Ompok platyrhynchus , Heok Hee Ng & Heok Hui Tan, 2004
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(Figs. 1 & 2a)
Type material. Holotype: ZRC 48678, male, 78.9 mm SL; Borneo: Brunei Darussalam, Temburong district: Temburong basin, Belalong sub-basin; Sungai Esu, about 15 minutes upstream of Kuala Belalong Field Studies Centre (04°32'17.9"N 115°09'35.2"E); H. H. Tan & K. K. P. Lim, 6 Oct 2001.
Paratype: ZRC 31807, cleared and stained, 55.0 mm SL; Borneo: Brunei Darussalam, Temburong District, Sungai Belalong at Kuala Belalong Field Study Center; S. C. Choy, 27 July 1992.
Diagnosis. Ompok platyrhynchus ZBK can be distinguished from all Southeast Asian congeners, except for O. hypophthalmus , O. rhadinurus ZBK and O. urbaini , in having 74-80 (vs. 40-70) anal-fin rays. Ompok platyrhynchus ZBK differs from O. hypophthalmus , O. rhadinurus ZBK and O. urbaini in lacking a distinct nuchal concavity (Fig. 2), and having a more slender body (13.5-17.7% SL vs. 18.9-24.5), shorter snout (37.1-38.1% HL vs. 39.4-47.5) and maxillary barbels (reaching to middle of pectoral fin vs. reaching to anterior third of anal fin), and more vertebrae (59-60 vs. 47-58).
Description. Biometric data in Table 1. Body laterally compressed; maximum body depth located at pelvic-fin origin; head as broad as body and depressed. Dorsal profile of body gently convex.
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 posteromedial to maxillary barbel base.
Mouth terminal; gape horizontal, small and extending halfway between maxillary barbel base and anterior orbital margin. Well-developed rictal lobes present, subtended by deep submandibular groove; upper rictal lobe without skin fold. Thin, broad supralabial fold extending from below orbit to maxillary barbel base.
Jaw teeth depressible and villiform. Premaxillary teeth in 4-5 irregular rows in narrow, gently curved rectangular bands. Dentary teeth in similar, slightly narrower bands narrowing posterolaterally, reaching from symphysis almost to mouth corners. First row of dentary teeth slightly visible when mouth is closed. Vomerine teeth in 2-3 rows in single ovoid patch straddling midline.
Two pairs of barbels, slightly flattened along entire length. Maxillary barbels reaching to middle of pectoral fin. Mandibular barbels (only outer pair present) reaching just beyond head. Eyes small, subcutaneous (without free orbital margin); located approximately midway on head and immediately behind supralabial fold. Dorsal orbital margin just visible dorsally; ventral quarter of orbital margin visible ventrally.
Gill membranes separate and overlapping, free from isthmus; gular fold well-developed and v-shaped. Branchiostegal rays 10 (1) or 11* (1). Gill rakers long and thin; anteriormost rakers on lower first arch widely spaced; 4+13 (1) or 4+14* (1).
Dorsal fin small, with i,1 (2) rays. Depressed pectoral fin to origin of anal fin; distal margin broadly convex, with rounded tip. Third branched pectoral ray longest and fin with 10 (1) or 14* (1) rays. Proximal two-thirds of first pectoral-fin element co-ossified into a slender spine. Spine with shallow oblique striae on dorsal and ventral surfaces and with 5 serrations on posterior edge spanning the distal end of the ossified and proximal end of the flexible distal tip. Axillary pore small, located just above pectoral spine base. Depressed pelvic fin reaching to second or third anal-fin ray; distal margin convex with i,7 (2) rays. Distal margin of anal fin straight, with 74* (1) or 80 (1) rays; separate from caudal fin. Integument over anal fin thickened proximally for two thirds of ray lengths; finray erector muscles attaching to base of fin rays, ventralmost extent of muscles defined by area of thickened integument. Caudal peduncle slender. Caudal fin deeply forked, lobes elongate and with rounded tips; upper lobe slightly longer; principal rays i,7,8,i (2).
