Hypergastromyzon sambas, Hui, 2021

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 : 350-352

Hypergastromyzon sambas , new species

( Figs. 5 , 11–13 )

Material examined. Holotype: MZB 17235, 1 ex., 32.4 mm SL; Borneo: Kalimantan Barat: Sambas basin, Bengkayang area ; aquarium trade; don. Sunbeam Aquarium , 24 August 2011.

Paratypes: MZB 17236, 5 ex. , ZRC 61380, 48 ex., 23.6–32.4 mm SL; same locality data as holotype . — ZRC 61383, 18 ex., 22.6–30.8 mm SL; Borneo: Kalimantan Barat: Sambas basin, Bengkayang area ; aquarium trade; don. Sunbeam Aquarium , 9 February 2007 . — ZRC 61384, 16 ex., 23.1–33.9 mm SL; Borneo: Kalimantan Barat: Sambas basin, Bengkayang area ; aquarium trade; don. Sunbeam Aquarium , 18 November 2008. — CMK 28828 , 7 ex., ZRC 61390, 10 ex., 21.6–33.1 mm SL; Borneo: Kalimantan Barat: Sambas basin, Bengkayang area ; aquarium trade; don. Sunbeam Aquarium , 17 October 2008 . — ZRC 61381, 7 ex., 24.5–32.2 mm SL; Borneo: Kalimantan Barat: Sambas basin, Bengkayang area ; aquarium trade; don. Sunbeam Aquarium , 8 September 2008 .

Other material: ZRC 61382, 14 ex., 24.6–30.8 mm SL; Borneo: Kalimantan Barat: Sambas basin, Bengkayang area ; aquarium trade; don. Sunbeam Aquarium , 26 March 2010 . — ZRC 61386, 3 ex., 23.1–27.5 mm SL; Borneo: Kalimantan Barat: Sambas basin, Bengkayang area ; aquarium trade; don. Sunbeam Aquarium , 28 October 2008 .

Diagnosis. Hypergastromyzon sambas can be distinguished from its congeners in having the unique combination of characters: lateral scales 58–78 [mean 71]; least slender caudal-peduncle depth (6.2–8.0% [mean 7.1] SL, vs. 5.2–6.8 [mean 6.3] in H. humilis , and 5.8–7.4 [mean 6.6] in H. abditus ); shortest caudal-peduncle length (3.2–6.4% [mean 4.8] SL, vs. 6.2–8.1 [mean 7.0] in H. humilis , and 4.3–7.4 [mean 5.9] in H. abditus ); shorter dorsal-fin base length than H. humilis (10.4–13.6% [mean 12.1] SL, vs. 12.0–15.2 [mean 13.7] in H. humilis ); shorter pectoral-fin length than H. humilis (36.7–42.4% [mean 39.1] SL, vs. 40.3–42.5 [mean 41.2] in H. humilis ); smaller eye diameter than H. humilis (3.4–5.6% [mean 4.6] SL, vs. 4.8–5.9 [mean 5.4] in H. humilis ); lower lip in ventral view with a median notch (vs. truncate of H. abditus and gentle downturned curve of H. humilis ).

Description. See Figs. 11–13 for general appearance, and Table 1 for meristics and morphometric data.

Body depressed, widest and deepest at posterior edge of pectoral-fin base, most narrow 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, a gentle downturned curve, 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 pocket on both sides of mouth, lower lip with notch at median, without tiny raised bumps, continuous to throat without discrete structures. Naris large, ca. ⅓ eye diameter, anterior to eye. Eye situated dorsally, posterior edge of eye located above pectoral-fin 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. Pectoral fins enlarged, forming a fan-like shape, total fin-ray count 24–25, posterior edge not overlapping with pelvic-fin origin. Pelvic fins enlarged and completely fused at the posterior edge, forming a U-shape, total fin-ray count 17–19 on each half. Belly naked. Anal pore situated nearer to pelvic-fin base than to anal-fin origin, not visible from ventral view (blocked by pelvic fins). 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, weakly tri-lobed, covering up to base of pelvic-fin ray 6. Anal fin small and triangular, adpressed extending past caudal-fin base. Caudal fin truncate. Dorsal fin small, triangular; origin posterior to pelvic-fin origin. Largest examined specimen 33.9 mm SL (ZRC 61384).

Total vertebral count: 30–31 (mode 30, n = 20).

Colouration in life. See Figs. 11 , 13 . Head very light brown with many dark brown blotches. Pupil of eye with gold ring. 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 thin 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 brown bar, interradial membrane hyaline. Caudal-fin base black, two distinct thick dark brown bars, at anterior half and median of fin, margin clear, interradial membranes hyaline. Anal fin with one brown bar, interradial membrane hyaline. Both pectoral and pelvic fins very light brown with one faint brown whorl. Pectoral-fin base with dark brown blotches. Supra-pelvic flap with faint dark brown blotches.

Some specimens covered with tiny black dots all over body and head (see Fig. 13 ).

Colouration in preservative. See Fig. 12 . Colouration as above, except bright colours are absent. Lateral line dark brown to black.

Sexual characters. Genital papillae indistinguishable between sexes. Males with enlarged tubercles on pectoral- and pelvic-fin rays, tubercles along proximal ⅔ to ¾ of fin ray, largest tubercles nearest to body, decreasing in size towards fin margin. Gravid females with ripe ova visible through belly (see Fig. 13 ); ovum estimated to be around 2 mm in diameter.

Distribution. Hypergastromyzon sambas appears to be endemic to the upper Sambas basin, which is a short coastal drainage flowing easterly from the Pueh Range into the South China Sea ( Fig. 6 ).

Field notes. Hypergastromyzon sambas was encountered as a contaminant in shipments of Gastromyzon imported from Indonesia from 2006 to 2011. From trusted sources, these fish were obtained from the upper Sambas drainage system. The main target for the trade is Gastromyzon , and this consists of G. stellatus , G. ctenocephalus , G. cf. viriosus , and occasionally G. ocellatus .

Etymology. Named for the Sambas River basin, where the fish inhabits in the upstream tributaries. Used as a noun in apposition.

Figures

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. 11. Hypergastromyzon sambas, ZRC 61381, ca. 30 mm SL, male (?); live colouration; composite of dorsal, lateral, and ventral views.

Fig. 12. Hypergastromyzon sambas, MZB 17235, 32.4 mm SL, holotype, female (?); composite of dorsal, lateral, and ventral views.

Fig. 13. Hypergastromyzon sambas. Top and middle: ZRC 61383, 28.0 mm SL, 31.6 mm SL; lateral views showing numerous tiny black spots. Bottom: ZRC 61381, 31.0 mm SL; ventral view of female showing ripe ova (estimated at 2 mm diameter).

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.