Several beaches in Spain — such as the beaches of Vivers in Guardamar del Segura, Santa Bárbara beach in La Línea de la Concepción, and Famara beaches in Lanzarote — were closed earlier this month after a tiny species of sea slug began to wash ashore in droves. These were blue dragons — barely four centimetres long but capable of delivering one of the most ferocious stings in the animal world.Their arrival, which has angered locals and ruined vacations during the peak summer tourism in Spain, has surprised everyone, including scientists. Blue dragons are typically found in tropical waters and have been rarely seen in the Mediterranean Sea.“We are all shocked… We’ve never had this little creature here before,” Alfredo Villalba Barreto, mayor of Haría in Lanzarote, told The New York Times.What are blue dragons?Blue dragons (Glaucus atlanticus) are a small type of sea slug that live on the ocean surface, drifting with the currents. While other sea slugs live on the seabed, blue dragons float upside down, like when we float on our backs. They can do so by swallowing and maintaining an air bubble in the stomach.The marine creature floats with its underside facing upwards in the water for a reason. It helps blue dragons “camouflage themselves with the sea’s blue colour, and on their underside they are white, blending with sunlight when seen from below. It’s a defensive strategy,” Juan Lucas Cervera, professor of Biology at the University of Cádiz (Spain), told the Spanish newspaper, El País.Blue dragons have another unique trait. They feed on jellyfish and venomous siphonophores such as the Portuguese man o’ war, and incorporate the stinging cells of the jellies into multiple finger-like structures protruding from their bodies. This gives blue dragons a potent form of weapon against their predators.Also, the venom becomes concentrated in blue creatures, and their sting can be more powerful than that of the jellyfish they consume.Story continues below this adA sting from a blue dragon can be extremely painful to humans and cause redness, swelling, nausea, and vomiting. However, it cannot lead to death.Also in Explained | US confirms its first human case of flesh-eating New World screwworm: What is this parasite?What led to the arrival of blue dragons in Spain?Blue dragons occur in the warm tropical waters of the Pacific, Atlantic, and Indian oceans. In recent years, they have been cited farther afield, in countries such as Australia, South Africa, and the United States.However, they have been rarely seen in the Mediterranean Sea. “The species was cited [in a scientific publication] in the second decade of the 20th century in the Balearic Islands, although the first record was in 1839 in the Canary Islands, but then a long time passed before it was detected again,” Cervera said.This has changed in the last five years. Scientists suggest that the blue dragons’ arrival could be linked to rising Mediterranean temperatures, which brought more of their food sources, like the Portuguese man o’ war, into the region, according to a report by The New York Times.Story continues below this adThe Mediterranean Sea is one of the fastest-warming water bodies — at the end of June, temperatures exceeded 28 degrees Celsius, with thermal anomalies of five degrees.