Lazy sunfish are actually active predators

If you spot an ocean sunfish (Mola mola) near the surface of the water, you might be amazed by its size. These are, after all, the biggest of all the teleost fish (the group of ray-finned fishes that includes many of the species we like to eat), and they can grow as large as 1,000 kilograms. You may also think the fish are nothing but lazy sunbathers. And that wouldn’t be an unreasonable guess — scientists used to think sunfish only drifted about in surface currents.

Then some researchers attached accelerometers to the fish and discovered they were active swimmers that could reach depths of 800 meters. But what were they doing in the deep?

To find out, Itsumi Nakamura and colleagues at the University of Tokyo attached instruments to seven sunfish caught in Funakoshi Bay on the east coast of Japan that could record the fish’s behavior and internal temperature. On some of the fish they also attached a still or video camera with a light to see and record what was in front of the fish — and what they were eating. The results of their study appear in the May issue of the Journal of Animal Ecology.

Over four to six days of recording, each fish traveled tens of kilometers; these were no lazy drifters. At night, they stuck close to the surface, within the top 20 meters of water. But during the day, the fish spent about 40 percent of their time in the top five meters of ocean and the rest diving to depths of up to 200 meters.

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.

The body temperature data showed that the sunfish lost a lot of body heat when they dove deep. When at the surface, the fish had body temps of 16° to 20° Celsius, similar to the ambient water temperature. When it dove, though, a fish’s body temp could decrease to as low as 12° C. Those hours lazing about in the sun, the researchers conclude, are needed to warm back up after a trip to the deep to catch a meal.