Zootaxa 2793: 56–62 (2011) www.mapress.com / zootaxa/ Copyright © 2011 · Magnolia Press ISSN 1175-5326 (print edition) Article ZOOTAXA ISSN 1175-5334 (online edition) Silurichthys ligneolus, a new catfish (Teleostei: Siluridae) from southern Borneo, Indonesia HEOK HEE NG & HEOK HUI TAN Raffles Museum of Biodiversity Research, National University of Singapore, 6 Science Drive 2 #03-01, Singapore 117546. E-mail: heokhee@nus.edu.sg; heokhui@nus.edu.sg Abstract Silurichthys ligneolus, a new species of silurid catfish, is described from blackwater habitats in southern Borneo. The new species can be distinguished from congeners in having a combination of: body depth at anus 9.1–11.0% SL, caudal peduncle depth 4.7–5.5% SL, eye diameter 6.7–9.8% HL, pelvic fins present, 44–45 anal-fin rays, 3–4 principal rays on the lower caudal-fin lobe, lower lobe of caudal fin indistinct, 46–48 vertebrae, one gill raker on the first branchial arch, and body uniformly brown. Based on the morphology of its caudal fin, S. ligneolus is hypothesized to be the sister taxon of S. sanguineus. Key words: Ostariophysi, Siluriformes, Kalimantan Tengah Introduction The silurid genus Silurichthys is restricted to the forest streams and freshwater swamps of Sundaic Southeast Asia. Members of the genus resemble the hillstream silurids of the genus Pterocryptis (their sister group, fide Bornbusch, 1995), but are distinguished from them by their confluent anal and caudal fins. The following combination of characters additionally diagnoses Silurichthys (fide Bornbusch, 1995): (1) metapterygoid contacting the hyomandibula only by a narrow extension that reaches to the anterodorsal corner of the hyomandibula; (2) ascending process of scaphium absent; (3) 9–11 branched caudal-fin rays; (4) upper hypurals fused to form a single plate; (5) lower hypurals fused to each other and, variably, to the parhypural. The only revision of the genus to date (Ng & Ng, 1998) recognizes eight valid species: S. citatus Ng & Kottelat, 1997; S. gibbiceps Ng & Ng, 1998; S. hasseltii Bleeker, 1858; S. indragiriensis Volz, 1904; S. marmoratus Ng & Ng, 1998; S. phaiosoma (Bleeker, 1851); S. sanguineus Roberts, 1989; S. schneideri Volz, 1904. During ichthyological surveys of the Kahayan River drainage in southern Borneo, four specimens of a Silurichthys species were collected and initially identified as S. sanguineus. A detailed study of this material and comparison with congeners revealed them to belong to an undescribed species. This unnamed species is described in this study as Silurichthys ligneolus, new species. Material and methods Measurements were made point to point with digital calipers and data recorded to tenths of a millimeter. Counts and measurements were made on the left side of specimens whenever possible. Vertebrae and median-fin rays were counted from radiographs, while paired-fin rays were counted under a binocular dissecting microscope. Subunits of the head are presented as proportions of head length (HL). Head length and measurements of body parts are given as proportions of standard length (SL). Measurements follow those of Ng & Ng (1998). An asterisk after a meristic value indicates that for the holotype; the number in parentheses after a count indicates frequency. Institutional acronyms follow Ferraris (2007). 