The largest spider crab in the British Isles. The common spider crab is red, brownish-red, or yellowish in colour and the body is up to 20 cm long. It has a circular to oval shell (carapace), broad at the back and narrow at the front. Strong tapering spines border the shell, while the rest of the shell bears smaller spines. The small eyes are either side of two distinct frontal spines. The legs are covered by hairs. The first pair of legs bear small claws while the rest taper to stout, dark tips. The shell is usually covered with algae.