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New research from scientists at the University of Tokyo has shed some light onto why sunfish spend so much of their time sunbathing at the ocean’s surface and what they hunt for in the deep sea.

Until now, scientists thought sunfish ate only jellyfish and were unsure why the large fish needed to spend so much time in the sun. When Japanese scientists attached thermometers and cameras to several of the species in order to collect data, they found out some surprising new facts.

Sunfish hunt at depths of about 150 to 250 feet below the surface during the day and, in addition to jellyfish, forage for siphonophores, a type of marine hydrozoan related to the jellyfish (a Portuguese man-of-war is one example). They swim back and forth between the surface and deep sea in order to regulate their body temperature, warming themselves in the sun before heading back down to hunt. Scientists were surprised at how quickly the sunfish’s body temperature increased in the sun, suggesting that “they have some physiological mechanisms to increase heat gain from the surrounding water.”

Sunfish are the largest of all the bony fish and can grow up to 10 feet in length and weigh as much as 5,100 pounds.

Read more about new sunfish discoveries here.