56 Accepted by R. Pethiyagoda: 3 Feb. 2011; published: 17 Mar. 2011 Silurichthys ligneolus sp. nov. (Fig. 1) Type material. Holotype: MZB 17184, 55.3 mm SL; Indonesia, Borneo: Kalimantan Tengah, Kahayan River drainage, Rungan River sub-drainage, Sungai Panta, blackwater river draining into Rungan River and its confluence, connected to Nyaru Menteng, 2°2'1.0"S 113°47'5.5"E; M. Kottelat & H. H. Tan, 5 March 2008. Paratypes: CMK 21943 (1), 27.5 mm SL; ZRC 52048 (2), 25.0–43.0 mm SL; Indonesia, Borneo: Kalimantan Tengah, Sebangau River drainage, Sebangau River, blackwater canal draining from forest, 2°17'56.7"S 113°52'22.4"E; M. Kottelat & H. H. Tan, 6 March 2008. Diagnosis. Silurichthys ligneolus can be distinguished from all congeners except for S. sanguineus in having 3–4 (vs. 5–7) principal rays on the lower caudal-fin lobe. It differs from S. sanguineus in having (vs. lacking) pelvic fins, fewer anal-fin rays (44–45 vs. 60) and vertebrae (46–48 vs. 57). The following combination of characters additionally distinguishes it from congeners: body depth at anus 9.1–11.0% SL, caudal peduncle depth 4.7–5.5% SL, eye diameter 6.7–9.8% HL, lower lobe of caudal fin indistinct, one gill raker on first branchial arch, and uniformly brown body. Description. Biometric data are given in Table 1. Body laterally compressed. Head somewhat depressed. Dorsal profile straight, descending gradually from dorsal-fin origin to snout tip. Anterior profile of snout rounded. Anterior pair of nostrils tubular and anteromedial to maxillary-barbel base. Posterior pair of nostrils bordered by fleshy dorsal and ventral membranes, situated posteromedial to maxillary-barbel base. Eyes small, subcutaneous; located in anterior half of head; visible dorsally, not visible ventrally. TABLE 1. Morphometric data for Silurichthys ligneolus (n=4). Holotype Range Mean±SD 26.8 26.8–28.7 27.7±0.87 % SL Predorsal length Preanal length 30.6 30.6–34.8 32.8±1.73 Prepelvic length 26.9 26.9–30.2 28.6±1.74 Prepectoral length 16.6 16.6–20.4 18.9±1.64 Length of dorsal-fin base 1.8 1.5–1.8 1.7±0.13 Anal-fin length 68.7 64.7–68.7 66.9±1.65 Pelvic-fin length 7.5 6.8–7.5 7.1±0.33 Pectoral-fin length 14.5 13.8–15.1 14.5±0.53 Pectoral-spine length 8.1 6.0–8.1 7.4±0.97 Caudal-fin length 30.9 25.1–30.9 28.2±2.56 Body depth at anus 11.0 9.1–11.0 10.2±0.88 Caudal peduncle depth 4.7 4.7–5.5 4.9±0.39 Head length 16.3 16.3–21.8 19.2±2.44 Head width 12.8 11.0–13.2 12.4±0.97 Head depth 9.4 8.2–11.2 9.7±1.24 34.4 33.3–37.3 34.9±1.71 % HL Snout length Interorbital distance 37.8 36.9–40.0 38.5±1.39 Eye diameter 6.7 6.7–9.8 8.0±1.39 Maxillary barbel length 243.3 146.7–243.3 201.3±54.21 Mandibular barbel length 111.1 102.0–118.3 112.0±7.36 Mouth subterminal; gape horizontal or very slightly oblique. Well-developed rictal fold present, consisting of large and fleshy upper lobe joined at corner of mouth with lower lobe; lower lobe subtended by short submandibular groove. NEW SILURICHTHYS Zootaxa 2793 © 2011 Magnolia Press · 57 FIGURE 1. Silurichthys ligneolus, holotype, MZB 17184, 55.3 mm SL; Borneo: Kahayan River drainage. Teeth villiform. Dentary teeth in slightly curved, elongate bands narrowing posteriorly, reaching from symphysis almost to mouth corners; premaxillary teeth in broader, slightly curved rectangular bands; vomerine teeth in a single crescent-shaped band. Maxillary barbels slightly flattened, reaching to anterior third of anal fin. One pair of mandibular barbels present; located slightly anterolateral to gular fold; barbels flattened for most of proximal length, reaching to middle of pectoral-fin base. Gill membranes separate, overlapping, free from isthmus. Branchiostegal rays 9 (4). Gill rakers short, small; 0+1* (2). Distal margin of dorsal fin pointed, with i,2 (4) rays; segments of first ray not co-ossified to form spine. Distal margin of pectoral fin broadly convex, with 7,i (2) or 8* (2) rays. Segments of the proximal two-thirds of first pectoral-fin element co-ossified, forming spine. Pectoral spine and articulated segments sexually dimorphic in mature individuals. Spine in males broad, somewhat flattened dorsoventrally, with 5–7 serrations on posterior edge, increasing in size distally; proximal articulated segments with 0–4 serrations on posterior edge. Spine in females and juveniles slender, without serrations on posterior edge