Silurichthys insulanus, Low & Ng & Tan, 2022

Low, Bi Wei, Ng, Heok Hee & Tan, Heok Hui, 2022, Two new species of blackwater catfishes (Siluriformes: Siluridae and Clariidae) from the Natuna Archipelago, Indonesia, Raffles Bulletin of Zoology 70, pp. 385-396 : 386-389

Silurichthys insulanus , new species

( Figs. 1 , 2 )

Silurichthys marmoratus (non Ng & Ng, 1998) – Tan & Lim, 2004: 109.

Holotype. MZB 17239, 41.9 mm SL; Pulau Natuna Besar: blackwater tributary of Sungai Sekeram, 3°50′29″N 108°03′47″E; D. Wowor et al., 18 March 2002.

Diagnosis. Silurichthys insulanus is distinguished from all congeners in having fewer branched dorsal-fin rays (1 vs. 2–3) and (except S. gibbiceps ) a distinctly humped (vs. evenly curving) nuchal profile ( Fig. 1 ). It is further distinguished from congeners by having the following unique combination of characters: eye diameter 8% HL; pectoral-fin length 16.0% SL; body depth at anus 16.0% SL; 54 anal-fin rays; caudal peduncle depth 5.7% SL; caudal fin asymmetrically forked, with 12 principal rays and upper lobe 1.1 times longer than lower lobe; and 50 vertebrae.

Description. Biometric data is given in Table 1. General body form as in Fig. 2 . Body laterally compressed. Head somewhat depressed. Dorsal profile straight, descending gently from dorsal-fin origin to snout tip. Lateral line complete, extending to middle of caudal-fin base; minute, posteroventrally-directed branches visible, decreasing in size posteriorly. 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 and situated posteromedial to maxillary barbel base. Eyes small, subcutaneous; located in anterior half of head; visible dorsally, but not ventrally.

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.

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 length, reaching to middle of pectoral-fin base. Gill membranes separate and overlapping, free from isthmus. Branchiostegal rays 14. Gill rakers short and small; 0 + 4.

Distal margin of dorsal fin pointed, with i,1 rays; segments of first ray not co-ossified to form spine. Distal margin of pectoral fin broadly convex, with 8,i rays. Segments of the proximal two-thirds of first pectoral-fin element co-ossified, forming spine. Pectoral spine slender, without serrations on posterior edge of spine proper and proximal articulated segments. Distal margin of pelvic fin convex, with i,5 rays. Distal margin of anal fin straight, with 54 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, ventral-most extent of muscles that of thickened integument. Caudal fin forked, with upper lobe 1.1 times longer than lower lobe; principal rays i,6,4,i. Vertebrae 13+37=50.

Preserved colouration: Flanks and thickened integument over anal fin brown, with numerous dark patches forming faint mottled pattern. Dorsal surface and sides of head brown with numerous scattered dark spots, fading to cream on ventral surfaces of head, breast, and belly. Dorsal, anal, and caudal fins brown, distal margins very slightly hyaline. Pectoral and pelvic fins light brown, with hyaline inter-radial membranes.

Distribution and habitat notes. Silurichthys insulanus , new species, is only known from Pulau Natuna Besar of the Natuna Archipelago, Indonesia, where it occurs in heath-peat swamp forest habitat with acidic (pH ~3.4), tannin-stained black waters. Syntopic species include Desmopuntius hexazona , Rasbora einthovenii , Trigonopoma gracile , Clarias rennyae , new species, Dermogenys colletei, Hemiramphodon pogonognathus, Brachygobius doriae , Betta aurigans , and Luciocephalus pulcher .

Etymology. The specific epithet comes from the Latin adjective insulanus , -a, -um, which means “of or belonging to an island”. The name is used in reference to the only location this species is known from (an island).

