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Gastromyzon monticola (non Vaillant) - Chin, 1990: SC-22 (part); Rahim et al., 2001: 126.
Material examined: SABAH (Mengalong River basin): HOLOTYPE : MUS uncatalogued, 38.8 mm SL; Sipitang: Sungai Malamum, ca. 9 km into track, tributary to Mengalong River ( 04º59.120'N115º37.581'E [50 m asl]); H. H. Tan, K. K. P. Lim & R. Goh, 9 Dec 2000 .
PARATYPES : MUS uncatalogued, 3 ex., 32.6-34.7 mm SL ; ZRC 47021, 4 ex., 32.1- 43.4 mm SL; same locality as holotype . SABAH (Padas River basin): ZRC 47022, 7 ex., 35.0-45.9 mm SL; Keningau: Sungai Agudon, mile 1 from Keningau to Crocker Range National Park headquarters, ( 05°21.206'N116°07.532'E [395 m asl]); H. H. Tan, K. K. P. Lim & R. Goh, 7 Dec 2000 .
NON-TYPE MATERIAL: SABAH (Petagas River basin): ZRC 47023, 3 ex., 37.7- 44.4 mm SL; Penampang: Penampang River, Sungai Moyog, Kampong Kibunut ( 05°53.301'N116°14.102'E [225 m asl]); H. H. Tan, K. K. P. Lim & R Goh, 10 Dec 2000 . BRUNEI DARUSSALAM ( Temburong district : Temburong River basin): UBD-B- 1a, 1 ex., 33.0 mm SL ; ZRC 38760, 2 ex., 33.3-37.8 mm SL; Sungai Belalong at Kuala Belalong; K. K. P. Lim et al., 14- 17 Jun 1995 . ZRC 47024, 5 ex., 31.0-34.4 mm SL; Sungai Belalong, in front and near to the Kuala Belalong Field Studies Centre ( 04°32'50.4"N115°09'27.6"E [80 m asl]); H. H. Tan & K. K. P. Lim, 4- 7 Oct 2001 . UBD- SC/92/7/F5a, 1 ex., 41.9 mm SL; Sungai Belalong, Kuala Belalong Field Study Centre; S. Choy, 27 Jul 1992 .
Diagnosis. Gastromyzon aeroides ZBK differs from its congeners in having the following unique combination of characters: angular gill slit; subopercular groove present and continuous to pectoral fin origin; body, including dorsum, brown; head dorsum with very fine cream reticulate pattern (similar to a cream head with brown spots and blotches); dorsal, caudal and anal fins blue in life; presence of sublacrymal groove; snout rounded when viewed dorsally; absence of secondary rostrum; absence of postoral pouch; scales absent on belly; 47-65 scales in lateral line (Sabah population: 56-65, Brunei population: 47-49); pelvic fin not overlapping level of anal fin origin, adpressed dorsal fin falling short of level of anal fin origin. Maximum size: 45.9 mm SL (ZRC 47022).
Description. General body shape and appearance as in Figs. 3-4. Meristic and morphometric data appear in Table 1. Head rounded in dorsal view; relatively long (27.9- 32.5 % SL) and wide (23.5-30.7 % SL, 83.1-100.0 % HL), head relatively flattened (head depth 12.6-15.2 % SL, 42.3-52.7 % HL); snout relatively long (snout length 59.6-70.3 % HL). Tubercles over entire head and anterior half of body. Sublacrymal groove present, not visible from side. Angular gill slit. Subopercular groove present and continuous to pectoral fin origin. Postoral pouch absent. No scales on belly. Posterior part of pectoral fin overlapping pelvic fin origin. Pelvic fin not reaching level of anal fin origin. Pectoral and pelvic fins with serrae on anteriormost rays. Dorsal fin situated at about mid body (predorsal length 53.8-60.4 % SL). Adpressed dorsal fin falling short of level of anal-fin origin. Deepest part of body at dorsal-fin origin (body depth at dorsal-fin origin 17.7-21.4 % SL). Anus situated just behind posterior margin of pelvic fin base. Caudal peduncle relatively deep (9.5-11.2 % SL) and short (6.8-9.6 % SL).
Body pigmentation and life coloration. See Fig. 3. Body uniform brown on dorsum and sides; a white spot on posterior edge of every body scale; ventrum cream. Head dorsum brown, with fine cream reticulate pattern (i.e. reticulations very dense, the head creamy with brown spots and blotches). Dorsal fin with 4 irregular black bars, the subdistal bar the most complete; interradial membranes and margin of fin blue; anterobasal black spot present. Caudal fin with 4-5 black bars, the interradial membranes and margin of fin blue (blue most intense in middle of fin). Anal fin with 2 faint black bars, the interradial membranes and margin of fin blue. Pectoral and pelvic fins brown with fine cream reticulate pattern and a brown margin. Pelvic axillary flap brown.
Colour in alcohol. See Fig. 4. Body black on dorsum and sides; ventrum cream. Head dorsum black. Dorsal fin grey with 4 irregular black bars, the subdistal bar most complete, interradial membranes and margin hyaline; fin with antero-basal black spot. Caudal fin grey, with 4-5 black bars, the interradial membranes and margin hyaline. Anal fin grey, with 2 faint black bars, the edges hyaline. Pectoral and pelvic fins greyish-black, with marked hyaline margin. Pelvic axillary flap dark grey.
Remarks. In addition to the characters mentioned in the diagnosis, G. aeroides ZBK can be differentiated from other congeners of the G. punctulatus group ZBK by the following characters: dorsal, caudal and anal fins blue in life (vs. yellow in G. punctulatus ZBK ); head rounded in dorsal view (vs. head truncate in G. punctulatus ZBK ); body plain (vs. spotted in G. punctulatus ZBK ); caudal peduncle depth greater than in G. punctulatus ZBK (13.0-15.1 vs. 12.0- 13.9 % BL).
The populations of G. aeroides ZBK in Brunei and Sabah exhibit some differences, the most prominent being the range of lateral scale counts. The Brunei population has 47-49 lateral line scales, whereas the Sabah population has 56-65 scales. However, based on present limited material available, the two populations cannot otherwise be readily discerned.
Etymology. From the Latin aeroides, meaning sky-blue. This is in allusion to the blue dorsal, anal and caudal fins in life. Used as a noun.
Field notes. Syntopic species of Balitoridae include: G. lepidogaster ZBK (Padas River basin); G. borneensis ZBK , G. lepidogaster ZBK (Mengalong River basin); and G. cranbrooki ZBK , G. lepidogaster ZBK , G. punctulatus ZBK , G. venustus ZBK and Neogastromyzon ZBK sp. (Temburong River basin).