Swimming at beaches along Guardamar del Segura in the Alicante province (Valencia region) was prohibited on Wednesday and Thursday morning after two examples of a venomous tropical sea slug were discovered in the sea.
The species, known as the blue dragon (Glaucus atlanticus), measures around four centimetres, sports six wing-like appendages and a striking blue colour. Its sting is notorious for being extremely painful and can lead to serious health issues in people.
With the summer season bringing large numbers of swimmers, the town council ordered a ban on entering the sea while specialists searched the coastline for additional specimens.
Mayor José Luis Sáez told the EFE news agency that ‘the beaches were closed on Wednesday and would remain so on Thursday while the situation was monitored’. Local outlets reported that the precaution covered the town’s entire 11-kilometre stretch of shore.
By Thursday lunchtime, Sáez confirmed via social media (see below) that the inspection had ended and the yellow flag was raised, allowing the public to return to the water. ‘Municipal services, police, and lifeguards remain attentive to the evolution of the situation,’ he wrote.
Authorities had cautioned swimmers not to touch the sea slugs if spotted — not even with gloves — and instead to notify lifeguards or police immediately.
The blue dragon feeds on creatures like the Portuguese man o’ war, storing their venomous cells and later deploying them itself, which makes encounters with humans potentially hazardous.
Stings can bring on nausea, severe pain and vomiting. Health guidance advises rinsing the wound with seawater and getting medical care as soon as possible.
These sea slugs were first seen again on Spain’s Mediterranean coast in 2021, after an absence of some three centuries. That summer, eight were found between Guardamar del Segura, Orihuela and Torrevieja. Another appeared off Javea in spring 2023, and yet another was located in Torrevieja waters that August.
Normally the blue dragon inhabits tropical and some temperate waters, only rarely turning up in the Mediterranean. According to Sáez, the specimens near Guardamar had most likely been carried in by strong currents.
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Bandera AMARILLA en las playas de Guardamar tras el cierre del operativo de vigilancia especial, por la presencia de dos ejemplares de Dragón Azul.
Los servicios municipales, policía y socorrismo, siguen atentos a la evolución de la situación. #Playas #MedioAmbiente #Seguridad pic.twitter.com/v3bPRRQwuD
— José Luis Sáez (@JoseLSaezPastor) August 21, 2025
Máxima precaución 👇https://t.co/nzE2Q3RjVY
— Guardamar del Segura (@Guardamar_) August 20, 2025
Bandera ROJA en las playas de Guardamar PROHIBIDO el BAÑO tras la aparición en Playa Vivers de dos ejemplares de Glaucus atlanticus, conocido como Dragón Azul. pic.twitter.com/Ksy21uADmO
— José Luis Sáez (@JoseLSaezPastor) August 20, 2025
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