A First for Fish
By Sage Wesenberg, Biochemistry 2019
All fish are cold-blooded in order to allow them to survive in cold ocean waters. Until now. The opah, or Lampris guttatus, has just been discovered as the world’s first fully warm-blooded or endothermic fish. In a recent research paper by Dr. Nicholas Wegner published in Science, researchers outlined the characteristics of this fish, as well as how its endothermy gives it significant advantages.
Water usually takes the heat right out of animals. By being cold-blooded, fish are able to stay the same temperature of the water that they swim in. This, however, limits their ability to function well when the water gets colder. This is called cardiovascular endurance, and is what their body can take based on temperature. However, some fish, like tuna and billfish, are able to raise their body temperatures for a short amount of time to help fuel their muscles for hunting purposes. Afterwards, however, they must go back to some warmer water to return to normal.
The opah, also called the moonfish, can keep its entire body warm despite the fact that it lives 50–200 meters below the surface in water about 50°F and colder. Round, and about the size of a car tire, the opah can weigh up to 200 pounds and is reddish-orange and silver colored with white spots. Its normal diet includes squid and other fish. The opah swims by flapping its large pectoral fins, which generates heat that can then be stored in body fat. Unlike other fish, it has a special structure of many small blood vessels in the gills which help conserve heat.
Most fish have a few larger vessels that bring blood to and from the gills, but the opah’s detailed network of smaller vessels is made up of paired veins and arteries, creating a “rete mirabile” or “wonderful net”. This can also be found in species other than fish and is used as a countercurrent heat exchanger, but the opah is the first fish to have the net around its gills. Around this, there is a thick layer of fat that is used for additional insulation. This system is important in many animals because it helps minimize heat that could be lost.
In 2012, in an annual ocean survey, some opah were caught off of the coast of California and the gills were given to Dr. Wegner for research. After seeing the different gill structure, researchers measured the opah’s body temperature in the ocean, and saw it to be about 5 degrees warmer than the surrounding the water. Then, the muscle temperature was measured to be about 10 degrees warmer than the surrounding water. All of this brought together the conclusion that the opah was a fully warm-blooded fish.
Being warm-blooded provides many advantages for the opah. Because it has a higher body temperature, the opah is a very powerful swimmer and has a higher endurance for temperature changes, as well as the distance it is able to swim. Opah are known to migrate very long distances and can quickly chase prey no matter where they go. Its brain and eyes are even warmer than the rest of the body, giving the opah strong vision and fast response time. All of this makes the opah a very fierce predator.
Being warm-blooded has for a long time set mammals apart in the world. Now we have discovered a fish that has adapted to being warm-blooded, and this characteristic will be able to give it several advantages in the ocean.