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Cypriniformes: Balitoridae
Gastromyzon extrorsus Tan, 2006
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Dusun (local lang.)
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Rokot, Rogot, Kirokot
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‘Gastromyzon’ is derived from Greek and means bellysucker; ‘gaster’ means belly, ‘myzo’ means to suck; gender: masculine.
The specific epithet ‘extrorsus’ is an adjective meaning towards the outside. This is an allusion to the distributional range of this species; it occurs in streams running towards the sea along the west face of the Crocker Range in Sabah, Borneo, Malaysia. All these streams running to the exterior of Sabah.
Largest reported specimen SL 76.7 mm (lot ZRC 46999; 14 ex.); within the type series (Tan, 2006).
Distinct secondary rostrum; truncate head profile when viewed from above; sublacrimal groove; complete mandibulary fold; body brown with up to 9 thin gold bars; head darker brown with golden reticulate pattern; 69-77 lateral line scales; stout dorsal body profil; head broad (26.6-28.0% in SL); very similar and frequently confused with G. borneensis Gϋnther, 1874 and G. introrsus Tan, 2006.
no characters known; females might grow larger.
Type locality is the Sungai Moyog, Penampang River, Petagas basin, Crocker Range west face, South China Sea slope, Penampang county, W Sabah, Borneo, Malaysia (EW-MAL 16-09).
Inhabiting well oxygenated, moderate to fast running streams over rock and boulder substratum; Petagas and Kimanis basins; down to 90 m asl; along the west face of the Crocker Range, W Sabah, Borneo, Malaysia (ME, pers. obs., June 2009).
Singly; immobile; territorial; specimen with 15 mm SL start showing agonistic behaviour; hierarchical fights for best feeding places; benthic; lithophilic; in EW-MAL 17-09 Kimanis River, Sabah, Borneo, Malaysia, syntopic with two balitorids, viz. G. aeroides Tan & Sulaiman, 2006 and Glaniopsis sp.
Clinging to the rock substratum grazing algae and oportunistically aquatic insect larvae; this refers to ‘aufwuchs’; grazing technic mainly top-down vertically, seldom left-right horizontally; graze marks distinct visible on rocks exspecially near to the water surface.
Type locality Sungai Moyog (EW-MAL 16-09) is a stream running over rock substratum and boulders with smooth surfaces, pebbles with fast current all over; high gradient; ca. 20 m broad and 1 m deep; 202 m asl, N 05º 53.320', E116º 14.054'; 26 ºC, 8.1 pH, 40 µS/cm, 20 mg/l (ppm; TDS) on 2009-06-07, 11:45.
Another biotope is the Kimanis River (EW-MAL 01-09, 17-09); a high gradient stream running over boulder and rock substratum, ca. 15 m broad and 1.5 m deep, riffle section 0.3 m deep; 90 m asl, N 05º 32.223', E115º 59.572'; 26.5 ºC, 7.6 pH, 140 µS/cm, 70 mg/l (ppm; TDS) on 2009-06-02, 10:30; 28 ºC, 8.1 pH, 60 µS/cm, 30 mg/l (ppm; TDS) on 2009-06-07, 15:00 (ME, pers. obs., June 2009).
IUCN Red List not evaluated.
Requires well aerated and biologically filtered water with a temperature of 24-28 ºC; strong current preferred; feeds upon Artemia, Spirulina tablets and algae; not bred yet in a home tank.
Large individuals possess a spectacular secondary rostrum covered with breeding tubercles all over. The definite reason for a evolution of a secondary rostrum is yet unclear. It may be hypothesized to be evolved for usage during agonistic territorial fights for best feeding places when the snout is rammed into the flanks of an opponent. Another hypothesis is that it serves for better stimulate spawning partners, since flank stimuli are known to be a part of the courtship in balitorid loaches (see OTT, 2002c).
Figure 1. Gastromyzon extrorsus; mature fish, dorsolateral view, 60 mm SL, caught 2009-06-02, not preserved, EW-MAL 01-09, Kimanis River, W Sabah, Borneo, Malaysia.
Figure 2. Gastromyzon extrorsus; mature fish, lateral view, caught 2009-06-02, not preserved, EW-MAL 01-09, Kimanis River, W Sabah, Borneo, Malaysia.
Figure 3. Graze marks of Gastromyzon spp.; 2009-06-02, EW-MAL 01-09, Kimanis River, W Sabah, Borneo, Malaysia.
Figure 5. EW-MAL 01-09, biotope of Gastromyzon spp.; 2009-06-02, Kimanis River, W Sabah, Borneo, Malaysia.
Figure 4. Gastromyzon extrorsus; mature fish, head, anterodorso-lateral view, tubercles tightly allocated on tip of rostra, caught 2009-06-02, not preserved, EW-MAL 01-09, Kimanis River, W Sabah, Borneo, Malaysia.
Figure 6. EW-MAL 16-09, Sungai Moyog, type locality of G. extrorsus; downstream view, 2009-06-07, W Sabah, Borneo, Malaysia
You can see more related photographs at the photo section
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