Chaetostoma trimaculineum is distinguished from all other congeners from Amazon Basin except C. anale by having three to four linear rows of distinct, round, naris-sized black spots on the median, mid-dorsal, and dorsal plate series of the trunk (vs. absence of black spots, larger than orbit, or also present on mid-ventral plate series). It also differs from all other Amazon Basin species except C. anale, C. daidalmatos, C. dermorhynchum, middle Ucayali populations of C. lineopunctatum (including type locality), C. stroumpoulos, and C. vagum by having distinct round black spots half naris diameter spaced less than one spot width apart on head (vs. absence of spots on head or white); from C. anale, C. carrioni, C. lexa, C. marmorescens, C. stroumpoulos, and C. vagum by having four branched rays on anal fin (vs. anal fin absent or with most frequently three or less or five branched rays); from C. branickii/taczanowskii, C. carrioni, C. lexa, C. loborhynchos, C. marmorescens, and C. microps by having a supraoccipital excrescence (vs. absence of excrescence); from C. changae, C. jegui, C. lexa, C. marmorescens, and C. microps by having eight branched dorsal-fin rays (vs. most frequently nine or sometimes ten); from C. anale, C. carrioni, C. daidalmatos, C. lexa, and C. microps by having more teeth per premaxillary ramus (an average of
112632, vs. typically, 80); from C. anale, C. carrioni, C. lexa, C. marmorescens, and C. microps by having more teeth per mandible (an average of 156645, vs. typically, 111); from C. breve, C. lineopunctatum, and C. microps by having lesser head depth (23.060.3% SL, vs. 23.3); from C. loborhynchos and C. marmorescens by having lesser cleithral width (32.760.8% SL, vs. 33.5); and from C. branickii/taczanowskii and C. lexa by having four or five evertible cheek odontodes (vs. six or more). Chaetostoma trimaculineum mostly resembles C. anale but can be distinguished by the tooth and anal-fin branched ray counts, by the absence of spots from most or all of the compound pterotic (vs. spots covering the pterotic), by having generally straighter rows of spots along the body, and by lacking spots from the mid-ventral plate series (vs. spots often present on at least some mid-ventral plates) (Ref. 104727).
Description: Dorsal-fin branched rays 8; anal-fin branched rays 4-5; pectoral-fin branched rays 6; pelvic-fin branched rays 5 (Ref. 104727).