Opahs closely resemble in shape the unrelated butterfish (family Stromateidae). Both have falcated (curved) pectoral fins and forked, emarginated (notched) caudal fins. Aside from being significantly larger than butterfish, opahs have enlarged, falcated pelvic fins with about 14 to 17 rays, which distinguish them from superficially similar carangids—positioned thoracically; adult butterfish lack pelvic fins. The pectorals of opahs are also inserted (more or less) horizontally rather than vertically. The anterior portion of an opah's single dorsal fin (with about 50–55 rays) is greatly elongated, also in a falcated profile similar to the pelvic fins. The anal fin (around 34 to 41 rays) is about as high and as long as the shorter portion of the dorsal fin, and both fins have corresponding grooves into which they can be depressed.