Opah is First Known Warm-Blooded Fish

Posted on May 15, 2015

Researchers from NOAA Fisheries has revealed that the opah (Lampris guttatus) is the first known fully warm-blooded fish. The warm heart gives the opah an advantage and makes it a high-performance predator in the cold ocean depths where other fish tend to be slow and sluggish due to the cold water.

An adult opah is about the size of a car tire. It swims through the water using its large pectoral fins like wings. The researchers found that the flapping of its fins also helps warm the opah's heart. This is due to a densely packed net of blood vessels. These blood vessels use venous blood heated by the swim muscles to warm cold arterial blood. The opah's retia mirabilia is stacked into heat-exchanging layers and insulated by fatty connective tissue.

Nicholas Wegner (pictured above) of NOAA Fisheries' Southwest Fisheries Science Center is the lead author of the research paper on the opah's warm-blooded nature. Wegner says in the announcement, "Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments. But because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid and can migrate long distances."

The researchers call the design of the opah's heart warming system a "counter-current heat exchange." They compare it to a car radiator where heat is conserved. The researchers say, "The unique location of the heat exchange within the gills allows nearly the fish's entire body to maintain an elevated temperature, known as endothermy, even in the chilly depths."

Wegner adds, "There has never been anything like this seen in a fish's gills before. This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it."

Opahs were equipped with an intramuscular temperature data logger so scientists could study their temperature. The photograph below shows researchers preparing an opah for release. BBC News reports that scientists found the opah's muscles were 5°C warmer (41°F) than the surrounding water and the head and heart were about 3°C warmer (37.4°F).

The research paper about the opah's warm heart was published here in the journal Science.



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