"Originally there were sardines in the area but over fishing caused the sardine population to collapse in the 1960s and 1970s," said Professor Victoria A. Braithwaite of Penn State University.

"The sardines never recovered and jellyfish became a huge and serious problem, eating what the sardines had eaten."

Jellyfish are considered a dead-end food source as they eat lots of small fish and other sea creatures -- but they have few predators.

However the bearded goby, Sufflogobius bibarbatus, a 4-to-6-inch long, 1.5 inch-wide fish, eats jellyfish.

Larger fish, sea mammals, and sea birds eat gobies -- putting jellyfish back into the food cycle.

"We don't know if they are eating dead jellyfish from the bottom, or if they are coming up to oxygen-filled layers to eat jellyfish, but they are eating jellyfish," Professor Braithwaite said.

Stranger still is the gobies' use of the dead zone in the area.

One reason there were so many sardines and now jellyfish is a large area of up-welling water off the southwest coast of Africa from Namibia to South Africa.

This deep cold water contains large amounts of nutrients.

When plankton eat the nutrients, their populations increase massively.

Excess nutrients and dead plankton then fall to the ocean floor.

"A horrible toxic sludge forms, and very few things can live in it except for some bacteria and nematodes," said Professor Braithwaite.

"Somehow the gobies can withstand the toxic environment, but we don't know exactly how they are doing it."

Gobies can cope without oxygen for hours at a time while they rest on the muddy seabed -- but remain alert.

"When we touch them with a rod, they show rapid escape responses," said Braithwaite.

For the goby, the toxic mud is a perfect hiding place as no predators are willing to enter that environment.

"It is a win-win situation where the gobies are using a resource that is usually a dead end in the ocean, the jellyfish," Professor Braithwaite said.

"And they are using the toxic mud as a refuge."

"Together this seems to explain why their population is growing despite the fact that they are now being the main prey species in this unusual ecosystem."