Can be diagnosed from species of the A. facetus, A. forquilha, A. kaaygua and A. scitulus species groups, and from A. acaroides, A. ykeregua, A. angiru and A. taura by having 12 caudal vertebrae (vs. 13 - 15) and 14 precaudal vertebrae (vs. 12 - 13); from species of the A. facetus, A. forquilha and A. kaaygua species groups by having three abdominal bars in all stages of life (vs. always four abdominal bars in juveniles, and adults with three bars in about 50% and four in about 50% of all specimens examined); from A. perdi by having 26 total vertebrae (vs. 25); from A. ykeregua by the absence of dark marks on the suborbital region (vs. presence); from A. agiru by having a conspicuous rounded caudal-fin base spot (vs. spot very narrow or absent); from all its congeners of the A. autrani species group (except A. macacuensis) by having posterior arm of trunk bar 7 wider than anterior one (vs. arms with the same width) from A. autrani, A. capixaba, A. macacuensis, A. macaensis, A. saquarema and A. ribeirae by having a complete red bar on posterior margin of caudal fin (vs. absence of red regions on caudal fin in A. autrani, A. capixaba, A. macacuensis, A. macaensis, A. saquarema and A. ribeirae; and two red regions on posterior margin of caudal-fin, one dorsal corner and other ventral corner in A. robustus and A. paraibae); from A. saquarema, A. muriae, A. robustus, A. barbosae, A. macacuensis, A. ipatinguensis, A. paraibae and A. ribeirae by having a longer caudal pedulce (caudal pedulce length 10.1 - 11.9 % SL in A. montanus sp. n. vs. 6.6 - 8.0% SL in A. ipatinguensis, 6.7 - 9.0% SL in A. saquarema, 5.1 - 7.9% SL in A. macacuensis, 5.5 - 8.7% SL in A. barbosae, 7.1 - 8.9% SL in A. muriae, 6.4 - 8.1% SL in A. paraibae, 7.4 - 9.2% SL in A. robustus and 6.6 - 9.1 % SL in A. ribeirae); from A. macaensis and A. saquarema by not having depression on snout (vs. depression on snout present in specimens above 30.0 mm SL, just above eyes); from A. macacuensis by having fewer teeth along posterior margin of ceratobranchial 5 (22 - 24 vs. 25 - 29); from A. macacuensis, A. macaensis and A. autrani by having trunk bars usually forked ventrally (vs. never forked ventrally); from A. mattosi by having anal-fin base squamation beginning at the sixth anal-fin spine (vs. anal-fin base squamation beginning at the third anal-fin spine); from A. tavaresi by having mouth isognathous (vs. prognathous) (Ref. 89866).