Glyptothorax intonsus: a new species of rheophilic catfish (Siluriformes: Sisoridae) from the Mekong River drainage in Laos

Abstract

We describe Glyptothorax intonsus, a new species of sisorid catfish from the Nam Mang watershed in the Mekong River drainage of Laos. The new species is distinguished from all Mekong drainage Glyptothorax by a combination of its very long nasal barbels, head and body proportions, features of the thoracic adhesive apparatus and lateral line, colouration and body tuberculation.

1. INTRODUCTION

Glyptothorax Blyth 1860 is one of the most speciose Asian catfish genera (with 148 species as of early 2026; unpublished data), comprising small‐ to medium‐sized sisorids primarily found in fast‐flowing hill streams and torrential rivers (Ng & Kottelat, 2016). These rheophilic catfishes are externally distinguished by their tuberculate skin, maxillary barbels posteriorly subtended by a broad skin flap and a generally ovoid thoracic adhesive apparatus (TAA) with longitudinally oriented striae. The geographic range of Glyptothorax extends from the rivers of Asia Minor eastward across the Indian subcontinent into Sundaic Southeast Asia and eastern China, featuring many geographically restricted taxa confined to no more than one or two adjacent river drainages (Ng & Kottelat, 2016).

The Mekong River drainage is one of the world's largest transboundary river systems and supports one of the most diverse freshwater fish assemblages globally (with ~900 species; Rainboth et al., 2012). High endemism in the Mekong's benthic hillstream fishes (90% of the estimated 150 benthic hillstream species known from the Mekong River drainage are found nowhere else; unpublished data), together with the river's extensive habitat heterogeneity and complex geological history, strongly suggests the Mekong drainage to be a major diversification centre for Glyptothorax. Currently, 13 Glyptothorax species are known from the Mekong River drainage (Ng, 2025; Ng & Kottelat, 2025): Glyptothorax carnatus Ng & Kottelat 2025; Glyptothorax deqinensis Mo & Chu 1986; Glyptothorax filicatus Ng & Freyhof 2008; Glyptothorax forabilis Ng & Kottelat 2017; Glyptothorax horai (Fowler 1934); Glyptothorax irroratus Ng & Kottelat 2023; Glyptothorax laosensis Fowler 1934; Glyptothorax longinema Li 1984; Glyptothorax macromaculatus Li 1984; Glyptothorax cf. platypogonides; Glyptothorax porrectus Ng & Kottelat 2017; Glyptothorax rhadinus Ng & Kottelat 2025; and Glyptothorax sulculus Ng 2025. Despite the recent increase in Glyptothorax species diversity reported from the Mekong drainage (Ng, 2025; Ng & Kottelat, 2017, 2023, 2025), the results of ongoing ichthyofaunal surveys point to substantial undocumented Glyptothorax diversity.

During surveys conducted in the Mekong drainage in Laos (Kottelat, 2000, 2001), specimens of an unnamed Glyptothorax species that exhibit a unique combination of morphological characters distinguishing them from all previously described congeners were collected. The aim of this study is to provide a formal taxonomic description of this species as Glyptothorax intonsus, new species, and to compare it with morphologically similar congeners.

2. MATERIALS AND METHODS

Specimens were collected using push nets and seines, killed using clove oil and fixed with 10% formalin. Measurements were taken point to point using digital callipers, and data were recorded to 0.1 mm. Where feasible, all features were measured or counted on the left side of specimens following the methods of Ng and Kottelat (2013), with the TAA length measured as the straight‐line distance from the anterior apex to the posteriormost point of the posteriormost stria along the midline and TAA width measured as the maximum transverse distance perpendicular to the longitudinal axis of the TAA. Measurements taken within the head are expressed as ratios of head length (HL). Head length and measurements of body parts are reported as ratios of standard length (SL). Asterisks after meristic values indicate the condition for holotype. Nomenclature for components of the TTA is as described by Ng and Kottelat (2016). Museum abbreviations follow Sabaj (2020).

3. RESULTS

3.1. Glyptothorax intonsus, new species

Figures 1, 2, 3 and 5; Table 1.

FIGURE 1.

Glyptothorax intonsus, holotype, MHNG 2814.066, 75.1 mm SL; Laos: Nam Mang watershed, dorsal, lateral and ventral views (in preserved condition).

FIGURE 2.

Schematic illustration of TAAs (thoracic adhesive apparatus) showing overall shape and pattern of striae for (a) Glyptothorax intonsus [MHNG 2814.066, holotype, 75.1 mm SL]; (b) Glyptothorax deqinensis (BMNH 1987.9.17.5, paratype, 74.0 mm SL); (c) Glyptothorax longinema (KIZ 741097, holotype, 67.0 mm SL). Images not to scale.

