The darkest depths of the oceans where no sunlight reaches can be pretty chilly for cold-blooded fish. However the moonfish is one of a kind, as it is able to maintain its own body heat.

Not much was known about this colourful, predatory fish other than its name until now, as scientists have discovered it doesn’t have to return to the surface to warm its body.

Big fish, little fish, warm-blooded fish

Mammals and birds traditionally are the planet’s only warm blooded animals, while fish and reptiles are ectothermic, or cold blooded. Some, such as tuna, are use ‘regional endothermy’, which requires returning to the surface to warm their swimming muscles, to give them a burst of speed when hunting.

Research by the National Oceanic and Atmospheric Administration (NOAA) has discovered that the opah, or moonfish, can circulate warm blood throughout its body. This gives it a competitive edge over others – predator and prey.

An opah fish is able to chase down fast prey like squid and can migrate long distances, as it is in charge of its own body temperature.

How can the moonfish be warm-blooded?

The research team measured the temperature of different body parts of freshly caught moonfish and implanted thermometers into fish returned to the water.

An opah fish’s muscles were about 5°C warmer than the water, even when they dived between 50m and 300m under the surface. The fish’s head and heart were also about 3°C warmer than the surrounding water.

A moonfish can weigh anywhere between 22kg to 270kg, therefore it must have highly sophisticated thermal engineering like a car radiator.

Researchers have found that when the moonfish flaps its large fins, they warm the body which helps blood circulate. When blood reaches the gills to collect oxygen, heat loss is minimised by blood vessels.

“There has never been anything like this seen in a fish’s gills before,” biologist Nicholas 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 moonfish doesn’t let changes in temperature or the weather stop it in its path!