World’s first fully warm-blooded fish

NOAA Fisheries biologist Nick Wegner holds an opah     (C)NOAA Fisheries

NOAA scientists have discovered the ocean's first fully warm-blooded fish. The opah, also known as the moonfish, maintains its body temperature by circulating warm blood through its body.
The silvery fish, roughly the size of a large automobile tire, is known from oceans around the world and dwells hundreds of feet beneath the surface in chilly, dimly lit waters. The opah roams the depths of the ocean, where most of its neighbors are slow and sluggish. Unlike its friends, the opah constantly flaps its fins, which heats up its body and fuels its metabolism. In turn, it also has a distinct predatory advantage, as the opah can move more quickly than other marine predators.
"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," said NOAA's Nicholas Wegner, the lead author of the new study. "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."

                                                  An Opah                                 (C)NOAA Fisheries

Wegner and his team unraveled the opah's mystery by attaching temperature monitors to opahs off of the western United States. As the fish dived to the depths of the ocean, their body temperature remained considerably warmer than the surrounding water temperature.
Other fish have developed limited warm-bloodedness (known as regional endothermy) to help expand their reach from shallower waters into the colder depths. But the opah's evolutionary lineage suggests that it evolved its warming mechanisms in the cold depths, where the fish can remain with a consistent edge over other competitors and prey. Recent research has found distinctive differences among opah from different parts of the world, and Wegner said scientists are now interested in comparing warm-blooded features among them.