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new species of bagrid catfish in the genus Mystus is described from the Mahakam River drainage in eastern Borneo. Mystus impluviatus closely resembles Mystus nigriceps and Mystus castaneus but differs in having a second posterior fontanel on the supraoccipital. It can be further distinguished from M. nigriceps and M. castaneus a combination of the following characters: maximum height of adipose fin 4.7–6.0% SL, length of adipose fin base 31.6–36.1% SL, long second and third dorsal fin rays, causing the dorsal fin to appear less rounded, maximum height of pectoral spine serrations 3.6–4.8% of spine length, a deeply forked caudal fin with a slender upper lobe, and a brown body with a distinct dark triangular patch on the base of the caudal peduncle.

 

THE genus Mystus Scopoli, 1777 is a group of bagrid catfishes found in streams and rivers throughout South and Southeast Asia. The genus Macrones was used by earlier authors, but Macrones Duméril, 1856, is preoccupied by Macrones Newman, 1841, in Coleoptera and Mystus Scopoli, 1777, was revived by Fowler (1928) as a replacement name. Mystus is poorly diagnosed and is polyphyletic (Mo, 1991). Twentytwo of the 45 nominal species of Mystus are found in Southeast Asia, 19 of which are currently considered valid.

The identity of Mystus nigriceps Valenciennes has been the subject of much confusion among ichthyologists (Ng, 2002). A recent study by Ng (2002) recognized two distinct species in the material generally identified as M. nigriceps: one in Java and the other in Sumatra, the Malay Peninsula, and northern and western Borneo. Ng restricted M. nigriceps for the Javanese species and described the other as Mystus castaneus. While comparing M. castaneus and material identified as M. nigriceps from Java and the Mahakam River drainage in eastern Borneo, differences were observed, which suggested that the Mahakam River material belong to an undescribed species.

MATERIALS AND METHODS
Measurements were made point to point with dial calipers and data recorded to tenths of a millimeter. Counts and measurements were made on the left side of specimens whenever possible. Sub units of the head are presented as proportions of head length (HL). Head length itself and measurements of body parts are given as proportions of standard length (SL). Measurements follow those of Ng and Dodson (1999); the maximum height of the pectoral spine serrations is measured following Ng and Tan (1999). Institutional abbreviations
follow Leviton et al. (1985) except for the Zoological Reference Collection of the Raffles Museum of
Biodiversity Research, National University of Singapore (ZRC).

DESCRIPTION
Mystus impluviatus n. sp. Figures 1, 2A, 3A, 4A, and 5A

Holotype.—CAS 97049, male, 87.0 mm SL; Borneo: Kalimantan Timur, small river on road 12.2 km from Sebulu crossroads towards forest and 59 km from Air Putih crossroads (08169S, 117809E); M. S. Christensen, H. Christensen, and H. Aschanuddin, 31 July 1982.

Paratypes.—CAS 94621, 2 juveniles, 20.3–22.8 mm SL; Borneo: Kalimantan Timur, beneath floating water hyacinth island about 6 m2, flooded swamp forest ca. 6 km SE of Kota Bangun (0°16'S, 116°34'E); M. S. Christensen et al., 1983. CAS 95038, 4 males, 76.2–119.6 mm SL, 1 female 80.8 mm SL; USNM 371267, 1 male, 83.0 mm SL, 1 female, 81.1 mm SL; Borneo: Kalimantan Timur, remnant pool in swamp forest, approximately 120 m2 along Sungai Kedang Kepala, ca. 15 km NNW of Muara Kaman (0°9'S, 116°43,E); M. S. Christensen and M. Schellenberg, November 1982. CAS 95035, 1 male, 98.9 mm SL; 1 female, 113.0 mm SL; MZB 10703, 1 female, 96.1 mm SL; Borneo: Kalimantan Timur, Belayan River (0°7'N, 116°22'E); M. S. Christensen et al., 10 August 1982. CAS 97045, 3 juveniles, 25.7–30.0 mm SL; Borneo: Kalimantan Timur, from small stream on logging trail going inland from southern junction of Sebulu, side road to stream approximately 4 km (0°16'S,117°0'E); M.S. Christensen and G.von Drabich, 31 October 1982.