Lateral line complete, extending to middle of caudal-fin base, with short branches along flanks directed posteroventrally. Urogenital papilla located immediately posterior to insertion of pelvic fin. Vertebrae 13+46=59 or 14+46=60*.
Coloration. In 70% ethanol: Body and head cream and diffusely pigmented. Light powdering of melanophores on all surfaces of head and body imparting a grayish color, somewhat less dense on belly and ventral surface of head. All fin rays with a light powdering of melanophores; fin membranes hyaline. Barbels with melanophores on dorsal half; ventral half unpigmented.
Color in life translucent (Fig. 1b)
Distribution. Known from the Temburong River drainage in northern Borneo (Fig. 3).
Habitat. The holotype of O. platyrhynchus ZBK was obtained from a small meandering stream about 3 meters at its widest and up to 1 meter deep. The water was clear and fast flowing over a rock and sand substratum, at parts bedrock that split the stratum and formed small cascades up to 3 meters high (Fig. 4). Syntopic fish collected include: Hampala bimaculata , Nematabramis steindachneri ZBK , Paracrossochilus acerus ZBK , Rasbora agyrotaenia , Tor tambra (Cyprinidae), Gastromyzon lepidogaster ZBK , Neogastromyzon ZBK sp., Parhomaloptera microstoma , Protomyzon ZBK sp. (Balitoridae), Pangio cf. mariarum (Cobitidae), Hemibagrus baramensis (Bagridae), Pterocryptis furnessi (Siluridae) and Macrognathus maculatus (Mastacembelidae).
Etymology. From the Greek platys, meaning flat, and rhynchos, meaning nose; in reference to the lack of a distinct nuchal concavity in this species. Used as a noun.
The highly elevated anal-fin ray counts of O. platyrhynchus ZBK easily distinguishes it from other species of Southeast Asian Ompok ZBK , except for O. hypophthalmus , O. rhadinurus ZBK and O. urbaini (Table 2).
The biometric differences between O. platyrhynchus ZBK and members of the O. hypophthalmus species group are summarized in Table 3. The differences observed in the body depth and snout length between O. platyrhynchus ZBK and O. hypophthalmus , O. rhadinurus ZBK and O. urbaini are not solely due to ontogeny. A bivariate analysis (ANCOVA) shows that the regression lines of the body depth (Fig. 5a) and snout length (Fig. 5b) on SL are significantly different (with P=0.0137, P<0.0001, and P=0.011 for body depths of O. hypophthalmus , O. rhadinurus ZBK and O. urbaini respectively and P=0.034, P=0.0347, and P<0.0001 for snout lengths of O. hypophthalmus , O. rhadinurus ZBK and O. urbaini respectively).
Steindachner (1901) described Ompok borneensis from the Baram River. The species is known only from the holotype whose current disposition is unknown. Because of the similarity in the freshwater ichthyofauna of the Temburong and Baram River drainages, some comments on the identity of O. borneensis and more detailed comparison with O. platyrhynchus ZBK is necessary. Ompok borneensis is treated as a senior synonym of Ompok jaynei Fowler, 1905 ZBK here. Steindachner (1901) noted that the two median caudal-fin rays were shorter and thinner than the ones immediately adjacent to it (“Die mittleren 2 caudalstrahlen sind bedeutend kürzer und zarter als die nahe gelegenen längsten strahlen der flosse.”), implying that the caudal fin is forked, and not rounded as illustrated in the original description (Steindachner, 1901: Pl. 18 Fig. 3). The caudal-fin shape, its confluence with the anal fin, the anal-fin ray count (Table 2) and the coloration (dark brown), indicate that O. borneensis is almost certainly conspecific with O. jaynei ZBK (also described from the Baram River). In addition to the more numerous anal-fin rays of O. platyrhynchus ZBK compared to O. borneensis (Table 2), the former species also has fewer dorsal-fin rays (2 vs. 4).
Singapore, National University of Singapore, Raffles Museum of Biodiversity Research, Zoological Reference Collection
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