Diagnosis: Hemibrycon sierraensis n. sp. is distinguished from all other species of the genus by having an iridescent red adipose fin in life (vs. hyaline or transparent in life, except H. divisorensis that has a reddish adipose fin) and by having a dark brown adipose fin in alcohol (vs. hyaline or transparent in alcohol); it differs from H. divisorensis, H. pautensis and H. santamartae in having the last dorsal-fin ray unbranched; it further differs from H. beni, H. boquiae, H. brevispini, H. colombianus, H. mikrostiktos, H. microformaa, H. metae, H. palomae and H. rafaelense in having a vertically elongate humeral spot that extends 3 to 4 scale rows below the lateral line series (vs. humeral spot roughly circular not extending ventrally 3 to 4 scales rows below lateral line series, or with inconspicuous, diffuse vertical extensions); it differs from H. pautensis by the number of scale series between the lateral line and the pelvic-fin insertions (4-5 vs. 6-7); it differs from H. divisorensis in the number of unbranched anal-fin rays (iii vs. iv), number of dentary teeth (10 vs. 13-16) and the number of maxillary teeth (6-7 vs. ten-13); in addition to the above characters, we found the following differences that distinguish this new species from those that occur in the same basin (and in allopatry with H. santamartae): fewer supraneurals (6 vs. 8), greater caudal peduncle depth (16.5-17.7 vs. 8.7-15.6% SL), longer head length (25.0-29.6 vs.18.4-25.2% SL) and smaller orbital diameter (23.9-34.8 vs. 33.1-45.7% SL) (Ref. 118119).

Description: Body slender and elongate (mean maximum body depth 31.4% SL); area between orbits slightly convex; dorsal profile of head straight to supraoccipital, then slightly convex from supraoccipital to dorsal origin and from last dorsal-fin ray to caudal peduncle, then straight to base of caudal fin; ventral profile of body convex from snout to base of pelvic fin, oblique along anal-fin base, then straight to base of caudal fin; head and snout large, mandibles equal; mouth terminal, lips soft and flexible, not covering outer row of premaxillary teeth; ventral border of upper mandible oblique; posterior edge of maxilla reaching anterior edge of orbit; opening of posterior nostrils vertically ovoid; premaxilla with 2 rows of teeth, outer row with 4 to 5 tricuspid teeth; internal row with 4, the 1st 3 pentacuspid and the last tricuspid, diminishing gradually in size laterally; maxilla extends beyond posteroventral edge of the 2nd infraorbital, and has 6-8 tricuspid teeth in a series not reaching anteroventral margin of that bone; 4 anterior most dentary teeth larger, with 5 cusps, followed by 6 medium-sized tricuspid teeth; lateral line complete, perforated scales 38(3), 39(15), 40(52), 41(4) or 42*(12); scale rows between dorsal-fin origin and lateral line 7*(40) or 8 (46); scale rows between lateral line and anal-fin origin 6*(28) or 7(58); scale rows between lateral line and pelvic-fin insertion 4(30) or 5*(56); predorsal scales 14(4), 15*(44), 16(30) or 17(8), arranged in regular series; anal-fin rays iii, 26*(28) or 27(58); anal-fin origin posterior to vertical through base of 1st dorsal-fin ray; pectoral-fin rays i,10*,i (46) or 11(40); dorsal-fin rays ii,7,i*; pelvic-fin rays i,6,i*; in both fins the last ray is simple; 1st unbranched ray approximately 1/2 length of 2nd ray, its tip reaching 1st bifurcation of 1st branched ray; pelvic-fin insertion anterior to vertical through dorsal-fin origin; caudal fin forked with short pointed lobes not covered with scales; total number of vertebra 40-41 (n=4); 6 infraorbitals; 1st infraorbital extending over dorsal surface of maxilla, bearing laterosensorial canal pores, its anterior portion has short process with blunt tip, that does not extend towards antorbital; its posterodorsal margin not modified; 7 supraneurals between head and 1st proximal pterygiophores of dorsal fin, with cartilage on upper and lower margins; 4 proximal radials, 3rd postcleitrum with a small laminar lateral process on medial surface; pelvic bone short, straight, blunt with cartilage at anterior tip; pelvic bone ischial process with cartilage; 5th and 6th hypurals united, posterior margin of hypurals without cartilage; medial and proximal pterygiophores of 1st 4 anal-fin rays fused (Ref. 118119).

Color in alcohol: body dark brownish-yellow, chromatophores more densely concentrated on dorsum, most intense on head and extending to anterior part of dentary; midlateral body with dark stripe from posterior margin of eye, interrupted at humeral spot and then continuing posteriorly onto middle caudal-fin rays; humeral spot vertically elongate, located near posterior margin of opercle, extending from 2nd to 5th scale of lateral line series and extending 3 to 5 scale rows below the lateral line series; ventral part of body light yellow; posterior margin of scales on dorsal region of body dark; dorsal and anal fin with strong concentration of chromatophores along distal margin; adipose dark brown; caudal-fin with dark chromatophores on middle rays; pectoral and pelvic fins as well as caudal-fin lobes hyaline; dorsal fin with chromatophores concentrated mostly on interradial membranes and distal margins of anterior rays (Ref. 118119). Color in life: counter-shaded and with silvery lateral stripe highlighted in iridescent yellowish-green, more conspicuous along dorsal margin of stripe; dorsal margin of opercle anterior to lateral stripe intense violet; dorsal margin of eye yellow; infraorbital along posterior margin of orbit violet, this color extending along dorsal half of opercle; head beneath orbit intense blue; scales on sides of body without melanophores, giving it a whitish or silvery appearance; dorsal region dark violet; wide dark humeral spot, conspicuous beneath silvery lateral stripe, extending through it but with less intensity; posterior part of caudal peduncle with dark midlateral stripe that extends on to middle caudal-fin rays; ventral margin of lateral stripe reddish on caudal peduncle; ventral posterior portion of caudal peduncle red; pectoral and pelvic fins hyaline, anal and dorsal fins with reddish bar crossing middle sections of rays, more notable in males, distal tips of dorsal and caudal fins dark; adipose fin iridescent red (Ref. 118119)