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.
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.
Hiding in plain sight
This discovery is surprising since the opah is large and conspicuous; indeed, it's already a favourite in fish markets and restaurants. Wegner and his colleagues deserve great credit for recognising and describing in detail the specialised gill heat exchangers that have been hidden right under the noses of fishermen and chefs for centuries.
The opah is shaped like a flattened disc with bright red fins. It grows up to two metres long and can weigh up to 80 kilograms
. It's a solitary fish, never caught in large numbers and is found in all oceans except polar seas. It swims by continuously flapping its pectoral fins in a similar way to the wings of a bird -- and it is the energy from these muscles that provides most of the heat.
It has long been known that certain high-performance fishes such as sharks, tuna and swordfish can warm some muscles, the brain or their eyes using a dense web of warm and cold heat exchanging blood vessels around the area in question. However their blood is still cooled to ocean temperature each time it passes the gills, as in all other fishes. With its heart and all its vital organs working at an elevated temperature, the opah is the first fish that can be regarded as truly warm-blooded.
It is intriguing to speculate whether this is a new evolutionary trend for fish that in future might emulate the warm-bloodedness of birds and mammals. For most fishes living in tropical seas this adaptation is not necessary; the warm water temperature is ideal for life. But for the opah, which wants to stay down deeper for longer in order to hunt squid in cold waters, the warm-blood adaptation helps it outcompete partially heated rivals like the Albacore tuna.
The mechanism can only work for large-bodied fish with space for insulation, meaning heat loss to the surroundings can be controlled. Even with specialised heat-retaining gills like the opah has, a small fish the size of a mouse would quickly cool down, the heat absorbing capacity of water is too great for any small animal to retain body warmth.
Even the opah is not able to compete with warm-blooded diving foragers such as penguins and seals, or whales in the polar seas. The fish is a zoological oddity belonging to a group that appeared in the last 100m years at the same time as mammals and birds evolved. We cannot know if the fossil species were warm-blooded and if we search further we may find other species with similar adaptations.