Spiny crustaceans cover the surface waters of St Ives, Cornwall (Image: BNPS)

Holidaymakers screamed as thousands of poisonous crabs invaded Cornish beaches as sea temperatures rose.

Spiny crustaceans were seen swallowing water from swallowtail butterflies on Porthwiden beach in St Ives to shed their shells before returning to their deep-sea breeding grounds.

Marine photographer Kate Lowe, who captured the gruesome images the same week a woman snorkeling off Penzance was bitten by a blue shark, said: in such numbers.

When I got to the beach, it seemed like there were a lot of black rocks under the sea.

“But it turns out there are thousands of crabs in just a few steps of water.

“It was amazing. It was only up to my knee. I was able to float on the water above them and avoid hitting them.”

“Many tourists screamed when they saw them, their shells just floating.

Thousands of poisonous crabs invade Cornish beaches

The sighting occurred the same week a female diver was attacked by a blue shark in Cornwall (Image: @cornish.coast/BNPS)

Immediately recognizable by their outstretched legs and claws, giant spider crabs are harmless to humans, but they are poisonous.

Many tourists screamed and bathers fled the sea, while others took advantage of Saturday’s crab beds to practice snorkeling.

After dropping the shell, the creature returns to a depth of 300 feet.

They often roll together in huge pools when on the surface to protect themselves from predators.

Alastair Grant, professor of ecology at the University of East Anglia’s School of Environmental Sciences, said “climate change is causing ocean temperatures to rise” and fish and shellfish are becoming more abundant as “expand their geographic range.”

Get in touch with our news team by sending an email to

For other stories like this, Visit our news page .