 Tuna and other fast @-@ swimming ocean @-@ going fish maintain their muscles at higher temperatures than their environment for efficient locomotion . Tuna achieve muscle temperatures 19 ° F ( − 7 ° C ) or even higher above the surroundings by having a counterflow system in which the metabolic heat produced by the muscles and present in the venous blood , pre @-@ warms the arterial blood before it reaches the muscles . Other adaptations of tuna for speed include a streamlined , spindle @-@ shaped body , fins designed to reduce drag , and muscles with a raised myoglobin content , which gives these a reddish colour and makes for a more efficient use of oxygen . In polar regions and in the deep ocean , where the temperature is a few degrees above freezing point , some large fish , such as the swordfish , marlin and tuna , have a heating mechanism which raises the temperature of the brain and eye , giving them significantly better vision than their cold @-@ blooded prey . 