of spine proper and proximal articulated segments. Distal margin of pelvic fin convex, with i,5,i (4) rays. Distal margin of anal fin straight, with 44 (2) or 45* (2) rays; joined to caudal fin for length of last anal-fin ray. Integument over anal fin thickened proximally for slightly more than half of ray lengths; fin-ray erector muscles extending along anterior edges of anal-fin rays, ventralmost extent of muscles that of thickened integument. Caudal fin very obliquely truncate, lower lobe not markedly distinct from upper lobe; principal rays i,6,3* (3) or i,6,3,i (1). Vertebrae 11+35=46* (1), 11+36=47 (1) or 11+37=48 (2). 58 · Zootaxa 2793 © 2011 Magnolia Press NG & TAN Coloration. In 70% ethanol: flanks and thickened integument over anal fin chestnut brown. Dorsal surface and sides of head uniformly chestnut brown, fading to dark yellow on ventral surfaces of head, breast and belly. Maxillary and mandibular barbels brown, fading to yellow distally. Dorsal, pectoral, pelvic and anal fins hyaline; anal fin with scattering of melanophores along fin rays imparting very diffuse brown color. Caudal fin largely hyaline, with scattered melanophores imparting diffuse brownish color at its base. Distribution. This species is known only from blackwater habitats in the Kahayan and Sebangau river drainages in southern Borneo (Fig. 2). Etymology. The specific epithet comes from the Latin ligneolus, the diminutive form of the Latin noun ligneus, meaning wood. This name is used in allusion to the slender, uniformly brown body of this species (resembling a small piece of wood). FIGURE 2. Map showing collecting localities of Silurichthys ligneolus. NEW SILURICHTHYS Zootaxa 2793 © 2011 Magnolia Press · 59 Discussion Bornbusch (1991) diagnosed the Pterocryptis+Silurichthys clade as having fewer gill rakers on the first branchial arch than other members of the Siluridae. This reduction is typically manifested as fewer rakers on both the epibranchial and ceratobranchial, although the reduction is more pronounced in the former. The reduction in the number of gill rakers within the Pterocryptis+Silurichthys clade can be further subdivided into two states. All Silurichthys have zero or one rakers (Ng & Ng, 1998), while all Pterocryptis typically have two or more rakers on the first epibranchial (Bornbusch, 1991; Ng, 1999; Ng & Freyhof, 2001; Ng & Chan, 2005). We therefore identify the presence of zero or one gill rakers on the first epibranchial as an additional synapomorphy that diagnoses Silurichthys. Silurichthys ligneolus not only shares the unusually reduced number (3–4) of principal rays on the lower lobe of the caudal fin with S. sanguineus, but also the unusual shape of the caudal fin. In both species, the lower lobe is not markedly distinct from the upper lobe and the fin appears very obliquely truncate (Fig. 3a; also see Ng & Ng, 1998: Fig. 3g), as opposed to other congeners (except S. citatus, which has a nearly truncate caudal fin), in which the upper and lower caudal fin lobes are easily distinguishable, with the upper lobe being of equal length, or longer than the lower lobe (Fig. 3b). We hypothesize here that the number of principal rays on the lower caudal-fin lobe and the shape of the caudal fin are synapomorphies that diagnose S. ligneolus and S. sanguineus as sister taxa. Although we have not directly verified the sexes of the type series of S. ligneolus, we note that sexual dimorphism of the form seen in S. ligneolus has been verified in congeners (Ng & Ng, 1998), as well as in other members of the Siluridae (Bornbusch & Lundberg, 1989; Bornbusch, 1991). We therefore feel confident in extrapolating the sexual dimorphism in pectoral-spine serrations seen in congeners and confamilials to S. ligneolus. Silurichthys ligneolus is found sympatrically and syntopically with