The opah, or moonfish, is the first known fully warm-blooded fish, according to a study published in the journal Science.
The determination helps to explain why opah are such high performance predators that have a keen sense of vision, swim speedily, react quickly, and have the stamina to chase down fast-moving prey.
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"Before this discovery I was under the impression this was a slow moving fish, like most other fish in cold environments," lead author Nicholas Wegner said in a press release. "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."
Wegner is a fisheries biologist at NOAA Fisheries' Southwest Fisheries Science Center in La Jolla. He first became aware that opah were unlike other fish when a colleague, Owyn Snodgrass, collected a sample of an opah's gill tissue.
Wegner noticed that the tissue had blood vessels to carry warm blood into the fish's gills. The blood vessels then wound around those carrying cold blood back to the body core after absorbing oxygen from water. Engineers call this a "counter-current heat exchange."
In this case, the car radiator-like system means that warm blood leaving the fish's body core helps to heat up cold blood returning from the respiratory surface of the gills where it absorbs oxygen.
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Opah live up to 1000 feet beneath the ocean's surface in very cold, dimly lit waters, making this discovery all the more remarkable.
The researchers found that the industrious fish constantly flap its fins, which generate body heat. The flapping also speeds up the opah's metabolism, movement and reaction times.
Temperature measurements found that the opah's body is about 41 degrees Fahrenheit higher than the surrounding chilly waters. While such a temperature is a drop in the bucket compared to the body temps of humans and other mammals, we are not living in a perpetually cold environment as these fish are.
"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."
Certain other fish, such as some sharks and tuna, have what's known as "regional endothermy," or limited warm-bloodedness. It allows them to stay active in colder depths, as well as shallower waters. But the fully warm-blooded opah are unlike all other fish, at least so far as we know it.
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"Nature has a way of surprising us with clever strategies where you least expect them," Wegner said. "It's hard to stay warm when you're surrounded by cold water but the opah has figured it out."
Opah, primarily caught in waters off of Hawaii, are becoming increasingly popular in seafood markets. The organization SeaChoice, however, warns that long-line fishing of opah can lead to the deaths of endangered turtles, seabirds, marine mammals, sharks and rays. Consumers are advised to avoid imported opah, and even opah caught by U.S. fisheries warrants "some concerns," according to the organization, which tracks fish populations and how fishing impacts other species.
The determination that opah are unlike all other fish, at least in terms of their blood and body temperature, will hopefully stimulate our efforts to keep populations of the unique fish healthy.
"Discoveries like this help us understand the role species play in the marine ecosystem, and why we find them where we do," said Francisco Werner, director of the Southwest Fisheries Science Center. "It really demonstrates how much we learn from basic research out on the water, thanks to curious scientists asking good questions about why this fish appeared to be different."