The king crab, or horseshoe crab, is not really a crab at all. In fact it is not even a crustacean but a very primitive arthropod which belongs to a group of its own. From fossil evidence we know that king crabs were abundant in seas about 175 millions years ago. The few species that survive today are virtually unchanged from those early ancestors and for this reason they are sometimes called ‘living fossils’.
King crabs have some unusual features. They are about a foot across, have a heavy domes shell divided by a joint across the middle, four pairs of walking legs and a long spiky tail. The bony mouth extends between the bases of the legs which help in chewing up the food. These characteristics, together with their gills and mouthparts, tell us that the king crab is actually descended from the forerunners of another group of arthropods adapted for life on land – the arachnids. These include the scorpions, spiders, mites and ticks, and daddy long legs or harvestmen.