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.

Though often confused with jellyfish, this species is a colony of polyps in which many animals with a specific function — floating, feeding, reproducing and more — join forces. The men-of-war “can even eat big fish,” said Ricardo Aguilar, director of research at Oceana, a marine conservationist group.

Its poison has more serious consequences than that of jellyfish, including abdominal and chest pain, arrhythmias and muscle spasms which can cause hospitalization and, rarely, in people with cardiovascular ailments, even death. On the surface, it appears harmless enough at only 6 to 8 inches in size — but dangling below are tentacles up to 100 feet long reaching out for prey.

“You can’t see it, but it’s there,” Aguilar warned. This species lives in colder, Atlantic water, but it has been discovered sailing on powerful currents through the Strait of Gibraltar and into Mediterranean. Jellyfish do not swim — they drift. And they travel en masse — more than 6-mile-long swarms have been sighted south of Ibiza, Gili said.

Attempts to prevent jellyfish from reaching the coast have been largely unsuccessful. Tentacles that break off in protection nets can still sting. The nearly enclosed and calm waters of the Mar Menor Lagoon make it an exception, where fishermen netted more than 4,000 tons in 2003 but less than 2 percent of that last year, or 87 tons. Local authorities say this is the desired natural level. Murcia's regional government attributes the reduction to the removal of adults from the population, curbing reproduction. 

The Ministry of Environment is going to hire fishermen to pick up jellyfish in Balearic Island coastal waters, but only when there are very large hordes. The difficulty is that sighting can be unreliable: A group of jellyfish may be spotted 20 miles off the coast, floating toward land, but changing currents may move them away by the time fishing boats with nets are positioned.

“Putting the jellyfish away does not solve the problem; the species unbalance continues,” Aguilar said. The increase of jellyfish “is a symptom of a serious illness,” he said, adding that the removal of jellyfish from the coast is “like putting Band-Aids” on a deeper problem. Jellyfish feed on fish eggs and larvae, which also puts fish — and the fishing industry — at risk.

There is no easy, quick solution. Scientists say longer-term plans geared toward sea life recovery, marine environment protection and prevention of climate change are needed. Coastal communities dependent on tourism revenues are on the front line — once a battalion of jellyfish reaches the coast, there is little to do but have lifeguards raise a red flag and call out over loudspeakers to advise swimmers.

So what to do if stung by a jellyfish? Do not rub the sting. Rinse profusely with salty water, and apply a bag of ice. As for vinegar, it helps with the Portuguese man-of-war but not with most jellyfish species. Once stung, bodies become more sensitive to jellyfish poison, and the next sting can be more dangerous.

Read more GlobalPost dispatches on Spain:

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