Found this small crab shell during a walk on the beach. It was more or less whole when found, but, in spite of trying to take care, it disintigrated. The photo is of what was left - the limbs and underneath part of carapace. It was mostly white and the carapace probably about 1 to 1 1/2 cm across, broader than it was long. The ends of the rear legs had a paddle like shape, so I assume this kind of crab could swim well. The top of the carapace was broken when found - perhaps eaten by something - or maybe it was the remains of a shed shell belonging to a live crab?
My immediate suggesting on looking at the photos, (pity it isn't complete), was a small crab called Portumnus latipes. But on reading your description you say that the carapace was broader than it is long. The proportions of Portumnus latipes are the other way round. So after a rethink I come up with Bathynectes longipes - found well below low water mark, and reported in the Bristol Channel area, south to the Isles of Scilly. This is one of the species in the family Portunidae, all of which have the dactylus (end claw) of the hind legs flattened to perform either a digging in sand, and/or swimming function.
Thank you very much, I had given up hope of having this question, and the other one about a shell with holes in it, answered. It was the paddle shape of the last pair of legs that made me suspect it wasn't one of the common shore crabs.
Do you have any advice on how best to stop any future find of this sort from disintegrating when you try to pick it up? Its a pity I didn't have my camera or coule have photographed it in situ.
My guess is that the crab was not completely dried out? Small crab shells, especially a shed one can be very fragile and being tumbled about even in a gentle rippling wave can't do it any good. Try scooping the fragile find, and the sand it is sitting on into shallow container to take it back. What I have done with delicate seaweeds and things like your fragile crab shell is carefully put them into a small wide mouthed bottle or jar of clean sea water. The water helps support the specimen and minimizes any further damage, especially if all the air is removed to eliminate sploshing about.
Keep hunting the tidelines.