At first I thought this was a tangle of fine twigs or maybe a Sea Spider (Pycnogonidae) but closer inspection revealed this to be a small species of spider crab – possibly Inachus phalangium (Fabricius).
This group of marine animals has four pairs of very long thin legs or pereopods and a pair of shorter, smaller, broader legs at the front. The carapace, the part of the shell covering the head and body, is pear-shaped and can be upto 20mm long- though the one I found was smaller. For more detailed information about this type of crab look at the web site of the Marine Life Information Network (marLIN) run by the Marine Biological Association of the United Kingdom.
These are slow moving creatures that live among the seaweeds and tufts of hydroids – some of which they may attach to themselves as a camouflage. Zooming in on the pictures I took, I could see pieces of green algae and other debris on the shell; it is this that makes the specimen difficult to identify because it is not possible to see the pattern of bumps or tubercles on the carapace that characterise each species.
You can see a few more details in the photograph below but the picture is not clear enough to positively name the animal. With the legs in the bent position as shown, the crab measured 10cm across. As the Spider Crab was still alive, I put it back in the water.
[This is a revised version of a post first published 14 March 2009].
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