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Just days later, when holidaymakers thought it was safe to go back in the water, a plague of poisonous crabs swept in on the tide to terrorise their summer breaks.
The giant mass of spider crabs has gathered in the shallow water off St Ives to shed their skins, creating a veritable carpet of the spiky, 10-limbed creatures.
However, swimmers courageous enough to risk either a sharp nip from one of the crabs or a sore foot from standing on them should find the spectacle anything but dangerous.
Despite their docile nature, more than a few visitors to Porthgwidden Beach were put off entering the sea by the gathering of the barbed decapods just below the surface.
A few brave souls, however, did dare to snorkel above them.
Kate Lowe, a marine photographer, captured the event with her camera. She said: “I go snorkelling most of the time throughout the year, but I have never seen spider crabs in such numbers.
“When we turned up at the beach, it looked as though there were lots of dark rocks under the surface. But it turned out that there were thousands of crabs just two or three steps into the water.”
She added: “It was just really incredible. They were only knee-deep. I was able to float on the water above them and tried not to step on them.
“A lot of the tourists were squealing at the sight of them.”
The mass gathering is thought to help the crabs protect themselves from predators while they wait for their new exoskeletons to thicken and toughen up.
Climate change making mass gatherings common
While it is not unusual to see them in British waters, mass gatherings such as this one are becoming more common in the summer because of rising sea temperatures from climate change.
The crabs are migratory and once their new shells are tough enough, they will disperse and disappear to depths of up to 300ft, leaving Cornish beaches quiet and claw-free.
European spider crabs are much smaller than their famed giant Japanese cousins, with their carapace reaching about eight inches in width and a claw-to-claw measurement of 20 inches.
The crustaceans are common in the Mediterranean Sea and can migrate up to 100 miles over the course of eight months.
Last Thursday, a snorkeler was bitten on the leg by a blue shark during a swim. The company behind the tour said that such incidents were “extremely rare”.
The crabs are not the first plague to strike the Cornish shores this summer. Last month, the Cornwall Wildlife Trust reported hundreds of octopuses swarming the county’s seas and devouring lobsters.
Local fishermen have been able to sell the octopus meat to Europe, where it is popular, to make up for their lost crustaceans.
There were also reports that the spider crabs were spreading as far north as Norfolk, which is usually well outside their traditional range.
Fishermen in the seaside town of Cromer, Norfolk, said they had caught dozens of the species where previously they had seen none.
Henry Randall, a local crab fisherman, said: “We don’t want to see spider crabs because we’re fishing for brown crabs. Are we going to see more and more every year? Are they going to take over? That’s the concern.”
Scientists were quick to quell the fears, however.
Alastair Grant, a professor of ecology at the University of East Anglia, said they were likely to “coexist with our more familiar Norfolk species, just as they do in South West England”.
That sentiment was backed up by Ron Jessop, a senior marine science officer at the Eastern Inshore Fisheries and Conservation Authority, who said: “Normally, spider crabs won’t outcompete brown crabs. Spider crabs are slow-moving, less robust and less aggressive than brown crabs.”