If you slice into a tuna like we did in Ichthyology class, here is what you’ll see: a layer of red muscle sandwiched between white muscle (which looks pink in this photo).   The oxygen-rich red muscle helps tunas power through the water.  Having red muscle located near the core of their body makes tunas unique from many other fishes, and allows them to keep their muscle at a higher temperature than the surrounding water.  Warmer muscle means faster contractions and increased power.  No wonder tunas are some hot-blooded fish!