I think this empty Common Spider Crab carapace (Maja squinado (Herbst.) is particularly pretty. It has been moulted by the animal which has outgrown its shell. Crab shells like this are a very common find on the strandlines of beaches. I found this one at Studland Bay in Dorset. It is a rather small carapace, only about 70 mm maximum, so a young individual. A large mature specimen would have a carapace 200 mm long by 150 mm wide.
The circular, convex shell is edged with strong tapering spines, with shorter, blunter spines and tubercles or bumps over the general external surface. This rough, spikey, outer layer is also covered with a brightly-coloured, irregular and abstract pattern in hues of red, orange and brown. Older crabs decorate and camouflage themselves by attaching pieces of weed, sponge and other sedentary organisms to the shell – but this one is still clean and lacks any encrustation.
The inside of the carapace is a real contrast to the outside. It is smooth, glossy, and white. Numerous pores or holes form an overall network. There is a pattern which is difficult to define. Maybe the pores relate to the spines and tubercles above. This internal layer is slightly translucent and almost like porcelain in texture when viewed normally. However, if the shell is held up to the light, the white inner layer seems to become transparent, so that the red patterns on the outer surface become hazily visible.
Revision of a post first published 16 March 2010
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