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!
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