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Mass strandings of highly venomous jellyfish have been reported from Cornish beaches in the past week.

Huge rafts of the mauve stinger jellyfish (Pelagia noctiluca) have been reported around the coast of Cornwall.

Rich Mulryne spotted thousands of the creatures at Poldhu beach on November 19, and there have also been reports from Tolcarne beach, Newquay, and Sennen.

Mauve stingers, an oceanic jellyfish that glows spectacularly in the dark, are the most venomous species in our waters. They grow up to four inches across, according to the Cornwall Wildlife Trust, but can reach 16in in northern waters. The deep bell is pinkish purple in colour and is speckled with wart-like spots that are actually stinging cells.

They have four frilly arms and eight trailing tentacles measuring up to six feet long.

The trust warned: “This jellyfish can have a potent sting (as a member of the trust found out surfing the other day) so be careful not to handle them – the stinging capsules still function automatically when an object comes into contact and even dead jellyfish can still sting.”

Dr Priscilla Licandro from the Sir Alister Hardy Foundation for Ocean Science in Plymouth said the jellyfish were common in the Atlantic and the Mediterranean said: “It wouldn’t kill people, but if you are allergic it would cause trouble.”

In 2007 a 10-mile square bloom of mauve stingers killed 100,000 salmon at a fish farm in Northern Ireland, causing damage estimated at £1million.

The jellyfish have few predators, apart from some turtles and some fish.

Population explosions, or blooms, used to occur roughly every 10-12 years and last for about four years, but the creatures were now common all year round, Dr Licandro said.

We think that this is the effect of the rise in sea temperatures recorded over the last few years, she said.

She said that for this species it was recommended to not use vinegar, freshwater or alcohol but rather to rinse with seawater or salty water and use cold or an icebag on the affected area.

The Marine Conservation Society said that as well as formal reports of mauve stinger jellyfish and the harmless “by the wind sailors” from several Cornish beaches, they had also been encountered in Devon and Dorset.