 

Fig. 1. Mystus impluviatus, paratype male, CAS 95035, 98.9 mm SL.

 

Diagnosis.—Mystus impluviatus differs from both M. castaneus and M. nigriceps in having a second posterior fontanel on the supraoccipital (Fig. 2). It further differs from M. castaneus in having a shallower adipose fin (4.7–6.0% SL vs 6.1– 7.0), smaller serrations on the posterior edge of the pectoral spine (maximum height of serrations 3.6–4.8% of spine length vs 6.9–8.0), and a more deeply forked caudal fin with a more slender upper lobe (Fig. 3). Mystus impluviatus further differs from M. nigriceps in having a longer adipose fin base (31.6–36.1% SL vs 26.3– 31.4), longer second and third dorsal fin rays, causing the dorsal fin to appear less rounded, and a brown body with a distinct dark triangular patch on the base of the caudal peduncle (vs greenish gray body with a diffuse dark triangular patch on the base of the caudal peduncle).

 

 

Fig. 2. Schematic illustration of dorsal views of heads of (A) Mystus impluviatus, CAS 97049, holotype, 87.0 mm SL; (B) Mystus castaneus and Mystus nigriceps (M. castaneus, UMMZ 155714, 77.9 mm SL illustrated), showing presence of second posterior fontanel on supraoccipital in M. impluviatus. Scale bar represents 10 mm.

Description.—Head depressed; dorsal profile slightly convex and ventral profile almost straight. Bony elements of dorsal surface of head covered with thin skin; bones readily visible, ornamented with numerous fine grooves. Eye ovoid, horizontal axis longest, with free margin; located entirely in dorsal half of head. Gill openings wide, extending from posttemporal to beyond isthmus. Gill membranes free from isthmus, with 9 (n 5 11) or 10 (n 5 4) branchiostegal rays. First branchial arch with 6 + 14 (n = 4), 5 + 20 (n = 2), 5 + 21 (n = 2), 6 + 19 (n = 1), 6 + 20 (n = 4) or 6 + 21 (n = 2) gill rakers.

Mouth subterminal, fleshy upper lip extending anteriorly beyond upper jaw. Oral teeth small and viliform, in irregular rows on all tooth-bearing surfaces. Premaxillary tooth band rounded, of equal width throughout. Dentary tooth band much narrower than premaxillary tooth band at symphysis, tapering laterally. Vomerine tooth band unpaired, continuous across midline; smoothly arched along anterior margin, tapering laterally to point extending posteriorly well past level of premaxillary band; band width narrower than premaxillary band at midline, widening laterally and then tapering to a sharp point posterolaterally.

 

 

Fig. 3. Schematic illustration of caudal fins of (A) Mystus impluviatus, CAS 95038, paratype, 83.0 mm SL; (B) Mystus castaneus, UMMZ 155714, 77.9 mm SL. Scale bar represents 5 mm.

 

 

Fig. 4. Map showing distribution of Mystus castaneus, Mystus impluviatus and Mystus nigriceps (c). Base map courtesy of W. J. Rainboth.

Barbels in four pairs. Maxillary barbel long and slender, extending beyond caudal fin. Nasal barbel slender, extending to halfway between posterior orbital margin and dorsalmost extent of gill opening. Inner mandibular-barbel origin close to midline; barbel thicker and longer than nasal barbel and extending just beyond base of last pectoral fin ray. Outer mandibular barbel originates posterolateral of inner mandibular barbel, extending to base of first pelvic fin ray.

Body slightly compressed, becoming more so toward caudal peduncle. Dorsal profile rising evenly but not steeply from tip of snout to origin of dorsal fin and sloping gently ventrally from origin of dorsal fin to end of caudal peduncle. Ventral profile slightly convex to anal fin base, then sloping slightly dorsally to end of caudal peduncle.
Skin smooth. Lateral line complete and midlateral in position. Vertebrae 20 + 17 = 37 (n = 10), 21 + 16 = 37 (n = 1), 20 + 18 = 38 (n = 2), 21 + 17 = 38 (n = 1), or 20 + 19 = 39 (n = 1).

