Trichomycterus steindachneri is diagnosed from most of its congeners, with the exception of T. cachiraensis, by having several broad foramina on cleithrum lamina (vs. cleithrum not pierced). It differs from most congeners except T. cachiraensis and T. brachykechenos by having a small posterior cranial fontanel, restricted to the parieto-supraoccipital (vs. a long somewhat rectangular fontanel extending anteriorly between frontal bones and posteriorly into anteromedian region of parietosupraoccipital). It can be distinguished from T. cachiraensis by having 40 or 41 free vertebrae (vs. 37-39), 14-18 ribs (vs. 12), a single upper hypural plate: 3+4+5 (vs. two hypural plates: 3,4+5), and by its coloration pattern consisting of ground color dark brown with a thin mid-lateral dark stripe (vs. ground colorlight brown with three lateral rows of small dark spots). It differs from T. brachykechenos by the possession of four rows of premaxillary and dentary teeth (vs. three), 13-14 opercular odontodes (vs. 8-11), 41-43 interopercular odontodes (vs. 22-30), 6-7 branchiostegal rays (vs. 8), anterior portion of sphenotic anterolaterally directed in dorsal view (vs. directed anteriorly), autopalatine with mesial margin straight(vs. concave), 12-16 teeth on ceratobranchial 5 (vs. 21-22), 11-14 teeth on upper dentigerous plate (vs. 22-25), infraorbital sensory pores i1 and i3 present (vs. absent), 40 or 41 free vertebrae (vs. 37 or 38), 14-18 ribs (vs. 12), first ray of pectoral fin slightly projected as a short filament(vs. not prolonged as a filament), pectoral fin with 7-8 branched rays (vs. 5-6), dorsal-fin origin at same level of pelvic-fin insertion (vs. at vertical through last third of pelvic fin), anal-fin origin posterior to dorsal-fin base(vs. at vertical through middle of dorsal-fin base), anal fin with seven basal radials (vs. six), procurrent caudal-fin rays 16-21 dorsally and 14-17 ventrally (vs. 15-16 and 10-13, respectively), and by the color pattern of lateral surface of body consisting of background dark brown with a thin mid-lateral dark stripe (vs. background light yellow with superficial layer of densely mottled dark brown). It further differs from T. dorsostriatus, which is also found in the río Meta system, by having 40 or 41 free vertebrae (vs. 37), dorsal fin inserted just posterior to neural spine of vertebrae 18 or 19 (vs. vertebra 16), anal-fin origin behind posterior end of dorsal-fin base, inserted just posterior to hemal spine of vertebrae 22-24 (vs. under posterior portion of dorsal-fin base, just posterior to vertebra 20), a single upper hypural plate (vs. two upper hypural plates), and by the color pattern consisting of a thin mid-lateral dark stripe (vs. a dark band or row of spots from just above the gill-opening to the base of the upper caudal-fin rays). It differs from T. migrans, the only other species described from western tributaries of the Orinoco (río Guaviare system) in Colombia, by its maxillary barbel reaching beyond pectoral-fin origin(vs. scarcely reaching this point), anal-fin origin entirely behind dorsal-fin base (vs. at vertical through base of last dorsal-fin ray), and caudal-fin margin slightly rounded (vs. deeply emarginated). It is readily distinguished from remaining species of Trichomycterus described or recorded from Colombia, except T. retropinnis, by having relatively rare pigmentation pattern composed of background dark brown with a single thin dark stripe along middle of side of body (vs. background light; either plain, with more than one stripe on lateral surface of body, with a single but wide mid-lateral band, or variably spotted or dotted). It differs from T. retropinnis by its pectoral-fin ray slightly projected beyond margin of fin as a short filament of less than 20% length of adjacent branched ray (vs. projecting as a long filament of ca. 40% length of adjacent branched ray), pectoral fin with 7-8 branched rays (vs. 6 branched rays), pelvic fin covering anus (vs. not reaching anus), relative position of the dorsal fin, expressed as dorsal-fin origin at same level of pelvic-fin insertion (vs. dorsal-fin origin behind distal margin of pelvic fin), and anal-fin origin entirely behind of dorsal-fin base (vs. anal-fin origin under anterior third of dorsal-fin base) (Ref. 102659).

Description: Dorsal fin with 2-4 (3) procurrent rays and ii, 7 principal rays; anal fin with 3-5 procurrent rays and ii, 5 or 6 principal rays; pectoral fin with i,7-8; pelvic fin with i,4 rays (Ref. 102659).