The yellowfin is a large tuna. Its body is strongly fusiform, and deepest under its first dorsal fin, while tapering considerably towards the caudal peduncle. Two dorsal fins are present. In adults, the second dorsal fin is very long, as is the anal fin, which is directly below the second dorsal. These fins become relatively longer in larger individuals. The pectoral fin is also long, reaching beyond the space between the dorsal fins. The caudal peduncle is very slender and includes three sets of keels. Seven to ten dorsal and ventral finlets are present. Scales are lacking behind the corselet, a band of large scales forming a circle around the body behind the head. A swim bladder is present. The eyes are small; teeth are small and conical. Similar species occurring in the same areas as yellowfin tuna include bigeye tuna (Thunnus obesus), blackfin tuna (T. atlanticus), and albacore (T. alalunga). The bigeye tuna has shorter dorsal and anal fins than the yellowfin tuna and is generally a heavier and deeper-bodies fish. Blackfin tuna have dorsal and anal finlets that are dusky in color, rather than bright yellow with black margins as seen in the yellowfin and bigeye tunas. Albacore can be distinguished from the yellowfin with the size of the pectoral fins. The pectoral fins of the albacore are much longer than the yellowfin tuna, reaching to the second dorsal finlet in most cases. Also the caudal fin of the albacore has a white posterior edge which is lacking on the yellowfin tuna. The body is metallic dark blue or greenish above, while the belly and lower sides are silvery white and crossed by many vertical, interrupted lines. Perhaps most distinctly, a golden stripe runs along the side. The second dorsal and anal fins and finlets are bright yellow, and the finlets are bordered by a narrow band of black.