The face of an ocean sunfish certainly leaves an impression.

So too does its size; they are the largest bony fish in the world, ranging from one to four metres across, and weighing up to a tonne.

And their name?

They are called sunfish because they spend almost half their day basking motionless at the ocean's surface, seemingly catching some rays. 

But until now it was not understood why they sunbathed, or exactly what they get up to in the deep ocean.

New research shows that they forage for marine hydrozoans (Greek for sea serpent), which are small groups of predatory animals related to jellyfish. The fish take siphonophores, a type of hydozoan, most frequently at depths of between 50-200m. After their hunt they go back to the surface to top up their body temperatures by basking in the sun.

It's the first evidence that sunfish graze on these creatures in deep water. It was also previously believed they only ate jellyfish.

A team led by Itsumi Nakamura of the University of Tokyo, Japan, caught several sunfish off the country's coast, around Funakoshi Bay, and attached thermometers to measure changes in their body temperatures.

They also attached cameras with lights on them, to gain insight into exactly what the fish were hunting. After four to six days these instruments naturally separated from the sunfish, allowing the researchers to collect and analyse the data.

The research, published in the journal Animal Ecology, uncovered that ocean sunfish swam back and forth between the surface and deep water during the day. Their cycles of diving deep and then warming at the surface helped to maximize their foraging time. Each time they "sunbathed" it regulated their body temperature.

Nakamura was also surprised to find just how quick the warming process was. "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."

This explains why larger sunfish can forage for longer, adds Nakamura.

It also provides a possible answer to why they have such a large body – as it serves to help them adapt to their hunting environment, losing heat very slowly.

The sea giants only hunt during the day. At night, they stayed at shallower depths of about 20m. 

The discovery is a reminder that the deep sea is "still a frontier" says Nakamura, and only by increasingly observing the ocean depths will we unlock it secrets.

Follow Melissa Hogenboom and BBC Earth on twitter