Dorsal fin with spinelet, spine, and 6 (n = 2) or 7 (n = 13) rays. Origin of dorsal fin slightly anterior to midline. Dorsal fin margin convex, usually with anterior branch of fin rays longer than other branches. Dorsal fin spine short, straight and slender, posterior edge with low irregular serrations.

Pectoral fin with stout spine, sharply pointed at tip, and 8 (n = 4), 9 (n = 2) or 9,i (n = 9)
rays. Anterior spine margin smooth; posterior spine margin with 10–12 small serrations along entire length. Pectoral fin margin straight anteriorly, convex posteriorly.

Pelvic fin origin at vertical through posterior end of dorsal-fin base, with i,5 (n = 15) rays and slightly convex margin; tip of adpressed fin not reaching anal fin origin. Anus and urogenital openings located at vertical through middle of adpressed pelvic fin. Males with a long, slender genital papilla extending to base of second anal fin ray.

Adipose fin with convex margin for entire length; anterior end reaching to base of last dorsal fin ray. Anal fin base ventral to posterior half of adipose fin, with iv,7 (n = 2), iv,8 (n = 9) or iv,9 (n = 4) rays and curved posterior margin.

Caudal peduncle moderately slender. Caudal fin deeply forked, with i,7,7,i (n = 2) or i,7,8,i (n = 13) principal rays; upper lobe slender and lanceolate, lower lobe pointed. Procurrent rays extend only slightly anterior to fin base. Morphometric data as in Table 1.

 

TABLE 1. MORPHOMETRIC DATA FOR Mystus impluviatus (n = 10). Range (mean ± SD) % SL

Head Length	27.3–29.2 (28.1 ± 0.70)
Head width	18.4–20.7 (19.4 ± 0.74)
Head depth	1.60–20.4 (17.6 ± 1.30)
Predorsal length	42.3–47.0 (38.7 ± 0.60)
Preanal length	69.8–73.1 (71.7 ± 1.87)
Prepelvic length	48.8–56.7 (51.4 ± 2.37)
Prepectoral length	23.8-28-9 (26.3 ± 1.60)
Body depth at anus	13.9–22.7 (18.2 ± 3.30)
Length of caudal peduncle	16.6–19.7 (18.2 ± 1.18)
Depth of caudal peduncle	7.7–10.0 (9.1 ± 1.08)
Pectoral spine length	17.7–22.2 (19.5 ± 1.25)
Pectoral fin length	18.8–23.6 (21.7 ± 1.80)
Dorsal spine length	10.5–14.7 (12.4 ± 1.30)
Length of dorsal fin base	14.4–17.2 (16.1 ± 0.92)
Pelvic fin length	16.1–19.0 (17.2 ± 0.90)
Length of anal fin base	9.3–13.5 (12.3 ± 1.09)
Caudal fin length	26.9–34.4 (29.7 ± 2.18)
Length of adipose fin base	31.6–36.1 (34.2 ± 1.46)
Maximum height of adipose fin	4.7–6.0 (5.6 ± 0.34)
Post-adipose distance	12.7–14.4 (13.4 ± 0.51)
% HL
Snout length	33.0–36.1 (34.9 ± 1.34)
Interorbital distance	33.3–40.0 (36.3 ± 2.21)
Eye diameter	18.9–21.1 (20.9 ± 1.09)
Nasal barbel length	64.9–86.9 (74.2 ± 8.32)
Maxillary barbel length	372.5–424.6 (394.5 ± 21.12)
Inner mandibular barbel length	96.2–121.8 (107.6 ± 8.16)
Outer mandibular barbel length	148.7–191.5 (176.2 ± 11.56)

 

 

Fig. 5. (A) Maximum height of the adipose fin (%SL) plotted against SL (mm) for Mystus castaneus and Mystus impluviatus; (B) Length of the adipose fin base (%SL) plotted against SL (mm) for M. impluviatus and Mystus nigriceps.

