Tympanopleura longipinna is distinguished from T. atronasus and T. cryptica by possessing much longer anal fin (32-42 anal-fin rays vs. 23-30). It further differs from T. atronasus in having 10-13 pectoral-fin rays (vs. 7-9), 19-25 gill rakers on the first arch (vs. 14-18), 13-15 preanal vertebrae (vs. 16-19), 40-43 total vertebrae, mode 43 (vs. 39-43, mode 41), 4-5 pleural ribs (vs. 7-8), distance from pelvic- to adipose fin origin 37.6-45.1% SL (vs. 31.2-38.1%), anal-fin base 33.9-39.9% SL (vs. 22.1-27.6% SL), and a generally lighter overall pigmentation pattern on the head, dorsum, and sides of the body, lacking the characteristic dark patches of melanophores concentrated on the flanks above the anal-fin base, the chin, and streaks in each caudalfin lobe usually present in T. atronasus. It can be further diagnosed from T. cryptica in having 10-13, mode 11, pectoral-fin rays (vs. 8-10, mode 9), 13-15, mode 14, preanal vertebrae (vs. 14-15, mode 15), 40-43, mode 43, total vertebrae (vs. 38-41, mode 38), preanal length 49.7-57.6% SL (vs. 59.6-66.0% SL), prepelvic length 38.8-48.4% SL (vs. 48.3-53.6% SL), distance from dorsal- to adipose-fin origin 46.5-54.0% SL (vs. 33.9-46.7% SL), anal-fin base length 33.9-39.9% SL (vs. 24.4-30.3% SL), eye diameter 11.6-18.5% HL (vs. 16.7-25.6% HL), and a generally lighter pigmentation pattern on the head, dorsum, body, and fins. It is distinct from T. brevis in having an overall greater number of anal-fin rays (32-42, mode 37 vs. 31- 36, mode 33) and total vertebrae (40-43, mode 43 vs. 38-41, mode 40), greater distance from pelvic- to adipose-fin origin (37.6-45.1% SL vs. 31.2-38.3% SL), shorter pectoral fin spine (15.9-18.2% SL vs. 19.1-24.4% SL), longer anal-fin base (33.9-39.9% SL vs. 26.6-33.9% SL), shorter head length (25.2-29.7% SL vs. 29.7-35.6% SL), and usually lighter igmentation pattern on the head, dorsum, sides of the body, and fins (vs. diffuse, dark tan to brown pigment extending well below the lateral line and paired fins with dark membranes). It can be diagnosed from T. piperata in having a greater number of pectoral-fin rays (10-13, mode 11 vs. 6-10, mode 9), more gill rakers on the first arch (19-25, mode 23 vs. 16-23, mode 19), fewer preanal vertebrae (13-15, mode 14 vs. 14-16, mode 15), more total vertebrae (40-43, mode 43 vs. 39-41, mode 40), greater body width at the pectoral-fin origin (21.1- 25.7% SL vs. 16.8-20.0% SL), smaller eye diameter (11.6- 18.5% HL vs. 24.3-35.7% HL), gas bladder with two short posterior diverticula (vs. diverticula absent), and lack of a dense band of pigmentation on the base of the caudal fin. It is distinguished from T. rondoni in having 32-42, mode 37, anal-fin rays (vs. 28-37, mode 31), 19-25, mode 23, gill rakers on the first arch (vs. 24-33, mode 29-30), 13-15, mode 14, preanal vertebrae (vs. 14-16, mode 15), 40-43, mode 43, total vertebrae (vs. 38-42, mode 40), predorsal length 29.5-36.1% SL (vs. 35.2-47.4% SL), distance from dorsal to adipose-fin origin 46.5-54.0% SL (vs. 38.1-46.8% SL), anal-fin base length 33.9-39.9% SL (vs. 23.1-32.5% SL), a cordiform gas bladder with two short posterior diverticula (vs. gas bladder elongated antero-posteriorly and with two longer, recurved posterior diverticula), and an overall light pigmentation pattern on the head, dorsum, sides of body, and fins (vs. dark pigmentation over most of body and fins with prominent spots or mottling) (Ref. 103256).