Spider crabs - spiny substrates on legs

We're all familiar to some extent with crabs and other crustaceans - the shell or carapace, the pointed walking legs, the first pair of legs with pincers and so on. Around the (mainly southern and western) coast of Britain, one large and distinctive species is the Spiny Spider Crab Maja squinado which can have a carapace up to 20cm long (not to mention the long legs), although the specimen below is about half this size. This is a fairly shallow-water species, found sublittorally to about 50m depth, but sometimes also in deep littoral pools low on the shore.

Dorsal view of the carapace showing bumps (tubercles) and spines, including the two larger frontal spines of the rostrum seen spreading apart at the top of the photo. These rostral spines are straight unlike those of M. verrucosa which are curved outwards. Note the growths of other organisms such as seaweeds, hydroids and sponges.

The spines have clear potential as protection against predators, and can also prove uncomfortable to the unwary paddler if a carapace is trodden on (fortunately they are not poisonous like some organisms such as weaver fish) and indeed may hide under loose substrates such as coarse sand. Their ability to hide is enhanced by the range of other organisms that use them as a substrate to grow on e.g. sponges, hydroids and seaweeds - this allows them to blend in with other substrates supporting similar organisms. For those of you who like fairy tales, think about Baba Yaga's walking hut from the traditional Russian story, or indeed the Studio Ghibli animation Howl's Moving Castle!

A close-up of the upper surface of the carapace.

Zooming in on the carapace, the tubercles can be seen to have tufts of bristles - a close look shows that where some have broken off, tiny holes remain in the surface of the carapace. Such bristles or hairs are also seen on the legs following a moult, though these are gradually rubbed off. More of these bristles later...

The inner surface of the carapace.

In contrast to the patchily pinkish, rough outer surface (could this texture be to aid the attachment of organisms?), the inner surface, with no need to be involved in camouflage, is smooth and white. The dimples show that the tubercles are hollow, reducing weight (even so, with their spindly limbs and large bodies, large spider crabs can't support their own weight out of water) while the wrinkled arc shows where muscles and other tissues were attached when the animal was alive.

Another view of the underside; this time some of the points of attachment for the mouthparts and related appendages. The basal segment of one such appendage remains - its pair shows an empty socket.

A close-up of one of the articulations on the underside of the carapace.

The first of these two photos shows the membrane forming part of the joint at the base of the appendage, but some hairs are also visible. The second photo shows such a brush of hairs in more detail. They are simple unjointed structures and are likely to be guard hairs with the function of keeping sand grains and other unwanted material out of delicate joints.

The V-shaped notch between the two rostral spines showing numerous bristles as well as the tiny holes where some are missing.

The spaces between spines around the edge of the carapace show similar bristles. These may again be there to trap unwanted material (and maybe they help with camouflage by breaking up the outline, especially if they collect seaweed fragments and similar), but the crab also has much tinier, finer sensory hairs (not visible here) which also need to be protected by guard hairs. Of course, crabs do not rely solely on sensory hairs - they also have compound eyes on mobile stalks.

One mottled compound eye on its mobile stalk. This mobility means the eye can be tucked away into a notch in the carapace to prevent damage.

A close-up of the eye showing individual lenses and the 'seal' around the edge. Red and green iridescence is also visible just below the centre of the photo.

The base of the eye-stalk again showing the guard hairs which help to protect joints from damage by unwanted materials.

This has been a fairly brief tour of spider crab morphology, even though it covers only the carapace - if I find some during a bout of beach-coming, I may write a follow-up covering legs and other appendages. Until then, enjoy the story so far and we'll see what the tide and strandline bring...


References

Crothers, J. & Crothers, M. (1988). A key to the crabs and crab-like animals of British inshore waters. Field Studies 5: 753-806. Revised version available here.
Gibson, R., Hextall, B. & Rogers, A. (2001). Photographic Guide to the Sea & Shore Life of Britain & North-west Europe. OUP, Oxford.
Hayward, P.J. & Ryland, J.S. (eds) (2000). Handbook of the Marine Fauna of North-West Europe (reprint with corrections). OUP, Oxford.