Color in alcohol.—Dorsal surface of head and body uniform medium brown, with a very faint dark gray humeral spot and a distinct oval or triangular dark gray spot on base of caudal peduncle.Ventral surfaces of head and body dirty white; adipose fin and fin rays of all fins brown; interradial membranes of all fins with scattered melanophores. Dorsal half of barbels brown at base, gradually turning to dark yellow on ventral half and tips.

Distribution.—Known from the Mahakam River drainage in eastern Borneo (Fig. 4).

Etymology.—From the Latin impluvium, which refers to a skylight; in allusion to the second posterior
fontanel on the supraoccipital of this species. Used as a noun in apposition.

DISCUSSION
The identity of M. nigriceps has been problematic (for a detailed discussion, see Ng, 2002), particularly so for the material from the Mahakam River. It has been variously identified as M. nigriceps by Christensen (1992) and as M. cf. cavasius by Kottelat (1994). Mystus impluviatus, M. castaneus, and M. nigriceps can be distinguished from all other Southeast Asian Mystus in having a moderately long adipose fin (29–37% SL) and an oval or triangular dark gray patch on the base of the caudal peduncle (all other Southeast Asian Mystus lack markings on the base of the caudal peduncle, except for M. bimaculatus Volz, 1904, which has a black vertical bar with a white vertical bar immediately anterior to it at the base of the caudal peduncle).

The difference in the height of the adipose fin between M. castaneus and M. impluviatus cannot be explained by ontogenetic change alone, as a plot of the maximum height of the adipose fin against SL (Fig. 5a) shows. A plot of the length of the adipose fin base against SL (Fig. 5b) also shows that ontogeny alone cannot explain the differences observed between M. nigriceps and M. impluviatus. Lines drawn through the plot of all data from both species as a single series have very low r2-values (0.0002 for the maximum height of the adipose fin and 0.0296 for the length of the adipose fin base).

Mystus impluviatus appears to be endemic to the Mahakam River drainage (or, at least, to eastern Borneo). This is not surprising, given that the Mahakam River was isolated from the other main Southeast Asian river drainages throughout the low sea level periods of the Pleistocene (Voris, 2000). Although the endemism of the Mahakam fish fauna is currently thought to be low (about 9%; Kottelat, 1994), recent studies have identified at least three more endemic species of catfishes (e.g., Ng, 2000, 2001; Ng and Ng, 2001). Therefore, the fish fauna of the Mahakam River deserves further study as it is likely that there are more endemic species in the river system.

COMPARATIVE MATERIAL
Mystus nigriceps: RMNH 2948, holotype, 119.3 mm SL; RMNH 3009, 199, 99.4 mm SL; Java. RMNH 15857, 105.3 mm SL; Java: Batavia (neotype of Bagrus micracanthus). MNHN 4369, 71.5 mm SL; Java (paralectotype of Bagrus keletius; photograph examined). CMK 9214, 7 ex., 63.9– 77.0 mm SL; Java: Java Timur, Kali Brantas basin, canal at Nggareman (Kecamatan Patianrowo, Kabupaten Kertosono) (7°34'S, 112°5'E). CMK 9231, 8 ex., 57.0–84.4 mm SL; Java: Java Timur, Kali Brantas basin, channelized stream through drained (formerly swampy) area at Campurdarat, S of Tulungagung (8°10'S, 111°20'E). MZB 10032, 2 ex., 78.1–79.5 mm SL; Java: Java Timur, Kabupaten Bojonegoro, Kecamatan Bojonegoro, Desa Bojonegoro, Bengawan Solo. MZB 10056, 10 ex., 56.1–86.1 mm SL; Java: Java Timur, Brantas River, Lengkong Dam at Mojokerto. MZB 10066, 4 ex., 77.6–110.5 mm SL; Java: Ciujung River, Kecamatan Pamarayan Serang. ZRC 43878, 4 ex., 67.5–81.0 mm SL; Java: Java Tengah, Citalahab next to rice field, probably draining into Citanduy about 20 km to Banjar.

