A marine conservationist filmed the extraordinary sight of thousands of spider crabs gathered together on the seabed off the British coast.
The crabs huddle together to protect themselves from predators while they moult, growing a new shell after their exoskeleton cracks open.
It is an annual occurrence, between late summer and early autumn, but is rarely seen.
Matt Slater, a marine conservation officer with Cornwall Wildlife Trust, filmed the crabs in knee-deep water during low tide at Falmouth beach in south-west England.
He told The National: "It was an absolute privilege to see so many crabs together in British waters. There were absolutely thousands of them and they were massive – shell of about 20 centimetres and arms of 70cm."
He said he had never seen anything on that scale before and that it was particularly unusual for them to be in shallow water.
"Fishermen sometimes see them off shore, but holidaymakers may not have been as keen on paddling if they'd known how close they were."
He was going for a swim on his day off but fortunately had his waterproof camera with him.
“I have seen spider crabs on every dive and snorkel I have done for the past four years, but I have never seen a group as large as this,” he said.
“Looking down at the mass of crabs scuttling on the seabed was a truly incredible experience.”
Cornwall Wildlife Trust described it as a “spectacular natural phenomenon”.
“This rarely seen event happens when males rally to protect themselves from predators as they moult,” the trust tweeted, asking people to send in their own sightings of groups of crabs.
Spider crabs, which can measure up to a metre across when legs and claws are included, are common in Cornish waters.
They have thrived in recent years as sea temperatures rise.