However, a few brave souls dared to snorkel over them.
Kate Lowe, a marine photographer, captured the event with her camera. She said: “I snorkel most of the time during the year but I have never seen it spider crabs in such numbers.
“When we showed up at the beach, it looked like there were lots of dark rocks under the surface. But it turned out that there were thousands of crabs just two or three steps down into the water.”
She added: “It was just really incredible. They were only knee deep. I was able to float on top of them and try not to step on them.
“Many of the tourists squealed 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 harden.
Climate change is making mass gatherings common
Although it is not unusual to see them in British waters, mass gatherings like this are becoming more common in the summer due to 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 clawless.
European spider crabs are much smaller than their famously 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 and can migrate up to 100 miles in eight months.
Last Thursday, a snorkeler was bitten in the leg by a blue shark during a swim. The company behind the ride said such incidents were “extremely rare”.
The crabs are not the first plague to hit the Cornish coast this summer. Last month, the Cornwall Wildlife Trust reported hundreds of squid swarming the county’s gardens and devouring lobsters.