Bigeye tuna look a lot like Yellowfin. They are hard to distinguish sometimes. They swim at greater depth then Skipjack and Yellowfin, and therefore have more fat to insulate them from the cold water. The body of the bigeye tuna is cigar-shaped (tapered at both ends). The head is pointed and the eye is relatively large. The color is dark metallic brownish blue to dark yellow on the back becoming gray or whitish below. There often is a bluish stripe on the side. Both bigeye and yellowfin tuna look similar, but yellowfin tuna have much shorter pectoral fins.
Here is some information for those of you interested in scientific terms and additional details:
Family: Scombridae Mackerels
Tunas Genus and Species: Thunnus obesus
Description:Tuna which cannot be distinguished by external characteristics can be positively identified by liver characteristics. Bigeye tuna livers are striated (covered with blood vessels) along the trailing edges, while yellowfin tuna livers are smooth. Small bigeye tuna also may be distinguished from albacore by the characteristics of the liver. The liver is heavily striated in the albacore while the bigeye tuna liver is only striated along the trailing edges.