Diagnosis: Eigenmannia guairaca can be distinguished from other species in the E. trilineata species group, except E. desantanai, E. microstoma, and E. muirapinima, by the ii,11–12 pectoral-fin rays (versus ii,13–14 in E. antonioi and E. pavulagem; ii,16–17 in E. matintapereira; ii,14–15 in E. trilineata; ii,15–17 in E. vicentespelaea; and ii,13–15 in E. waiwai); Eigenmannia guairaca differs from E. desantanai, E. microstoma, and E. muirapinima by the number of total anal-fin rays, 151–170 (versus 170–198 in E. desantanai and E. muirapinima; and 173–207 in E. microstoma); Eigenmannia guairaca can be further distinguished from E. desantanai and E. muirapinima by the 5 or 6 endopterygoid teeth (versus 14–15 in E. desantanai and 8 or 9 in E. muirapinima); Eigenmannia guairaca differs from E. desantanai by the dentition pattern of the premaxilla with 9 or 10 teeth distributed in 2 rows (outermost row with 4 teeth, innermost row with 5 or 6 teeth) [versus 21–23 teeth distributed in 2 rows (outermost row with 10 to 12 teeth; innermost row with 9 to 13 teeth)]; Eigenmannia guairaca can be further distinguished from E. microstoma by the suborbital depth, 22.2–27.5% HL (versus 29.9–40.8%); the length of anterodorsal process of the maxilla equal to 50% of the width of posterior nostril (versus equal to the width of the posterior nostril); and the length of the coronomeckelian bone equal to 20% of the length of Meckel’s cartilage (versus 45% of the length of Meckel’s cartilage); Eigenmannia guairaca can be further distinguished from all species in the E. trilineata species group, except E. microstoma and E. pavulagem, by 15 precaudal vertebrae (versus 13 or 14 in E. antonioi, E. muirapinima and E. vicentespelaea; 11 or 12 in E. desantanai; 13 in E. matintapereira and E. trilineata; and 12 or 13 in E. waiwai) (Ref. 115689).

Description: body elongate and laterally compressed; dorsal profile of body nearly straight from rear of head to posterior 1/3 of anal fin, and then posteroventrally aligned with distal portion of caudal filament; ventral profile of body posteroventrally aligned from anterior margin of dentary to 1st anal-fin ray, and then posterodorsally aligned with last anal-fin ray; ventral profile of caudal filament 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, 9(1) or 10(1), distributed in 2 rows [outermost row with 4(2) teeth; innermost row with 5(1) or 6(1) teeth]; maxilla with sickle-shaped anterodorsal process equal to 50% of width of posterior nostril; dentary teeth 17(1) or 19(1) distributed in 2 rows [outermost row with 10(1) or 11(1) teeth; innermost row with 7(1) or 8(1) teeth]; dentary teeth all similar in size; coronomeckelian bone equal to 20% length of Meckel’s cartilage; endopterygoid with 5(1) or 6(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 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 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), 119(1), 120(1), 122(2), 124(2), 132*(1), or 143(3) perforated scales to vertical through end of anal fin; longitudinal series of scales above lateral line, 9*(5), 10(5), or 11(1); scales over anal-fin pterygiophores approximately 1/2 the size of others; pectoral-fin rays, ii,11(3) or ii,12*(8); distal margin of pectoral fin slightly rounded; tip of pectoral fin reaching vertical through base of anal-fin rays 16–20; analfin 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 25% LEA in sexually mature specimens; precaudal vertebrae 15(2); anterior vertebrae 13(2); transitional vertebrae 2(2); displaced haemal spines 3(1) or 4(1) (Ref. 115689).

Coloration in alcohol: background colour pale brown; dorsal region of head dark brown; gradually becoming lighter ventrally; lips and suborbital region yellowish; dorsal region of body brown; gradually becoming lighter to 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 20–30 to posterior 1/3 of body; inferior medial stripe moderately thick, 2 scales deep, extending from vertical between base of anal-fin rays 12–23 to posterior 1/3 of body; anal-fin base stripe thick, 2 scales deep, extending from vertical between base of analfin rays 1–16 to last anal-fin ray; pectoral and anal fins hyaline, with scattered tiny chromatophores on interradial membranes (Ref. 115689).