Hypergastromyzon abditus, 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 : 345-349

Hypergastromyzon abditus , new species

( Figs. 5 , 7–10 )

Material examined. Holotype: MZB 17233, 37.2 mm SL; Kalimantan Tengah: Katingan basin, Mendawai sub-basin; stream at km 57 logging road at buffer zone of Bukit RayaBukit Baka National Park ; at deep pool (ca. 2–3 m depth) at first cascade series, about 15 m high, about 800 m upstream, 00°45.011′S, 112°17.183′E, 211 m asl; H. H. Tan et al., 11 August 2007.

Paratypes: MZB 17234, 1 ex. , ZRC 61397, 2 ex., 31.1–35.9 mm SL; same locality data as holotype . — ZRC 61400, 4 ex., 36.0– 40.3 mm SL; Kalimantan Tengah: Katingan basin, Mendawai sub-basin; Sungei Baha’e , km 64 logging road at buffer zone of Bukit Raya-Bukit Baka National Park , 00°47.593′S, 112°19.220′E, 218 m asl; H. H. Tan et al., 12–13 August 2007 .

Diagnosis. Hypergastromyzon abditus can be distinguished from its congeners in having the unique combination of characters: large number of lateral scales (62–74 [mean 71]); highest number of pectoral-fin rays (26–28, mode 27, vs. 24–25 for other two species); slender caudal-peduncle depth (5.8–7.4% [mean 6.6] SL); short caudal-peduncle length (4.3–7.4% [mean 5.9] SL); short dorsal-fin base length (10.6–14.4% [mean 12.2] SL); shortest pectoral-fin length (37.2–40.9% [mean 38.5] SL, vs. 40.3–42.5 [mean 41.2] in H. humilis , and 36.7–42.4 [mean 39.1] in H. sambas ); smallest eye diameter (3.6–5.3% [mean 4.5] SL, vs. 4.8–5.9 [mean 5.4] in H. humilis , and 3.4–5.6 [mean 4.6] in H. sambas ); lower lip with truncate anterior margin (vs. gently curved in H. humilis and bilobed with median cleft in H. sambas ).

Description. See Figs. 7–10 for general appearance, and Table 1 for meristic and morphometric information.

Body depressed, widest and deepest at posterior margin 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, lower lip in ventral view with truncate anterior profile, mouth width about ¼ of head width, two pairs of distinct but short rostral barbels, one pair of relatively longer maxillary barbels, margin of both upper and lower lips entire; shallow pocket on both sides of mouth, lower lip without tiny raised bumps, continuous to throat without discrete structures. Naris large, ca. ⅓ eye diameter, anterior to eye. Eye situated dorsally, middle 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 26–28, posterior edge just overlapping with pelvic-fin origin. Pelvic fins enlarged and completely fused at the posterior edge, forming a U-shape, total fin-ray count 19–20 on each half. Belly naked. Anal pore situated nearer to pelvic-fin base than to anal-fin origin, just 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 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 40.3 mm SL (ZRC 61400).

Total vertebral count: 31–32 (mode 31, n = 8).

Colouration in life. See Fig. 9 . Head 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 saddlelike bars edged with black, extending to mid-body, maybe extended downwards but not to venter. Middle of body with clear patternless border dorsal and ventral to lateral line, lateral line black. Venter cream. Dorsal fin with one faint black bar, interradial membrane hyaline. Caudal-fin base black, one distinct thick black bar, at median of fin, border clear, interradial membranes hyaline. Anal fin with one black bar, interradial membrane hyaline. Both pectoral and pelvic fins anterior ⅔ dark brown with yellowish-brown margin. Pectoral-fin base light brown with irregular dark brown blotches. Supra-pelvic flap with three black blotches, first at anterior portion, second at middle and third at posterior margin.

Colouration in preservative. See Figs. 7 , 8 , 10 . Colouration as above, except bright colours are absent. Lateral line dark brown.

Sexual characters. Genital papillae indistinguishable between sexes. Males with enlarged tubercles on pectoral- and pelvic-fin rays, tubercles along whole fin ray, largest tubercles nearest to body, decreasing in size towards fin margin. Large male (ZRC 61400, 40.3 mm SL) with tubercles on venter of caudal peduncle.

Distribution. Hypergastromyzon abditus appears to be endemic to the upper Katingan basin, which drains southeasterly from the Schwaner Range ( Fig. 6 ). The current known distribution of H. abditus is directly adjacent to that of H. humilis , which is northwards in the Melawi sub-basin. Field notes. See Tan (2009) for a list of syntopic species. From the locality of the type series above a small cascade, only the following syntopic fish species were observed: Cyprinidae — Paracrossochilus acerus and Tor sp. ; and Gastromyzontidae — Gastromyzon cf. psiloetron . Downstream of the cascade, the following fish species were observed: Cyprinidae — Barbodes sealei ; Balitoridae — Pseudohomaloptera batek ; and Channidae — Channa limbata .

Etymology. From the Latin abditus , meaning hidden, concealed. This is in reference to its cryptic colouration and ability to blend into its environs. 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. 7. Hypergastromyzon abditus, MZB 17233, 37.2 mm SL, holotype, male; composite of dorsal, lateral, and ventral views.

Fig. 8. Hypergastromyzon abditus, ZRC 61400, 40.3 mm SL, paratype, male; close-up of lateral view of anterior half of body showing tubercles on head, body, and pectoral fin.

Fig. 9. Hypergastromyzon abditus, ZRC 61397, ca. 38 mm SL, female; freshly preserved colouration; composite of dorsal, lateral, and ventral views.

Fig. 10. Hypergastromyzon abditus, ZRC 61397, 38.0 mm SL, female; composite of dorsal, lateral, and ventral views.

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.