Diagnosis: Characidium summus is a unique species among congeners by lacking an adipose fin and having a complete lateral line; can be further distinguished from congeners, including C. vestigipinne, that have complete lateral line and adipose fin absent, vestigial or reduced by the absence of dark bars or spots on head, body or fins, except for a dark blotch immediately on the rear of opercle, a narrow midlateral dark stripe (less than half scale deep), and a dark basicaudal spot (Ref. 104900).

Description: body fusiform and moderately compressed; greatest body depth at vertical through dorsal-fin origin; dorsal profile convex from upper lip to vertical through nares, slightly convex to nearly straight from this point to end of occipital process, slightly convex to nearly straight from this point to origin of dorsal-fin base, slightly convex along dorsal-fin base, and nearly straight from end of dorsal-fin base to origin of anteriormost dorsal procurrent caudal-fin ray; ventral profile straight to slightly convex from anterior portion of head to pelvic-fin origin, straight from latter point to origin of anal fin, straight and posterodorsally slanted along anal-fin base and straight along caudal peduncle; snout triangular-shaped in lateral view; mouth subterminal, slightly more dorsal than ventral edge of orbit; distal tip of maxilla surpassing anterior margin of orbit; orbit rounded, horizontal length longer than snout length; cheek thin, its depth around a 3rd to a quarter of orbit diameter; nares separated; anterior naris with raised margins; posterior naris considerably closer to orbit than to anterior naris, without skin flaps; supraorbital reduced to a minute ossification on anterodorsal portion of eye; nasal bones restricted to the ossified canal, without lateral lamella; parietal fontanel limited anteriorly by frontals; parietal branch of supraorbital canal present, not reaching frontal-parietal border; epiphyseal branch of supraorbital canal absent; dentary teeth in 2 rows; outer series with 10(1) or 12(1) conical teeth; teeth decreasing in size from symphysis; inner series, with around 15 minute conical teeth inserted on edge of replacement tooth trench; dentary teeth usually inclined anteriomedially; distal portion of symphyseal tooth overlapping its contralateral in some specimens; premaxilla with a single series of 7(2), 8*(12), 9(2), or 10(1) conical teeth, decreasing in size from symphysis; maxillary edentulous; ectopterygoid teeth 6(1), conical, distributed in 1 series; mesopterygoid teeth absent; scales cycloid; circulii absent and around 25 radii present on posterior field of scales; lateral line complete, perforated scales 35(3), 36*(11), or 37(3); total horizontal scale rows above lateral line 4*(15) or 5(2); horizontal scale rows below lateral line 5*(14) or 6(2); scales along middorsal line between supraoccipital and origin of dorsal fin around 15, but mostly disordered; scales rows around caudal peduncle 14*(17); axillary scale absent; isthmus completely scaleless, but area between pectoral fins covered by scales; pseudotympanum present, represented by small muscular hiatus at vertical through anterior portion of the swimbladder; hiatus situated between ribs of 5th and 6th vertebrae; dorsal-fin rays ii,9(3), 3,i,8(1), or 3,i,9*(13); distal margin of dorsal fin rounded; dorsal-fin origin at middle of standard length and slightly anterior to vertical through pelvic-fin origin; adipose fin absent (20); pectoral-fin rays highly variable iii,6,i(2), iii,6,ii(2), iii,7,i(2), iii,8,i(1), iii,8,ii(1), iv,6,i(1), iv,6,i(2), iv,7(4), or 4,i,8*(1); 1st and 2nd branched pectoral-fin rays longest; posterior tip of pectoral fin not reaching pelvic-fin insertion; pelvic-fin rays i,6,i*(16), or i,7(1); 2nd and 3rd branched pelvic-fin rays longest; posterior tip of pelvic fin not reaching anal-fin origin; anal-fin rays ii,6*(15), iii,6(1), and iv,5 in the CS specimen; posterior margin of anal fin rounded; caudalfin rays i,7,8,i(1), i,8,8,i*(15), or i,9,9,i(1); caudal fin forked, lobes somewhat pointed, of similar size; dorsal procurrent caudal-fin rays 9(1); ventral procurrent caudal-fin rays 9(1); total number of vertebrae 36(1); precaudal vertebrae 21(1); caudal vertebrae 15(1); supraneural bones 4(1), similar in size; epural bones 3(1); uroneural bones 1(1); branchiostegal rays 4(1), 3 connected to anterior ceratohyal, 1 connected to area between anterior and posterior ceratohyal (Ref. 104900).

Coloration: Color in alcohol: ground color of head and body light brown; head with minute melanophores homogeneously distributed over dorsal and ventral portion, not forming spots; body similarly colored, usually slightly darker on dorsal half; some specimens with inconspicuous reticulate pattern on dorsal half of body due to light concentration of melanophores on posterior margin of scales; dark midlateral narrow stripe (less than half scale deep) usually extending from rear of opercle to caudal peduncle, but not reaching its end or the basicaudal spot; dark blotch immediately at rear of opercle, over 2 1st scales of the lateral line and formed by pigment on skin; blotch somewhat round in most specimens; smallest specimen examined without dark stripe and anterior blotch; vertical dark bars on body absent (n=19); 1 specimen with very inconspicuous vertical blotches below dark midlateral stripe on 1st half of body; basicaudal black spot conspicuous and well defined, somewhat vertically elongated, occupying area on the base of 4–6 caudal-fin rays; small specimens with basicaudal spot distinctly black; fins similarly colored to each other, with melanophores over rays; interradial membranes hyaline; paired and anal fins somewhat less pigmented on its distal half (Ref. 104900).