mm SL for female, 55.0 mm SL for male.

Dorsal fin with 4 unbranched and 8½ branched rays; distal margin slightly concave. First branched ray longest. Pectoral fin with 1 unbranched and 10 (2), 11 (9) or 12* (1) branched rays (including small last ray, usually unbranched), rounded, reaching about halfway of distance to pelvic-fin base; rays without filamentous extensions. See below for sexual dimorphism. No axillary pectoral lobe. Pelvic fin with 1 unbranched and 7* (11) or 8 (1) branched rays (including small last ray, usually unbranched); reaching about to or slightly beyond anus, halfway to anal-fin origin; triangular, posterior margin rounded; origin below base of last unbranched dorsal-fin ray to base of 2nd branched ray; axillary lobe present, entirely free, conspicuous. Anus situated about 1.8–2.4 eye diameter in front of anal-fin origin, about at extremity of pelvic fin. Anal fin with 3 unbranched and 5½ branched rays; distal margin straight. Caudal fin with 9+8 branched rays, forked, tip of lobes rounded, subequal, length of upper lobe about 1.5–1.7 times length of median rays.

Body entirely covered by scales. Scales embedded. Lateral line almost complete, with 88–93 pores, missing on last 3–8 scales. Cephalic lateral line system with 6 supraorbital, 4 + 10–11 infraorbital, 9 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 1.5–2 times wider than long ( Fig. 3a ). Lips thin. Upper lip with median notch, with a few wrinkles (especially near corner of mouth). Lower lip continuous, with narrow median notch; median part with 3–7 shallow and narrow sulci, with a few wrinkles at angle of mouth. Processus dentiformis present. Tip of lower jaw not exposed. A shallow median concavity in lower jaw. Inner rostral barbel reaching beyond corner of mouth, almost vertical of anterior margin of eye; outer one reaching below anterior half of eye. Maxillary barbel reaching at most beyond posterior margin of eye. Intestine with a loop behind stomach ( Fig. 3b ). Air bladder without posterior chamber in abdominal cavity. Two halves of bony capsule of anterior chamber of air bladder connected by a short manubrium.

Sexual dimorphism. Males with suborbital flap, swollen, globulous, with tubercles along free posterior edge ( Fig. 4a ). Pectoral fin of male with modified rays and covered by small tubercles on rays and interradial membranes ( Fig. 4b ). Unbranched and first branched rays thickened. Branched rays 1–4 extended posteriorly over dorsal surface of membrane by a thickened unculiferous pad (sensu Conway et al. 2012); rays and unculiferous pads covered with densely set tubercles. Tubercles gradually less numerous and pads gradually narrower on posterior rays, only a small row of smaller tubercles along posterior edge of ray 5 and behind. Branches of branched rays with secondary branching near tip, missing on anterior branch of first ray. Membranes between branched rays at most as wide as width of branch; secondary branches adjacent, without membranes. Smallest male with distinct modification of pectoral-fin rays 36.5 mm SL. Pectoral fin of female without widened ray, membranes wider, secondary branches located more proximally than in male, with membranes between secondary branches. Ripe females deeper bodied.

Coloration. After one month in formalin. Head and body background colour: dorsal half pale yellowish brown, ventral half whitish. A black midlateral stripe from tip of snout through eye to upper extremity of gill opening, then along lateral line to middle of caudal-fin base, ending in a more intense black mark slightly below middle of base of fin, on base of lower rays of upper lobe and three upper rays of lower lobe. A little blackish spot at base of unbranched ray of lower caudal-fin lobe. Faint grey margin on end of caudal peduncle. Narrow black saddles on back, usually regularly shaped; in some specimens irregularly shaped and set or even dissociated into pairs of spots; total 14–19 saddles, 5–6 in front of dorsal-fin origin, 4–6 along dorsal-fin base and 4–7 behind dorsal fin, last one as a short stripe extending posteriorly to base of unbranched rays of upper lobe of caudal-fin.

All fins hyaline, with a few black pigments on membranes between branches near branching points and on rays (on edges and between segments) near primary and secondary branching points. Dorsal fin with a small patch of blackish pigments at base of each ray. In pectoral fin, pigments denser along posterior edge of unbranched ray and branched rays 1–4, appearing as thin stripes.

