According to a recent report in the journal Science, scientists from the National Oceanic and Atmospheric Administration (NOAA) have discovered the world’s first known fully warm-blooded fish. Collectively known as endotherms, warm-blooded creatures can maintain their internal body temperature despite the surrounding environment’s temperature. It was previously thought that only mammalian or avian species were endothermic, but the deep-water fish known as an opah, or moonfish, has proven an exception. Opahs are large, rust-colored fish that can weigh up to 200 pounds (90 kilos) and are found in the temperate and tropical waters of most oceans. They spend the majority of their time at depth, and are most commonly found in 165 to 1,300 feet (50 to 400m) of water.
NOAA’s recent revelation came after scientists analyzed gill-tissue samples from opahs caught off the west coast of the United States. The research team knew that the opahs generated muscle heat by constantly flapping their sickle-shaped pectoral fins, but weren’t sure how they managed to maintain that heat until lead scientist Nicholas Wegner recognized an unusual design in the fishes’ gill tissue. Upon closer investigation, it became clear that the blood vessels responsible for transporting warm blood from the opah’s core to its gills were wrapped around those that carried cold, oxygen-laden blood in the opposite direction. The contact between the two types of blood vessel allows the cold blood returning from the gills to be heated, thereby enabling the opah to maintain a body temperature that exceeds the water’s temperature by approximately 41F (5C) at all times. This design is known as a counter-current heat-exchange system, the best manmade example of which is a car’s radiator.
“There has never been anything like this seen in a fish’s gills before,” says Wegner. “This is a cool innovation by these animals that gives them a competitive edge.”
Some fish, including tuna and certain sharks, are able to generate heat in specific areas of their bodies, for example in their eyes, brains and swimming muscles. Yet these species cannot survive for extended periods in extremely cold, deep water. After a time, organs that are unable to thermoregulate (including their hearts) slow down so much that the fish must return to the shallows to warm up, which is not the case for the opah. The NOAA research team attached monitors to live opahs, which sent back temperature data as the fish dived to extreme depths. The data showed that regardless of whether the fish were swimming at 150 feet (45 m) or 1,000 feet (300 m), their internal temperatures remained a constant 41F (5C) above the temperature of the surrounding water.
The ability to maintain their body temperature gives the opah an incredible advantage over other deep-dwelling species, most of which become quite sluggish in an attempt to conserve energy in the cold water. Wegner, who previously thought the opah to be as slow-moving as its peers, says that “because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid, and it can migrate long distances.” Opahs swim faster, react more quickly and have better sight and brain functioning than other deep-sea species, simply because they are warm-blooded. Perhaps most importantly, they (unlike tuna and sharks) do not need to return to the surface to warm up. Instead of wasting energy migrating to and from deep water, they are able to spend extended periods of time at depth, where their chosen prey is most prolific.
The ability to stay at depth should mean that opahs remain out of reach of most recreational anglers, and yet NOAA research surveys off the California coast have seen a significant increase in the number of opahs caught in recent years. It is not known whether this increase represents a population growth for this species, or whether their behavior is changing as a result of unseen environmental factors. NOAA hopes to conduct further research on opah populations in other areas of the world.
“Nature has a way of surprising us with clever strategies where you least expect them,” says Wegner. “It’s hard to stay warm when you’re surrounded by cold water, but the opah has figured it out.”
