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Safety in numbers as crabs gather to shed their shells

CRAB INVASION 15Credit Dan Bolt

 

These spectacular images capture the incredible underwater scene as thousands upon thousands of giant spider crabs gather in a mass aggregation - just a few hundred yards from a popular Devon beach.

1Left, credit Terry Griffiths; right, Dan Bolt

Underwater photographers Dan Bolt and Terry Griffiths captured the incredible spiny spider crab 'invasion' while scuba diving at Babbacombe Beach, Torquay.

'Everywhere we looked were crabs, upon crabs, upon crabs, three or four individual-deep in places like something out of a horror or alien film,' explained Dan, 42, of Exeter, Devon.

'Swimming over the seabed before we reached the site everything was normal, but as we approached the area we saw one or two individuals, then three or four, then 10 or 12.

'It takes a couple of seconds for your brain to actually make sense of the fact that the sea-floor is actually moving with thousands of spider crabs aggregating to moult and mate.'

This wonder of nature takes place anually in late summer/early autumn at a number of locations in the waters around the British Isles and is thought to offer the crustaceans a 'safety in numbers' strategy to avoid predation while shedding their outer shells during the moulting process.

CRAB INVASION 6Credit Neil Hope

The formidable-looking spiny spider crab (Maja brachydactyla) is one of the largest species of crab in the UK with a carapace (shell) of up to 20cm and arms of up to 50cm in length. Ranging in colour from orange to brown and red during the breeding season, these predatory crustaceans will feed on anything from seaweed to starfish, sea urchins and small fish.

Dan and dive buddy Terry Griffiths have been diving together for over a decade and have witnessed this natural phenomenon on a number of occasions in the past.

CRAB INVASION 7Credit Neil Hope

'The aggregation happens most years, but in a different spot. Each year we look for it, but sometimes we hear on the diver grapevine that we've been looking in the wrong area and we soon manage to locate it. To see such a density of marine life in a world of depleting fish-stocks is a joy to behold.'

Dr Keith Hiscock, a Fellow of the Marine Biological Association, Plymouth, witnessed the same gigantic gathering – also known as a mound. 'These aggregations are so dense that it is almost impossible to see the seabed and are observed in the shallows of bays and inlets during summer and early autumn.' explained Dr Hiscock.

'The largest aggregation where a count was made was of about 50,000 individuals off the coast of Dorset in 1997 and the consensus among scientists seems to be that the crabs aggregate when moulting. There is mating in the aggregations but reproduction doesn't seem the primary reason.'

CRAB INVASION 20Credit Dan Bolt

Once the month-long moulting session comes to an end the seabed is left littered with discarded shells - including a large number of casualties which offer easy pickings for other marine life.

'Because shells are cast when growth occurs, there's a great deal of shell debris in places but also many intact dead crabs and in this instance I observed a large number of bass patrolling the area,' Hiscocksaid, 'possibly ready to consume those crabs that had died.'

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