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Blue turquoise waves lap at white sand on the Spanish island of Formentera in the Mediterranean Sea. Sweaty tourists from all over Europe cram the beach. But on this particular afternoon, no one dares take a cool dip in the water.

The reason? It's what Spaniards call "medusas" — named after the monster from Greek mythology, with a woman's face and venomous snakes for hair. In English, they're called jellyfish.

Gabrielle Amand's son was a recent victim of one. He's wrapped in a towel, sitting under an umbrella on the shore.

"It's very small, very small. ... It hurts a lot. He cried a lot," says Amand, whose family is on holiday from France. "He doesn't want to go in again."

Santiago Sanchez and his college buddies from Madrid have been coming to Formentera for summer vacation for 14 years. They charter a boat, sleep out at sea and swim into sheltered crystal-clear coves. Unfortunately, so do the jellyfish.

"I was swimming from the boat, and I think I passed by around five of them. Just small ones, but I think I got stung on my arm. It's a little bit hot, and it feels like scratching all the time, but it's not too bad," Sanchez says.

Will he go back in the water?

"I think I'd rather go back in the dinghy," he replies.

The Rising Jellyfish Tide

There's been a spike in the number of jellyfish on Mediterranean beaches this summer. Scientists blame overfishing — and possibly climate change. The British government has put out a warning to its citizens vacationing near those waters.

Up to 150,000 people are treated for jellyfish stings on Mediterranean beaches each year, and that number is on the rise. Along some stretches of Spain's coast, scientists have spotted huge, mile-long blooms of jellyfish, sometimes with 30 to 40 animals per square yard of sea.

Stefano Piraino, a marine biologist at Italy's University of Salento, recently completed a flyover of 200 miles of Mediterranean coast to monitor growing jellyfish populations. He thinks he knows the reason.

"Overfishing is one reason. Because if we take out the fishes from the oceans, we leave more food in the environment, and jellyfish are very smart," he says. "They can multiply very easily in a very short time — much faster than any vertebrate, any fish in the sea."

Piraino says he believes research will eventually show that climate change is also to blame.

"Many of the species we are observing have a faster growth rate with increased temperature," he says. "There are a number of alien species coming from the Red Sea, so tropical and subtropical species that have entered the Mediterranean Sea from the Suez Canal."

That includes the Rhopilema nomadica, a stinging jellyfish that's been sighted in the eastern Mediterranean near the warm-water gateway of Egypt's Suez Canal. The European Union considers it one of the most invasive marine species in the region.

Tourists Rethink Mediterranean Getaways

Most jellyfish in the Mediterranean do not sting, and stings aren't life-threatening. But they can hurt tourism — a major source of revenue for Spain and other coastal countries. Britain's foreign office put out a jellyfish warning to its citizens holidaying on the Med this summer.

The Amand family, from France, is even thinking of cutting short their vacation.

"We want to stay 14 days, but ... if we can't swim every day, then we won't come back," Gabrielle Amand says.

In addition to red, yellow and green flags that indicate whether it's safe in general to swim, Spain has also introduced jellyfish warning flags on some beaches. Scientists on Israel's Mediterranean coast are experimenting with sound frequencies that could disrupt jellyfish patterns.

But Piraino, the Italian scientist, says he doesn't want to scare tourists; he wants to recruit them. He's developed a smartphone app on which people can report jellyfish outbreaks.

"If you are on the beach and you see some jellyfish, you can even send us a picture. In the last three years, we received around 10,000 records from citizens," Piraino says. "You can imagine, if the same campaign would have been carried out by scientific boats or personnel, it would have cost an enormous amount of money."

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