Fig. 3. (A) Diagram of a yellowfin tuna (Thunnus albacares) in cross section or steak view indicating the structural components of the blood vessels that make up the specialized supply for the internal red muscle. The right half of the diagram is similar to Fig. 1C to show the muscle anatomy and allow comparison. In the left half, the muscle anatomy has been removed and the superimposed vascularity is displayed. Arterial vessels are in red and venous vessels are in blue. In yellowfin, the deep red muscle is supplied by large lateral vessels that lie close to the skin, at the margins of the superficial red muscle and the skin. In addition, there is a central rete composed of vessels attached to the posterior cardinal vein and dorsal aorta, from which vessels also enter the deep red muscle. (B) Diagram of the anatomy of a bluefin tuna (Thunnus thunnus) in steak view similar to that in A. Like the yellowfin tuna, the bluefin tuna has peripheral retia, but it lacks the central rete. Importantly, comparison of these figures shows that the vascularity of the deep red muscle is diverse among the heterothermic fish, even between these closely related tuna.

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