Birds and mammals aren't the only vertebrate animals with warm hearts, it seems. There's also the opah, a deep-water fish that uses heat from its muscles to keep its heart — and the rest of its body — warmer than surrounding waters.
The opah uses an ingeniously stacked network of blood vessels at its gills to retain the heat generated by the flapping of its brilliantly-colored fins, according to a new study in the 15 May issue of Science.
Nick Wegner holds a recently caught opah. | NOAA Fisheries/ Southwest Fisheries Science Center
A few other large fish like tuna and some sharks can temporarily warm a few muscles or organs as they chase after prey, but the opah is unique in that it can warm up its entire body. This feat gives the fish a competitive advantage in cold and deep waters, say NOAA fisheries biologist Nicholas Wegner and his colleagues.
By keeping its heart and brain warm, the opah can react faster to prey and swim stronger and longer than other fish living and hunting at similar ocean depths. "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 said.
Fish lose much of their body heat when their blood comes in contact with their cold surroundings at the gills, where the fish draw oxygen from the water. The opah counteracts this heat loss with networks of intertwined blood vessels called retia mirabilia or "wonderful nets."
In the opah, veins carry blood warmed by muscle work back out toward the gills. These veins are wrapped around arteries that carry colder blood that has been oxygenated at the gills. The densely packed network of vessels conserves body heat by transferring some of the veins' warmth to the arterial blood. Aided by insulating layers of fat at the gills, the warmed arterial blood is distributed throughout the whole body.
Other animals use retia mirabilia to adjust the temperatures of specific parts of the body. Dogs, for instance, have retia mirabilia in their heads to keep the brain from overheating, and penguins have retia mirabilia that reduce heat lost through their flippers.
NOAA biologist Owyn Snodgrass inserts a device to track an opah's internal and external temperatures after its release. | NOAA Fisheries West Coast/ CC 2.0
The NOAA research team examined the opah species Lampris guttatus for this study, but Wegner says there are probably several genetically distinct species of opah, all of which may be able to warm their whole bodies through retia mirabilia.
Sometimes called moonfish, opah has become an increasingly popular catch since the 1980s, when Hawaiian fishers began selling it to use in sushi and sashimi. The fish, about the size of a car tire or manhole cover, are found in tropical and temperate ocean waters throughout the world.
Researchers don't know much about the opah's biology or how sustainable its populations might be, so studies like the one by Wegner and colleagues are helping to fill in basic data on the fish.
"If you asked me a couple years ago if it was possible for a fish to warm its entire body, I would have said it was extremely unlikely," said Wegner. "However, this discovery shows just how little we know and how much there remains to be discovered."
[Credit for associated teaser image: NOAA Fisheries West Coast/ CC 2.0]