Glyptothorax irroratus, a new species of rheophilic catfish from the Mekong River drainage (Actinopterygii: Siluriformes: Sisoridae)

ABSTRACT Glyptothorax irroratus, a new species of sisorid catfish from the Mekong River drainage in Laos and China, is described. It differs from its Indochinese congeners in having both large and small tubercles arranged irregularly on the lateral surfaces of the body and by combinations of colour pattern, morphometry (with particular regards to the eye, body depth, adipose fin and caudal peduncle) and thoracic adhesive apparatus morphology. http://www.zoobank.org/urn:lsid:zoobank.org:pub:1031A8CE-F51D-4954-A812-14EE132371BA


Introduction
The family Sisoridae consists of rheophilic catfishes distributed throughout the entire southern arc of the Asian continent (Ng 2015). The family is most species-rich in Indochina (here defined as continental Southeast Asia from the Irrawaddy River drainage eastwards to the Red River drainage and southwards to the Malay Peninsula), where 120 out of about 260 species, representing approximately 45% of its species diversity, are known to occur. Glypotothorax is the most speciose sisorid genus, with more than 80 species identified. Glyptothorax species inhabit swift-flowing streams and rivers with a sandy or rocky substrate, with their most distinctive feature being a thoracic adhesive apparatus (consisting of longitudinal or oblique folds of skin arranged in a roughly elliptical field on the thoracic region) that assists the catfish in adhering to the substrate against strong currents.
Although many new Indochinese sisorid species continue to be identified only from recently collected material (e.g. Suvarnaraksha 2020; Shangningam and Kosygin 2022), a few others have been recognised from previously misidentified material deposited in collections (e.g. Ng and Kottelat 2021). One such misidentified species is the dark-bodied Glyptothorax from the middle Mekong River drainage in Laos, which has been variously identified as G. trilineatus (e.g. Taki 1974), G. zanaensis (e.g. Kottelat 2001) or G. deqinensis (e.g. Rainboth et al. 2012). This study clarifies the identity of this Glyptothorax species, describing it as a new species (G. irroratus) herein.  Kottelat et al., 20 March 1996. CMK 20017 (27)

Diagnosis
Glyptothorax irroratus differs from Indochinese congeners in having both large and small tubercles arranged irregularly on the lateral surfaces of the body (vs all tubercles of approximately equal size). It is further distinguished from Indochinese congeners except for G. granulus and G. zanaensis in having a uniformly dark grey to black body with only a very faint pale mid-dorsal stripe (vs dark grey to black body with pale mid-lateral and mid-dorsal stripes in G. coracinus, G. deqinensis, G. forabilis, G. granosus, G. interspinalum, G. lanceatus, G. laosensis, G. longicauda, G. longinema, G. porrectus, G. schmidti, G. trilineatus and G. yuensis, and body with pale or dark bands, patches or spots in other species). The following unique combination of characters further distinguishes G. irroratus from Mekong River congeners: eye diameter 9-12% HL; interorbital distance 25-31% HL; head length 23.6-27.8% SL; head depth 13.1-15.7% SL; anteromedial striae in TAA absent; short, triangular medial pit in TAA extending about one-fifth TAA length; TAA width 10.1-12.0% SL; 7-14 serrae on posterior margin of pectoral spine; dark nuchal plate; neural spines of vertebrae between dorsal and adipose fins distally expanded; body depth at anus 12.2-16.0% SL; uniformly dark-coloured adipose fin, with pale distal margin; length of adipose-fin base 12.5-17.0% SL; post-adipose distance 13.4-18.9% SL; caudal peduncle depth 6.3-7.6% SL (2.6-3.1 times in its length and 1.8-2.1 times in body depth at anus); and caudal peduncle length 17.8-21.4% SL.

Description
Morphometric data in Table 1. Head depressed; body subcylindrical. Dorsal profile rising evenly from tip of snout to origin of dorsal fin, then sloping gently ventrally from origin of dorsal fin to end of caudal peduncle. Ventral profile straight to anal-fin base, then sloping gently dorsally from anal-fin base to end of caudal peduncle. Anus and urogenital openings located at vertical through posterior quarter of adpressed pelvic fin. Skin tuberculate, with tubercles of uneven sizes on sides of body and caudal peduncle. Vertebrae between dorsal and adipose fins with distally expanded neural spines. Lateral line complete and mid-lateral.
Head depressed and broad, triangular when viewed laterally. Snout prominent. Anterior and posterior nares large and separated only by base of nasal barbel. Gill opening broad, extending from ventral margin of post-temporal to isthmus. First branchial arch with 3 + 6* (2) rakers. Bony elements of dorsal surface of head covered with thick, tuberculate skin. Eye ovoid, horizontal axis longest; located entirely in dorsal half of head.
Barbels in four pairs. Maxillary barbel long and slender, extending to base of second pectoral-fin ray. Nasal barbel slender, extending nearly to anterior orbital margin. Inner mandibular-barbel extending to midway between its base and that of pectoral spine. Outer mandibular barbel extending nearly to base of pectoral spine.
Mouth inferior, premaxillary tooth band partially (approximately half) exposed when mouth is closed. Oral teeth small and villiform, in irregular rows on all tooth-bearing surfaces. Premaxillary teeth appearing in single broad semilunate band. Dentary teeth in a single crescentic band, consisting of two separate halves tightly bound at midline.

