Pictures shared on social media have captured the swarming crustaceans that massed in the shallows of Porthgwidden Beach, St Ives.
The crabs are there to shed their shells before returning to their breeding ground as part of the species’ annual migration pattern to seek out warmer waters.
When the crabs moult, they exit through the back of their shell, leaving behind the whole exoskeleton including the legs and eye stalks, which can look just like an intact crab.
One snorkeller who dived down to get a glimpse of the spectacle said she had never seen spider crabs accumulate in such numbers.
Kate Lowe, a marine photographer who snorkels at Porthgwidden throughout the year, 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.”
It is believed mass gathering helps the crabs protect themselves from predators while they wait for their new exoskeletons to thicken and toughen up.
The crustaceans have a venomous bite which can kill their predators – but is harmless to humans.
Despite the reported boom in numbers of spider crabs in British waters, there is little market in the UK for spider crab meat, despite it being considered a delicacy in European countries including Spain and France.
But the National Coastwatch St Ives told Cornwall Live: “They are just the shells of a crab, they are not dead.
“They shed shells for new ones.”
Last July, a congregation of spider crab carcasses washed up on the beaches of Anglesey off north Wales.
Photographs similar to those out of Cornwall today captured the ghastly tableau where the legs, claws and carapaces of washed-up spider crabs had gathered.