Etheostoma marmorpinnum is distinguished from all other members of the species complex

by higher percentage of belly covered by scales (60–80% vs. 10% or less); higher percentage of body area along the first dorsal-fin base covered with scales (100% vs. 70% or less); dark distinct marbling in second dorsal fin of nuptial males (vs. lighter diffuse marbling or marbling absent); narrower band width for caudal fin (range = 12–15% of fin length vs. 15–25%) and anal-fin (range = 29–33% vs. 33–58%); more scales

around caudal peduncle (25 vs. 23 or 24); and higher first dorsal fin (D1H, =117 vs. 105 or less). The species is further distinguished from E. percnurum by fewer caudal fin rays (15 vs. 18); narrower distal band on pectoral fin (range = 17–20% vs. 27–32% of fin length) and second dorsal fin (14–21% vs. 23–25% of fin

height); and by prominent tessellation of medial region of caudal fin of nuptial males (vs. uniformly dusky). Further distinguished from E percnurum and E. sitikuense by an intermediate number of pored lateral-line scales (27 vs. 22 or 33 respectively). Means of other measurements were also informative in distinguishing E. marmorpinnum from other members of the complex (Ref. 79849).