The opah has also been called the moonfish due to its unusually round, silvery body. But the opah's shape isn't its only unusual trait. In a surprise finding, San Diego researchers have discovered that it's also warm-blooded.
In a study published Thursday in Science, researchers with the National Oceanic and Atmospheric Administration in La Jolla describe the opah as the first example of a fish that can warm its entire body above the temperature of surrounding waters.
Researchers reeled in more than 20 opah off the coast of San Diego for the study. Temperature measurements revealed that all parts of the fish were warmer than its environment. Scientists already know that tuna and certain species of shark can selectively warm specific body parts, but the opah's self-heating extends throughout its entire body.
"It's the first fish that actually has a warm heart," said study author and fisheries research biologist Nick Wegner. "This is significant because it allows the fish to stay at depth in cold water and function at a higher level."
Over the last three years, Wegner has been catching lots of opah during surveys for the Southwest Fisheries Science Center. He's not sure why his catch of opah has been increasing in California. He said it's common in Hawaii, where it's often sold for meat.
At first Wegner thought they were probably sluggish creatures like most of the fish in similarly deep, chilly waters. But he's learned that the opah — despite looking like a manhole cover that sprouted fins — is actually a fast and formidable predator. He thinks the opah's advantage is its elevated body temperature.
"At warmer temperatures, muscles contract faster and they have more power," he said. "It can swim around faster, respond faster and see better than these animals that are the same temperature as that cold water."
The researchers dissected some of the opah for clues about how its body temperature is regulated. They found an intricately woven circulatory system. The opah's veins and arteries overlapped tightly. That density allows blood in the veins, warmed by vigorous muscle movement, to transfer heat into the arteries, chilled by oxygen entering the blood from cold surrounding waters through the fish's gills.
Warm blood is often thought to be a unique property of mammals and birds. But Wegner said marine biologists can now point to the opah as one major exception.
