Your school science teacher was wrong about all those necessarily cold-blooded fish. There's now at least one that joins the ranks of mammals and birds among the world's warm-blooded creatures. Research from NOAA Fisheries indicates that the opah, or moonfish, which swims in the dark, chilly depths of the ocean, is the first completely warm-blooded fish.
The silverish, tire-sized fish glides around by rapidly flapping its red fins, warming its blood and circulating it through its body in the process. This is different from other cold-blooded swimmers that tend to move around slowly and sluggishly to conserve energy, according to a paper on the finding published Tuesday in the journal Science.
"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," said lead author and fisheries biologist Nicholas Wegner of NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, Calif. "But because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid and can migrate long distances."
Key to the opah's elevated body temperature is the unique design of its gill tissue, in which blood vessels carrying warm blood wrap around those that carry cold blood back to the center of the body after gathering oxygen from the frigid water. It's what's known as “counter-current heat exchange,” an efficient way of conserving heat similar to the way a car radiator works.
"There has never been anything like this seen in a fish’s gills before," Wegner said. "This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it."
The opah doesn't keep its body at a toasty 98 degrees Fahrenheit like we humans, but it was found to maintain its body temperature even when the water around it became colder suddenly. The researchers found that it kept its muscle at a temperature about 9 degrees (F) warmer than the surrounding water. While some fish warm up parts of their body for brief periods of action, the opah appears to be the first to keep its entire a body at a consistently warmer temperature than that of its environment.
And, in case you were wondering, the opah is said to have a rich flavor and is becoming increasingly popular in California seafood markets.
