White muscle contracts anaerobically, using glucose for fuel and producing lactate. Red muscle contracts aerobically, using lipid for fuel and producing CO 2 . The Q 10 Rule states that for a 10 degree rise in temperature, enzymatic reactions increase their rate by a factor of three. Muscle contracts more rapidly at a higher temperature but delivers the same power. A fish 10 degrees warmer would theoretically be able to swim three times as fast.
Fish breathe by gills, which are super-efficient diffusion exchangers.
However, heat diffuses 10X as rapidly as does oxygen, so any exchanger which brings the blood into close contact with water will also bring it to the approximate temperature of the water. As the blood circulates down the center of the fish and out through the muscles it cools them so that a typical fish body will be within a few tenths of a degree of the ambient temperature. There can be no warm-blooded fish. Where a typical fish transports most of the blood through the dorsal aorta, then out to the muscles, in the Lamnid sharks and tunas, the majority of the blood flow is by way of the cutaneous artery and returned by the cutaneous vein, both of which lie just under the lateral line. The red muscle is sunken deep into the interior of the fish.