Distribution: tropical and subtropical seas. The 2 dorsal fins separate and depressible into grooves. Finlets following anal and posterior dorsal fins. Origin of anterior dorsal fin well behind the head. Pectoral fins high on body. Pelvic fins below pectorals, with 6 fin rays. Gill membranes not united to isthmus. Small cycloid scales present. Two keels on slender caudal peduncle. Thunnus and close relatives with a specialized vascular system for heat exchange; the evolution of this and related adaptations for endothermy are discussed in Brock et al. 1993 Science 260:210-214. Thunninae=ISSCAAP 36; Scombrinae=ISSCAAP 37.