Bluefin tuna does not experience water temperature like most fish: thanks to a network of blood vessels called rete mirabile, it produces and maintains its own body heat. Its swimming muscles, eyes and brain remain up to 10°C warmer than the surrounding water, even in the cold waters of the North Atlantic.
A counter-current exchange system
The heat produced by the contraction of the deep red muscles would normally be evacuated by the blood which returns to the gills to oxygenate on contact with cold water. The rete mirabile prevents this loss: the hot veins exiting the muscles and the cold arteries entering them are joined in a tight bundle, allowing heat to pass directly from one to the other before the cooled blood reaches the central organs. This system, called regional endothermy (tuna also remains a fish with generally variable blood, not a true homeotherm like mammals), also concerns the eyes and the brain thanks to a specific thermogenic organ close to these organs.
This muscular heat confers a decisive advantage: warmer fibers contract more quickly, which allows bluefin tuna to maintain sustained swimming speeds and dive several hundred meters deep, where the temperature can drop more than 15°C compared to the surface, without losing power or visual acuity.
Another strategy: biological antifreeze
Fish from polar waters, like the nototheniids of Antarctica, have evolved towards a radically different solution since they live permanently below 0°C: they produce anti-freeze glycoproteins circulating in their blood, which attach to nascent ice micro-crystals and block their growth, thus preventing the formation of destructive ice in their tissues. Unlike bluefin tuna, which fights the cold by generating heat, these species tolerate cold by neutralizing its most dangerous physical effects, illustrating two distinct evolutionary pathways for surviving in icy water.
