(See "'Immortal' Jellyfish Swarm World's Oceans.")

In the 1960s and '70s, overfishing off the southwestern coast of Namibia (see map) wiped out sardines, allowing two opportunistic jelly species, the many-ribbed jellyfish and the sea nettle, to move in.

The jellies would "hoover up" just about "anything they could get their tentacles on," said study co-author Victoria A. Braithwaite, a fisheries biologist at Penn State University.

And, thanks to their painful stings, the voracious jellyfish are generally not on other animals' menus.

"We tend to think of jellyfish as being a dead end, because very few organisms eat them," Braithwaite said. (See related pictures