When a pair of curious-looking sea slugs were found this month on the beach near Guardamar del Segura, few in the resort town in southern Spain noticed.
But then, last week, the tiny blue creatures — capable of delivering one of the most ferocious stings in the animal world — began washing ashore in droves.
“They kept appearing, one after the other, after the other, and at times on a massive scale,” said José Luis Sáez, the town’s mayor.
The arrival of the tiny marine mollusks, known as Glaucus atlanticus or blue dragons, is raising concern across Spain. The animals have so far shuttered a handful of beaches across four regions, angering locals and spoiling vacations at the height of the busy summer tourist season.
More than 1,000 miles from Guardamar del Segura, another mayor on the Canary Islands was also managing an invasion of the noxious slugs.
“We are all shocked,” Alfredo Villalba Barreto, the mayor of Haría, said. He closed two beaches last week after a young child was sent to the hospital for a suspected blue dragon sting. “We’ve never had this little creature here before.”
Scientists are also concerned about the presence of the blue sea dragons, which are typically found in warm tropical waters. Their encroachment into the Mediterranean and nearby waters is an undesirable — and possibly long-term — effect of climate change.
For now, however, local officials say they are simply trying to understand what they’re dealing with.
“It’s a very strange animal,” said Manuel Ballesteros Vazquez, a professor emeritus of marine zoology at the University of Barcelona and an expert on sea slugs.
While most sea slugs live on the seabed, the blue dragon floats on top of the water, trapping air in its stomach to float, Mr. Vazquez said. It also has a rare superpower: It can feed on jellyfish and siphonophores, including the venomous Portuguese man-of-war, and incorporate the stinging cells of the jellies into the tips of its fingerlike appendages and fire them at predators or prey.
Because the venom becomes concentrated in the blue dragons, their sting can be even more powerful than that of the jellyfish they eat, Mr. Vazquez said. While death from a blue dragon prick is unlikely, it can still cause intense pain, along with redness, swelling, nausea and vomiting.
“I absolutely would not touch one,” Mr. Vazquez said. (If you do get stung, experts advise rinsing the area in salt water, followed by hot water to ease the pain. Avoid urine, alcohol or ice, which can make the sting worse.)
While blue dragons are typically found in tropical waters in the Atlantic, Pacific and Indian Oceans, in recent years they’ve been spotted farther afield, in countries like Australia and South Africa, as well as in states including Florida, North Carolina and Texas.
They have rarely been found in the Mediterranean, Mr. Vazquez said. But five years ago, that began to change, as warming waters brought more of their food sources, like the Portuguese man-of-war, into the region.
The Mediterranean is among the fastest-warming bodies of water in the world, with water temperatures hitting record highs in June and July this year, according to Mercator Ocean International. The warming has led to an increase of species not typically seen in some of its regions, including lionfish in Malta and Portuguese man-of-war in the south of France.
In Guardamar del Segura, the blue dragon invasion has been accompanied by swarms of blue button jellyfish, officials said, and in Lanzarote, locals are also dealing with an outbreak of foul-smelling, warm-water seaweed.
“We are seeing the other side of climate change,” Mr. Barreto said.
Officials have now begun to comb the shores of Lanzarote in the morning looking for blue dragons, Mr. Barreto said, and they plan to send out warnings when large quantities of the creatures are spotted.
In Guardamar del Segura, officials collected some of the dozens of blue dragons that washed ashore last week and sent them to biologists in Valencia to extract their genetics and determine their diets. They’re hoping to understand what the animals are feeding on and how dangerous they might be for locals.
“We still don’t know exactly what we’re dealing with here,” Mr. Sáez, the mayor, said.
“But given the warming of the Mediterranean,” he added, “we’re not ruling out that in the coming years we will once again confront situations that we’ve never dealt with.”
Jonathan Wolfe is a Times reporter based in London, covering breaking news.
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