Nannostomus rubrocaudatus is similar to and closely related to Nannostomus marginatus and Nannostomus mortenthaleri. They differ from them in that adult males have a bright purplish-red color between the dark stripes on their sides. The middle dark stripe extends to the base of the anal fin. The dorsal fin is predominantly red in the males, in the females it is a smaller red spot. Nannostomus marginatus is clearly distinguished from the other two in that it lacks the red color on the males’ bodies, but N. mortenthaleri and N. rubrocaudatus are less easily distinguished. Males of N. rubrocaudatus differ from those of…