FIGURE 3.

Glyptothorax intonsus, holotype, MHNG 2814.066, 75.1 mm SL; Laos: Nam Mang watershed, lateral view showing colouration soon after capture.

FIGURE 5.

Schematic illustration of TAAs (thoracic adhesive apparatus) showing overall shape and pattern of striae in Glyptothorax intonsus: (a) MHNG 2814.066, holotype, 75.1 mm SL; (b) CMK 13310, paratype, 80.1 mm SL. Images not to scale.

TABLE 1.

Morphometric data for Glyptothorax intonsus (n = 5).

	Holotype MNHG 2814.066	Range	Mean ± SD
Standard length (mm)	75.1	41.5–80.1
In % SL
Predorsal length	36.5	33.3–36.9	35.2 ± 1.54
Preanal length	68.3	64.3–68.3	65.8 ± 1.54
Prepelvic length	48.9	47.0–49.2	48.2 ± 0.87
Prepectoral length	19.0	18.4–20.5	19.2 ± 0.77
Length of dorsal‐fin base	12.5	11.2–12.8	12.1 ± 0.60
Dorsal‐fin spine length	13.8	11.8–13.8	13.2 ± 0.83
Length of anal‐fin base	15.6	15.4–17.2	16.1 ± 0.72
Pelvic‐fin length	17.6	17.1–19.3	17.9 ± 0.84
Pectoral‐fin length	23.2	22.4–23.2	22.7 ± 0.32
Pectoral‐fin spine length	14.0	13.3–14.7	14.1 ± 0.54
Caudal‐fin length	28.2	24.6–29.6	27.7 ± 1.94
Length of adipose‐fin base	13.4	11.6–13.8	12.9 ± 0.93
Dorsal to adipose distance	25.7	23.9–26.6	24.9 ± 1.17
Post‐adipose distance	18.0	17.3–18.0	17.6 ± 0.28
Length of caudal peduncle	19.3	16.8–20.3	18.7 ± 1.59
Depth of caudal peduncle	7.9	6.6–8.1	7.5 ± 0.61
Body depth at anus	13.4	12.5–15.2	13.4 ± 1.05
Head length	26.2	25.5–27.0	26.2 ± 0.55
Head width	17.2	16.8–18.3	17.7 ± 0.68
Head depth	12.4	11.6–12.6	12.2 ± 0.39
TAA length	16.1	15.9–17.1	16.4 ± 0.47
TAA width	9.5	9.0–10.8	9.5 ± 0.74
In % HL
Snout length	49	46–51	48.4 ± 1.9
Interorbital distance	22	22–24	23.4 ± 0.8
Eye diameter	9	7–9	8.4 ± 0.8
Nasal barbel length	37	37–45	39.8 ± 3.2
Maxillary barbel length	102	91–107	100.8 ± 6.2
Inner mandibular barbel length	38	36–41	38.4 ± 2
Outer mandibular barbel length	54	46–58	54.2 ± 4.9

urn:lsid:zoobank.org:act:E5F25A0A‐69E2‐44FA‐A8AA‐7BF2BC0AF109.

3.2. Holotype

MHNG 2814.066, 75.1 mm SL; Laos: Vientiane Province, small forest stream along road from Ban Pak Leuk to Ban Longxam, 18°27′05″N, 103°04′06″ E; headwater in Nam Leuk catchment, Nam Mang watershed; M. Kottelat et al., 25 February 1997.

3.3. Paratypes

CMK 13310 (3), 41.5–80.1 mm SL; ZRC 69399 (1), 65.5 mm SL; data as for holotype.

3.4. Diagnosis

Glyptothorax intonsus is distinguished from all Mekong drainage Glyptothorax by the following: nasal barbels reaching at least to the anterior orbital margin, head width 16.8%–18.3% SL, head depth 11.6%–12.6% SL, lateral line without short branches visible, length of adipose‐fin base 11.6%–13.8% SL, caudal peduncle depth 2.0–2.2 times in body depth at dorsal‐fin origin and a uniformly almost black body.

