How Large Predatory Game Fish Use Body Heat To Their Advantage –
Including a Tuna Photo Gallery
By April Huffman, Msc
From a young age, we were all taught that animals generally fall into two categories; warm-blooded and cold-blooded. Personality associations are often associated with these groups, hence phrases like “cold-blooded killer”. We were also taught that fish, snakes, turtles, and frogs are all examples of cold-blooded animals. However, in some of our most prized game-fish species, the opposite is true.
The basic difference between an animal that is warm-blooded (or endothermic) versus an animal that is cold-blooded (exothermic) is its ability to maintain its core body temperature. What this means is that warm-blooded animals can maintain a constant body temperature despite the external temperature of their environment. Humans fall into this category along with all other mammals and birds. The body temperature of cold-blooded animals is controlled by the temperature of their environment.
Fishery scientists consider many species of large, predatory game fish to be warm-blooded. These species include tunas, mako and great white sharks, and marlins. But what makes these fish so different from all the others? The actual physical structure of their circulatory systems is dramatically different from other fish. The blood vessels of these fish have a counter-current exchange system and a special structure called a “retia mirabilia”, which literally means “wonderful net”. Imagine the closed circulatory system of a tuna. As the tuna swims, heat is produced as a byproduct of active muscles. The muscles involved in the swimming are rich in blood and blood vessels that provide oxygen to the muscles as they rapidly move. The heat given off by all this activity in most fishes is lost as the blood moves throughout the body, especially near the skin surface, which is much cooler than the muscle because of its close contact with the colder ocean water. But in tuna and other warm blooded fish, the heat carried within the blood as it passes through active muscle tissue is not lost. These vessels, as they leave the muscle layers, pass through the retia mirabilia, which is similar to a honeycomb of blood vessels. Other blood vessels, returning from colder areas of the body, such as the skin, also pass through this retia mirabilia. As the cooler and warmer blood vessels pass through this structure, they are brought so close together that the blood heat from the warmer areas is exchanged across the walls of the blood vessels to the colder blood. These warm-blooded fish are not considered to be truly warm-blooded as mammals, because they cannot maintain a constant body temperature. However, the countercurrent exchange of the body heat produced in the swimming muscles is so efficient that the fish cannot be considered truly cold-blooded either. For example, in skipjack tuna, 95% of the heat produced in the swimming muscles is conserved. A grouper can only keep its body temperature about 0.3 degrees Celsius above its surrounding water temperature, while an albacore tuna can raise its body temperature to about 12 degrees Celsius above its surrounding water temperature.
So why are they warm-blooded? With very little exception, the warm-blooded fishes are all fast-swimming, blue-water predators or species that dive to catch prey. Also, there are similar heat exchange systems in other parts of the bodies of some predatory fish, such as the brains and eyes of swordfish, shortfin makos, and porbeagle sharks. Scientific research is concluding that these circulatory system adaptations of these predators allow them to have an advantage over their prey. For instance, warmed swimming muscles provide a boost to a tuna trying to catch its prey in cold water, since the warmer muscles can contract faster, allowing the tuna greater swimming thrust. Eyes and brains that are kept warmer are of great advantage to fish, like swordfish, which must dive great depths after their prey through thermoclines of cold water. Eyes are known to be enormously important for billfish hunting, and the vision function of these fish is not impaired by diving since they are kept at a warmer temperature.
The next time you hook a tuna or a billfish, take a moment to appreciate what an efficient “warm-blooded killer” Mother Nature has designed.