Colouration
In 70% ethanol: dorsal and lateral surfaces of head, and body very dark brown, fading to beige on ventral surfaces. A faint thin, cream mid-dorsal stripe extending from base of last dorsal-fin ray to dorsal procurrent caudal-fin rays; stripe broken up into conjoined series of cream patches (delineating distally expanded neural spines) in some individuals; stripe absent in some individuals. Pectoral and pelvic fins with dark brown on base of fin rays and hyaline posterior margin. Anal fin dark brown, with hyaline posteroventral corner, sometimes with a paler or hyaline median band. Dorsal fin dark brown, with hyaline posterior margin, sometimes with a paler or hyaline median band. Adipose fin dark brown, with hyaline distal margin. Caudal fin with dark brown fin rays, hyaline interradial membranes and tip of lobes cream. Maxillary and nasal barbels dark brown dorsally, beige ventrally. Mandibular barbels beige. In life: As above, but body uniform dark brown to black, mid-dorsal stripe not or poorly distinct. Edge of fins and tip of caudal-fin lobes hyaline to yellow.

Habitat
Glyptothorax irroratus is apparently the most rheophilic species of the genus in the middle and lower Mekong drainage. It has been collected in rapids, waterfalls and other habitats with strong current and with stone, boulder and rocky bottom (Figure 3). It occasionally occurs in areas with weaker current, but always over a stony substrate.

Distribution
Glyptothorax irroratus is known from the middle Mekong River drainage in China and Laos (Figure 4). The record farthest upriver is a single specimen from Xishuangbanna (China), in Nam Xing River (Buyuan watershed, 21°53ʹ23"N, 101°19ʹ41"E; CMK 23958). In Laos, we have seen material from the Nam Phak, Nam Ou, Nam Xuang, Nam Khan, Nam Ngum, Nam Mang, Nam Ngiep, Nam Kading and Xe Bangfai watersheds. It is expected in the Mekong drainage in Thailand and the headwaters of Nam Ou in Dien Bien province in Vietnam. Its presence farther downstream in the Mekong drainage cannot be excluded. Glyptothorax irroratus possibly also occurs in the Chao Phraya drainage in Thailand, with a single specimen collected from Chiangmai showing the tubercles, TAA, expanded neural spines and colour pattern diagnostic for G. irroratus (CMK 4099, 19°30ʹ29"N, 98° 57ʹ31"E).

Etymology
The Latin adjective irroratus (-us, -a, -um) means covered with dew; in zoology used to mean besprinkled with drops or particles. This is used in allusion to the irregular tuberculation on the sides of the body and caudal peduncle.

Discussion
As pointed out in previous studies, most Glyptothorax species are restricted to two or three adjacent river drainages or watersheds at most (Ng andKottelat 2016, 2017). Therefore, we restrict comparisons to congeners within Indochina (continental Southeast Asia from the Irrawaddy River drainage eastwards to the Red River drainage and southwards to the Malay Peninsula). Only 16 of the 35 other Indochinese species of Glyptothorax have a uniformly dark body (and typically with pale mid-lateral and mid-dorsal stripes): G. coracinus,  , G. forabilis, G. granosus, G. granulus, G. interspinalum, G. lanceatus, G. laosensis,  G. longicauda, G. longinema, G. porrectus, G. schmidti, G. trilineatus, G. ventrolineatus, G. yuensis and G. zanaensis. The remaining 19 congeners have an irregularly coloured body, often with pale or dark bands, patches or spots. Glyptothorax irroratus is unique among Indochinese Glyptothorax in having both large and small tubercles arranged irregularly on the lateral surfaces of the body. This character, however, is more obvious in some populations (e.g. in Nam Ngiep, Nam Theun and Nam Ou watersheds). In other watersheds (especially Nam Ngum), it is clearly distinct in specimens above about 50 mm SL, while it is usually indistinct in smaller specimens. We have no information suggesting this varies seasonally. Apart from the unusual tuberculation on the sides of the body and caudal peduncle, G. irroratus differs from its Indochinese congeners with a uniformly dark body, except for G. granulus and G. zanaensis, in lacking (vs having) a pale mid-lateral stripe on the body. Glyptothorax irroratus is further distinguished from G. granulus (from the Irrawaddy River drainage in India) in having the dorsal spine smooth (vs serrated) posteriorly and a longer dorsal to adipose distance (22.5-27.2% SL vs 19.7-22.8). Glyptothorax zanaensis was described from the Salween (Nujiang) River drainage in China (Wu et al. 1981), and the similarity in colour pattern between it and G. irroratus is one reason, among others, why the two were previously thought to be conspecific (see chresonymy). However, the specimens we examined and the redescription of G. zanaensis by Jiang et al. (2012) indicates that this species does not possess tubercles of unequal sizes on the flanks as seen in G. irroratus. Furthermore, G. zanaensis possesses anteromedial striae in the TAA (vs absent in G. irroratus).