In life: body pale yellowish brown above black midlateral stripe, contrasting with silvery white below midlateral stripe.

Notes on biology. A dissected female (CMK 21521, 57.2 mm SL) had ripe ovaries with eggs 0.8–0.9 mm diameter. ‘Nemacheilus’ argyrogaster was collected in stretches of streams with moderate to fast current, with pebble to stone bottom ( Fig. 5 ). Females reach a larger size than males, up to 76.9 mm SL. The largest examined male is 55.0 mm SL and the smallest with developped suborbital flap and modified pectoral-fin rays is 36.5 mm SL. In the two large samples examined (MHNG 2785.032, CMK 21663, ZRC 61628, n=32; CMK 21521, n=34), the adult male: female sex ratio is about 2:5.

At 5 of 13 sampling sites, ‘N.’ argyrogaster was collected in syntopy with the botiid Ambastaia nigrolineata (Kottelat & Chu) ( Fig. 6 ), whose juveniles have a colour pattern partly similar to that of ‘N.’ argyrogaster , composed of black midlateral and middorsal stripes.

Distribution. ‘Nemacheilus’ argyrogaster is presently known only from the Xe Kong watershed in the Mekong drainage in Laos. The species is expected to be present also in Cambodia, at least in Srepok and Sesan watersheds.

Etymology. From the Greek ἀργύρΕΙΟΣ (argyreios, silver) and γαστήρ (gaster, belly), reference to the striking silvery white belly in life. A noun in apposition.

Remarks. As mentioned in the diagnosis, ‘N.’ argyrogaster is distinguished from most other species of Nemacheilidae in Southeast Asia in having an obvious bold black midlateral stripe separating the yellowish brown dorsal part of the body from the silvery whitish ventral part. A somewhat similar colour pattern is observed in N. binotatus ( Fig. 7 ) known from the Chao Phraya and Mae Khlong drainages. The two species are distinguished by the presence in males ‘N.’ argyrogaster of modified pectoral fins with conspicuous tubercles on anterior rays and wide unculiferous pads (vs. minute tubercles, narrow unculiferous pads), the mid-dorsal series of saddles (vs. a black stripe from top of head to caudal-fin base), 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). Data on N. binotatus are from Kottelat (1990) and re-examination of specimens cited therein.

‘Nemacheilus’ argyrogaster 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 (e.g., Baird et al. 1999: 70; Rainboth et al. 2012: pl. 33 fig. 685). The two species are distinguished by the presence in males ‘N.’ argyrogaster of modified pectoral fins with conspicuous tubercles on and behind anterior branched rays (vs. pectoral fins not modified, tubercles minute and few, on a single row along posterior margin of branched rays 1–3), suborbital flap well developed (vs. rudimentary), the midlateral stripe very regular and present even in the smallest specimens (vs. small specimens [below about 35 mm SL] with small bars on the flank, connected with the saddles on the back; the bars then become fainter, restricted to course of lateral line, and connected by a midlateral stripe; vestiges of the bars persist and the stripe is irregular). Further, the general appearance is more compact in ‘N.’ argyrogaster (compare figs. 1–2 and 8) and it has a somewhat smaller eye (eye diameter 21–25 % of dorsal head length, vs. 24–29). ‘Nemacheilus’ argyrogaster is recorded (at least up to now) only from the Mekong drainage downstream of Khone falls. Data on N. longistriatus are from Kottelat (1990) and additional examined material.

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). No topotypical specimen could be examined. From the original description, it has the anus closer to the anal-fin origin (about 1 eye diameter, vs. 1.8–2.4 in ‘N.’ argyrogaster ) and consequently the pelvic fin does not reach the anus (vs. reaches). Nemacheilus nandingensis was treated as a synonym of Pteronemacheilus meridionalis (Zhu) by Yang (in Chu & Chen 1990: 49) and as a valid species of Schistura by Zhu (1989: 54), Kottelat (2012: 113) and Chen (2013: 296). A species apparently similar to N. nandingensis , or the same species, is present in the Mekong drainage in Xishuangbanna (Yunnan) ( Fig. 9 ); 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 anus is positioned about 2 eye diameters in front of the anal-fin origin and the mouth has the characters of Nemacheilus (see below) and definitively neither the conspicuous median interruption in the lower lip nor the deep sulci in its median parts typical of Schistura s.l. I consider it to be a species of Nemacheilus .