S. phaiosoma in the Kahayan River drainage. In addition to the number of principal rays on the lower lobe of the caudal fin (as outlined in the Diagnosis) and the shapes of the caudal fin discussed in the preceding paragraph, it can be distinguished from S. phaiosoma in having a more slender body (9.1–11.0% SL vs. 12.3–17.5) that is uniformly brown (vs. usually slightly mottled; compare Figs. 1 and 4), a smaller eye (6.7–9.8% HL vs. 10.9–17.5), fewer anal-fin rays (44–45 vs. 49–56) and vertebrae (46–48 vs. 49–52). Silurichthys ligneolus additionally differs from S. citatus in having a more slender body (depth at anus 9.1–11.0% SL vs. 14.9–16.4) and caudal peduncle (4.7–5.5% SL vs. 6.5–7.6) and fewer anal-fin rays (44–45 vs. 56), and from S. gibbiceps and S. hasseltii in having a more slender body (depth at anus 9.1–11.0% SL vs. 12.3–20.6) and caudal peduncle (4.7–5.5% SL vs. 5.8–8.8), a smaller eye (6.7–9.8% HL vs. 9.5–17.8), fewer anal-fin rays (44–45 vs. 46–56) and vertebrae (46–48 vs. 48–52). It is additionally distinguished from S. indragiriensis in having a more slender body (depth at anus 9.1–11.0% SL vs. 12.4–18.3), smaller eye (6.7–9.8% HL vs. 10.3–14.8) and uniform (vs. mottled) coloration, from S. marmoratus in having a more slender body (depth at anus 9.1–11.0% SL vs. 14.1–20.3), caudal peduncle (4.7–5.5% SL vs. 6.3–9.5) and uniform (vs. mottled) coloration, and from S. schneideri in having fewer anal-fin rays (44–45 vs. 58–68), gill rakers on the first branchial arch (1 vs. 7– 8) and vertebrae (46–48 vs. 53–55). Comparative material Silurichthys phaiosoma: ZRC 52049 (2), 81.0–98.9 mm SL; Borneo: Kalimantan Tengah, Kahayan River drainage, Rungan River, Nyaru Menteng, 2°2'S 113°47'E. ZRC 52050 (2), 32.5–49.9 mm SL; Borneo: Kalimantan Tengah, Sebangau River drainage, Sebangau River, blackwater canal draining from forest, 2°17'56.7"S 113°52'22.4"E. ZRC 52051 (11), 57.7–74.9 mm SL; Borneo: Kalimantan Tengah, Kahayan River drainage, Sungai Tahai, flowing into Rungan River, ca. 30 km outside Palangkaraya, 2°1'40.4"S 113°45'50.9"E. ZRC 52052 (25), 35.7–72.2 mm SL; Borneo: Kalimantan Tengah, Kahayan River drainage, junction of Sungai Tahai with Danau Tahai, 2°1'7.9"S 113°46'37.4"E. See Ng & Ng (1998) for a list of additional material examined. 60 · Zootaxa 2793 © 2011 Magnolia Press NG & TAN FIGURE 3. Caudal fins of: a. Silurichthys ligneolus, holotype, MZB 17184, 55.3 mm SL; b–c. typical condition for most other species of Silurichthys (b. S. phaiosoma, ZRC 52052, 51.1 mm SL; c. S. marmoratus, paratype, ZRC 25941, 53.6 mm SL). Split in between the last and penultimate anal-fin ray seen in the holotype of S. ligneolus is an artifact. Images not to scale. NEW SILURICHTHYS Zootaxa 2793 © 2011 Magnolia Press · 61 FIGURE 4. Silurichthys phaiosoma, ZRC 52052, 51.1 mm SL; Borneo: Kahayan River drainage. Acknowledgments We thank the following for access to material under their care: James Maclaine (BMNH), David Catania (CAS), Patrice Pruvost (MNHN), Renny Hadiaty (MZB), Ernst Mikschi (NMW), Martien van Oijen (RMNH), Jeffrey Williams (USNM), Ronald Vonk (ZMA), and Kelvin Lim (ZRC). Thanks to Maurice Kottelat, Patrick Yap and Hendra Tommy, for field assistance and logistical support, and to two anonymous reviewers for suggestions to improve the manuscript. Financial assistance for field work conducted by the second author is gratefully acknowledged from the Raffles Museum of Biodiversity Research, National University of Singapore research grants R264-001-004-272 and R-154-000-318-112. Literature cited Bornbusch, A.H. (1991) Redescription and reclassification of the silurid catfish Apodoglanis furnessi Fowler (Siluriformes: Siluridae), with diagnoses of three intrafamilial silurid subgroups. Copeia, 1991, 1070–1084. Bornbusch, A.H. 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