Engkaria eubranchus ( Roberts, 1991 )

Hui, Tan Heok, 2021, Hypergastromyzon revisited, with descriptions of a new genus and two new species (Teleostei: Gastromyzontidae), Raffles Bulletin of Zoology 69, pp. 336-363 : 353-358

Engkaria eubranchus ( Roberts, 1991)

( Figs. 14–20 )

Gastromyzontinae sp. nov. – Cramphorn, 1982: 26, 38 (list).

Hypergastromyzon eubranchus Roberts, 1991: 334 , fig. 1; Kottelat et al., 1993: 75, pl. 25; Kottelat & Lim, 1995: 236; Doi, 1997: 19 (list); Tan, 2006: 203, figs. 129, 130; Kottelat, 2013: 194.

Material examined. Holotype: BMNH 1984.11.15:1, 30.7 mm SL; Sarawak: Batang Ai , Lupar basin, near Wang Mepai; J. Cramphorn, 1982.

Paratypes: BMNH 1984.11.15 : 2, 35.6 mm SL; Sarawak: Batang Ai , Lupar basin, below confluence with Batang Enkori; J. Cramphorn, 1982 . — BMNH 1984.11.15 : 3, 16.8 mm SL; Sarawak: Batang Ai , Lupar basin, Sungei Delek, Wong Nanga Jeliak; J. Cramphorn, 2–5 July 1982 . — CAS 55889, 1 ex., 30.9 mm SL; Sarawak: Batang Ai, Lupar basin, confluence of Sungai Delok; J. Cramphorn, 1982 .

Other material: ZRC 37806, 3 ex., 36.0–41.0 mm SL; CMK 10833, 2 ex., 32.1–35.1 mm SL; Sarawak: Batang Lupar basin, Sungai Engkari ; R. Stuebing , 14 August 1993 . — ZRC 37802, 1 ex., 31.9 mm SL; Sarawak: Batang Lupar basin, Sungai Engkari ; R. Stuebing , 7 August 1993 . — CMK 10846, 1 ex., 37.4 mm SL; Sarawak: Sarikei district , Sungai Sekarang ; R. Stuebing , 8 October 1993 . — ZRC 60479, 5 ex., 37.9–39.3 mm SL; Sarawak: Sri Aman Div : Lupar basin; Sungei Engkari mainstream, adjacent to Nanga Segerak Field Station, 01°24.813′N, 112°00.253′E, 318 m asl; H. H. Tan et al., 20 September 2018 . — ZRC 60390, 1 ex., 36.9 mm SL; Sarawak: Sri Aman Div : Lupar basin; Sungei Segerak, feeder stream to Sungei Engkari, 01°24.813′N, 112°00.253′E, 318 m asl; H. H. Tan et al., 21 September 2018 . — ZRC 60401, 8 ex., 30.6–35.8 mm SL; Sarawak: Sri Aman Div : Lupar basin; Sungei Kaup, tributary of Sungei Engkari, 01°24.084′N, 111°59.494′E, 246 m asl; H. H. Tan et al., 26 September 2018 . — ZRC 60468, 31 ex., 31.0– 41.1 mm SL; Sarawak: Sri Aman Div : Lupar basin; Sungei Engkari mainstream, ca. 500 m – 1 km upstream of Nanga Segerak Field Station, just past Sungei Tapayang (source of freshwater for field station), 01°24.875′N, 112°00.379′E, 289 m asl; H. H. Tan et al., 22 September 2018 . — ZRC 60418, 20 ex., 29.7–38.8 mm SL; Sarawak: Sri Aman Div : Lupar basin; Sungei Engkari mainstream, ca. 500 m – 1 km downstream of Nanga Segerak Field Station; 01°24.881′N, 111°59.960′E, 388 m asl (start), 01°24.845′N, 112°00.060′E, 305 m asl (lunch stop); H. H. Tan et al., 23 September 2018 . — ZRC 60354, 17 ex., 29.5–37.1 mm SL; Sarawak: Sri Aman Div : Lupar basin; Sungei Segerak, feeder stream to Sungei Engkari; ca. 500 m upstream of THH18-31 (01°24.813′N, 112°00.253′E, 318 m asl); H. H. Tan et al., 24 September 2018 . — ZRC 60337, 11 ex., 31.2–38.0 mm SL; Sarawak: Sri Aman Div : Lupar basin; Sungei Engkari mainstream, ca. 200–400 m downstream of Nanga Segerak Field Station, before first series of cascades; including a small feeder stream; H. H. Tan et al., 25 September 2018 .

