Opah: The world’s first fully warm-blooded fish

By: Parimal M. Rohit

SAN DIEGO — No one will ever confuse the opah as human, even if the moonfish was recently identified by the National Oceanic and Atmospheric Administration (NOAA) as the first fully warm-blooded fish.

According to research published by NOAA Fisheries last month, the opah, similar to mammals and birds, circulates heated blood throughout its body. Being a warm-blooded fish means opahs have a competitive advantage in colder water. Opahs can also swim faster and see more clearly than its cold-blooded counterparts, allowing the large and colorful pelagic organism to be a high-performance predator, NOAA researchers stated.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments. 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,” said Nicholas Wegner, a fisheries biologist with NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla.

Wegner was the lead author of a new paper published by NOAA Fisheries identifying opahs as warm-blooded fish.

The team of NOAA researchers discovered opah uses its gills to conduct a “heat exchange,” regulating the cold blood entering its system as it absorbs oxygen from the water. Combined with the unique location of its gills, opahs are able to maintain an elevated temperature throughout their entire body, even while traveling through colder depths. Helping the fish species stay warm is fatty tissue surrounding the gills, heart and muscles. The fatty tissue helps the opah generate internal heat and keep it insulated from colder water temperatures.

“There has never been anything like this seen in a fish’s gills before. 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,” Wegner said.

Helping opahs maintain warmer blood temperatures: its external fins. NOAA’s researchers said opahs constantly flap their fins similar to how a bird flaps its wings, helping the fish species maintain high metabolism and speedy reaction times. Generally speaking, fish in colder depth are “slow and sluggish,” the NOAA research team stated.

Wegner and his colleagues found opahs generally have, on average, a muscle temperature about 41 degrees Fahrenheit higher than the surrounding water temperature as it swims between 150 and 1,000 feet below the surface.

Researchers added the opah, unlike sharks or tuna, are capable of keeping its entire body warmer than its surrounding environment. Sharks and bluefin tuna also travel at fast speeds while underwater but are only capable of keeping body parts warm; internal organs, such as a shark’s heart, does not stay warm at lower depths.

“Nature has a way of surprising us with clever strategies where you least expect them. It’s hard to stay warm when you’re surrounded by cold water but the opah has figured it out,” Wegner said.

Recent NOAA surveys indicate anglers have reeled in more opah off the California coast in recent years.

“Current conditions may be favoring the fish, or their population may be growing,” a NOAA statement explained. “Opahs are not usually targeted by fishermen off California but local recreational anglers and commercial fisheries occasionally catch the species.”

For more information about the opah species, visit http://goo.gl/Fr8875.

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