Hypergastromyzon humilis Roberts, 1989

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 : 338-344

Hypergastromyzon humilis Roberts, 1989

( Figs. 1–5 )

Hypergastromyon humilis Roberts, 1989: 92 , fig. 72; 1991: 334; Kottelat et al., 1993: 75, pl. 25; Doi, 1997: 19 (list); Tan, 2006: 201, fig. 128; Kottelat, 2013: 194.

Material examined. Holotype: MZB 3980 , 34.5 mm SL; Kalimantan Barat: Kapuas basin: Sungai Tamang, small forested stream flowing into Sungai Pinoh , opposite mouth of Sungai Kelawi , 0°35′S, 111°44′E; T. R. Roberts & S. Woerjoatmodjo, 26 July 1976.

Paratypes: CAS 49333, 1 ex., 34.8 mm SL, paratype; same collection data as holotype .

Other material: CMK 10574, 4 ex., 23.6–31.8 mm SL; Borneo: Kalimantan Barat: Sungai Melawi basin; Sungai Kelawai between Nanga Pintas and about 3 km upstream, 0°36′49″S, 111°47′22″E; M. Kottelat et al., 17 September 1993. — ZRC 61394 , 1 ex., 31.0 mm SL; Borneo: Kalimantan Barat: Kapuas basin, Melawi sub-basin; Sungei Elar Hulu ; river along border of buffer zone with village and Bukit Raya-Bukit Baka National Park ; 00°35.370′S, 112°14.354′E, 231 m asl; H. H. Tan et al., 12 August 2007 .

Diagnosis. Hypergastromyzon humilis can be distinguished from its congeners in having the unique combination of characters: lowest number of lateral scales (57–69, mean 62; vs. 62–74 [mean 71] in H. abditus and 58–78 [mean 71] in H. sambas ); fewer pectoral-fin rays than H. abditus (25–26, mode 25; vs. 26–28, mode 27); most slender caudalpeduncle depth (5.2–6.8% [mean 6.3] SL, vs. 5.8–7.4 [mean 6.6] in H. abditus , and 6.2–8.0 [mean 7.1] in H. sambas ); longest caudal-peduncle length (6.2–8.1% [mean 7.0] SL, vs. 4.3–7.4 [mean 5.9] in H. abditus , and 3.2–6.4 [mean 4.8] in H. sambas ); longest dorsal-fin base length (12.0–15.2% [mean 13.7] SL, vs. 10.6–14.4 [mean 12.2] in H. abditus , and 10.4–13.6 [mean 12.1] in H. sambas ); longest pectoralfin length (40.3–42.5% [mean 41.2] SL, vs. 37.2–40.9 [mean 38.5] in H. abditus , and 36.7–42.4 [mean 39.1] in H. sambas ); largest eye diameter (4.8–5.9% [mean 5.4] SL, vs. 3.6–5.3 [mean 4.5] in H. abditus , and 3.4–5.6 [mean 4.6] in H. sambas ); lower lip in ventral view with gently curving corners towards vent (vs. truncate in H. abditus and notched in H. sambas ).

Description. See Figs. 1–4 for general appearance, and Table 1 for meristics and morphometric data.

Body greatly depressed, widest and deepest at posterior edge of pectoral-fin base, narrowest at caudal peduncle. Lateral line distinct and complete, running along median of body to caudal-fin base. Head depressed, rounded in dorsal profile, inferior mouth. Mouth simple with entire lower and upper lips, without modifications or specialised organs, lower lip in ventral view with gently curving corners towards vent, mouth width about ⅓ of head width, two pairs of distinct but short rostral barbels, one pair of relatively longer maxillary barbels, margin of both upper and lower jaws entire; shallow the posterior edge, forming a U-shape, total fin-ray count 19 on each half. Belly naked. Anal pore situated nearer to pelvic-fin base than to anal-fin origin, not visible from ventral view. Both pectoral- and pelvic-fin anterior rays dorsal surface with tubercles, evenly distributed along fin ray, roughly half of ray length. Supra-pelvic flap present, bi-lobed, covering up to base of pelvic-fin rays 6–7. Anal fin small and triangular, adpressed just reaching caudal-fin base. Caudal fin truncate. Dorsal fin small, triangular; origin posterior to pelvic-fin origin. Largest examined specimen 34.8 mm SL (CAS 49333).

