Researchers from Murdoch University in Perth were not quite sure what they were seeing when they first photographed the activity, in 2007, in which dolphins would shake conch shells at the surface of the ocean.

"It's a fleeting glimpse — you look at it and think, that's kind of weird," said Simon Allen, a researcher at the university's Cetacean Research Unit.

"Maybe they're playing, maybe they're socializing, maybe males are presenting a gift to a female or something like that, maybe the animals are actually eating the animal inside," he added.

But researchers were more intrigued when they studied the photos and found the back of a fish hanging out of the shell, realizing that the shaking drained the water out of the shells and caused the fish that was sheltering inside to fall into the dolphins' mouths.

A search through records for dolphins in the eastern part of Shark Bay, a population that has been studied for nearly 30 years, found roughly half a dozen sightings of similar behavior over some two decades.

'Tantalizing possibility'
Then researchers saw it at least seven times during the four-month research period starting this May, Allen said.