Did you know there’s a fish that can keep its body warm hundreds of meters deep in the ocean? Meet the opah, also called moonfish. A fascinating study from 2015 revealed it as the first fully warm-blooded fish ever discovered. Just like mammals, the opah circulates warmed blood throughout its body, offering several advantages in the chilly depths of the sea.
Typically, fish are cold-blooded, adjusting their body temperature to match their surroundings. This makes sense since they extract oxygen from seawater, which is usually cold. But the moonfish flips that expectation on its head. Researchers found that the opah maintains a body temperature nearly 5°C (9°F) warmer than the ocean around it.
Scientists studied the anatomy of the opah and found something unique in its gills: a special network of blood vessels. This acts like a heat exchange system. Warm blood from the body warms up the colder blood returning from the gills, preventing heat loss. “This is a cool innovation by these animals that gives them a competitive edge,” said fisheries biologist Nicholas Wegner from NOAA Fisheries.
Previously, scientists only knew of fish that could generate warmth in specific body parts, mainly during movement. The moonfish is different; it’s fully warm-blooded, making it an agile predator that can chase faster prey, like squid.
Interestingly, since that revelation, the opah has gained a new reputation. Many thought it was a slow-moving fish, typical of deep-sea dwellers. Wegner was surprised too, noting, “Because it can warm its body, it turns out to be a very active predator that chases down agile prey and can migrate long distances.”
The opah’s unique traits challenge what we thought we knew about fish. So far, no other fish have been found to share its ability to be entirely warm-blooded. However, recent studies show that the moonfish is more diverse than we believed. Instead of just one species, there could be at least five, though only Lampris guttatus has this extraordinary ability.
In short, the opah is a remarkable creature—a warm-blooded fish that thrives in deep waters, changing the way we view marine life. For more insight into oceanic wonders like this, check out studies from NOAA and other scientific resources.
