Environmentalists have been amazed at the rarely-seen spectacle of thousands of spider crabs piling on top of each other.
Marine Conservation Officer Matt Slater says he has never seen a sight like it.
"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.
"Looking down at the mass of crabs scuttling on the seabed was a truly incredible experience."
The mass aggregation of male crabs was filmed in knee-deep water just a few metres from a popular Falmouth beach at low tide.
It looks unusual, but this mass pile-on happens every year between late summer and early autumn to protect the male crabs as they shed their shells.
Cornwall Wildlife Trust says it protects the crabs from the threat of predators as they are extremely vulnerable during the moulting process, which sees them crack open their exoskeletons.
Matt Slater said most locals would have no idea this was happening on their shores.
"It goes to show how important our Cornish seas are and why we all need to look after them better,” he said.
Spider crabs are a common species in Cornish waters and are recognisable from their long, spiny legs and claws which can span up to one metre. Their numbers are on the increase as a direct result of climate change and warming sea temperatures.
Cornwall Wildlife Trust says it would love beachgoers to film any spider crab sightings, including those in larger groups or aggregations, and share with the Trust via its ORKS app or online.