Discovered. A fish with a warm heart

New research demonstrates a remarkable adaptation in a fish. It has a warm heart

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.

The mysterious warm-blooded opah
Researcher and lead author, Nick Wegner, holding a specimen of the mysterious warm-blooded opah Photograph: NOAA Fisheries, Southwest Fisheries Science Center

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.

The opah's eye
“The elevated body temperature of opah should...enhance temporal resolution and neural conductance for the eye and brain,” report researchers. Photograph: NOAA Fisheries, Southwest Fisheries Science Center

“With a warm body core and heart, and even warmer cranial region, opah have the capacity for enhanced physiological function in their deep, cold habitat,” the researchers write.

Henry Nicholls

The GuardianTramp

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