3.5. Description

Morphometric ratios are as documented in Table 1. Body subcylindrical; greatest body depth at dorsal‐fin origin, body not noticeably deeper anteriorly (caudal peduncle depth 2.0–2.2 times in body depth at dorsal‐fin origin). Predorsal profile moderately arched, rising evenly to dorsal fin then sloping gently ventrally beyond. Ventral profile with minimal curvature to anal‐fin origin, then sloping gradually upwards to caudal‐fin base. Anus located at vertical through posterior quarter of adpressed pelvic fin; urogenital opening located about midway along distance between base of last pelvic‐fin ray and anal‐fin origin. Skin on sides of body and caudal peduncle densely tuberculate. Lateral line complete and midlateral, without visible short branches.

Head depressed, broader than deep, skin densely tuberculate; interorbital surface flat to slightly convex. Snout broad, rounded in dorsal view and with inferior mouth. Eyes small, elliptical, positioned dorsally and at about midway along longitudinal axis of head; completely covered by translucent skin. Anterior and posterior nares separated only by nasal‐barbel base. Gill opening wide, reaching from ventral edge of post‐temporal to isthmus.

Barbels in four pairs; long, slender and flattened. Maxillary barbel extending to middle of pectoral‐fin base. Nasal barbel reaching at least to anterior orbital margin. Inner mandibular barbel extending to two thirds of distance between its base and that of pectoral spine. Outer mandibular barbel just reaching base of pectoral spine.

Mouth inferior, crescentic, moderately wide; upper jaw projecting beyond lower; lips thick, fleshy, papillate. Premaxillary tooth band nearly fully exposed when mouth is closed. All tooth‐bearing surfaces with small and villiform oral teeth arranged in irregular rows. Premaxillary teeth arranged in a wide semilunate band. Dentary teeth in a crescentic band, consisting of two bilaterally symmetrical halves meeting at symphysis.

TAA consisting of striae in an elongate pentagonal field (Figure 2a) extending from isthmus to slightly beyond posterior limit of pectoral‐fin base; anterolateral edges straight. Width of TAA 1.7–1.9 times its length, and lacking distinct medial pit.

Dorsal fin located above anterior third of body, with I,6 (5) rays; fin margin concave; spine slightly curved, posterior margin with two to four weak serrations. Adipose fin small, positioned nearly vertically opposite anal fin, with straight anterior and gently convex posterior margins.

Caudal fin strongly forked, with lobes of nearly equal length and i,7,8,i (5) principal rays. Procurrent rays symmetrical and projecting marginally anterior to fin base. Caudal peduncle depth 2.4–2.6 times its length.

Anal fin with nearly straight anterior and slightly indented posterior margin; positioned at 33%–37% of SL from snout tip and with iv,9* (3), iv,9,I (1) or iv,10 (1) rays. Anal‐fin origin positioned slightly anterior to vertical through adipose‐fin origin.

Pelvic fin with straight posterior margin and i,5 (5) rays; positioned at 47%–49% of SL from snout tip. Pelvic‐fin origin very slightly anterior to vertical through posterior extremity of dorsal‐fin base; tip of adpressed fin not reaching anal‐fin origin.

Pectoral fin with I,8,I (1), I,9* (2), I,9,I (1) or I,10 (1) rays; somewhat wedge‐shaped with gently convex anterior and concave posterior fin margin. Pectoral spine robust, with smooth anterior margin, and posterior margin bearing 9–12 serrations; distal serrations smaller.

3.6. Colouration

In 70% ethanol: dorsal and lateral regions of head and body chocolate brown, progressively fading to cream ventrally. A narrow, thin cream stripe running mid‐dorsally from base of terminal dorsal‐fin ray to dorsal procurrent caudal‐fin rays; some individuals without stripe. Pectoral and pelvic fins with chocolate‐brown fin rays and hyaline interradial membranes. Anal‐fin rays chocolate brown, with more intense colouration in anterior half; interradial membranes without pigmentation. Dorsal‐fin rays chocolate brown, with more intensely brown curved wedge‐shaped marking on its medial third, and a thin hyaline distal edge. Adipose fin chocolate brown, with clear posterodistal edge. Caudal fin with chocolate‐brown fin rays; interradial membranes and tip of lobes hyaline. Dorsal surfaces of maxillary and nasal barbels chocolate brown, fading ventrally to cream. Mandibular barbels cream.

In life and shortly after fixation (Figure 3): body blue black, ventral surface of head and body yellow brown. Membranes of fins yellow.

3.7. Distribution and habitat

Glyptothorax intonsus is known from the Nam Mang watershed within the Mekong River drainage (Figure 4). Its type locality (shared with Rhyacoschistura suber Kottelat 2000; Kottelat, 2000, 2019) is a small stream on the boundary between the Nam Ngum and Nam Mang watersheds, which drains into the Nam Mang watershed [~440 m a.s.l. (above sea level) according to Google Earth]. The only known locality was (in dry season 1997) a very small creek in the forest, with only little water and moderate current, flowing between rocks and roots, which is unlikely to be the primary habitat of the species. In 2014 the same creek was dry. The species was not observed at 15 other sites in the Nam Mang watershed visited in 1997.

