Mola mola spend a large portion of their lives in temperate and tropical ocean waters, but because they spend a great deal of their time in very deep water — they can swim down to 2,000 feet — they’re susceptible to disorientation and even death when exposed for long periods to water colder than 50 ̊F. It’s long been believed that the mola basked at the water’s surface to catch some rays and warm themselves — hence their common name, ocean sunfish.

On the ocean’s surface, the world’s heaviest fish — the average weight of adult molas is 2,200 pounds — like to swim on their sides; scientists think this thermal recharging follows dives into deeper, colder water.

But until now it was not understood why they sunbathed, or exactly what they do in the deep ocean. New research by a team led by Itsumi Nakamura of the University of Tokyo, Japan, has shed some light on the mola’s behavior and foraging. Nakamura’s team caught several sunfish off Japan’s Funakoshi Bay and attached thermometers to measure changes in their body temperatures.

“Our research suggests the sunfish can increase heat gain during warming, though its mechanism is still a mystery,” says Nakamura. “Their body temperature changes three times faster during surface warming than during foraging in cold water.”

The researchers also attached cameras to the fish. The findings, which were published in the journal Animal Ecology, showed that ocean sunfish traveled between the surface and deep water during the day. Each time the fish spent time at the surface, the thermometers recorded higher body temperatures. The conclusion: The “sunbathing” helps the fish regulate their body temps and prolong their stay in deeper water.

Nakamura theorizes that this behavior helps explain why sunfish have such a large body: Larger sunfish were able to hunt longer, and they lost heat at slower rates than smaller ones.

Another surprise: Molas — which were previously known to eat only jellyfish — also hunt for marine hydrozoans, predatory animals related to jellyfish.

What’s next for the team? “Deep sea is still a frontier,” says Nakamura. “Pelagic predators with electronic devices will become live [autonomous underwater vehicles] to observe organisms living in water column.”