Researchers at the NOAA Fisheries discovered that the opah or moonfish is warm-blooded.
Not all fish are cold blooded. Researchers at the National Oceanic and Atmospheric Administration (NOAA) Fisheries discovered that the opah, also known as moonfish is fully warm-blooded. The opah is roughly the size of a large car tire roams oceans around the world at a depths of hundreds of feet beneath the surface in chilly, dimly lit waters. It swims by rapidly flapping its large, red pectoral fins like wings through the water.
NOAA Fisheries biologist Nick Wegner holds an opah
Fish that typically inhabit such cold depths tend to be slow and sluggish, conserving energy. The opah in contrast can move quick because of its warm blood. That warm-blooded advantage turns the opah into a high-performance predator that swims faster, reacts more quickly and sees more sharply, said fisheries biologist Nicholas Wegner of NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, Calif., lead author of a new paper published 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,” Wegner said. “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 realized the opah was unusual when a coauthor of the study, biologist Owyn Snodgrass, collected a sample of its gill tissue. Wegner recognized an unusual design: Blood vessels that carry warm blood into the fish’s gills wind around those carrying cold blood back to the body core after absorbing oxygen from water.
The design is known in engineering as “counter-current heat exchange.” In opah it means that warm blood leaving the body core helps heat up cold blood returning from the respiratory surface of the gills where it absorbs oxygen. Resembling a car radiator, it’s a natural adaptation that conserves heat. The unique location of the heat exchange within the gills allows nearly the fish’s entire body to maintain an elevated temperature, known as endothermy, even in the chilly depths.
“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 researchers collected temperature data from opah caught during surveys off the West Coast, finding that their body temperatures were regularly warmer than the surrounding water. They also attached temperature monitors to opah as they tracked the fish on dives to several hundred feet and found that their body temperatures remained steady even as the water temperature dropped sharply. The fish had an average muscle temperature about 5 degrees C above the surrounding water while swimming about 150 to 1,000 feet below the surface, the researchers found.
While mammals and birds typically maintain much warmer body temperatures, the opah is the first fish found to keep its whole body warmer than the environment.
A few other fish such as tuna and some sharks warm certain parts of their bodies such as muscles, boosting their swimming performance. But internal organs including their hearts cool off quickly and begin to slow down when they dive into cold depths, forcing them to return to shallower depths to warm up.
“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.”
NOAA research surveys off California have caught more opah in recent years, but biologists are not sure why. Current conditions may be favoring the fish, or their population may be growing. Opah are not usually targeted by fishermen off California but local recreational anglers and commercial fisheries occasionally catch the species. The opah’s rich meat has become increasingly popular in seafood markets.
“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.”
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