Remarks. In addition to the number of branched dorsal-fin rays and the nuchal profile, Silurichthys insulanus is further distinguished from S. citatus in having a longer pectoral fin (16.0% SL vs. 12.6–14.2), a more slender caudal peduncle (5.7% SL vs. 6.5–7.6) and an asymmetrically-forked caudal fin with distinct lobes (vs. obliquely truncate without distinct lobes), from S. exortivus in having (vs. lacking) dorsal and pelvic fins, and more principal caudal-fin rays (7+5 vs. 4+5), from S. gibbiceps in having a smaller eye (diameter 8% HL vs. 10–14), a longer pectoral fin (16.0% SL vs. 13.0–14.8), a more slender caudal peduncle (5.7% SL vs. 6.3–8.8), more branchiostegal rays (14 vs. 10) and gill rakers (0+4 vs. 0+2), and from S. hasseltii in having a smaller eye (diameter 8% HL vs. 11–18). It further differs from S. indragiriensis in having a smaller eye (diameter 8% HL vs. 10–15) and a shorter upper lobe of the caudal fin relative to the lower lobe (1.1 times vs. 1.4–1.6), from S. ligneolus in having a deeper body (depth at anus 16.0% SL vs. 9.1–11.0), more anal-fin rays (54 vs. 44–45), vertebrae (50 vs. 46–48), and principal caudal-fin rays (7+5 vs. 7+3–4), and an asymmetricallyforked caudal fin with distinct lobes (vs. obliquely truncate without distinct lobes), and from S. marmoratus in having a smaller eye (diameter 8% HL vs. 10–13), a longer pectoral fin (16.0% SL vs. 12.4–14.9), and a more slender caudal peduncle (5.7% SL vs. 6.3–9.5). Silurichthys insulanus is also distinguished from S. phaiosoma in having a smaller eye (diameter 8% HL vs. 11–14) and a shorter upper lobe of the caudal fin relative to the lower lobe (1.1 times vs. 1.4–1.6), from S. sanguineus in having a deeper body (depth at anus 16.0% SL vs. 9.2) and caudal peduncle (5.7 vs. 4.2), fewer vertebrae (50 vs. 57), more principal caudal-fin rays (7+5 vs. 7+4), and an asymmetrically-forked caudal fin with distinct lobes (vs. obliquely truncate without distinct lobes), and from S. schneideri in having a longer pectoral fin (16.0% SL vs. 12.7–14.8), and fewer anal-fin rays (54 vs. 58–68) and vertebrae (50 vs. 53–55).

Comparative material. Silurichthys exortivus: MZB 17240, 60.6 mm SL; Borneo: Kalimantan Timur, Mahakam River drainage, Belayan River system, REA Plantations , Long Buluh Damai Estate , 0°14′29″N 116°19′14″E.

Silurichthys gibbiceps: ZRC 40328 (5 paratypes), 47.2–75.2 mm SL; Borneo: Kalimantan Tengah, Barito basin, Sungei Paku –merah, 0°35′10″N 115°11′24″E.

Silurichthys marmoratus: ZRC 40107 (9), 34.5–52.4 mm SL; Borneo: Brunei Darussalam, Belait District, peat swamp from next to Sungai Sepan. ZRC 40110 (1), 56.5 mm SL; Borneo: Brunei Darussalam, Tutong District, Sungai Ratuan Uluh , 200 km behind base camp at Tasik Merimbun , 4°34′53″N 114°21′24″E. ZRC 41849 (1), 97.8 mm SL; Borneo: Sarawak, Serian market.

Additional comparative material used is listed in Ng & Ng (1998) and Ng & Tan (2011).

Figures

Fig. 1. Lateral views of head region of A) Silurichthys insulanus, new species, MZB 17239 holotype, 41.9 mm SL; B) S. marmoratus, ZRC 40107, 41.7 mm SL, Brunei; C) S. gibbiceps, ZRC 40328 paratype, 47.2 mm SL, Barito (not to scale).

Fig. 2. Lateral view of Silurichthys insulanus, new species, MZB 17239 holotype, 41.9 mm SL (composite of the same fish, top with black background, bottom with white background).