In a recent discovery that goes against everything we were taught growing up; birds and mammals are warm-blooded, while reptiles, amphibians and fish are cold-blooded, scientists have discovered that the Opah is the worlds first fully warm-bodied fish off the west coast of North America.
In a paper published today in Science, researchers from the National Oceanic and Atmospheric Administration (NOAA) describe the unique mechanism that enables the Opah (Lampris guttatus), a deepwater predatory fish, to keep its body warm. The secret generating its own body heat lies in a specially designed set of blood vessels in the fish's gills. Unlike tuna, sharks, and sailfish species which are partially endothermic (heterothermic), the Opah is the first fully endothermic fish discovery.
The tire-sized Opah, also known as the moonfish or sunfish, has relatively small red fins adorning its large, round body, which can grow up to 6 feet (1.8 meters) long. These fins, which flap rapidly as the fish swims, turn out to be important in generating body heat for the Opah, often found swimming with tuna schools.
Fish, amphibians and reptiles belong to a group called ectotherms meaning that these animals do not produce heat to maintaining a constant and normally high body temperature (as is the case for birds and mammals). Instead they rely on the environment and their own behaviour to control their temperature. For fish, this means that body temperature varies directly with water temperature.
Endothermy is the ability to maintain body temperatures that are higher than environmental temperatures. Tuna are endothermic and therefore are able to migrate over huge distances and make deep vertical dives in order to catch prey and avoid predators while maintaining a high over-all body temperature.