FIGURE 4.

Map showing type locality of Glyptothorax intonsus.

3.8. Etymology

The Latin adjective intonsus (‐a, ‐um) means ‘unshorn, unshaven, bearded’, with the name used in reference to the long barbels of this species compared to most Mekong congeners.

4. DISCUSSION

Considering the highly restricted distributions of Glyptothorax species [with nearly all species typically found in only one or two (adjacent) river drainages], we here restrict comparisons of the new species to only congeners known from the Mekong River drainage. G. intonsus is distinguished from all Mekong drainage Glyptothorax except for G. deqinensis and G. longinema by very long nasal barbels (reaching at least to vs. not reaching anterior orbital margin). It differs from both G. deqinensis and G. longinema by a narrow, slender head (width 16.8%–18.3% SL vs. 19.2–26.9, depth 11.6%–12.6% SL vs. 12.8–18.4), broader striae in the TAA (Figure 2), a slender body relative to the caudal peduncle (caudal peduncle depth 2.0–2.2 times in body depth at dorsal‐fin origin vs. 2.4–2.6) and the lateral line without (vs. with) short branches visible, further differing from G. longinema by a shorter adipose‐fin base (11.6%–13.8% SL vs. 13.1–21.6). Glyptothorax intonsus is further distinguished from all Mekong drainage Glyptothorax except for G. deqinensis, G. forabilis, G. irroratus, G. laosensis, G. longinema and G. porrectus by a uniformly almost black body (vs. a non‐uniformly coloured body with pale or dark bands, patches or spots in other species).

Among all the Mekong Glyptothorax with a uniformly dark body, G. intonsus differs from G. forabilis by a smaller interorbital distance (22%–24% HL vs. 25–31) and a deeper body relative to the caudal peduncle (caudal peduncle depth 1.7–1.9 times body depth at anus vs. 1.6–1.7), and from G. irroratus by a slender head (depth 11.6%–12.6% SL vs. 13.1–15.7), a shorter pectoral spine (13.3%–14.7% SL vs. 16.1–20.5) and tubercles of approximately the same size (vs. of varying sizes) on the lateral surface of the body. It is distinguished from G. laosensis by a shorter post‐adipose distance (17.3%–18.0% SL vs. 18.0–23.7) and by the absence of a prominent pale midlateral stripe (vs. stripe usually present), and from G. porrectus by the absence (vs. presence) of a medial pit in the TAA, by the presence of a deeper body (depth at anus 12.5%–15.2% SL vs. 8.7–12.4) and a shorter, deeper caudal peduncle (length 16.8%–20.3% SL vs. 21.7–25.8, depth 6.6%–8.1% SL vs. 5.8–6.7, 2.4–2.6 times its length vs. 3.3–4.1) and by the absence (vs. presence) of a prominent pale midlateral stripe.

Against the predominantly banded species of Mekong Glyptothorax, G. intonsus is further distinguished from G. filicatus, G. horai and G. macromaculatus by a shorter, narrower head (length 25.5%–27.0% SL vs. 27.6–29.3, width 16.8%–18.3% SL vs. 22.4–23.4) and a narrower TAA (width 1.7–1.9 times its length vs. 1.1–1.4) without (vs. with) anteromedial striae. It is additionally distinguished from G. filicatus by a slender caudal peduncle (depth 6.6%–8.1% SL vs. 8.4–8.8, 2.4–2.6 times its length vs. 1.9–2.4).

Glyptothorax intonsus further differs from G. carnatus, G. cf. platypogonides and G. sulculus by a smaller eye (7%–9% HL vs. 10–15), a slender head (depth 11.6%–12.6% SL vs. 13.7–17.2), a slender body relative to the caudal peduncle (caudal peduncle depth 2.0–2.2 times body depth at dorsal‐fin origin vs. 2.5–3.6) and a longer dorsal‐to‐adipose distance (23.9%–26.6% SL vs. 15.6–23.8). It additionally differs from G. cf. platypogonides by the absence (vs. presence) of a medial pit in the TAA, and is further distinguished from G. rhadinus by a smaller eye (7%–9% HL vs. 12–18) and a broader TAA (width 1.7–1.9 times its length vs. 1.9–2.2), the absence (vs. presence) of a medial pit and a longer dorsal‐to‐adipose distance (23.9%–26.6% SL vs. 15.5–20.0).

