Astyanax eremus belongs to A. scabripinnis species complex . It differs from all members of this species complex by having a subterminal mouth in specimens larger than 4.82 cm SL (vs. mouth terminal in all species). It is further distinguished from all species from this complex, except A. guaricana, A. gymnogenys, A. laticeps, A. obscurus, A. paranae, A. pirabitira A. scabripinnis, A. serratus, and A. varzeae by having 39-41 lateral line scales (vs. 31-38). It can be diagnosed from A. gymnogenys A. laticeps, A. obscurus, A. scabripinnis and A. serratus by its shallower body depth (27.3-31.3% vs. 35.7-39.0%, 35.3-40.6%, 31.6-40.8%, 33% and 34.2-39.7% of SL, respectively). It differs from A. paranae by its longer snout length (21.8-26.8% vs. 16.0-20.4% of HL); from A. guaricana by its shorter interorbital width (23.5-28.4% vs. 32.7-40.9% of HL) and shorter head length (27.1-32.5% vs. 23.9-26.6% of SL); from A. varzeae by its shorter interorbital width (23.5-28.4% vs. 29.8-37.7% of HL), by relatively shorter caudal peduncle length (13.3-16.3%, modally 14.9%, vs. 10.5- 13.9%, modally 12.4%, of SL) and relatively longer snout length (21.8-26.8%, modally 24.5%, vs. 16.4-23.3%, modally 20.1%, of HL); from A. pirabitira by having four to five cusps on the second to fourth tooth in the inner premaxillary series (vs. seven cusps). It can be distinguished from A. burgerai, A. intermedius, A. jacobinae, A. leonidas, A. microschemos, A. ojiara, A. turmalinensis, A. laticeps, A. obscurus, A. pirapuan, A. rivularis, A. serratus, and A. troya by the shape of humeral spot (straight, very narrow and height, occupying two or two and a half scales wide and about six scales height, vs. wide and short with curved or rounded portions, occupying three or more scales wide and less than six scales height in the former seven species and with upper portion wide and something rounded with a vertical projection in A. laticeps, A. obscurus, A. pirapuan, A. serratus, and A. troya) (Ref. 96865).