Pseudancistrus sidereus is diagnosed by a unique modification of the ventral plates on the caudal peduncle. In loricariids, the plates of the ventral row on the caudal peduncle are typically bent at an approximately 90° angle to follow the contour of the body. The bend is often the site of a slight keel formed from one or more rows of slightly longer odontodes. In P. sidereus, the keel is accentuated by having the dorsal laminae of the plates strongly concave. Although some loricariids may have the dorsal laminae slightly concave, it is much more pronounced in P. sidereus. Pseudancistrus sidereus can be separated from all other species of the Ancistrini by the presence of the keel mentioned above and by the presence of a single large white to yellow spot located at the center of the posterior lateral plates. The only species with a similar coloration are some Hypancistrus and some Panaque, both of which have far fewer than 25 teeth per jaw ramus (vs. much more than 25 teeth), some other species of Pseudancistrus which have hypertrophied odontodes along the snout in males and females (vs. no hypertrophied snout odontodes), and have the dorsal fin reaching at least the preadipose

plate when depressed (vs. about two plates anterior to preadipose plate); and some Hemiancistrus and Peckoltia which generally have the spots much more diffuse (vs. borders of spots distinct) and have the dorsal fin reaching at least the preadipose plate when

depressed (vs. about two plates anterior to preadipose plate) (Ref 53964).