The opah is a big fish, with a warm heart in world first

15 May 2015

In a first for marine biology, the large, photogenic opah fish has been found to have a big secret: it’s the only fish known to be warm blooded and capable of heating its entire body.

Until now, it was accepted that underwater creatures were cold-blooded animals whose temperature would change depending on the temperature of the water than surrounds it due to the extreme cold that is experienced at most depths.

As for whether there were any warm-blooded animals in the depth of the ocean, the only previous examples were found to be partially warm-blooded animals, including the tuna fish and swordfish, which would warm parts of their body for say, swimming faster.

Wonderful net of blood vessels

However, according to National Geographic, a team of researchers came across a number of opah during an expedition, and on further inspection, the fish has a vast array of red and blue blood vessels woven throughout its gills.

Known in Latin as ‘retia mirabilia’ – or wonderful nets — the regular blood flow acts as an insulation from the deep cold of the ocean by heating the one part of the fish’s body that has to take in large quantities of cold water.

Opah’s hot blooded, check it and see

It’s also helped by a thick layer of fat the opah fish maintains to lock in a regular heat of 5C.

The team has since published a paper on its findings, having confirmed that it’s a warm-blooded fish with the help of a thermometer placed inside a caught opah fish, which has shown that all of its internal organs are warm, including its heart, which is a first for marine life in itself.

This, Nicholas Wegner from the National Oceanic and Atmospheric Administration (NOAA) said, helps them swim deeper than any other predator in its class.

It’s also rather fast: “Just from looking at it, I really thought it was a slow, sluggish, deep-water fish that doesn’t do very much. But all indications are that this is a very fast fish and an active predator. We’ve put some tags on them to show that they migrate thousands of kilometres,” said Wegner.

Opah stamp image via Shutterstock

Colm Gorey was a senior journalist with Silicon Republic