They are blueish grey in colour with a flattened body and nippers.Ther are not a true crab as they have 3 pairs of walking legs instead of 4 pairs.
They are found under rocks in the low tide zone.
Half crabs feed by casting out net-like mouth parts and sieve plankton and detritus from the water. They are predominantly filter feeders but will scavenge at times of no plankton.
Half crabs will readily shed nippers and legs as an escape response - like a lizard drops it's tail.
Between the Tides : Bradstock