Empty Masked Crab shell, Corystes cassivelaunus (Pennant), dorsal surface, with acorn barnacles attached, picked up from the strandline on the beach (1)

Empty Masked Crab shells, Corystes cassivelaunus (Pennant), picked up from the seashore strandline. These are the remains of small crabs: the length of the carapace is usually no more than 40mm; and length is greater than the width. The shell in life and when fresh is reddish in colour grading to yellowish-white but these shells are bleached and faded.

The surface of the carapace is coved overall with tiny granular markings. It is thought that the larger fainter grooves and ridges sometimes look like a face – hence the reference to ‘masked’ in the name. In Picture 1 the markings are obscured by acorn barnacles. Barnacles can attach to the crab shell while the animal is alive; and they can also settle on the shell after it has been shed through moulting or when the animal dies.

The antennae are longer than the carapace. When the two antennae are brought together, the hairs interlock to form a tube through which the crab can draw down water to breath when it is buried in sand.

Like other crabs, this species has five pairs of legs – but many of the jointed legs are missing in these empty crab shells. Each pair of legs has become adapted for different specialist purposes. For example, the first pair of legs are the chelipeds and have pincers at the end. In the male Masked Crab the chelipeds are twice the length of the other legs. Pictures 3 & 4 in this post clearly show a female Masked Crab with a short first walking leg and the rounded hinged abdominal segments; and pictures 1 & 2 are probably a female crab too. 

For photographs of living specimens, see the earlier post Masked Crabs living at Rhossili.

Empty Masked Crab shell, Corystes cassivelaunus (Pennant), ventral surface, with acorn barnacles attached, picked up from the strandline on the beach (2)

Empty Masked Crab shell, Corystes cassivelaunus (Pennant), dorsal surface, picked up from the strandline on the beach (3)

Empty Masked Crab shell, Corystes cassivelaunus (Pennant), ventral surface, picked up from the strandline on the beach (4)

COPYRIGHT JESSICA WINDER 2011

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