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. 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.
The opah (Lampris guttatus) has evolved a unique solution to this problem. The fish has a special insulated network of blood vessels between the heart and the gills. These vessels act as a heat exchanger in which warm blood from the heart reheats oxygenated blood leaving the gills before it goes to the body. In this way heat is retained and not dissipated into the ocean.
This enables the opah to maintain a body temperature 5°C higher than the surrounding water and to dive 500 metres below the surface without cooling down. An insulating layer of fat in the skin keeps the heart, brain, muscles and vital organs warm.