Diagnosis: Ancistrus maximus is distinguished from all congeners, except A. dolichopterus, A. fulvus and A. latifrons, by having more branched rays in the dorsal fin (8 vs. 7); Ancistrus maximus is distinguished from A. dolichopterus by its color pattern, overall body color dark brown with reddish-orange spots almost half the size of pupil in adults and with reddish-orange bands on distal portion of dorsal and caudal fins only in juveniles vs. overall body color black or dark-gray with numerous white dots and with white band on distal portion of dorsal and caudal fins throughout life; Ancistrus maximus differs from A. latifrons by the absence of hypertrophied ('spiny') odontodes in lateral plates (vs. presence of elongate and strong 'spiny' odontodes later­ally oriented in A. latifrons); Ancistrus maximus differs from A. fulvus by dorsal-spine length 27.9-34.5 % SL (vs. 25.7); pectoral-spine length 30.7-42.6 % SL (vs. 26.6) and by total lateral median plates 24-25 (vs. 22) (Ref. 103398).

Description: Large-sized Ancistrus, with largest specimen examined 200.8 mm SL; head and body broad and relatively deep; body rounded anteriorly in dorsal view; becoming gradually narrow from cleithrum to caudal fin; greatest width of body at cleithrum; in lateral view, dorsal profile straight from snout tip to supraoccipital, gently convex from supraoccipital to dorsal-fin origin, and nearly straight and gradually sloping down to 1st dorsal procurrent caudal-fin ray; ventral profile straight from snout tip to pelvic-fin insertion, then ascending gently to insertion of first lower caudal-fin ray;

snout rounded with broad naked margin in males reaching from upper lip to pre-nasal, frontals and preopercular; narrower in females; adult males with well developed, branched tentacles along lateral border of snout and longitudinally aligned along mesethmoid, bifurcating caudally to nares; tentacles smaller and less numerous in females and juveniles, limited to lateral border of snout; females with mesethmoid forming elevated median dorsal ridge on plated snout; males with similar but inconspicuous elevation on plated area in front of eyes; eyes large and laterally positioned on head; interorbital area slightly convex; exposed part of opercle roughly triangular with posterior portion slightly elongated; opercle covered with odontodes, larger on its posterior border; dermal plates of postopercular area small and numerous; hypertrophied cheek odontodes large, strong and few (5-11); fleshy and thick odontode sheath, covering proximal half of cheek odontodes, sometimes leaving its distal 3rd exposed; oral disk circular, lips almost completely covered with small round papillae (larger on proximal region of lower lip); lips smooth around maxillae; lower lip large but not reaching pecto­ral girdle; maxillary barbel moderate, slightly smaller than orbital diameter; sometimes branched, and with large portion free from lower lip; mandibular tooth row short; teeth thin, delicate and bifurcated, mesial cusp larger; lateral cusp half length of mesial cusp; tooth crown bright red; only 1, small median buccal papilla, positioned between premaxillae; branchial opening moderate; supraoccipital large, flat, roughly heptagonal in shape, without any process, ending posteriorly in an almost straight line; dorsolateral region of trunk completely covered by plates, except at dorsal-fin base; only 2 pairs of predorsal plates between supraoccipital and nuchal plate; 1st pair composed by large, almost fused, roughly triangular plates limiting supraoccipital posteriorly; 2nd pair comprised of short and wide plates; 5 series of lateral plates, reduced to 3 on caudal peduncle; ventral surface entirely devoid of plates from snout to anal-fin insertiori; base of 1st anal-fin pterygiophore sometimes covered by skin and preanal plate element sometimes present; a single plate (sometimes absent) between adipose fin and first procurrent caudal-fin ray; 7 to 8 oblong plates on caudal-fin base; caudally oriented short odontodes present on all body plates, except for very hypertrophied odontodes on cheek plates, and slightly larger on opercle, pectoral-fin spine, and on distal border of body plates; origin of dorsal fin at midportion of body, between pectoral and pelvic-fin insertion; dorsal fin II+8 in all specimens examined; spinelet present, dorsal-fin locking mechanism developed; dorsal fin long and very high; its distal border slightly convex; tips of last branched rays reaching adipose-fin insertion when adpressed; 5 to 6 plates separating dorsal and adipose fins; adipose fin well developed, its base containing 3 to 4 plates; preadipose plate present; adipose fin with strong spine; its distal tip reach­ing first caudal-fin procurrent ray; caudal fin i,14,i; caudal fin truncated or slightly emargin­ated, with distal border of caudal-fin rays almost straight, dorsal lobe subequal to inferior lobe or with just the lower unbranched ray slightly longer than others; pectoral fin 1,6, large, arrow­shaped, reaching or surpassing urogenital opening when ad pressed, in mature males; pectoral-fin spine straight, thin, longer than branched rays and covered by large odontodes in mature males; pectoral-fin with anterior branched rays much more developed than posterior ones (first branched ray 1.9-2.9 times larger than last branched ray); pelvic fin i,5; reaching mid portion of anal-fin rays when adpressed; anal fin i,4; small, not reaching vertical of adipose-fin inser­tion; pelvic and anal fins more convex in females, with branched rays slightly longer than un­branched rays (Ref. 103398).

Color in life: overall body color dark brown at dorsum, sides and abdomen; numerous pale orange to reddish-orange dots of similar size over head, trunk and fins; ventral surface with dots only at anterior portion of head and around vent; pale orange to reddish-orange dots almost same size as pupil in juveniles, becoming more numerous and proportionally smaller in adults; juveniles with narrow distal white band on caudal and dorsal fins, followed by a subdistal reddish-orange band; bands absent in adults (Ref. 103398). Color in alcohol: overall body color gray-brown; small pale orange to reddish-orange dots barely visible; white to clear distal bands on caudal and dorsal fins of juveniles (Ref. 103398).