Warm Blooded Fish Discovered

warm blooded fish

Researchers have discovered the first fully warm blooded fish, called the Opah. It is commonly known as a sunfish, moonfish or Jerusalem haddock.


This fish inhabits deep water and lives at a depth of hundreds of metres under the surface where it is cold and there is very little light. Given this environment, its warm blood gives it an advantage over the cold blooded rivals. Fish that live in this cold deep water tend to be slow and sluggish, but the opah is active and flaps its fins to heat its body. With a warmer body, it results in a faster metabolism, faster movement and faster reaction times.

Chilled out

Prior to the discovery, scientists were of the opinion that this was a slow moving fish in these depths, similar to other fish in this environment. Because of the warm body of the fish it tends to be a very agile predator than can hunt down prey like squid and migrate over significant distances.

The team

The research was undertaken by the National Marine Fisheries in the USA, and led by Nicholas Wegner. Wegner noticed there was something different about the opah when they discovered unusual blood vessels in its gills. Through closer inspection, he discovered air flowing around those blood vessels carrying cold blood back to the body. This process acts a bit like a heat exchanger and transfers warmth from the warm blood to the colder blood.

Why warm blood

This process means that the fish can keep its body at higher temperatures than the surrounding environment in a similar way to mammals. A few other fish like tuna and some sharks were already known to be able to heat specific parts of their bodies. This is used to increase the performance of their muscles and their swimming speed. However, neither of these fish are as effective as the opah at improving their performance.

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