Serrapinnus potiguar can be diagnosed from all congeners by the shape and arrangement of the ventral procurrent caudal-fin rays of mature males, where the hypertrophied elements are scimitar-shaped and arranged in a semi-circle through the ventral profile of the caudal peduncle (vs. hypertrophied elements rod-shaped or pointed distally, arranged parallel to each other). It differs further by having 5 cusps in the premaxillary and dentary teeth (vs. 7 to 9 in S. calliurus, S. micropterus, and S. piaba, 9 to 11 in S. sterbai, and 10 to 12 in S. gracilis and S. littoris), terminal mouth (vs. slightly superior mouth in S. microdon), an incomplete lateral line (vs. complete lateral line in S. heterodon), dorsal fin mostly hyaline (vs. dorsal fin with an anterior and proximal dark blotch in S. notomelas), and by lacking black spot in the posteroventral region of the abdomen (vs. present in S. kriegi) (Ref. 96867).

Description: dorsal-fin rays ii,9; anal-fin rays ii-iv,17-20; pectoral-fin rays i, 10-12; pelvic-fin rays

i,7-8; scale rows from lateral line to pelvic-fin origin 4 or 5; abdominal vertebrae 15; and caudal vertebrae 18 or 19 (Ref. 96867).