Astyanax bagual is distinsguisded from all congeners occurring in rio Uruguay basin, laguna dos Patos and rio Tramandaí systems by having the following characters: first humeral spot vertically elongate with upper portion wider than lower portion and slightly directed forward

(vs. presence of oval spot with two brown vertical bars in humeral region in A. jacuhiensis, oval spot in A. laticeps, and forming a uniform wedge in A. eigenmanniorum, A. henseli, A. saguazu and A. stenohalinus; two humeral spots (vs. one in A. eigenmanniorum, A. aff. fasciatus (Cuvier), A. jacuhiensis, and A. laticeps); 24-28 branched anal-fin rays (vs. 18 or less in A. brachypterygium and A. cremnobates, 18-23 in A. procerus, and A. Xiru, 20-22 in A. paris, 16-21 in A. obscurus, 15-23 in A. laticeps, 15-20 in A. pirabitira, 20-23 in A. ojiara; 22-24 in A. douradilho; 2-4 maxillary tricuspid teeth (vs. one in A. aramburui, A. dissensus, and A. aff. fasciatus); 37-39 perforated scales along the lateral line (vs. 34-36 in A. paris); and 14 scale rows around caudal peduncle (vs. 15-16 in A. stenohalinus). In addition, A. bagual differs from A. douradilho and A. saguazu by body depth (37.9-45.9% of SL, mean = 43.3%, vs. 34.4-41.0% of SL, mean = 37.0% and 30.9-39.0% of SL, mean = 35.6%); and from A. saguazu and A. stenohalinus by preanal distance (62.4-69.1% of SL, mean = 65.5% vs. 56.0-61.2% of SL, mean = 58.7% and 58.2-62.7% of SL, mean = 59.8%), respectively; and from A. stenohalinus by the anal-fin origin located at vertical through last dorsal-fin ray insertions vs. before a vertical through last dorsal-fin ray insertions. In addition, A. bagual differs from all congeners (including the species complex A. bimaculatusand A. scabripinnis) by the following features: two vertically elongated humeral spots present (the anterior humeral spot with upper portion wider than lower portion and slightly directed forward); 24-28 branched anal-fin rays; 37-39 perforated lateral line scales; outer row of premaxilla with tricuspid teeth, inner premaxilla row with pentacuspid teeth, 2-4 maxillary tricuspid teeth; small bony hooks on all fins of mature males; conspicuous caudal peduncle spot, body depth (37.9-45.9% of SL); preanal distance (62.4-69.1% of SL); anal-fin base length (29.7-35.6% of SL); and orbital diameter (29.5-38.2% of HL) (Ref. 103251).

Description: Dorsal-fin rays i,10; anal-fin rays iii or iv, 24, 25, 26, 27, or 28; pectoral-fin rays i,11,12, or 13; pelvic-fin rays i,7; 5 or 6 scale rows between lateral line and pelvic-fin origin (Ref. 103251).