Mystus castaneus: ZRC 41848, holotype, 121.4 mm SL; Borneo: Sarawak, Serian market, from Sungai Sadong. ZRC 28176, 1 paratype, 77.6 mm SL; Malaysia: Johor, Kota Tinggi, Sungai Mupor. ZRC 39419, 3 paratypes, 124.0–134.5 mm SL; Sarawk: Serian market. ZRC 40487, 8 paratypes, 86.5–114.1 mm SL; Borneo: Brunei, Tutong district, Sungai Merimbun, outflow from Tasik Merimbun draining into Sungai Tutong. ZRC 40490, 6 paratypes, 92.7–143.8 mm SL; Borneo: Sarawak, Serian market. UMMZ 155689, 1 ex., 101.9 mm SL; UMMZ 155713, 1 ex., 58.7 mm SL; UMMZ 155714, 6 ex., 38.8– 87.8 mm SL; Sumatra: Musi River, Muara Klingi.

ACKNOWLEDGMENTS
I am grateful to D. Catania (CAS), M. Kottelat (CMK), I. Rachmatika, and R. Hadiaty (MZB); M. van Oijen (RMNH), D. Nelson (UMMZ), and K. Lim (ZRC) for access to material under their care, and to W. Rainboth for permission to use the map of Southeast Asia. This work was funded by support from the Rackham School of Graduate Studies of the University of Michigan.

LITERATURE CITED
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FOWLER, H. W. 1928. Further notes and descriptions of Bombay shore fishes. J. Bombay Nat. Hist. Soc. 33:100–119.
KOTTELAT, M. 1994. The fishes of the Mahakam River, east Borneo: an example of the limitations of zoogeographic analyses and the need for extensive fish surveys in Indonesia. Trop. Biodiv. 2:401–426.
LEVITON, A. E., R. H. GIBBS, JR., E. HEAL AND C. E. DAWSON. 1985. Standards in herpetology and ichthyology.
Part I. Standard symbolic codes for institutional resource collections in herpetology and ichthyology. Copeia 1985:802–832.
MO, T.-P. 1991. Anatomy and systematics of Bagridae (Teleostei) and siluroid phylogeny. Theses Zool. 17: 1–216.
NG, H. H. 2000. Bagrichthys vaillantii (Popta, 1906), a valid species of bagrid catfish from eastern Borneo (Teleostei: Siluriformes). Zool. Med. 73:327–332.
———. 2001. Clarias microstomus, a new species of clariid catfish from eastern Borneo (Teleostei: Siluriformes). Zool. Stud. 40:158–162.
———. 2002. The identity of Mystus nigriceps (Valenciennes in Cuvier & Valenciennes, 1840), with the description of a new bagrid catfish (Teleostei: Siluriformes) from Southeast Asia. Raffles Bull. Zool 50:161–168.
———, AND J. J. DODSON. 1999. Morphological and genetic descriptions of a new species of catfish,
Hemibagrus chrysops, from Sarawak, East Malaysia, with an assessment of phylogenetic relationships (Teleostei: Bagridae). Ibid. 47:45–57.
———, AND P. K. L. NG. 2001. A revision of the akysid catfish genus Acrochordonichthys Bleeker. J. Fish Biol. 58:386–418.
———, AND H. H. TAN. 1999. The fishes of the Endau drainage, Peninsular Malaysia with descriptions of two new species of catfishes (Teleostei: Akysidae, Bagridae). Zool. Stud. 38:350–366.
VORIS, H. K. 2000. Maps of Pleistocene sea levels in Southeast Asia: shorelines, river systems and time durations. J. Biogeog. 27:1153–1167.

FISH DIVISION, MUSEUM OF ZOOLOGY, UNIVERSITY OF MICHIGAN, 1109 GEDDES AVENUE, ANN ARBOR, MICHIGAN 48109-1079. E-mail: [email protected]. Submitted: 6 Oct. 2002. Accepted: 6 Nov. 2002. Section editor: D. G. Buth.