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

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.

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.

“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.”

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.

Matt continues: “We hope this mass sighting is a sign that spider crab populations are healthy. We would love to know if there are more aggregations like this forming around the coast or if this is a one off.”

Cornwall Wildlife Trust is encouraging members of the public to record any spider crab sightings, including those in larger groups or aggregations, with the Trust via its ORKS app or online at www.erccis.org.uk/share-sightings.