Diagnosis: Distichodus atroventralis is distinguished from D. affinis, D. altus, D. decemmaculatus, D. noboli, D. notospilus and D. teugelsi by the higher number of lateral line scales, 61-71 vs. 37-46; from D. antonii by the inferior mouth vs. terminal, and a lower number of dark vertical bars, 6-9 vs. 9-14; from D. fasciolatus by the higher number of pelvic-fin rays, 11, exceptionally 10, vs. 10, exceptionally 11, the lower number of dark vertical bars, at least in specimens smaller than about 150 mm standard length, 6-9 vs. 13-20, and the blackish pelvic fin, at least in specimens smaller than about 200 mm standard length vs. whitish-yellow; from D. langi by fewer dorsal-fin rays, 22-24 vs. 26-28, a lower number of pectoral-fin rays, 17-20 vs. 21, a shallower head, 35-53% of head length vs. 67-69, and fewer dark vertical bars, 6-8 vs. 13; from D. lusosso by the inferior mouth vs. terminal, and the short snout vs. elongated; from D. maculatus by the absence of dark blotches all over the body vs. dark blotches all over the body; and from D. sexfasciatus by the higher number of teeth in the outer row of the upper and lower jaw, 18-36 and 20-34 vs. 12-14 on both jaws, and the greenish to light brown and yellowish body colouration vs. orange-reddish (Ref. 122092). It is distinguished from its most similar congener, D. kasaiensis, by having fewer dorsal-fin rays, 22-24 vs. 27-29, more teeth in the upper jaw, 18-36 vs. 15-25 (overlap due to positive allometry), and by having a shorter dorsal-fin base, 20.1-28.8% of standard length vs. 24.9-33.7% (overlap due to positive allometry); and from D. ingae by having fewer dorsal-fin rays, 22-24 vs. 26, and shorter dorsal fin, 12.0-21.9% of standard length vs. 23.6-24.2%, anal fin, 9.9-15.7% of standard length vs. 16.9-17.5%, and pectoral fin, 14.2-20.5% of standard length vs. 21.6-22.7% (Ref. 122092).
Description: Body elongated and compressed; dorsal profile straight from tip of snout to above posterior border of eye, then convex to end of dorsal-fin base, straight between dorsal and adipose fins, and concave between adipose and caudal fins; ventral profile straight from tip of snout to level of posterior eye border, concave afterwards until end of anal-fin base, and convex between latter and caudal fin (Ref. 122092). Mouth inferior; 18-36 teeth in the outer row of the upper jaw with the number of teeth increasing in size (Ref. 122092). Caudal peduncle from deeper than long to slightly longer than deep (Ref. 122092). Dorsal fin with straight to slightly convex distal margin and its origin well in front of pelvic-fin origin; adipose fin at equal distance from dorsal and caudal fins; only proximal two-thirds of adipose fin scaled; anal fin with concave distal margin; length of pectoral- and pelvic fin rays decreasing from last unbranched ray to last branched ray; tip of pectoral fin just or just not reaching vertical through dorsal-fin origin; tip of pelvic fin may or may not reach vent, but never passes it; caudal fin forked; lobes generally rounded although sometimes slightly pointed, with lower lobe slightly larger than upper lobe; caudal fin, except for its distal fifth, almost entirely covered with small scales (Ref. 122092).
Colouration: In preserved specimens, overall body colour light brown-yellowish with whitish belly; small specimens of less than about 20 mm standard length with 6-9 continuous or interrupted blackish vertical bars; bars fade with size and are completely absent in larger specimens; black caudal spot vaguely visible or absent; dorsal fin with whitish rays connected with translucent membranes, with scattered small, black melanophores of irregular size; proximal two thirds of adipose fin with same colour as body, generally without thin dark distal edge; pectoral fin whitish at base and more translucent distally; pelvic fin entirely blackish in small specimens less than about 200 mm standard length, becoming less dark in larger specimens, but always remaining dark at their distal border; caudal fin slightly darker than basic colour, sometimes with distal end of rays blackish; preservation may cause dark vertical bars and pelvic fin colour to fade and even disappear (Ref. 122092). In life, overall colour dark brown to greenish, with a lighter belly; otherwise, colour pattern similar to preserved specimens (Ref. 122092).