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Just when you thought it was swimming season ...

Jellyfish invade Mediterranean beaches, alarming scientists and sunbathers.

ALICANTE, Spain — Sunlight glimmers through their translucent tentacles as they drift among swimmers, often unnoticed until it's too late: the piercing pain of a jellyfish sting.

Tons of these creatures are increasingly haunting the Mediterranean coast every summer. While certainly an annoyance for beach-goers — and, with the appearance of the Portuguese man-of-war, at times a serious danger — scientists say it goes beyond that.

Jellyfish are the ocean’s messenger. “The sea is telling us, ‘Look what I got here, look how you’re treating me,’” said Josep Maria Gili, a scientist from the Ocean Science Institute in Barcelona. 

Overfishing reduces the natural predators of the jellyfish and decreases the numbers of those that compete with them for food. Dumping organic waste into rivers increases food supplies. And the more jellyfish there are, the greater the possibility they end up near shore. Warmer water for longer periods of time means a more comfortable environment too.

Fresh water serves as a natural barrier against a jellyfish invasion: Rain creates a not-welcome zone around the shore and rivers bring melted snow to the sea. But burgeoning coastal Mediterranean communities, along with industry and agriculture, siphon off fresh water before it gets there.

Gili, a marine biologist, is studying the connection between the jellyfish arrival and climate conditions. He said it's nearly impossible to predict how many will come, but in Mar Menor, a small Mediterranean lagoon off the southeastern Spanish coast of Murcia, one of the few calculations available puts the number of jellyfish at between 40 million and 100 million during one summer.

Jellyfish do not attack, but contact prompts a painful injection of poison.

“I rub it with sand, and my mother used to pour vinegar on us,” said Gema Tello, enjoying a day on the beach.

“I’d have no idea what to do if my children were stung,” said a concerned Silvia Barreda. A couple of weeks after dozens of stings on a nearby beach made national television newscasts, these two friends brought their four children, ranging from 21 months to 5 years, to the shallows of La Villa Joiosa, in the Mediterranean province of Alicante.

Further north on the beaches of Catalonia, more than 20,000 people received medical assistance for jellyfish stings last summer. Gili suspects that number represents only half the victims; the other half manages the pain in extreme discomfort with home remedies.

And now, the Mediterranean Sea, which draws hordes of tourists to its temperate waters each year, holds a sinister surprise: Portuguese men-of-war have been sighted near the Spanish coast.