Diagnosis: distinguished from all congeners, except M. cosmops, by the presence of the red pigmentation on the upper lip, eye with green to blue bright pigments, and for the transparency of the opercle, which allows a view of the pinkish red gill filaments; preserved specimens of M. uirapuru can be distinguished from the remaining congeners, with the exception of M. cosmops, M. dyktiota, M. forestii and M. pyrophthalma, by the presence of an incomplete lateral line (vs. complete lateral line in the remaining Moenkhausia species) and by the presence of wavy longitudinal stripes between scale rows along lateral sides (vs. absence of wavy longitudinal stripes between scale rows along lateral sides in all remaining Moenkhausia species, with the exception of M. agnesae); easily distinguished from M. cosmops by the presence of dark (lightbrown to greenish in life), wavy longitudinal lines between scale rows along lateral portions of body (vs. wavy longitudinal stripes absent), caudal fin with a narrow dark stripe on middle rays (vs. narrow dark stripe on middle rays absent), and by the number of longitudinal scales (28-30, vs. 23-26); can be diagnosed from H. skolioplatus, a similar and putatively related species by the absence of a longitudinal blurred, curved dark stripe on the lower portion flank (vs. longitudinal blurred, curved dark stripe present), a large dark blotch on the caudal peduncle of similar size to the diameter of the eye (vs. caudal peduncle blotch, when present, less than half the diameter), absence of red pigmentation on the peduncle (vs. red pigmentation present over much of upper and lower half of caudal peduncle in living specimens), and by the number of horizontal scale rows between the lateral line and the origin of the dorsal fin (4 vs. 5) (Ref. 119236).