Diagnosis: can be distinguished from other species in the E. trilineata species group, except E. microstoma and E. trilineata, by the mouth width 20.0–25.1% HL (versus 13.1–18.4% in E. desantanai; 12.9–17.5% in E. guairaca; 12.6–16.1% in E. matintapereira; 13.2–18.1% in E. muirapinima; 10.8–19.0 in E. pavulagem; 9.5–17.2% in E. vicentespelaea; and 9.5–14.6% in E. waiwai); Eigenmannia antonioi differs from E. microstoma and E. trilineata by the suborbital depth 18.4–27.8% HL (versus 29.9–40.8% and 32.5–46.6%, respectively); by the dentition pattern of the premaxilla with 8 to 12 teeth distributed in 2 rows (outermost row with 3 to 6 teeth, innermost row with 4 to 6 teeth) [versus 16 teeth distributed in 3 rows (outermost row with 5 teeth, median row with 6, innermost with 5 teeth) in E. microstoma, and the 31–33 teeth distributed in 4 rows (outermost row with 8 or 9 teeth, 2nd row with 5 or 6, 3rd row with 10; innermost with 7 or 9 teeth) in E. trilineata] and by the length of anterodorsal process of maxillary corresponding to 50% of the width of the posterior nostril (versus equal to the width of the posterior nostril in E. trilineata and E. microstoma); Eigenmannia antonioi can be further distinguished from E. microstoma by the length of the coronomeckelian bone 20% of the length of Meckel’s cartilage (versus 45% of the length of Meckel’s cartilage in E. microstoma); distinguished from E. trilineata by the depth of the posterodorsal expansion on infraorbitals 1 + 2, which approximately equals the total length of infraorbitals 1 + 2 (versus less than 50% of the length of infraorbitals 1 + 2 in E. trilineata), and by 8 or 9 endopterygoid teeth (versus 17 in E. trilineata) (Ref. 115689).

Description: body elongate and laterally compressed; dorsal profile of body nearly straight from rear of head to vertical through middle of anal fin, and then posteroventrally aligned with distal portion of caudal filament; ventral profile of body slightly concave along anterior half of abdominal cavity, then posterodorsally aligned with last analfin ray; ventral profile of caudal filament straight; greatest body depth at vertical through distal margin pectoral fin; head laterally compressed, with greatest width at opercular region and greatest depth at posterior margin of supraoccipital; dorsal profile of head convex from upper lip to vertical through branchial opening; ventral profile of head slightly concave from anterior margin of lower lip to branchial opening; snout rounded in profile; upper lip slightly overlapping lower lip; premaxillary teeth 8(1), 9(2), or 12(1); distributed in 2 rows [outermost row with 3(1), 4(1), 5(1), or 6(1) teeth; innermost row with 4(2) or 6(2) teeth]; maxilla with sickle-shaped anterodorsal process equal to 50% of width of posterior nostril; dentary teeth 8(1), 11(1), 14(1), or 15(1), distributed in 1 or 2 rows [outermost row with 6(2), 7(1), or 8(1) teeth; innermost row with 5(2) or 7(1) teeth]; dentary teeth increasing abruptly in size from the 4th or 5th teeth of outermost row towards rictus; coronomeckelian bone equal to 20% length of Meckel’s cartilage; endopterygoid with 8(3) or 9(1) teeth in single row; mouth rictus at vertical through anterior nostril or in region between nares; anterior naris tube-like, with posterior margin located at vertical through posterior margin of rictus or in median portion of rictus; posterior naris elliptical, without tube, located closer to anterior margin of eye than snout tip; eye approximately circular, covered by skin, laterally located on anterior half of head; antorbital and infraorbitals 1–4 in form of enlarged, partial cylinders with slender osseous arches; 5th and 6th infraorbitals slender and tubular; depth of posterodorsal expansion on infraorbitals 1 + 2 approximately equals total length of infraorbitals 1 + 2; branchial opening moderately elongate; branchial membrane joined to isthmus; anus and urogenital papilla shifting anteriorly ontogenetically; anus and urogenital papilla at vertical through posterior margin of orbit in mature specimens; cycloid scales present from immediately posterior to head to distal portion of caudal filament; lateral line complete, with 113(1), 115(1), 120(2), 121(1), 122(2), 123(3), 124(4), 126(2), 127(3), 128(4), 130(2), 131(1), or 132*(3) perforated scales to vertical through end of anal fin; longitudinal series of scales above lateral line, 8(18), 9*(8), or 10(3); scales over anal-fin pterygiophores approximately 1/2 size of others; distal margin of pectoral fin slightly rounded; tip of pectoral fin reaching vertical through anal-fin rays 16–19; anal-fin origin immediately posterior to vertical through pectoral-fin base; distal margin of anal fin slightly convex; caudal filament cylindrical, tapering gradually distally, relatively short and approximately 30% of LEA in mature specimens; precaudal vertebrae, 13(1) or 14(3); anterior vertebrae, 11(3) or 12(1), transitional vertebrae, 2(2) or 3(2); displaced haemal spines, 3(4) (Ref. 115689).

Coloration in alcohol: background colour brown; dorsal region of head dark brown; gradually becoming lighter ventrally; lips and suborbital region light brown; dorsal region of body dark brown, gradually becoming lighter in region overlying anal-fin pterygiophores; 4 longitudinal dark stripes along body; lateral-line stripe thin, 1 scale deep, extending from 1st perforated lateral line scale to distal portion of caudal filament; superior medial stripe thick, 3 scales deep, tapering from vertical between base of anal-fin rays 21–33 to posterior 1/3 of body; inferior medial stripe moderately thick, 2 scales deep, extending from vertical between base of anal-fin rays 15–31 to posterior 1/3 of body; anal-fin base stripe thick, 2 or 3 scales deep, extending from vertical between base of 9th and 20th anal-fin ray to last anal-fin ray; pectoral and anal fins hyaline, with scattered tiny chromatophores on interradial membranes (Ref. 115689).