The Opah

Researcher and lead author, Nick Wegner, holding a specimen of the mysterious warm-blooded opah Photograph: NOAA Fisheries, Southwest Fisheries Science Center. We usually think of fish as cold-blooded creatures and 99.9% of them are. But the opah, a deep-sea fish, has distinctly warm blood, according to new research.

Most fish have a body temperature tracks that of the water around them. A few, like tuna, can effect small localised increases in body temperature. But a mysterious deep-sea fish known as the opah (Lampris gutattus) appears to be the first fish known to be fully warm-blooded.

Between 50 and 100 m beneath the ocean surface, there is a sudden decrease in the temperature of the water. Most fish that live below this so-called thermocline have a relatively inactive lifestyle, but the opah has found a way to get a thermal edge over its drifting competitors. Rather than undulating its body, the opah propels itself along with a flapping of its wing-like pectoral fins, a continuous action that raises the temperature of the pectoral muscles by around 5 oC above the ambient, report researchers in the journal Science.

It’s a big muscle, comprising around 1/6th of the opah’s body mass and more than 1/3rd of its total propulsive musculature. The pectoral muscle is insulated from the water by thick layer of fatty connective tissue and, with a complex network of blood vessels preventing heat loss through its gills, the opah has found a way to get warm blood circulating throughout its body. This is particularly noticeable around the eye and brain, which is warmer still.