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 and the males lack modified pectoral-fin rays. Besides, S. rubrimaculata has up to 6 saddles on the back and a maximum known size of 28 mm SL, and S. pawensis has 6–7 weakly marked saddles on the back, no suborbital flap, 7–8 + 7 branched caudal-fin rays (data from Bohlen & Šlechtová, 2013).

Specimens of a Nemacheilus superficially similar to N. masyae Smith and N. pallidus Kottelat , ( Fig. 10 ) were also collected in the Xe Kong watershed and confused in the field with ‘N.’ argyrogaster . The two are sympatric but not syntopic. Nemacheilus aff. pallidus was observed only in the Xe Pian (a tributary of Xe Kong). It too, has a midlateral stripe, thinner than in ‘N.’ argyrogaster , including in juveniles, without indication that it would be derived from a row of coalescent blotches; there is a conspicuous black spot at the base of the caudal fin, a black spot on the anteriormost branched rays of the dorsal fin at about ¼ of their length, and rows of spots on the dorsal and caudal fins; the upper lobe of the caudal fin is distinctly longer than the lower one, the pectoral-fin rays have long projections, and there are numerous small tubercles on the middle and posterior part of the body. Males also have a suborbital flap, with a few minute tubercles, and there are a few minute tubercles along the anterior 2 branched pectoral-fin rays.

It is not clear to which species these specimens belong, and in fact they may represent a distinct, unnamed species. There is variability in both N. masyae and N. pallidus as described by Kottelat (1990: 55, 63), who identified all his material from the Mekong drainage as N. pallidus . Since then, numerous and better preserved specimens have become available. This material allows us to better understand variability within the two species. It appears that N. pallidus is restricted to the Chao Phraya drainage; the specimens from the Mekong drainage in Kottelat (1990 [including fig. 36b from Nam Mun], 2001 [fig. 257 shows a fish from Nam Mang]) are not N. pallidus (in prep.).

Page et al. (2020) treated N. pallidus as a synonym of N. masyae , based on their interpretation of the colour pattern and morphometric characters that had been considered diagnostic by Kottelat (1990), supplemented by a molecular analysis. Because they did not see differences in morphometry and because their interpretation of elements of colour pattern placed the sequenced specimens of their N. pallidus into their N. masyae clade, making it non-monophyletic, they concluded that N. pallidus is a junior synonym of N. masyae . However, their morphometric and colour pattern data are not presented in a way that allows comparison by colour pattern or by geographic origin, or with the data presented in Kottelat (1990). The morphometric data of all their N. pallidus and N. masyae are lumped in Table 2 or divided into two groups in a PCA, and there is no discussion of colour pattern variability, except for the statement “the putative differences [...] do not hold up when specimens are examined from across the range of the putative species ( Fig. 7 )” (p. 399). In fact, their figure 7 shows material only from peninsular Thailand, the Mae Khlong drainage and the upper Chao Phraya drainage, which is only a fraction of the range; it does not show specimens from the Mekong drainage and south-eastern Thailand. Further, the usefulness of the tree is limited because the geographical origin of some samples is not mentioned in the list of examined material.

For these reasons, I am unable to follow the conclusions of Page et al. (2020). Their analysis may show that the characters listed to diagnose N. pallidus and N. masyae are insufficient (or not detailed enough), but it shows neither that the two are synonyms, nor that a single species is involved. There are differences in colour pattern between specimens of N. pallidus and N. masyae in the figures in Kottelat (1990: figs. 29a, 36) and, despite their statement, Page et al.’s (2020) figure 7d of a topotype of N. pallidus shows a fish very distinct from their N. masyae from Peninsular Thailand (their figures 7a–b) and Mae Khlong drainage (their figure 7c) (compare size, shape, position and numbers of bars and saddles, and body shape). As mentioned above, N. pallidus from the Mekong in Kottelat (1990: fig. 36b) is misidentified.

Taking into account the variability in N. masyae and N. pallidus as understood in Kottelat (1990) and mentioned above, and the non-monophyly retrieved in Page et al.’s tree, the hypothesis that more than two species are involved should have been investigated. Preliminary observations (unpublished) suggest that N. pallidus is restricted to the Chao Phraya drainage (widely spaced narrow saddles, midlateral row of spots, stout caudal peduncle), that N. masyae is possibly restricted to the Malay Peninsula (bolder saddles and midlateral blotches, slenderer caudal peduncle), that the populations from the Mae Khlong are potentially distinct from both N. pallidus and N. masyae and possibly close to populations from south-eastern Thailand (midlateral row of blotches fusing into a stripe with growth or age), and that 2 or 3 species are present in the Mekong drainage.

