A new species of sisorid catfish of the genus Exostoma from the Salween drainage, Yunnan, China

A new species of the sisorid catfish genus Exostoma Blyth, 1860 was collected from two hill-stream tributaries of the Nujiang (Salween River) drainage in Gaoligong Mountain, south-western Yunnan Province, China from 2003 to 2006 and from two tributaries of the Salween River in Cangyuan County, Lingcang Prefecture, Yunnan Province, China (in 2007) and in Yongde County, Lingcang Prefecture, Yunnan Province, China (in 2015). Exostoma gaoligongense sp. nov. is the 10th species of the genus and is most similar to E. vinciguerrae in morphology but can be distinguished by pelvic fin reaching anus vs. not reaching; maxillary barbels just reaching or slightly surpassing pectoral-fin origin vs. surpassing pectoral-fin origin or even reaching posterior end of gill membrane; abdominal vertebrae 23-25 vs. 25-27; length of dorsal fin/dorsal to adipose distance 90.3%-287.0% vs. 59.2-85.7. A key to Exostoma spp. is provided.


MATERIALS AND METHODS
Fishes were collected with an electroshocker or purchased from local fishermen, fixed in 10% formalin, and transferred to 75% ethanol for long-term storage, or fixed in 75%-100% ethanol. Measurements were made point to point with digital calipers and recorded to 0.1 mm. Counts and measurements of paired structures were made on the left side of specimens, except counts of pectoral-fin rays were on both sides when available. Counts and measurements followed Hubbs & Lagler (1958) and Ng & Kottelat (1999). Vertebral counts were made from radiographs following the methods of Roberts & Ferraris (1998). Morphology of teeth on jaws follows Steinitz (1961). Fishes were cataloged in the Kunming Natural History Museum of Zoology, the Kunming Institute of Zoology, the Chinese Academy of Sciences, Kunming, Yunnan, China (KIZ) and in the California Academy of Sciences, San Francisco, California, USA (CAS). Symbolic codes for institutions are those given by Leviton et al. (1985). Some comparative specimens were kept in the Southeast Asia Biodiversity Research Institute (SEABRI), Chinese Academy of Sciences, Nay Pyi Taw, Myanmar. Description: Morphometric and meristic data are given in Table  1. Head and body slightly rounded and depressed. Head medium size, with minute papillae scattered on dorsal and lateral surfaces and on maxillaries. Snout blunt and depressed.

Exostoma
Rostral cap with non-prominent groove in middle of anterior edge, turning to pelvic side and forming prominent preoral groove with papillae on surface. Mouth inferior, opening transversely. Tooth bands on upper and lower jaws visible when mouth in normal position or when made to close. Premaxillary with two semicircular-shaped tooth bands, contacting at posteromedial margins. Dentary with two distinctly separate tooth bands. Oar-shaped teeth prominent on both jaws, and pointed conical small teeth along the posterior borders of the teeth patches. Lower lip thin, flat, split into two big lateral lobes and two small median lobes by notches, posterior margin without fimbriate projections. Postlabial groove on lower jaw present, not interrupted in middle. Middle lobe of upper lip with papillae. Pelvic side of rostral barbel and lateral lobe of lower lip with feather-like wrinkles. Anterolateral portion of lateral lobe of lower lip connected with maxillary barbel through a membrane. Eyes tiny, subcutaneous, on dorsal side of head. Distance from posterior edge of eye to upper corner of gill opening almost equal to distance from posterior edge of eye to posterior edge of anterior nostril. Four pairs of flattened barbels. Nasal barbels long, in mid-point from anterior edge of eye to snout tip, tips reaching middle of or surpassing posterior edge of