The shape of the TAA, and to a lesser extent the orientation of the striae within the TAA, has long been used as a character to diagnose Glyptothorax species (e.g. Ng, 2005; Ng & Kottelat, 2016; Ng & Kottelat, 2025). The utility of this character in this role has been reported by us (Ng & Kottelat, 2016) and confirmed here by our examination of intraspecific variation in TAA morphology for G. intonsus. A comparison of conspecific individuals reveals that although minor variation exists in details such as the degree of striae bifurcation, the fundamental shape and architecture of the TAA remain consistent within species. The TAA in the type series of G. intonsus shares an elongate outline (i.e. with nearly straight lateral margins; Figure 5). This contrasts with the broader, more spatulate TAA observed in G. deqinensis (Figure 2b) and the nearly cordiform TAA observed in G. longinema (Figure 2c). The overall differences in TAA geometry exceed the intraspecific variation observed and remain consistent across species. Thus, when emphasis is placed on the gross morphological pattern rather than fine structural details, the shape of the TAA can effectively diagnose Glyptothorax species.

Colour pattern serves as an important diagnostic character in Glyptothorax, and field observations by one of us (M.K.) suggest it may correlate with ecological preferences. Uniformly dark‐bodied species (such as G. irroratus and G. laosensis) typically inhabit upper river reaches with swift currents, associating with large stones, boulders, rocks (often black basalt outcrops) and waterfalls. In contrast, banded species (such as G. carnatus and G. cf. platypogonides) are generally found in lower reaches, often near shores, in riffles and among branches of fallen trees. However, this ecological segregation is not absolute; up to three species (including both dark‐bodied and banded forms) may co‐occur at a single collection site. Phylogenetic studies (Boyd et al., 2023; Jiang et al., 2011) suggest that colour pattern may carry phylogenetic signal: clades E and F of Jiang et al. (2011) comprise banded species, whereas clade H comprises dark‐bodied species; similarly, the two major clades identified by Boyd et al. (2023) consist predominantly of dark‐bodied and banded species, respectively. However, taxon sampling in phylogenetic studies of Glyptothorax remains limited, preventing us from drawing definitive conclusions about the evolutionary significance of colour pattern.

5. COMPARATIVE MATERIAL

G. deqinensis: KIZ 1974000937–1974000938 (2 paratypes), 89.2–89.7 mm SL; China: Yunnan Province, Deqin County, Yanmen. KIZ 1980001799 (1 paratype), 85.1 mm SL; China: Yunnan Province, Zidongjiang. BMNH 1987.9.17.5 (1 paratype), 74.0 mm SL; China: Yunnan, Weixi County, Baijixun Township.

G. longinema: KIZ 741097 (holotype), 67.0 mm SL; KIZ 740198–740200 (3 paratypes), 55.1–58.9 mm SL; China: Yunnan Province, Nujiang Prefecture, Lushui County, Liuku Township, Bijiang. KIZ 1981001381–1981001383 (3), 82.3–89.1 mm SL; China: Yunnan Province, Nujiang Prefecture, Lushui County. KIZ 2006000865 (1), 87.8 mm SL; KIZ 2006000870 (1), 95.2 mm SL; KIZ 2006000878 (1), 89.0 mm SL; China: Yunnan Province, Baoshan Prefecture, Longlin County, Sanjiangkou. KIZ 2002002337–2002002341 (5), 71.8–91.0 mm SL; China: Yunnan Province, Lincang Prefecture, Yongde County, Nanting River. CMK 22499 (5), 54.5–71.9 mm SL; Laos: Vientiane Province, Nam Lao, downstream of bridge at 7.6 km on road from Ban Xamthong to Ban Longcheng, a stream entering a cave ~2 km downstream of bridge, 950 m a.s.l., 19°09′32″N, 102°52′51″ E.

The reader is referred to Ng (2025) and Ng and Kottelat (2025) for additional material examined.

AUTHOR CONTRIBUTIONS

Conceptualization, data curation, formal analysis, investigation, visualization and writing – original draft: Heok Hee Ng. Acquisition of specimens, field observations, data curation, formal analysis, resources, validation and writing – review and editing: Maurice Kottelat.

FUNDING INFORMATION

No funding was received.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

Ng, H. H. , & Kottelat, M. (2026). Glyptothorax intonsus: a new species of rheophilic catfish (Siluriformes: Sisoridae) from the Mekong River drainage in Laos. Journal of Fish Biology, 109(1), 542–548. 10.1111/jfb.70409

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.