Fish
Opah
Lampris guttatus · also called opah, moonfish, kingfish
The opah is a fish shaped like a manhole cover and coloured like a sunset, and it holds a distinction no other fish can claim: it is genuinely warm-blooded, keeping its whole body — heart, brain, and all — several degrees above the cold ocean around it.
Overview
Opah live in the open ocean of every sea, at depths of roughly 50 to 500 metres — the dim, cold twilight zone. They are solitary and rarely seen, hunting squid and small midwater fish. The body is a deep, rounded, silver-and-rose disc flecked with white spots, with crimson fins, and typical adults run about a metre across and 20–40 kilograms.
The only fully warm-blooded fish
A 2015 study showed the opah keeps its entire core, including the heart and brain, about 4–5 °C warmer than the water — whole-body endothermy, unique among fish. Other fast predators like tuna and mako sharks only warm parts of themselves and keep the heart cold.
How it stays warm
The heat is produced by the opah’s unusual swimming style: instead of beating its tail, it flaps its wing-like pectoral fins continuously, and that constant muscle work generates warmth. A heat-exchanger in the gills — warm outgoing blood running alongside cold incoming blood — recovers the heat that would otherwise be lost when blood is cooled while collecting oxygen, and fat insulates the vessels. The result is a predator that can stay fast and sharp in cold deep water where its prey and rivals are sluggish.
Status
The opah is assessed as Least Concern. There is no directed fishery for it — nearly all are caught as bycatch on tuna and swordfish longlines — and it is found across all oceans, though its biology is still not well understood.
Opah: common questions
Is the opah really warm-blooded?
Yes, and it is the only fish known to be warm-blooded throughout its body. Some fast predators — tunas, the mako and great white sharks, billfish — are "regional endotherms" — they keep specific parts warm, such as the swimming muscles, the eyes, or the brain, but their hearts and much of their body stay at water temperature. The opah, discovered to be fully endothermic in a 2015 study, keeps its entire core — including the heart and brain — roughly 4 to 5°C warmer than the water around it, all the time. This lets it stay fast, alert, and well-coordinated in the cold deep water where sluggish prey and slow competitors give it an edge.
How does the opah heat itself?
The heat comes from its muscles. Unlike most large fish, which power through the water by beating the tail, the opah swims by continuously flapping its large, wing-like pectoral fins, and this constant muscular work generates warmth. To keep that heat instead of losing it at the gills — where blood is normally cooled to water temperature as it picks up oxygen — the opah has a specialised heat-exchanger (a rete mirabile) in the gill tissue. There, warm blood leaving the body core flows alongside cold blood coming in from the gills, and the warmth passes across, so the returning blood is pre-heated. The gill vessels are also insulated with fat.
What does an opah look like and how big is it?
An opah is a striking fish — a deep, almost circular, laterally flattened body like a giant plate, coloured silvery-blue to rose-red and covered in small white spots, with bright crimson fins and jaws and large round eyes. Typical adults are about a metre across and 20 to 40 kilograms, and the biggest recorded are around 2 metres and 90 kilograms. Despite the size it is rarely seen, because it lives in the open ocean at moderate depths and does not come to the surface.
Is the opah good to eat, and is it fished?
Yes, opah is prized as a food fish, with rich, fatty, pink-to-orange flesh often compared to a cross between tuna and salmon, and different parts of the body have different textures and colours. There is no large directed fishery for it, however; almost all opah reaching markets are caught incidentally on longlines set for tuna and swordfish. Because it is not targeted and turns up across all the world's oceans, it is currently assessed as Least Concern, though its biology is still poorly known.