Fisheries biologist Nicholas Wegner of NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, California and colleagues are the first to prove that at least one warm-blooded fish swims the oceans of the world. The opah or moonfish is the only known warm-blooded fish ever found.
The opah normally inhabits ocean waters at the depth of several hundred feet. The low temperature at these depths would predict that the opah was cold blooded but the fish is a very fast and agile predator. The first hint that the opah was warm-blooded was the fast movement of the animal’s pectoral fins relative to other fish that live at the same depths.
The scientists examined the opah’s gills and found that heat exchange that produces the distribution of warm blood in the fish occurs in the gills. The mechanism has never been seen in any other fish. The rapid movement of the opah’s pectoral fins also helps move warm blood from the internal organs and heart of the fish throughout the body of the fish.
The researchers measured the blood temperature and body temperature of several opahs on dives as deep as 1,000 feet below the surface of the ocean. The body temperature of the fish remained constant during dives to extreme depths. The muscle temperature of the fish stayed 41 degrees Fahrenheit above the temperature of the surrounding waters. The opah is the first fish that has been proven to be warm-blooded. Being warm-blooded provides the fish with competitive advantages including speed and the ability to swim long distances. The adaptation has allowed the fish to inhabit most of the world’s oceans.