Diagnosis. See generic diagnosis.

Description. See Figs. 14–20 for general appearance, and Table 1 for meristics and morphometric data.

Body depressed, widest and deepest at area anterior to dorsal-fin origin, most narrow at caudal peduncle. Lateral line distinct and complete, running along median of body to caudal-fin base. Head depressed, bluntly wedge-shaped in dorsal profile, inferior mouth. Mouth simple, a gentle downturned curve, mouth width about ¼ of head width, two pairs of distinct slender rostral barbels, 1 pair of short maxillary barbels, margin of both upper and lower jaws entire; relatively deep pocket on both sides of mouth, lower lip with cleft at median, continuous to throat without discrete structures. Naris large, ca. ⅓ eye diameter, anterior to eye. Eye situated dorsally, anterior to pectoral-fin origin, eye diameter less than half of opercle opening. Opercle opening angular, originating above pectoral-fin ray 5. Tubercles present and evenly distributed over head and anterior-dorsal part of body and extending to supra-pectoral region. Pectoral fins enlarged, forming a fan-like shape, total fin-ray count 21–25, posterior edge just overlapping with pelvic-fin origin. Pelvic fins enlarged with incomplete fusion leaving posterior ½–⅔ margin free, forming a U-shape, total fin-ray count 17–19 on each half. Belly anterior portion naked, scaled at area bordered where pelvic fin is fused to body. Anal pore situated nearer to pelvic-fin base than to anal-fin origin, not visible from ventral view (blocked by pelvic fins). Pectoral fin with densely packed tiny tubercles on the anterior 5th to 6th rays; rest of tubercles are larger and single file along posterior rays. Pelvic fin with anterior ⅔ covered with larger tubercles along proximal part of rays. Supra-pelvic flap present, bi-lobed, covering up to base of pelvic-fin ray 8. Anal fin small and triangular, adpressed extending past caudal-fin base. Caudal fin broadly emarginate. Dorsal fin small, triangular; origin posterior to pelvic-fin origin. Largest examined specimen 41.1 mm SL (ZRC 60468).

Total vertebral count: 27–28 (mode 28, n = 20).

Colouration in life. See Figs. 14 , 15 . Head light brown with many dark brown blotches and reticulated pattern. Pupil of eye with gold ring. Dorsum of body light brown with 6–7 large saddle-like dark brown bars or rounded blotches, extending to mid-body. Middle of body with many gold tiny spots, lateral line with gold dots. Lower half of body with many small dark brown blotches. Venter cream, shimmery gold over belly area. Caudal peduncle usually with two dark brown blotches continuous to venter. Dorsal fin with two black bars, interradial membrane hyaline. Caudal-fin base median black with top and bottom white wedges, two distinct thick dark brown bars with red edges, at anterior half and median of fin, margin clear, interradial membranes hyaline. Anal fin with one brown bar near distal edge, interradial membrane hyaline. Both pectoral and pelvic fins light brown with 3–4

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large dark brown blotches, evenly spaced. Pectoral-fin base light brown with corresponding dark brown blotches to those on fin. Supra-pelvic flap with two dark brown blotches, one at anterior and other at distal half.

Colouration in preservative. See Figs. 16 , 20 . Colouration as above, except bright colours are absent. Well-demarcated dark brown blotches faded and muted to grey.

Roberts (1991) mentioned the juvenile pattern being different from that of adults (based on a 16.8 mm SL specimen, BMNH 1984.11.15:3); however, he did not provide any illustrations. The Natural History Museum (London) has provided an online resource for its zoological collection, and the data resource (including image and radiograph) of the juvenile specimen is available (https://data.nhm.ac.uk/object/ efbf1c6d-d51e-4903-8c21-70dd1fbc4038/1620259200000).

