The Opah: First Warm-Blooded Fish Discovered

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Researchers at the National Oceanic and Atmospheric Administration announced on Thursday, May 14th that they have identified the world’s first known warm-blooded fish – the opah.

The NOAA scientists discovered that the opah (also known as the moonfish) circulates heated blood through its body much like mammals by flapping its fins as it moves around. They determined the opah can actually flap its winglike pectoral fins to create forward motion. Moreover, the large pectoral muscle is insulated from the cold water the fish lives in by a natural layer of fat.

This new discovery about the opah was reported in the May 14th edition of the academic journal Science.


Endothermy is the metabolic production and retention of heat to warm body temperature above ambient, and characterizes birds and mammals including humans. Warm-blooded creatures have enhanced physiological function compared to cold-blooded creatures.

This new NOAA study highlights the first discovery of whole-body endothermy in a fish, the opah (Lampris guttatus). The opah creates heat through constant “flapping” of its winglike pectoral fins, and minimizes heat loss through a series of counter-current heat exchangers located in the gills. Unlike cold-blooded fish, warm-blooded fish like moonfish distribute warm blood throughout the body, improving physiological performance and protecting  internal organ function while swimming in the cold, deep ocean waters.

More on discovery of the Opah

In order to study the opah, the NOAA researchers attached temperature sensors and satellite tags to a number of the moonfish (which grow to be larger than manhole covers) that permitted them to track the movements of the fish for eight months. The scientists monitored the body temperature as the fish dove down into cooler parts of the water.

Scientists found that no matter what temperature the fish were swimming at, the fish constantly stayed five degrees cooler than the surrounding water.

Of note, the researchers also determined that the blood vessels in the gill tissue of the opah are structured so that the vessels transporting cool blood from the gills make contact with those that move the warm blood in the opposite direction. This process effectively warms the incoming blood.

The fish are able to increase the temperature of its heart by flapping its fins, which helps the fish dive into the deeper, colder depths of the ocean and remain there for longer periods of time. The scientists point out that other fish such as tuna or lamnid sharks must return to the surface regularly to keep warm and rest before another dive.

The deep diving opah on the other hand are known to spend nearly all of their foraging hours at depths between 160 and 1,300 feet (50 and 400 meters) without making trips back to the surface to warm up.

Opah live in various oceans across the globe in both tropical and temperate waters, They are frequently caught as by-catch by professional fishermen fishing in the deep ocean for tuna and billfish, based background on data from the NOAA. However, since the opah are solitary fish that don’t swim in schools, they are not caught in large  numbers in most areas.

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