Spider crabs swarm beach… just when locals thought it was safe to go back in the water

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.

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