The juvenile lacks the defined blotches on the bodies of adults and exhibits a dark brown longitudinal swath from behind the eye to caudal-fin base; with lighter brown dorsum and cream venter. The only fin pattern discernible is two distinct dark brown bars on the caudal fin, with a hyaline margin. The differences between juvenile and adult colour patterns in members of Gastromyzon and Neogastromyzon have been covered in detail by Tan (2006). Juveniles of sarawakensis, O. waandersii , Tor tambra , T. tambroides ; Danionidae — Rasbora fasciata ; Cobitidae — Pangio piperata ; Nemacheilidae — Nemacheilus kapuasensis ; Balitoridae — Homalopteroides weberi ; Gastromyzontidae — Gastromyzon megalepis , G. stellatus , Neogastromyzon chini , Parhomaloptera microstoma ; Bagridae — Hemibagrus bongan , H. fortis , Leiocassis poecilopterus ; Sisoridae — Glyptothorax major ; Clariidae — Clarias planiceps ; Mastacembelidae — Macrognathus circumcinctus , Mastacembelus unicolor ; and Channidae — Channa lucius .

In Sungei Engkari, Engkaria eubranchus was obtained only in the main stream, exclusively from the riffle zone. It was absent from the slower flowing and smaller feeder streams. During the visit in September 2018, no juveniles were obtained, and the total size range of individuals collected was 29.5–41.1 mm SL. Mature males could be discerned by the presence of ridged tubercles on the opercle, but this was not readily observed in the field. As the rocky substratum was mostly dark grey or black, fishes were darkly pigmented, appearing almost black when freshly obtained.

Gastromyzon in general have a more reticulated pattern that matures into a more uniformly coloured pattern. Juveniles of Neogastromyzon have a more distinct barred pattern that matures into a more reticulated pattern.

The difference in juvenile colouration pattern can be perhaps used as part of the genus diagnosis; however, lacking fresh material or a larger series of juvenile specimens, this is left for future studies/surveys to confirm.

Sexual characters. Genital papillae indistinguishable between sexes. Males with densely packed minute tubercles on pectoral fin, tubercles along proximal ⅔ to ¾ of fin ray ( Fig. 18 ). Larger tubercles on rest of pectoral and pelvic fins. Males also with tubercles and ridges of tubercles on sub-orbital and opercle respectively ( Fig. 19 ). The head shape of male and female is slightly different as evident in Fig. 17 . Male with a more pointed snout and female with a more rounded snout. Mature females also appear more robust ( Figs. 15 , 17 ).

Distribution. Engkaria eubranchus appears to be endemic to the Engkari River, which drains into Batang Ai reservoir, which is part of the Lupar basin ( Fig. 6 ).

Field notes. Syntopic fishes from the main stream of Sungei Engkari include the following: Cyprinidae — Barbodes banksi , Barbonymus collingwoodii , Hampala bimaculata , Lobocheilos cf. erinaceus , L. ovalis , Osteochilus When Cramphorn (1982) conducted her fish survey, it was from May to August 1982, from the lower Batang Ai to Sungei Ekori [= Engkari]. From her report, only three sites yielded Engkaria (identified as Gastromyzontinae sp. nov.)—A2, A3, and A4. She did note that certain sites had thousands of Gastromyzon present. Her local Iban helpers helped to catch the fish, primarily using cast nets (which is not the most efficient way to catch smaller benthic fishes).

Figures

Fig. 14. Engkaria eubranchus, ZRC 60479, ca. 38 mm SL, male; live colouration; composite of dorsal, lateral, and ventral views.

Fig. 15. Engkaria eubranchus, ZRC 60479, ca. 40 mm SL, female; live colouration; composite of dorsal, lateral, and ventral views.

Fig. 16. Engkaria eubranchus, ZRC 60479, 41.1 mm SL, male; composite of dorsal, lateral, and ventral views.

Fig. 17. Engkaria eubranchus, ZRC 60468, 40.1 mm SL, female; composite of dorsal, lateral, and ventral views. Note abrasion marks on dorsal surface due to artefact from collection.

Fig. 18. Oral views of Engkaria eubranchus. Top: ZRC 60479, 41.1 mm SL male; bottom: ZRC 60468, 40.1 mm SL female.

Fig. 19. Engkaria eubranchus, ZRC 46068, 39.0 mm SL, male. Top: lateral view of head region showing tuberculation pattern on opercle; bottom: close-up of dorsal view of left pectoral fin showing tuberculation pattern (not to scale).

Fig. 20. Engkaria eubranchus, ZRC 60468, 36.8 mm SL; ventral view of fused pelvic fins, showing posterior half of last rays free; with view of anal pore.

Fig. 6. Map of Borneo showing distribution of H. humilis (black circle), H. sambas (black square), H. abditus (white circle), and E. eubranchus (black pentagon). Each symbol may represent more than one location.