Species Focus - Common Spider Crab - Maja brachydactyla

Ah, the North Welsh summer. Nothing beats the sun, the sands at Traeth Llanddwyn and the annual migration of hundreds of crabs!

Known as the Common Spider Crab, Maja brachydactyla is our largest British crustacean, easily identifiable by its spiny rounded carapace, brown/reddish colour and very long legs (hence the arachnological component). These crabs are usually denizens of the deep during the winter, however come summer hundreds of these crabs congregate in the rocky/sandy shallows anywhere from extreme low water to around 50m.

Although sightings of live Spider Crabs are uncommon without treading water or donning a snorkel and fins, you are likely to have seen bits of their exoskeleton washed up on beaches like Newborough throughout summer. Don’t be alarmed if you find a great number of discarded carapaces, claws or legs though, as the reason for their great migration is safety in numbers. These Spider Crabs congregate not only to mate, but to form protection during the moulting process; shedding their old exoskeleton so that the new soft one can expand and grow before hardening is the part of their life cycle most sensitive to predation, hence the crustacean congregation!

For the foragers out there, spiny spider crab can also be good and sustainable eating. Spider Crab stocks have a low vulnerability to overfishing compared to many UK marine species with the claws and legs being where the good bits are. Just remember if you do wish to partake in the sea’s edible bounty it’s a maximum of 5 crabs per day (including any other edible species), a minimum carapace size of 130mm between the eyes to the back and no berried females. Egg carrying ‘berried’ females should be put back to maintain the population.

Any shells and other crab bits found along the strandline should be reported via the Beached! Project on the ORS.