Breathing style.
The reason why fast-swimming fish are forced to breathe in the water is simple. Only in this way can their breathing frequency be closely related to their speed.
Because the faster the speed, the more oxygen you need to consume. This breathing method can get more oxygen at the faster speed, but other breathing methods can't.
The bright red meat of tuna.
The color of red fish comes from myoglobin, an oxygen-binding molecule. The expression of myoglobin in tuna is much higher than most other fish, so it is red.
After all, people need a lot of oxygen to run a marathon, so it is reasonable to need more myoglobin.
Tuna can preserve the heat generated by metabolism.
Most fish are cold-blooded animals, and their body temperature changes with the change of water temperature, but tuna also has its own body temperature, but it is not kept at a constant temperature like birds and mammals.
They can always maintain the core body temperature of 25℃-33℃ in water at 6℃. I can only describe it as "core temperature" here, because it is not the whole body temperature, but the temperature in some key parts.
Their hearts are actually not very hot, because cold blood always enters their hearts, but their skeletal muscles, eyes and brains are warm.
The sources of these temperatures are not generated actively like us, but generated by the body during swimming, and then concentrated in these tissues that need a lot of oxygen to maintain the temperature through complicated processes.
This way reduces energy consumption, thus enabling them to swim faster and hunt in colder environments.