G. deqinensis
Among species with a uniformly dark body and a pale mid-dorsal stripe, G. irroratus is further distinguished from G. coracinus (from the rivers draining the western face of the Cardamom Mountains in south-western Cambodia) in having a short, triangular medial pit in the TAA extending about one-fifth the TAA length (vs long, lanceolate medial pit extending about half length of TAA), a more tapering body manifested in the larger ratio between the body and caudal peduncle depths (caudal peduncle depth 1.8-2.1 times in body depth at anus vs 1.4-1.6) and a larger eye (9-12% HL vs 6-7); from G. deqinensis (from the Mekong River drainage in China) in having more serrae on the posterior margin of the pectoral spine (7-14 vs 5-8) and distally expanded (vs pointed) neural spines of the vertebrae between the dorsal and adipose fins; and from G. forabilis (from the Xe Nam Noy watershed on the Bolaven Plateau, southern Laos) in having (vs lacking) a medial pit in the TAA and a more tapering body manifested in the larger ratio between the body and caudal peduncle depths (caudal peduncle depth 1.8-2.1 times in body depth at anus vs 1.6-1.7). It further differs from G. granosus (from the Salween River drainage in China) in having a dark (vs pale) nuchal plate, the dorsal spine smooth (vs serrated) posteriorly, and a wider TAA (10.1-12.0% SL vs 8.7-10.3); from G. interspinalum (from the Red, Ma and Ca river drainages in north-eastern Laos and northern Vietnam) and G. lanceatus (from the Salween River drainage in China) in having more widely set eyes (interorbital distance 25-31% HL vs 20-22). Glyptothorax irroratus is further distinguished from G. laosensis (from Mekong and Chao Phraya drainages) in having a shorter postadipose distance (13.4-18.9% SL vs 18.0-23.7); from G. longicauda (from the Irrawaddy River drainage in northern Myanmar and China) in having a deeper caudal peduncle (6.3-7.6% SL vs 5.0-5.6); and from G. longinema (from the Salween and Mekong river drainages in China) in having a dark (vs pale) nuchal plate, the dorsal spine smooth (vs serrated) posteriorly, a uniformly dark-coloured adipose fin (vs anterior third of adipose fin pale) and a more slender caudal peduncle relative to its length (caudal peduncle depth 2.6-3.1 times in its length vs 1.8-2.4). It further differs from G. porrectus (from the Xe Nam Noy watershed on the Bolaven Plateau, southern Laos) in having a shorter caudal peduncle (17.8-21.4% SL vs 21.7-25.8), deeper head (13.1-15.7% SL vs 10.6-11.6) and body (depth at anus 12.2-16.0% SL vs 8.7-12.4); from G. schmidti (from the Malay Peninsula and Sumatra) in having a shorter post-adipose distance (13.4-18.9% SL vs 18.9-22.0); and from G. trilineatus (from Salween and Irrawaddy drainages) in having a short, triangular medial pit in the TAA extending about one-fifth the TAA length (vs long, lanceolate medial pit extending about half the length of the TAA). Glyptothorax irroratus is further distinguished from G. ventrolineatus (from the Irrawaddy River drainage in India) in having a longer adipose-fin base (12.5-17.0% SL vs 9.3-11.1) and lacking (vs having) a pale midventral line extending from just anterior to the pelvic-fin base to the ventral procurrent caudal-fin rays; and from G. yuensis (from the Irrawaddy River drainage in Myanmar) in lacking (vs having) anteromedial striae in the TAA, the dorsal spine smooth (vs serrated) posteriorly, and a longer adipose-fin base (12.5-17.0% SL vs 7.6-10.0).
Four other species of Glyptothorax are known from the Mekong River drainage: G. filicatus, G. horai, G. lampris and G. macromaculatus. Besides the colour pattern, G. irroratus differs from G. filicatus and G. horai in having a shorter head (23.6-27.8% SL vs 27.6-32.1) and a more slender caudal peduncle (6.3-7.6% SL vs 8.4-9.9) and in lacking (vs having) anteromedial striae in the TAA; from G. lampris in having a short, triangular medial pit in the TAA extending about one-fifth its length (vs long, lanceolate medial pit extending about half the length of the TAA); and from G. macromaculatus in having a shorter head (23.6-27.8% SL vs 27.4-34.4 mm SL) and distally expanded (vs pointed) neural spines of the vertebrae between the dorsal and adipose fins.