Tuna are one of the top ocean predators. Few fish out run them as they scythe through the oceans. The key to the tuna's place at the pinnacle of the ocean community is their warm muscles. Retaining heat generated by their red muscle through a network of blood vessels, known as retes, which transfer heat from the blood as it leaves the warm internal muscles to cold blood returning from the animal's surface, tuna are able to maintain a body temperature that is 12°C higher than the surrounding water. Most attempts to understand how this super-predator maintains its high body temperature have focused on retention of the heat generated by the fish's red muscle. `However, the white muscle fibre portions of the myotomes... account for approximately 45 to 55%of the total body mass,' says Hans Malte from the University of Aarhus. Could white muscle be contributing the fish's higher body temperature? Knowing that rete-like structures had been identified in tuna white muscle in the 1960s,Malte and his colleagues Jess Boye, Michael Musyl and Richard Brill decided to build a mathematical model of heat movements in bigeye tuna, to see whether white muscle counter current heat exchangers may contribute to the fish's phenomenal thermal efficiency(p. 3708).
Building models of a tuna fish in an environment that remained at a constant temperature and of another fish that made deep sea excursions to the chilly waters 400 m down, the team found that the only way that the tuna could maintained their body temperature was by recycling 96% of the heat from their white muscle, and 99% from their red muscle. And when they tested the effect of taking away the white muscle heat exchanger and leaving the fish with only its red muscle heat exchanger, the mathematical fish's temperature never reached the 32°C that had been recorded in the wild. The white muscle exchanger seemed to be essential for the fish's warm temperature.
Finally, the team ran a calculation that showed that the fish may actively regulate their body temperatures by adjusting the amount of blood that they send through heat exchanger tissues to warm and cool as they move through the water column.
`Our model shows that the presence of a functional rete in the blood supply to the white muscle is necessary to achieve realistic model outputs,' says Malte and he is keen to find out more about the fish's white muscle heat exchangers to understand how this top predator keeps warm.