You're going to have stop calling people 'cold fish': THIS one is HOT-BLOODED

So what does ‘warm’ actually mean here, anyway?

Deepwater opah fish. Image credit: National Oceanic and Atmospheric Administration

The world's first warm-blooded fish has been spotted splashing about as bold as brass in Pacific waters, to the delight of Ichthyologists marine boffins – and no doubt of metaphor-busters – around the world.

Scientists at the US National Oceanic and Atmospheric Administration (NOAA) have discovered that the opah, or moonfish, is completely warm-blooded, just like mammals and birds.

While generally the ability to engage in this form of thermo-regulation is lacking in other vertebrates, some highly active fish can temporarily warm their swim muscles.

The study revealed that the opah – a large deepwater fish – can use heat generated from its swim muscles to warm both its heart and brain.

"This ability increases its metabolic function in cold, deep waters, which will help the fish compete with other, colder-blooded species," said the editor's summary of the study, published in Science, and catchy titled Whole-body Endothermy in a Mesopelagic fish, the Opah, Lampris Guttatus.

The study highlighted an ongoing issue regarding how scientific the description of "warm-blooded" may actually be.

While mammals and birds are known to maintain a stable temperature in a relatively standard way, most often through increasing their metabolic rate, other forms of keeping warm have provoked scientists to increasingly use this means of thermo-regulation to differentiate between species in this regard.

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Swordfish, tuna and several species of sharks, for instance, are known to have particular circulatory mechanisms that will maintain their organs above the ambient temperature of their surroundings. As with the opah, this allows for very effective predation in the colder depths of the ocean.

Talking to Science, Diego Bernal, a fish physiologist at the University of Massachusetts, Dartmouth, said: "It's a remarkable adaptation for a fish." The magazine's explained:

Water will take the heat right out of most creatures. So fish typically remain the temperature of the water they swim in. And that, in turn, limits their biological functions in colder water, especially cardiovascular endurance.

There are partial exceptions ... but they must return to warmer waters to bring their core temperature back to normal.

Very little is known about the reclusive and tubby-looking fish. Measuring about a meter long, it is known to hunt squid as well as other fish, typically at depths of 50-200m below the surface, where it propels itself through the 10C Pacific waters by flapping its pectoral fins.

The discovery came off of the back of a fishing expedition by Owyn Snodgrass, a fisheries biologist with the NOAA. He'd caught some opah off the coast of California, as part of a regular survey, and generously offered the gills to his colleague Nicholas Wegner, a fish physiologist. "I’m kind of known as the gill guy," Wegner told Science.

The gills had been sunk into a plastic bucket of preservative for a few months before Wegner pulled them out for a look. "I noticed right away that there was something unique," he said.

Fish have only a few large blood vessels that bring blood to and from the gills, where tiny vessels pick up oxygen from the surrounding water. The opah, however, had an elaborate network of tiny blood vessels, in which arteries lay next to veins in tightly packed arrays.

This arrangement of twinned arteries and veins is known as a rete mirabile, or "wonderful net," and often functions as a counter-current heat exchanger in other species, according to Science. Vessels carrying warm blood transfer heat to the cold blood in vessels that has return from the fish's extremities.

This anatomical trick is known to help aquatic birds minimise heat loss when their feet are in cold water, and some whales have similar heat exchangers in their tongues. However, the opah is the first fish discovered with a rete mirabile around its gills.

Wegner also discovered that the gills' heat exchanger is wrapped in a centimeter-thick layer of fat, presumably for insulation, which is extraordinary in fish.

The aquatic boffins decided to measure the body temperature of opah at sea, and after hauling the fish aboard found that the average body temperature was roughly 5C warmer than the water from which they were caught. QED. ®

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