On Tuesday, authorities in Palma de Mallorca confirmed that several creatures had also been washed up dead on Molinar beach where red flags have now been raised.
An 11-year-old boy was hospitalized after being stung by a Portuguese man o’ war as he played at the water’s edge on La Marina beach in Elche on Sunday afternoon.
His mother immediately sought help from lifeguards stationed at the beach, who used tweezers to pluck off the potentially deadly stinging tentacles from the child’s arm.
He complained of feeling pain in his stomach and neck and was taken to Elche hospital for observation and was discharged several hours later.
The dangerous stingers have been sighted along a stretch of coastline from El Campello to the north of Alicante and Pilar de La Horadada, which is almost at the border with the Murcia region.
All beaches along the 118 kilometres of coastline between the two have been red flagged as a result of the stingers apart from San Juan and El Postiguet in the bay around Alicante itself where no Portuguese man o’war have yet been sighted.
Beaches in Benidorm were also closed on Monday when man o'war were detected.
The man o'war - not strictly a jellyfish but a floating colony of microscopic hydrozoans - has tentacles that can reach 30 yards long and are barbed with a sting that typically cause painful welts lasting up to three days.
In some cases the sting can cause an allergic reaction and in rare cases, heart failure.
Even when washed up on the sand the stings still contain venom.
Up until a decade ago, they were rarely sighted in the Mediterranean but have become a more common occurrence.
Spanish authorities are already preparing defences to tackle the swarms of Mauve Stinger jellyfish, bright purple invertebrates which emit a yellow glow at night, that annually plague the beaches of the Mediterranean.
Over the last several years there has been a huge rise in numbers due to the effects of global warming and over-fishing of their natural predators and each summer tens of thousands of holidaymakers are forced to seek treatment for minor stings.
How to avoid being stung by a Portuguese Man O'War and what to do if you are.
Photo: Tim Snell/Flickr
Rule No. 1: Don't touch it
Man-of-war fish have stinging cells that are still active and capable of stinging even after the creature is dead and washed up on shore. So don't touch it.
Rule No 2: Listen to lifeguards
If the red flag is raised then don't go into the water. If you break that rule you may face a fine or even be stung by one of the creatures lifeguards are trying to protect you from.
If you are in the water then you may be stung without even seeing the culprit. The Man-of-war looks like a floating blue plastic bag but they have practically invisible tentacles that typically extend over 2.4 meters (8ft) under the surface of the water and sometimes stretch to 10 meters..
Rule No. 3: Get help if you are stung
If you are stung then leave the water immediately and look for a lifeguard. If there are no lifeguards on the beach, wash the wound with salt water and remove any tentacles stuck to the skin by dousing the site with sea water.
Never use fresh water as that can actually cause the stinging cells, or cnidae, left on your skin to discharge, which could make the sting worse..
Don’t touch the tentacles with your bare hands, and don't scrape them off your skin with objects like a credit card or razor, because that could increase pressure around the sting and make it worse.
Some experts suggest cooling the area down by applying ice for around five to ten minutes. You can also try wetting the wound with vinegar or baking soda dissolved in water.
Protect the area by using an antiseptic to avoid infections.
If the pain continues, make sure you consult a doctor.
Rule No. 4: Don't pee on it
Professor Josep Maria Gili from Barcelona's Institute of Marine Sciences warns against the common myth that urinating on a jellyfish sting can help ease the pain.
"That is absolutely false," he told The Local.
"It could be effective for some fish bites but never for jellyfish."
In fact, it might actually make the stings worse.
Rule No. 5: Seek medical attention
If you get stung by a man-of-war and you have trouble breathing, this could be a result of severe envenomation, and you'll need to see a medical professional immediately.
Extremely painful stings also require medical attention, as do stings that make you feel dizzy or disoriented.