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The new ocean predator: Jellyfish?

As sharks and tuna are fished out, jellyfish take over.

Scientists are seeing evidence of exploding jellyfish populations as the creatures rush in to fill the voids left by overfishing.

Editor's note: This story is part of a project spearheaded by GlobalPost's Study Abroad team and summer interns. They spent the summer learning about the world's endangered oceans and their work is displayed in this interactive graphic.

BOSTON — Of all the oceans' predators, jellyfish have the least ... of everything: With no teeth, fins or brains, they catch only whatever unlucky animals drift into their path.

But to the animals they hunt, jellyfish and the many related species of comb jellies are a menace. Ranging from several feet long to nearly microscopic, their venom often kill creatures hundreds of times their size, including, in rare cases, humans.

As scientists study jellyfish more closely, a new creature is emerging: More than just a nuisance, jellyfish are viewed as barometers of ocean health and possibly a powerful force affecting seawater itself.

Scientists remain cloudy about the role of jellyfish in the ocean ecosystem, but the creatures are undeniably hardy. Unlike sharks, orcas and other aggressive carnivores, jellyfish thrive in ecosystems damaged by human activity. From the Gulf of Mexico to the Sea of Japan, oceanographers have found a common symptom among places where overfishing, chemical pollution and rising sea temperatures have killed off other species: more jellyfish.

"There are suggestions that the whole oceans are turning to jellyfish," said Larry Madin, the director of research at the Woods Hole Oceanographic Institution.

Madin said he finds that theory "premature," but anecdotal evidence shows jellyfish species invade places where fish, coral and other marine animals once thrived. They procreate more quickly in warmer water, tolerate pollution and escape commercial fishing nets that decimate almost every other marine species.

Off the northern and southern coasts of Europe, an invasive carnivore called Mnemiopsis leidyi, also known as the American comb jelly, has punished economically vital fish stocks by devouring the smaller fish and algae that form the base of the food chain. Originally found off the eastern coasts of North and South America, comb jelly now drift as far north as the Baltic Sea.

The sudden spread of the non-native species "surprised nearly everyone," researchers from the Finnish Institute of Marine Research said in a 2007 report. Until an expedition found areas of the polluted, brackish sea filled with the non-native species, scientists thought nothing like it could survive the waters' frigid temperatures.

Off coast, jellyfish do little direct damage to people, but the story changes near beaches where the human population has likewise exploded. Off the coasts of Australia, more than 70 people have been killed by jellyfish from 1882 to 2005.

The common box jellyfish and deadly irukandji found off the northern coast of Queensland have forced local officials to place nets around some crowded beaches.