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Goofy Looking Ocean Sunfish Are Actually Active Swimmers and Predators

The idea that these giant fish are lazy is just wrong

An ocean sunfish hangs out with pilotfishes (Richard Herrmann/Visuals Unlimited/Corbis)
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The largest bony fish in the world is the ocean sunfish or the Mola mola. These odd-looking creatures really look like someone was making a fish, and forgot to add the back end and tail. Or like the back half of the animal was bit off by a shark. These strange swimmers can grow to be 10 feet long and weigh 5,000 pounds. To see one sail by, large and serene, is awe-inspiring enough at the aquarium. In the open ocean their calmness, buggy eyes and open mouths make them seem even more goofy and gentle.

Researchers used to think that the giant fish were only lazy sunbathers, writes Sarah Zielinski for Science News, because they were only observed drifting along near the surface. But now, they're realized that was missing a big part of the picture. After attaching accelerometers to some sunfish, scientists recorded dives down to more than 2,600 feet. To figure out what the fish were doing down there, researchers led by Itsumi Nakamura of the University of Tokyo outfitted seven sunfish with more instruments including cameras with light sources, accelerometers and thermometers. They published their findings in the Journal of Animal Ecology

The fish traveled tens of miles a day. At night they did hang out near the surface but during the day they made frequent deep excursions, mainly to about 320 to 650 feet down. 

While they were down in the deep, they were hunting. Zielinski reports:

The cameras revealed that the fist were chasing after a variety of jellyfish and jellyfish-like creatures, mostly siphonophores. But the fish didn’t necessarily eat all of these creatures. When one sunfish approached a jellyfish, it ate only the gonads and oral arms; these bits are more nutritious than the bell.

Body temperature measurements indicate that the fish might return to the surface to warm up. For BBC Earth, Melissa Hogenboom reports that the researchers were surprised at how quickly the fish could increase their body temperature upon the return. "Beyond our assumption, their body temperature increased rapidly during surface warming, suggesting they have some physiological mechanisms to increase heat gain from the surrounding water," the scientists write. 

So the sunfish do sunbathe. But they aren’t quite as lazy as they might seem.

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