Total vertebral count: 30–31 (mode 30, n = 5).

Colouration in life. See Fig. 3 . Head light brown with many dark brown blotches. Eye with gold iris. Opercle with iridescent gold. Dorsum of body light brown or yellowish-brown, with many scattered darker brown bars and blotches. Dorsum with up to eight dark brown saddlelike bars, extending to mid-body. Middle of body with dark brown indistinct reticulated pattern, partly extending below lateral line, lateral line cream. Venter cream. Dorsal fin with one faint dark brown bar, interradial membrane hyaline. Caudal-fin base black or dark brown, two distinct black or dark brown bars, at anterior half and median of fin, margin clear, interradial membranes hyaline. Anal fin with one dark brown bar, interradial membrane hyaline. Both pectoral and pelvic fins golden-brown with 2–3 dark brown whorls. Pectoral-fin base with continuous or interrupted dark brown outline. Supra-pelvic flap with two dark brown blotches, first at anterior half, second at posterior margin.

Colouration in preservative. See Figs. 1 , 4 . Colouration as above, except bright colours are absent. Lateral line dark brown or cream, potentially due to different locality populations or artefact of preservation.

pocket on both sides of mouth, lower lip with many tiny raised bumps, continuous to throat without discrete structures ( Fig. 5 ). Naris large, ca. ⅓ eye diameter, anterior to eye. Eye situated dorsally, middle of eye located above pectoralfin origin, eye diameter less than opercle opening. Opercle opening lunate, located above pectoral-fin rays 6–7. Tubercles present and evenly distributed over head and anterior-dorsal part of body and extending to supra-pectoral region ( Fig. 2 ). Pectoral fins enlarged, forming a fan-like shape, total fin-ray count 25–26, posterior edge just overlapping with pelvicfin origin. Pelvic fins enlarged and completely fused at Sexual characters. Genital papillae indistinguishable between sexes. Males with enlarged tubercles on pectoral- and pelvic-fin rays, tubercles along proximal ⅔ to ¾ of each fin ray, largest tubercles nearest to body, decreasing in size towards distal-fin margin.

Distribution. Hypergastromyzon humilis appears to be endemic to the Melawi sub-basin, which drains northwesterly from the Schwaner Range into the Kapuas River at Sintang ( Fig. 6 ).

Field notes. Hypergastromyzon humilis occurs in fastflowing hill streams with clear, well-oxygenated water, running over rock and gravel bottom. Syntopic fish species observed together with H. humilis (ZRC 61394) include the following: Cyprinidae — Garra borneensis , Paracrossochilus acerus , and Tor sp. ; Gastromyzontidae — Gastromyzon praestans and G. ridens ; Sisoridae — Glyptothorax major ; and Mastacembelidae — Mastacembelus unicolor .

Figures

Fig. 1. Hypergastromyzon humilis, CMK 10574, 31.5 mm SL, male; composite of dorsal, lateral, and ventral views.

Fig. 2. Hypergastromyzon humilis, CMK 10574, 31.5 mm SL, male; close-up lateral view of anterior half of body showing tubercles on head and pectoral fin.

Fig. 3. Hypergastromyzon humilis, ZRC 61394, 31.1 mm SL, female (?); live colouration; composite of dorsal, lateral, and ventral views.

Fig. 4. Hypergastromyzon humilis, ZRC 61394, 31.1 mm SL, female (?); composite of dorsal, lateral, and ventral views.

Fig. 5. Oral views. A, Hypergastromyzon humilis, CMK 10574, 31.5 mm SL; B, Hypergastromyzon abditus, MZB 17233, 37.2 mm SL; C, Hypergastromyzon sambas, MZB 17235, 32.4 mm SL.

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.