The opah, or moonfish, a large colourful fish living across the world's oceans, has been found to have a warm heart and maintain a high body temperature, according to a report in the journal Science. It's a zoological curiosity and a remarkable evolutionary development for fish.
In the cold darkness of the deep sea there is a clear advantage to being warm-blooded and able to move faster than all the other creatures in order to hunt them down or to avoid being eaten. Mammals such as seals or whales exploit this to great effect. They take a big breath and dive down, insulated from the cold by a thick layer of blubber, to snatch live food such as squids, fish and shrimps from the depths.
Until now it was thought that fish couldn't keep warm in this way because instead of breathing air they extract oxygen directly from the water through their gills. The advantage of this is obvious: fish can stay underwater indefinitely. However, although their blood may be warmed by muscle activity on every circuit of the body as it comes gushing out of the heart it goes directly into the gills and is instantly cooled to ocean temperature.
The gills are intricate oxygen exchangers. A tiny membrane one thousandth of a millimetre thick is all that separates the blood and the sea, which ensures instant transfer of oxygen into the red blood cells. Heat flows faster than oxygen, so no matter how much heat the fish might be generating, its blood is automatically chilled with every heart beat.