The rotund, silvery opah looks less like a deep-sea predator than a Mylar balloon, with curved pectoral fins that flap like wings. Its chest muscles account for almost a fifth of its body mass and, cleverly marinated, can pass for beef. But biologists with the National Marine Fisheries Service have now discovered the oddball opah’s most distinctive feature: It is the only fish known to be fully warmblooded.

“The coolest part—well, not cool in terms of temperature, but the neatest part—is that the opah has a warm heart,” says Kenneth Goldman, an Alaska shark biologist. Scientists have long known that some fish, including select species of billfish, shark and tuna, are partially warmblooded. In 1835, British physician John Davy noted that a tuna’s blood temperature was “much the same, or little less than the blood of a pig.” That was a bit of an overstatement. Most partially warmblooded fish stay just a few degrees above the surrounding water temperature. But that’s enough to give them a predatory edge, relative to their “thermoconformist” peers.

Warmer fish can expand their range, in latitude and depth, and cruise faster because of increased red muscle output, benefits brought to an extreme in birds and mammals, whose stable body temperatures might have led to the development of complex central nervous systems. While mammals make meta­bolic heat even at rest, fish mostly keep warm through active movement. Thus the opah’s juiced-up pecs.

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