Peckoltia lujani is distinguished from P. pankimpuju by having well developed color and eyes; from all congeners except P. braueri, P. capitulata, P. compta, P. greedoi, P. oligospila, P. otali, and P. stimulata by having the abdomen largely naked posterior to the pectoral girdle (vs. only small naked patches at insertions of pelvic fins); from all congeners except P. furcata, P. greedoi, P. pankimpuju, and P. sabaji by having the dentaries meet at an angle greater than 90°; from P. ephippiata and P. greedoi by having large spots or blotches on the posterolateral surface of head and nape (vs. very small, very faint spots); from P. ephippiata by lacking slight keels on the lateral plates, particularly the median series (vs. slight keels present), by the presence of bands in the dorsal fin (vs. dorsal fin with light rays and dark membranes), by having fewer teeth, 20-37 dentary, 23-45 premaxillary (vs. 39-72 dentary, 41-73 premaxillary in P. ephippiata); from P. greedoi by having the pectoral-fin spine relaxed position only slightly dorsally, pointing maximally to dorsal insertion of caudal fin (vs. angled dorsally, pointing at insertion of dorsal fin) and pectoral-fin spine reaching less than one plate of the ventral series beyond the pelvic base when adpressed ventral to pelvic fin (vs. two or more). Peckoltia lujani can be diagnosed from Etsaputu by possessing more than six evertible cheek odontodes, the largest of which extending posterior to the eye (vs. six or fewer, the largest not extending beyond the exposed portion of the opercle). It can be separated from Hemiancistrus (except ‘H.’ landoni) and Ancistomus by having prominent dorsal saddles (vs. dark or light spots or entirely dark); and from all Hemiancistrus and Ancistomus by the presence of bands in the caudal fin and absence of free spots (vs. absence of bands or presence of some free spots) and bands in the dorsal fin (vs. spots or no markings).

Peckoltia lujani occurs in the same region as P. caenosa and identification of these two species when juveniles is difficult. Peckoltia lujani can be distinguished from adult P. caenosa by the absence of vermiculations on the abdomen and head, and from all P. caenosa by having the dentaries meet in a broad arc that is greater than 120° (vs. meeting at an angle less than 90°), and by having fewer teeth (all except one specimen with 24-37 dentary teeth and 22-45 premaxillary teeth [16 and 19 respectively in aberrant specimen] vs. 10-18 dentary teeth and 11-21 premaxillary teeth) (Ref. 99824).

Description: Dorsal fin ii,7; anal fin i,4; pectoral fin i,6; pelvic fin i,5 (Ref. 99824).