Spectacular spider crabs sighted in Cornish shores

4 min


Thousands of migrating crabs

Tourists and locals are recounting the incredible sight of seeing thousands of spectacular, but harmless, spider crabs off the Cornish shores in recent weeks. 

The migration of these creatures prompted Cornwall’s Wildlife Trust to encourage beachgoers to report sightings of the crustaceans so the charity can monitor their movements.  

The team is also keen to debunk the myth that the animals are ‘venomous’ as people flock to beaches to see the underwater spectacle for themselves. 

One of the charity’s volunteers even filmed the stunning ‘carpet of crabs’ during one of their incursions into shallower waters. 

The migratory animals head closer to the shoreline so they can shed their shells in relative safety.

Being away from deeper waters means they can avoid predators who may harm them as their new shells grow and become hardened. 

Once their exoskeletons have toughened up then they can head back out to sea where they can live in depths of up to 300 feet. Environmentalists say that seeing these incredible crabs up close is a real treat.

‘The spider crab mass moulting looked like something you’d see in a tropical country, yet it’s happening right here in Cornwall!’ 

‘I feel so lucky to have witnessed it first-hand,’ says Katie Maggs, a BSAC snorkel instructor and volunteer for Mounts Bay Marine Group. 

‘Unusually spectacular’

‘People often say to me they would never get to see our incredible marine life without me sharing my videos,’ continues Katie, whose footage of the event has gone ‘viral’ on social media platforms.  

‘I genuinely feel so lucky to be able to show people my snorkelling finds and I hope that it encourages people to want to protect our seas and learn more about them.’

The crustaceans have been spotted carpeting the sea floor at multiple popular tourist destinations in recent weeks.

The spider crab aggregations, or gatherings, used to be rare to see in UK waters, but Cornwall Wildlife Trust’s marine experts are describing this summer as being ‘unusually spectacular’ for sightings.

‘I’ve spent my whole career trying to get people to appreciate amazing marine animals like spider crabs,’ says Matt Slater, Marine Conservation Officer at Cornwall Wildlife Trust. 

‘Reports of them being venomous are simply untrue and could damage their reputation. These animals are truly unique and are completely harmless to humans.’

‘Despite the many gatherings we’ve seen in places like St Ives, it’s not that common to witness this kind of behaviour.’ 

‘I saw it for the first time in Falmouth last year and it was an unbelievable experience! Please go out, enjoy our coastline responsibly and admire these sensational spider crab displays should you be so lucky to see one.’

A healthy sign for the spider crab population 

Cornwall Wildlife Trust says it had just one confirmed gathering of the Spiny spider crab in 2021.

This year, the crabs have been seen rallying together by divers and snorkellers across the Duchy in Newquay, St Ives, Falmouth, St Austell Bay and on the Lizard Peninsula. 

Spider crabs are known to resemble the arachnids they are named after. Their long, spiny legs and claws span up to one metre and give them their spider-like look.

This year has seen the natural phenomenon being reported in Torbay and Wales as spider crab numbers continue to boom in UK waters.

Cornwall Wildlife Trust is encouraging members of the public to record any spider crab sightings with them, including those in larger groups or aggregations, to help them identify patterns in their behaviour and migrations.

‘We hope that these mass sightings are a sign that spider crab populations are healthy,’ says Matt. 

‘We would love to learn of more gatherings taking place around Cornwall’s coastline. Our seas are full of surprises and there’s still so much we don’t know about them.’

To record a wildlife sighting with Cornwall Wildlife Trust, visit the Online Recording Kernow and Sicily website: https://erccis.org.uk/share-sightings

Footage and photography: Spider crab aggregation image by Katie Maggs. Video by Katie Maggs. Imagery and video kindly supplied by Cornwall Wildlife Trust.

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