Diagnostic characters of Nemacheilus sensu Kottelat (1990: 43) are the forked to deeply forked caudal fin; the lips thin, usually smooth, the lower lip continuous along its anterior margin or with a slight median incision; the suborbital flap usually present; and, in most species, the pale background body colour. Kottelat (1990: 43) identified three groups of species in Nemacheilus and a few species that could not be placed in any group. While ‘N.’ argyrogaster shares the characters diagnostic of Nemacheilus , it does not fit in any of these groups. It lacks the fimbriate posterior margin of the pectoral fin and the tubercles on scales of body seen in group A (e.g., N. masyae , N. pallidus ); it lacks the acuminate scales on the caudal peduncle and lower lip with deep folds and sulci of group B (e.g., N. selangoricus , N. spiniferus ); and it has pectoral-fin modifications unknown in group C (e.g., N. binotatus ). This suggests that ‘N.’ argyrogaster might not be closely related to the other Nemacheilus , for which reason it is only tentatively placed in that genus.

Comparison material. List not comprehensive. 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 between 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 small 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. cf. pallidus: CMK 21246, 21, 46.7–53.8 mm SL; Attapeu Prov.: Xe Pian , about 2 km upstream of Ban Mai.— CMK 21280, 3 , 43.7–47.1 mm SL ; Attapeu Prov.: Xe Pian , about 3 km downstream of Ban Mai.— CMK 21330, 14 , 40.1–54.8 mm SL ; Attapeu Prov.: Houai Pin, a tributary of Xe Pian, entering it about 5 km downstream of Ban Mai.

N. zonatus: CMK 19373, 13, 25.1–30.2 mm SL; Laos: Khammouan Province: Xe Bangfai at Ban Tha Ponsaoe .

Figures

FIGURE 1. ‘Nemacheilus’ argyrogaster, MHNG 2785.032, holotype, 63.0 mm SL; Laos: Mekong drainage: Xe Kong watershed.

FIGURE 2. ‘Nemacheilus’ argyrogaster, Laos: Mekong drainage: Xe Kong watershed. a, CMK 15650, 48.0 mm SL; b, CMK 15672, 47.5 mm SL; c, CMK 15819, 62.1 mm.

FIGURE 3. Nemacheius argyrogaster, CMK 21663; a, 60.5 mm SL; mouth; b, 52.5 mm SL; digestive tract.

FIGURE 4. Nemacheius argyrogaster, CMK 21663, 49.4 mm SL, male sexual dimorphism; a, suborbital flap; b, left pectoral fin, dorsal view.

FIGURE 5. Laos: Xe Kong province. a, confluence of Xe Kong river and Houai Pheung stream, type locality of ‘Nemacheilus’ argyrogaster, CMK 21663; b, Houai Pheung stream about 100 m upstream of confluence with Xe Kong, locality of CMK 21684. 26 fish species were collected with N. argyrogaster at both sites.

FIGURE 6. Ambastaia nigrolineata, CMK 27801, 34.0 mm SL, juvenile; Laos: Mekong drainage: Nam Gnouang.

FIGURE 7. Nemacheilus binotatus, CMK 16044, 34.6 mm SL; Thailand: Chao Phraya drainage: Nan River.

FIGURE 8. Nemacheilus longistriatus, Mekong drainage: a, CAS 62547, holotype, 53.2 mm SL; Thailand: Mekong at Chiang Khan; b, CMK 5402, paratype, 45.1 mm SL; Thailand: Mekong at Chiang Khan; c, CMK 23085, 37.5 mm SL; Laos: Xe Bangfai.

FIGURE 9. Nemacheilus aff. nandingensis, CMK 23949, 56.6 mm SL; China: Xishuangbanna: Mekong drainage: Nam Xing.

FIGURE 10. Nemacheilus aff. pallidus, CMK 15687, 53.5 mm SL; Laos: Mekong drainage: Xe Kong watershed, Xe Pian.