The crabs were knee-deep in water as they cracked open their exoskeletons to grow new outer shells near Falmouth beach, a popular tourist destination. The process is known as moulting and usually happens annually between late summer and early autumn.
The spectacle involves crabs rallying together in their thousands to protect themselves from predators.
One of Cornwall Trust’s conservation officers said this leaves the crabs very vulnerable, and can cause problems if they are disturbed by visitors in large numbers.
This is why the exact location of the siting has not been revealed.
It comes after COVID-19 cases doubled since last week, with 770 cases per 100,000 people.
Head of Visit Cornwall, Malcolm Bell, urged visitors to keep the region safe.
He said: “We are asking people not to come unless they have booked ahead and request they take a lateral flow test before, during and after their stay so that people can be safe and help us to manage the current spike.”
The mass gathering of crab spiders usually goes unnoticed, but Matt Slater, Cornwall Wildlife Trust’s marine conservation officer, managed to capture the event on camera.
He 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.
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In recent years, they have become even more common due to the warming of sea temperatures, a direct impact of climate change.
Mr Slater continued: “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, with the Trust via its ORKS app or online at www.erccis.org.uk/share-sightings.
Concerns over tourists come after 4,700 cases may be linked to the Boardmasters festival held in Newquay.
About 800 of those positive cases are people who live in Cornwall.
But Virologist Dr David Strain said music festivals were “something that is important for the mental health of a younger generation who have missed out on so much”.
He added that 18-25-year-olds had a “very low risk of being hospitalised” with coronavirus, but the “real test” of how the festival had impacted young people would be the number of long Covid cases that emerged.