The generic name ‘Sinogastromyzon’ is composed of ‘sino’ for China and ‘gastromyzon’ which is derived from Greek and means bellysucker; ‘gaster’ means belly, ‘myzo’ means to suck; gender: masculine.

The specific epithet ‘tonkinensis’ derived from Latin and means ‘from Tonkin’.

45-50 mm SL, KIZ 737149-155, 7 exp. (Kottelat & Chu, 1998).

49.4 mm SL, 56.4 mm TL, EPC0499; 47.1 mm SL, 55.1 mm TL, EPC0500 (ME, 2010-01-16).

D II,9; A I,5; P XI-XIII,14-15; V IX-X,12.

Body depth 16.4-16.6% SL; head length 20.6-21.1% SL; head width 26.5-27.4% SL; head depth 12.3-12.5% SL; caudal peduncle depth/length 0.7-0.9; 51-55 lateral line scales, lateral line follows the shape of pectoral.

Caudal emarginate, with 2 broad dark bars, margin transparent; dorsal with 1 medium, broad, dark stripe on yellow-beige ground, margin transparent, origin behind pelvic origin; unpaired fins with yellow-beige ground; pectoral with 7-12 dark brown blotches irregularly distributed, may be vanished, origin below eye; suprapelvic, anxillary belt (flap) long, posteriorly not free, but connected to the upper surface of pelvic, yellow-beige ground; pelvic posteriorly completely fused (no notch); dorsal line with 6-8 dark brown blotches from dorsal origin to peduncle; dorsal line in front of dorsal origin with irregular pattern of dark brown blotches; subopercular groove continuous, extending to ventral surface; rounded dorsal snout profile, margin yellowish in some specimen; head with irregular dark brown blotches, partially interconnected; head covered all over with whitish, small breeding tubercles in some specimen (probably males); body ground grey to light brown in formalin, light brown when alive; anus fully covered by pelvic, location midway to anal fin or close to pelvic base.

S. tonkinensis is easily confused with S. lixianjiangensis Li, Chen & Yang, 2009 and S. multiocellum Nguyen, 2005. In the Lixianjiang River in Mojiang County and on the boundary of Jiangcheng and Lvchun County, Red River basin, China, all the three similar species occur syntopic. They are reliably and conveniently distinguishable by the mouth structure and anus position; S. tonkinensis possesses 2 pair of long, thin barbels at the rictus (vs. 2 pair of long, thick barbels in S. multiocellum; vs 2 pair of minute barbels in S. lixianjiangensis), the mouth is wide (39.8–48.3% of head width; vs. 34.6–40.1% in S. lixianjiangensis; vs. 25.0–33.2% in S. multiocellum), anus covered by pelvic (midway to anal or close to pelvic vs. close to anal and visible in S. lixianjiangensis and S. multiocellum) (ME, 2009-01-23).

Type locality is the lower Red River basin, Vietnam (Pellegrin & Chevey, 1935).

Inhabiting riffles with clear, well oxygenated, fast running water over boulder substratum; Menyejiang River 勐野江, tributary to Lixianjiang River 李仙江, Red River 红河 basin, Puer County, Yunnan Province, China (biotope EW-CN 66-09, ME, pers. obs., 2009-12-30, 2010-01-06). 

IUCN Red List not evaluated.

Stream set up with highly oxygenated water; additional pumps for current generation; boulders with smooth surface as major decoration.

Food: Tablets with Spirulina content, Daphnia spp., Moina spp., rarely Grindal worms; Tubifex should be avoided.

No captive breeding reported yet.

There is another very similar species which also occurs in the Red River basin, but just reported from Vietnam; S. rugocauda. It is easily distinguished by the number of lateral line scales: S. tonkinensis has 49-51 (although my counts are higher) to 38-44 in S. rugocauda.