Diagnosis: Eigenmannia desantanai can be distinguished from other species in the E. trilineata species group by the inferior medial stripe, which is 1 scale deep (versus 2 or 3 scales deep); Eigenmannia desantanai can be further distinguished from the other members of the species group, except for E. waiwai, by the 11 or 12 precaudal vertebrae (versus 13 or 14 in E. antonioi; 15 in E. guairaca, E. muirapinima, and E. vicentespelaea; 13 in E. matintapereira; 14 or 15 in E. microstoma and E. trilineata; and 13–15 in E. pavulagem); Eigenmannia desantanai can be differentiated from E. waiwai by the terminal mouth (versus subterminal); the orbital diameter 14.5– 19.6% HL (versus 22.6–28.8%); the length of the anterodorsal process of the maxilla equal to 50% of the width of the posterior nostril (versus 1.5 times the width of the posterior nostril); the depth of the posterodorsal expansion on infraorbitals 1 + 2 approximately equal to the total length of infraorbitals 1 + 2 (versus less than 50% of the length of infraorbitals 1 + 2); and by the dentition pattern of the dentary with 21–23 teeth distributed in 2 rows (outermost row with 10 to 12 teeth; innermost row with 9 to 13 teeth) [versus 37 or 38 teeth distributed in 4 rows (outermost row with 7 teeth; 2nd with 11–15 teeth; 3rd with 8 to 15; innermost row with 4 to 8 teeth)] (Ref. 115689).

Description: body elongate and laterally compressed; dorsal profile of body slightly convex from rear of head to vertical through middle of anal fin, and then posteroventrally aligned with tip of caudal filament; ventral profile of body slightly concave from anterior margin of dentary to 1st anal-fin ray, then posteroventrally aligned with last anal-fin ray; ventral profile of caudal filament nearly straight; greatest body depth at vertical through distal margin of pectoral fin; head laterally compressed with greatest width at opercular region and greatest depth at posterior margin of supraoccipital; dorsal profile of head slightly 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 24(1) or 25(1) distributed in 4 rows [outermost row with 5(2) teeth; 2nd row with 6(1) or 8(1) teeth; 3rd row with 6(1) or 7(1) teeth; innermost row with 7(1) or 8(1) teeth]; maxilla with sickle-shaped anterodorsal process equal to 50% width of posterior nostril; dentary teeth, 21(1) or 23(1) distributed in 2 rows [outermost row with 10(1) or 12(1) teeth; innermost row with 9(1) or 13(1) teeth]; dentary teeth all similar in size; coronomeckelian bone equal to 20% of length of Meckel’s cartilage; endopterygoid with 14(1) or 15(1) teeth in 2 series; mouth rictus at vertical through anterior nostril or in region between nares; anterior naris tubelike, 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 1/2 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 equal to total length of infraorbitals 1 + 2; branchial opening moderately elongate; branchial membrane joined to isthmus, extending to inferior margin of branchial aperture; 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 112(6), 113(1), 118(1), 120(3), 121(1), 125(1), 128*(4), 130(1), or 132(2) perforated scales to vertical through end of anal fin; longitudinal series of scales above lateral line, 8(7), 9(6), or 10*(7); scales over anal-fin pterygiophores approximately 1/2 size of others; pectoral-fin rays, ii,12*(18), ii,13(1), or ii,14(1); distal margin of pectoral fin slightly rounded; tip of pectoral fin reaching vertical through base of anal-fin rays 16–20; anal-fin origin immediately posterior to vertical through pectoral-fin base; total anal-fin rays, 170–198 (185*, N = 20); distal margin of anal fin approximately concave; caudal filament cylindrical, tapering gradually distally, relatively short, and approximately 25% LEA in mature specimens; precaudal vertebrae 11(1) or 12(1); anterior vertebrae 9(2), transitional vertebrae 2(1) or 3(1); displaced haemal spines 3(2) (Ref. 115689).

Coloration in alcohol: background colour dark yellow; 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 moderately thick, 2 scales deep, tapering from vertical between base of anal-fin rays 20–28 to posterior 1/3 of body; superior medial stripe hardly discernible in specimens over 85.0 mm LEA; inferior medial stripe thin, 1 scale deep, extending from vertical through base of anal-fin rays 13–17 to posterior 1/3 of body; anal-fin base stripe thick, 2 scales deep, extending from vertical between base of anal-fin rays 9–14 to last anal-fin ray; pectoral and anal fins hyaline, with scattered tiny chromatophores on interradial membranes (Ref. 115689).