eye. Maxillary barbels broadly connected to lower part of head by fold of skin, tips just reaching or slightly surpassing pectoral-fin base. Mandibular barbels, two pairs. Outer mandibular barbels with rough surface, situated on underside of head, bases covered by lower lip, projecting laterally, longer than inner pair. Inner mandibular barbels short, in notches along posterior margin of lower lip. Upper corner of gill opening level to ventral side of eye, lower corner extending to ventral side of pectoral fin. Predorsal part of body rounded and deep, back slightly convex, abdomen flat, smooth, without papillae, gradually compressed posteriorly. Pectoral fin round, first ray broad, flattened with numerous plicae on ventral surface, tip of fin nearly reaching to, reaching to, or extending beyond dorsal-fin origin. Dorsal fin without spine, distal margin truncate, its origin above tip of pectoral fin. Predorsal length greater than distance from dorsal-fin origin to adipose-fin origin. Tip of depressed dorsal fin not reaching adipose-fin origin. Adipose fin long, its origin above or slightly anterior of anus, confluent with caudal fin. Pelvic-fin origin under fourth branched dorsal ray, its tip sometimes reaching anus, first ray broad, flattened with numerous plicae on ventral surface. Anus and urogenital opening about two-thirds distance from pelvic-fin insertion to anal-fin origin. Anal fin distal margin rounded slightly. Distance from anal-fin origin to caudal-fin base almost equal to distance from anal-fin origin to pelvic-fin origin. Caudal fin emarginate, lower lobe slightly longer than upper lobe. Lateral-line complete, originating at upper corner of gill opening, arching slightly upward above pectoral-fin base, sloping downward until pelvicfin origin (on holotype), then curving upward slightly pelvic-fin base (on holotype), thereafter straight on midline of body and of caudal peduncle.
Coloration of live and preserved specimens: Head and back gray. Gill membrane transparent. Body gray above lateral-line, lighter below lateral-line lighter on caudal peduncle, abdomen yellowish. Non-prominent black stripe along dorsal mid-line and lateral-line. Dorsal-fin membrane yellow and rays light gray. Adipose fin dark gray at base, transparent and yellowish in the margin. Pectoral-fin and pelvic-fin bases darker, fin rays yellow and transparent at the edge. Unbranched dorsal-fin and anal-fin rays grayish. Caudal-peduncle base black, posterior margin of the black blotch concave, middle part of caudal fin yellow, distal part dark gray. Margin of upper and lower lobes gray. The grayish coloration of live specimens changes to brownish tones after preservation.
Allometry: Juveniles smaller than 39.0 mm SL have a more depressed head; smaller eye, eye diameter 6.7%-8.2% HL vs. 11.0%-14.3%, interorbital width 4.5-4.8 times of eye diameter vs. 2.1-2.9; bigger head, head width much larger than body width vs. head width almost equal to body width; longer adipose-fin base and origin of adipose fin more anterior, consequently distance between dorsal fin and adipose fin is shorter; longer pectoral fin, its tip surpasses dorsal-fin origin; slender caudal peduncle, caudal-peduncle length 2.6-2.8 times its depth vs. 1.9-2.2.
Distribution: Currently only known from four tributaries of the Salween River drainage in Yunnan, China (Figure 3), including the Manggang River ( Figure 4) and Tangxi River, tributaries of Nu River (Nujiang), Xinya River, a tributary of Nanting River, and Nanzha River, a tributary of Nangun River. Probably also exists in other mountain streams in east slope of Gaoligong Mountain that drain into the Nujiang. The four known locations are close to China-Myanmar border, it hopefully also occurs in Salween Drainage of Myanmar.

