Diagnosis: Nannocharax hastatus is distinguished from all congeners except N. maculicauda and N. ocellicauda by the possession of a largely unpigmented body except for a dark line along the bases of the dorsal and anal fins and a longitudinally elongate, ovoid, dark spot on the posterior portion of the caudal peduncle and bases of the central caudal-fin rays; the elongate form of the head and body are also unique to these three species in the genus (Ref. 96084). Nannocharax hastatus differs from N. maculicauda in the numbers of scale rows around the caudal peduncle, 18-20 vs. 14-16; and pored scales along the lateral line, 6-9 vs. 55-61 (Ref. 96084). Nannocharax hastatus is distinguished from N. ocellicauda in the number of scales along the lateral-line scale series, 59-68 vs. 50-56; and scale rows around the caudal peduncle, 18-20 vs. 13-16 (Ref. 96084).

Description: Body distinctly elongate and laterally compressed; snout anteriorly rounded in lateral view (Ref. 96084). Dorsal profile of head slightly convex from tip of snout to tip of suppraoccipital spine; dorsal profile of body overall convex from tip of supraoccipital spine to dorsal-fin origin; sometimes slightly concave immediately anterior to dorsal-fin origin; profile straight to slightly convex along dorsal-fin base; straight from insertion of last dorsal-fin ray to mid-length of caudal peduncle, and straight to slightly concave anterior to insertion of dorsal procurrent caudal-fin rays (Ref. 96084). Ventral profile of head convex along anterior portion of dentary; slightly convex from that point to isthmus; ventral profile of body slightly concave from isthmus to pelvic-fin insertion; straight in region of pelvic-fin insertion; straight to slightly concave from pelvic-fin insertion to anal-fin origin; straight to slightly convex along anal-fin base; and slightly concave along caudal peduncle; ventral surface of body transversely flattened between pelvic-fin insertions (Ref. 96084). Head pointed in lateral profile; eye large and laterally positioned, pupil ovoid with anterior emargination of iris; mouth terminal, located on horizontal through middle of eye (Ref. 96084). Premaxilla with one tooth row of 7 elongate, bicuspid, slightly distally expanded teeth; premaxillary teeth gradually decreasing in size posteriorly; contralateral premaxillae attached medially through syndesmotic articulation; dentary with one row of 7 elongate, bicuspid, slightly distally expanded teeth; dentary teeth gradually decreasing in size posteriorly; contralateral dentaries medially attached through immovable, syndesmotic articulation; dentary movably articulated posteriorly with anterodorsal region of anguloarticular and lacking laterosensory canal segment (Ref. 96084). Maxilla toothless, transversely flat and posteriorly expanded; ventral margin of maxilla slightly convex and posterior border rounded; maxilla extending posteriorly to vertical through anterior margin of posterior naris; middle and posterior portions of maxilla situated medial to anterior portion of first infraorbital when mouth closed (Ref. 96084).

Distal profile of dorsal fin stright to slightly convex; branched dorsal-fin rays progressively decreasing in size posteriorly; unbranched dorsal-fin rays 2-3; branched rays 9-11 (Ref. 96084). Pectoral fin pointed in profile; pectoral-fin rays i,7-10 (Ref. 96084). Distal profile of pelvic fin straight to slightly convex; pelvic-fin rays i,7-8 (Ref. 96084). Distal profile of anal fin straight to slightly concave; unbranched anal-fin rays 2-3; branched rays 11-12 (Ref. 96084). Forked caudal-fin lobes short, distally pointed and of similar size; caudal-fin rays i,17,i (Ref. 96084). Scales ctenoid with serrations formed by separate, distally narrowing ossifications along free margin of each scale (Ref. 96084). Lateral line straight, but incompletely pored with 59-67 scales; posteriormost one or two scales extending beyond point of flexure at posterior margin of hypural plate; anteriormost 6-9 scales pored; scale rows between dorsal-fin origin and lateral line 6-8; scale rows between pelvic-fin origin and lateral line 6-7; scales from posterior terminus of dorsal-fin base to adipose fin 17-22; scales between anus and anal-fin origin 2-4; scale rows around caudal peduncle 18-20 (Ref. 96084).

Colouration in alcohol: Overall color of head and body whitish to very light tan (Ref. 96084). Dorsal region of head and snout, maxilla, and first infraorbital darker than remaining portions of head and body; lateral portion of snout and maxilla dark pigmented; body with sparse, scattered, dark chromatophores more densely distributed over dorsal and posterolateral region of body; chromatophores sometimes more concentrated along posterior margin of scales; dark chromatophores forming longitudinal dark lines along bases of anal and dorsal fins; dark blotch surrounding anus and urogenital papillae usually merges posteriorly with dark line along anal-fin base; posterior portion of caudal peduncle with longitudinally elongate, ovoid, dark spot extending posteriorly onto basal 1/5 of interradial membranes of middle caudal-fin rays (Ref. 96084). Dorsal, pectoral, pelvic, anal, and caudal fins with sparse dark chromatophores positioned on interradial membranes along ray margins (Ref. 96084).