A deepwater fish joins mammals, birds in the warm-blooded club

Move over, mammals and birds, and make room for a fish called the opah in the warm-blooded club. Researchers said in the journal Science on Thursday that this deepwater denizen is the first fish known to be fully warm-blooded, circulating heated blood throughout its body, enabling it to be a vigorous predator in frigid ocean depths. Tuna and certain sharks can warm specific regions of their body such as swimming muscles, brain and eyes in order to forage in chilly depths but must return to the surface to protect vital organs such as the heart from the effects of the cold.

With a more whole-body form of endothermy, opah don’t need to return to surface waters to warm and can thus stay deep near their food source continually.

fisheries biologist Nicholas Wegner of the U.S. National Oceanic and Atmospheric Administration’s National Marine Fisheries Service

The opah, also called the moonfish, internally generates heat through constant flapping of wing-like pectoral fins, with an average muscle temperature about 7 degrees to 9 degrees Fahrenheit (4-5 degrees Celsius) above the surrounding water temperature at the time. The opah boasts a unique structure that prevents this heat from being lost to the environment.

Science warm blooded fish