Etymology:
The specific name is an adjective that refers to the Gaoligong Mountain in which the type locality is located, and the suffix agrees in gender with the generic name Exostoma (gender neuter).

Notes on biology:
This species was collected from shallow water (<1 m deep) in a fast flowing stream with clear water. Water temperature was 18.8 °C, water pH 6.95, conductivity 45.6 μS/cm. The bottom substrate was boulders, cobbles, gravel, and sand with many diatoms that made the rocks slippery. This species was obtained from fast water and small waterfalls. The new species of Exostoma seems to have much lower tolerance to either low dissolved oxygen or to stress from electrofishing than Pseudexostoma brachysoma Chu, 1979, which occurs in the same habitat. After shocking sampling on 7 October 2003, all the Exostoma were dead, whereas all the individuals of P. brachysoma survived until the next morning. Exostoma gaoligongense is most similar to E. vinciguerrae ( Figure 5) for similar body shape, color pattern, and overlapped fin ray counts, number of vertebrae, and metric characters, but the former can be distinguished from the latter by the following characters: pelvic fin reaching anus vs. not reaching; pectoral fin slightly longer, extending to vicinity of dorsal-fin origin vs. not reaching to dorsal-fin origin; maxillary barbels just reaching or slightly surpassing pectoral-fin origin vs. surpassing pectoral-fin origin or even reaching posterior end of gill membrane; principal caudal-fin rays usually 17 vs. 14-15; abdominal vertebrae 23-25 vs. 25-27 (data from Ng & Vidthayanon, 2014 Exostoma gaoligongense shares gill opening extending to ventral surface of the body with E. labiatum and E. stuarti. It can be further distinguished from E. labiatum and E. stuarti by adipose fin confluent with the upper procurrent caudal-fin rays vs. free from it. It can be further distinguished from E. labiatum by branched pectoral-fin rays 9-11 vs. 12-13. Exostoma stuarti has a distinctly arched dorsum and only three branched anal-fin ray relative to all congeners. Exostoma gaoligongense can be further distinguished from E. stuarti in pelvic fin reaching anus vs. not reaching, shorter caudal-peduncle, it length 18.8%-22.3% SL vs. 14.2-15.0, and relative more anterior position of anal-fin, anal to caudal distance/pelvic to anal distance 88.2%-116.7% vs. 79.6-82.6.   Hora & Silas (1952) recognized five valid species among the nominal species of Exostoma at that time, and of the five, one is included in a different genus now, namely Pseudexostoma yunnanensis. Exostoma labiatum was described from the Mishmis Hills of Assam [larger area of earlier times], India (McClelland, 1842) or known as Danbajiang, Tibet, China (Wu & Wu, 1992), and later was reported from other sites in the Brahmaputra and Chindwin river drainages of India (Hora & Mukerji, 1935;Hora & Silas, 1952), Brahmaputra River drainage of Tibet (Wu et al., 1981;Wu, 1985;Wu & Wu, 1992), and rivers in the Irrawaddy River drainage of Yunnan (Chu & Mo, 1999;Chu et al., 1990;Wu et al., 1981). Exostoma berdmorei was described from Tenasserim, southern Myanmar (Blyth, 1860;Hora & Silas, 1952), the second collection was from Dawna Hills, Kawkareik, Sukli, Kayin State, Myanmar, in 31 October 1934 (Fishbase-on-line). A small collection of Dr. Tyson Roberts kept in CAS from Salween drainage was examined for this study. From its forked caudal-fin, apparent small gill opening and only 14 branched caudal-fin rays, these fishes match very well characters of E. berdmorei in Hora & Silas (1952). It differs from description of Hora & Silas (1952) in the following characters: nasal barbel reaches eye or not vs. not
Exostoma stuarti was described from the Nam-Yak River at Tanja in northern Myanmar (from a single specimen) and later was reported from the Chatrickong River drainage in Manipur, India (Vishwanath et al., 1998;Keishing & Vishwanath, 1999). The first author collected 9 specimens from Tanjar Stream, Putao, 26 December 2015. The data of E. stuarti in this study was based on this collection. The description of E. vinciguerrae was based on one specimen from the Putao Plains, Kachin, northern Myanmar (Regan, 1905) and later was reported from Pazi Monghong, Hsipi State, and northern Shan States, northeastern Myanmar and Manipur, India (Hora and Silas, 1952;Vishwanath et al., 1998). Five collections from Zeyar and Ponyin streams, from 9 to 14 December, 2015, Hponkanrazi Wildlife Sanctuary, Malihka Drainage, Putao, collected by the first author and others were identified as E. vinciguerrae.
It seems Hora (Hora & Mukerji, 1935) assigned both Exostoma from Brahamaputra and Chindwin drainages (a big upper tributary of Irrawaddy) to E. labiatum, but later he (Hora, 1923;Hora & Silas, 1952) defined those from Brahamaputra as E. labiatum, while those from Upper Burma (Irrawaddy) as E. vinciguerrae. Wu et al. (1981) pointed out that according to the key of Hora & Silas (1952), E. labiatum and E. vinciguerrae differed in the shape of caudal fin (deeply emarginate vs. shallow emarginate) and number of branched pectoral-fin rays (12 vs. 10). Wu et al. (1981) compared specimens from Chayu (=Zayu) and Motuo (=Medog), the upper Brahmaputra River drainage in Tibet with some from Tengchong and Yingjiang, the Irrawaddy River drainage in Yunnan and found there were no prominent differences between them. Within-population variability in the caudal fin concavity (varied from shallow to deep) and branched pectoral-fin rays (mostly 10, a few 11) indicated that these characters will not distinguish the species reliably. From their observations, Wu et al. (1981) concluded that E. vinciguerrae was most likely a synonym of E. labiatum. Chu et al. (1990) pointed out that in specimens from the Irrawaddy River drainage in Yunnan the adipose fin can be confluent with the caudal fin or free from it; they questioned the use of caudal fin concavity and confluence of adipose and caudal fins by Hora & Silas (1952) and treated both E. vinciguerrae and E. stuarti as synonyms of E. labiatum.
Recent workers (Lalramliana et al., 2015;Ng & Vidthayanon, 2014;Talwar & Jhingran, 1991;Tamang et al., 2015;Vishwanath & Joyshree, 2007) all treated E. labiatum and E. vinciguerrae as distinct species. Lalramliana et al. (2015) pointed out "material from the Brahmaputra River drainage in Arunachal Pradesh and Nagaland identified as E. labiatum by Hora & Silas (1952) differs from the holotype of E. labiatum in having the adipose fin confluent with (vs. separate from) the upper procurrent caudal-fin rays, suggesting that it may represent an unnamed species". Seeing discrepancy in description of E. labiatum between Wu & Wu (1992) from upper Brahmaputra and Indian ones from lower, it implies that the identification of E. labiatum from both upper Brahmaputra and Irrawaddy drainages of China should be verified carefully in the future. Herein we follow these recent workers and confine E. vinciguerrae (and E. chaudhurii) in Malihka Drainage of Myanmar, and confine E. labiatum in Brahmaputra Drainage.
Based on all the results mentioned above, an artificial key to genus Exostoma is tentatively provided as follows: