Are all fish cold-blooded?

There is a species of fish, the Opah fish, which is the only one with such a characteristic.

It is equipped with thermoregulatory mechanisms that make its internal temperature independent of that of the water. It generates heat through its pectoral muscles, which are on average 5 degrees warmer than the rest of the body.

The Opah, instead of having a temperature equal to that of the water like any other fish, can be as much as 10 degrees warmer than its surroundings.

The heat generated by the muscles is preserved by a thick layer of insulating fat, and a structure called rete mirabile.

This is an elaborate network of blood vessels that preserves the heat from inside the body and uses it to warm incoming cold blood.

It is not the only animal to have such heat exchange systems, which are also common in penguins, tuna and marine mammals. The real peculiarity is that it is the only fish that possesses it near the gills.

This helps Opah to survive in cold water and to improve its performance and resistance when hunting.