"Hard to mistake with any other loricariid other than perhaps Acanthicus hystrix when adult. Quite slender in relation to body length, even delicate in appearance when very small, although as they grow they become much more robust. Small specimens have spots over the whole body, which become bands on the caudal filaments. These spots become relatively smaller and fewer in number with growth, they are reduced to remaining on the caudal peduncle and sometimes on the leading rays of the pectoral fins. Larger adults lack white spots. When adult the pectoral fins can be in excess of 15 cm in length, giving a remarkable front profile. Acanthicus adonis is distinguished from all other congeners in having a juvenile coloration of white spots, a more pointed (vs. rounded) snout, and reduced odontodal growth on the snout margin and the preoperculars. The original description also alludes to additional color differences (pitch black for A. adonis and dark brown for all other Acanthicus)."
quote from planetcatfish. red bit is the bit under dicussion, and the orange bit highlighted next to it is what i love in the adults, as it adds more to how they move