This species is distinguished from all congeners in the rio Uruguay basin, laguna dos Patos and rio Tramandaí systems by having the following characters: two humeral spots (vs. one in Astyanax eigenmanniorum, A. sp. aff. fasciatus, A. jacuhiensis, and A. laticeps); first humeral spot vertically elongate with upper portion wider portion narrow and curved, with a pronounced constriction at the junction of both portions (vs. present of oval shape spot with two brown vertical bars in humeral region in A. jacuhiensis, oval shape in A. laticeps and forming a uniform wedge in A. eigenmanniorum and A. henseli); anal fin with 8-23 branched rays (vs. 18 or less in A. rachypterygium and A.cremnobates, 23 or more in A. aramburui, A. paris, A. saguazu, and A. stenohalinus); one maxillary tooth (vs. 2-5 in A. henseli); maxillary teeth tri- to pentacuspid (vs. hexa- to heptacuspid in A. dissensus and A. ojiara); and body depth 38.3-46.0% of SL (vs. smaller than 38% of SL in A. brachypterygium, A. cremnobates, A. obscurus, and A. xiru). It is most similar to A. xiru in color pattern, but it can be differentiated by the pelvic-fin length 16.1-18.7% of SL (vs. 13.6-17.1%), dorsal-fin length 24.1- 28.7% of SL (vs. 21.1-25.4%) and absence of bony hooks on the anal and pelvic fins of males (vs. presence) (Ref. 94153).
Description: Anal-fin rays iii-v,19, 20, 21, 22, or 23 (Ref. 94153).