Astonishing scenes on Anglesey 'nudist beach for crabs' littered with thousands of shells

A wash-up of spiny spider crabs at Aberffraw in July 2024. Beach walkers who come across them have been warned to be careful as their spiky shells can be razor-sharp
-Credit: (Image: Tina Bradbury)


Beach walkers on Anglesey were left wide-eyed after coming across one of Britain’s greatest natural spectacles. Littering the sand along the entire mile-long shoreline were what appeared to be thousands of alien-looking crabs.

Last week's mass wash-up turned Aberffraw’s Traeth Mawr (Big Beach) into what seemed to be a giant graveyard. According to one visitor, “layers” of reddy-orange crab shells were also deposited along a riverbank towards the village some 500 metres inland.

All had jagged spikes and most were encrusted with algae, giving them a green, hairy look. “You wouldn’t believe how scary they were,” said one dog walker.

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The unnerving sight was the latest in a series of crab wash-ups on the beach this year. In the latest incident, most were empty shells but visitors reported seeing scores of intact crabs as well. Marine experts were quick to allay fears and explain it was a natural, annual occurrence.

The shells belonged to spiny spider crabs which congregate off-shore in vast numbers, moulting as they grow. “It may look alarming but there is nothing to be concerned about,” said Anglesey Sea Zoo owner Frankie Hobro. “It is in fact an amazing natural phenomenon.”

Spiny spider crabs typically migrate to shallower waters during the warmer months to feed. As they grow, they need to moult their outer shells, which carpet the seabed. Storms, high tides and strong currents drive the discarded shells to shore.

Vast numbers of the crabs are often seen offshore in Devon and Cornwall, often massing just yards from beaches. But some of the most impressive wash-ups have occurred at Aberffraw, which has begun drawing visitors hoping to witness the remarkable spectacle for themselves. The North Wales Live Whatsapp community for top stories and breaking news is live now - here’s how to sign up

Hundreds of spider crabs mass at a Plymouth marina
Hundreds of spider crabs mass at a Plymouth marina -Credit:Plymouth Live/SWNS

Last week’s recent crab shell deposit was likely to have been the last for this year, as coastal waters will now start cooling. It brought to an end a successful season for Anglesey’s spider crabs, which mop up dead or decaying fish, invertebrates and algae from the seabed.

Frankie said: “The more food they find, the faster they grow and the more moults they need,” she said. “Full-sized adult crabs moult once or twice a year. If juveniles find good feeding grounds they can grow fast and moult every few weeks. So what's being seen off Anglesey is a sign of a healthy ecosystem.

“People often panic when they see lots of crab shells like this, thinking it’s a mass die-off. We need to educate people that it is nothing to worry about – it’s just that they’ve all grown and moulted at the same time. It’s actually a positive thing!”

A huge deposit of spider crab shells at Traeth Mawr
A huge deposit of spider crab shells at Traeth Mawr -Credit:Richard Lee

While beach wash-ups can be impressive, the debris left behind is only half the story. Moults earlier in the summer were probably females shedding their shells. Only once the breeding season has ended do larger males follow suit, ready for the winter months: these are likely to have accounted for last week’s wash-up.

Put them together and underwater scenes will be twice as spectacular for those prepared to look for them. According to Frankie, thousands of spider crabs can gather in feeding grounds. They've been likened to the "Red Army" on the march and divers report seeing scenes "like something from a science fiction film". Not for nothing have the crabs been described as the wildebeests of Britain’s waters.

For Traeth Mawr to be such a good wash-up site suggests not only plentiful food offshore: the beach probably also has the right marine topology and currents to gather up discarded shells. It is, as one online wag put it, like a “nudist beach for crabs”.

A spiny spider crab at Anglesey Sea Zoo
A spiny spider crab at Anglesey Sea Zoo -Credit:Anglesey Sea Zoo

Frankie said: “When they moult their outer case, the animal squeezes out of a tiny little corner at the bottom of the shell, leaving it intact. It’s actually a very clever process. The crabs absorb extra water from around them, so their new exoskeleton is bigger than they actually need. This allows for extra growing time before they need to moult again.”

Although Aberffraw’s crab carpets are impressive, they are, for the most part, lifeless. But danger still lurks. The crabs’ bodies can be up to 5-20cm long, with a claw-to-claw span of up to half a metre, and they are covered in razor-sharp spines.

“As their name suggests, their shells can be very spiky,” warned Frankie. “So avoid treading on them with bare feet – it can really be quite painful!” Sign up for the North Wales Live newsletter sent twice daily to your inbox

If you find unusual creatures on Anglesey’s shoreline, you can report them to Anglesey Sea Zoo (01248 430411) even if you think they're dead. Often its experts can nurse marine creatures back to health – provided strandings are reported rapidly and are left untouched.

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