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Biologist Nicholas Wegner with an opah. Source: NOAA Fisheries/Southwest Fisheries Science Center

Scientists Identify the World’s First Warm-Blooded Fish

One of the things we thought we knew about fish has been turned on its head. Fish, it turns out, are not all cold-blooded creatures.

Scientists have discovered that one fish in particular, the “opah”, has a unique talent. It can consistently maintain a body temperature which is about 5 degrees warmer than its surrounding environment. While there are other fish which can temporarily heat parts of their bodies, as some sharks do their muscles when speeding after prey, they are not able to heat their entire bodies. In general, their body temperature is the same as the surrounding water. It is for this reason that, while they may dive down to deeper waters to hunt, they must still regularly return closer to the surface to warm themselves.

Nicholas Wegner, whose research team made the discovery, is a scientist from the National Oceanic and Atmospheric Administration (NOAA). He has been studying the opah fish, and his first surprise came at finding that the animal is not the slow, ambling animal of prey that it looks to be. The opah, in spite of its awkward, though attractive looks (large, oval, flat, with red-orange fins and a speckled body), is an active predator. Rather than ambushing its prey, as most deep-ocean fish do, it chases them down. “Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” Wegner has stated.

Cold-blooded fish which live at such depths (45-300m below the surface) are typically loathe to expend that kind of energy when hunting. This gives the opah an edge. By first using its constantly flapping wing-like fins to produce heat, the opah speeds its metabolism and maintains a quick reaction reflex. This is not what makes the fish unique, however, since the muscles of all fish generate small amounts of heat, which then dissipates into the surrounding water. What is unusual about the opah is how this heat is retained. Whereas other fish rapidly lose the heat in their gills, where the blood runs so close to the water, the opah’s gills have a unique arrangement of blood vessels that manage to use the animal’s warm, deoxygenated blood to heat the cooler, oxygen-carrying blood before it travels through the rest of the body. The system, called “ has been likened to a car radiator. Fatty tissue around the gills, heart and muscle then serve to insulate the body, keeping the warmth from disappearing too quickly.

By keeping the blood warm, the fish thus maintains warmth in its muscles, brain, and even its heart. This last organ’s warmth is the most unusual aspect, unknown in any other fish.

While the opah’s body heat is next to nothing when compared with warm-blooded land mammals such as humans, 5 degrees celsius above the surrounding water is considerable for a deep-water fish. The NOAA has said that it is now interested in examining opah specimens from around the world, in order to compare them and find out more about how their warming mechanisms evolved.

As Wegner has said, “Nature has a way of surprising us with clever strategies where you least expect them”.

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