A mass aggregation of marine creatures off the Cornish coast had visitors in awe of the rare phenomenon.

Huge spider crabs have been witnessed gathering in their thousands in a rarely-seen event by one of Cornwall Wildlife Trust’s Marine Conservation Officers.

The mass aggregation of male crabs was filmed in knee-deep water just a few metres from a popular Falmouth beach at low tide.

Falmouth Packet: Snorkeller and spider crabs. Picture: Matt Slater & Cornwall Wildlife TrustSnorkeller and spider crabs. Picture: Matt Slater & Cornwall Wildlife Trust

This undersea spectacle, which takes place annually between late summer and early autumn, involves crabs rallying together to protect themselves from the threat of predators.

This is because they are extremely vulnerable during the moulting process, as they crack open their exoskeletons and grow a new outer shell.

Falmouth Packet: Mass aggregation of spider crabs. Picture: Matt Slater & Cornwall Wildlife TrustMass aggregation of spider crabs. Picture: Matt Slater & Cornwall Wildlife Trust

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Falmouth Packet: Moulted spider crab. Picture: Matt Slater & Cornwall Wildlife TrustMoulted spider crab. Picture: Matt Slater & Cornwall Wildlife Trust

Matt Slater, Cornwall Wildlife Trust’s Marine Conservation Officer who filmed the incredible gathering, said: "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.

Falmouth Packet: Aggregation of spider crabs from above. Picture: Matt Slater & Cornwall Wildlife TrustAggregation of spider crabs from above. Picture: Matt Slater & Cornwall Wildlife Trust

"Our seas are full of surprises - most locals would have no idea that one of the world’s great wildlife aggregations is occurring not too far from where they sleep.

"It goes to show how important our Cornish seas are and why we all need to look after them better."

Falmouth Packet: The aggregation took place off the coast of Falmouth. Picture: Matt Slater & Cornwall Wildlife TrustThe aggregation took place off the coast of Falmouth. Picture: Matt Slater & Cornwall Wildlife Trust

The formidable-looking spiny spider crab is a common species in Cornish waters, known for its characteristically long, spiny legs and claws which can span up to one metre.

Populations appear to have thrived in recent years as a direct result of climate change and warming sea temperatures.