Heather Hamilton, 26, made the discovery when she was swimming from a beach near St Ives.
As a regular diver, she’s used to seeing weird and wonderful creatures, but Heather was astonished by the hundreds of crabs just metres beneath her, describing the scene as “a little chaotic.”
Heather, who videos her underwater explorations for her Instagram account @cornwallunderwater, found the spider crabs gathering in the shallows on July 10.
“They weren’t in the shallows right off the beach, so you’d have been unlikely to step on one,” she said.
The phenomenon happens when spider crabs gather to moult their shells for the summer.
Heather captured photographs of the hundreds of spider crabs in Cornwall. (Image: Heather Hamilton / SWNS)
“I knew they were around, and I really wanted to see them.
“But I had to wait for a calm day without wind so the visibility was better,” said Heather, from Truro, Cornwall.
“When the conditions were right, I swam from St Ives beach.
“It was quite a long swim around some rocks. They were in about three metres of water and that was high tide.
“It was so shallow you could see them from the surface, you didn’t even need to dive down.
“They gather about this time of year around summertime to moult their shells.
“The reason they gather in large numbers is to protect themselves from predators, because their new shells are super soft, so they’re vulnerable to predators trying to eat them.”
Heather Hamilton was amazed by the experience (Image: Heather Hamilton / SWNS)
Heather, who’s been snorkelling her whole life, describes the spectacle as “wonderful.”
“I think it was very special, I’ve been lucky I’ve seen it before, but not in these bigger numbers,” she said.
“It was a bit overwhelming, there was a lot going on.
“It was quite shallow water so you don’t want to kick your fin and then visibility go bad.”
Heather says the crabs pose absolutely no risk to people.
“They’re not right in the shallows, so you’re unlikely to step on one,” she said.
“But their shells will be washing up on beaches, and often people mistake the phenomenon for hundreds of dead crabs.”
Spider crabs in Cornwall (Image: Heather Hamilton / SWNS)
Heather set up her Instagram account to share the wonders of the UK seas, and her following has grown quickly.
“There’s a stigma of UK seas being murky and not a lot of life, but there’s so much to see and so much habitat to explore,” she said.
“I’ve become a bit obsessed with sharing it with people.
“I rarely go into the sea without my camera now.”
Heather has seen and documented all kinds of underwater spectacles, from sea salps to octopus blooms.
“My best diving experiences have all been in UK seas,” she says.