Diving with British Marine Life: the European Spider Crab

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If you’ve dived in the UK chances are you’ve encountered a European spider crab (Maja brachydactyla). While often overlooked, these crabs display a wide range of behaviours making them brilliant photography subjects and great characters to observe on a dive.

M. brachydactyla is found on the South and West Coast of England and Ireland and is the largest spider crab in the UK. European spider crabs are characterised by their red-yellow body colouration and triangular carapace bearing two distinct frontal spines between the eyes. The claws are relatively narrow with white tips, while their walking legs have dark tips.

Their carapace is covered in tiny barbs, which they use to fix an array of algae in order to ‘decorate’ a fresh moult. Such behaviour is well documented in majid crabs and helps to camouflage their bright exoskeletons. Paul Naylor, marine biologist, photographer and author of Great British Marine Animals, showcases this decorative behaviour on his Vimeo profile.

As well as actively masking their exoskeleton, spider crabs play host to a wide range of epibionts. Sessile organisms such as barnacles, anemones and tunicates settle onto their carapace in their larval stage. This relationship is known as mutualistic; the host benefits from protection from predators via camouflage, and the epibionts (an organism that lives on the surface of another organism) are exposed to sediment resuspended by the host, protection from slow moving predators, and better dispersion of offspring.

Documentary worthy aggregations of spider crabs also occur close to shore around the UK; but surprisingly these dense aggregations, also known as mounds, are driven by moulting rather than breeding. I am yet to witness this phenomenon, but look forward to the day I do!


Hear more from Georgie here: https://georgiebullphotography.home.blog/

Georgie Bull

Georgie Bull

Georgie is a Marine Biology and Coastal Ecology student at Plymouth University and an active diver in the South West of England. This year she will be completing the HSE Scuba qualification with the University in the hope that scuba will become part of her future career. She is particularly interested in native species and has a soft spot for elasmobranchs and molluscs.

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