Portuguese Man-of-War
© Jim Simmen
Portuguese Man-of-War Jellyfish

Holidaymakers are being warned to be vigilant when they take to the water and beware of the stinging menace in the shallows.

For the first time in a decade the potentially deadly Portuguese Man o' War, which are not strictly jellyfish but floating colonies of microscopic hydrozoans, has been spotted close to the beaches of the Costa del Sol.

With tentacles sometimes more than 30 yards long, which are barbed with a sting 10 times stronger than an ordinary jellyfish, it presents a more dangerous threat than the annual jellyfish invasion of Mediterranean beaches.

In extreme cases, the sting can cause heart failure in victims who are allergic to it.

Scientists fear the creatures could spread along the coast of Spain and invade waters around the Balearic Islands after venturing away from its north Atlantic habitat and through the Strait of Gibraltar.

"The Portuguese Man o' War hasn't been seen in the Mediterranean for a decade, and its appearance off the Spanish coast could herald a process of colonisation, which has happened with other invading species," said Xavier Pastor, the European director of the Oceana ecological campaigning group. "We can't say with any certainty what has caused so many sightings but we are talking about thousands."

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.

Scientists predict that the numbers of jellyfish swarming off the coast of Spain this summer will be higher than ever and new measures have been introduced to protect unsuspecting bathers.

In the northeastern region of Catalonia, where some 20,000 people received treatment after being stung last summer, authorities are using satellite images to track dangerous hoards of the slimy creatures.

When they seem dangerously close to shore, fishing boats will be sent to scoop up the gelatinous masses and divert them away from popular beaches.

Spain's environment ministry has launched a campaign that will see pamphlets distributed alerting bathers of the dangers of jellyfish and what measures to take in case of a sting.

"It's a case of swim at your own risk," said Mr Pastor. "The measures in place are as useful as treating cancer with a sticking plaster. Until we tackle the environmental issues that are causing the proliferation of jellyfish in Mediterranean waters the problem will only get worse."