Crabs are very familiar denizens of any coastal area. Some form the basis for very important food fisheries, while others are notorious for invading the homes of less robust species, while others still are quiet occupants of the shore, neither commercially viable or invasive. The most common crab species seen on British shorelines include the shore crab, the edible crab, the common spider crab, the velvet swimming crab, and the masked crab.
The shore crab, or Carcinus maenas, is a mottled brown-to-green crab with five spines going down either side of the carapace behind the eyes. It is a medium-sized crab, with adults being from 2.5 inches to 4 inches in length. The shore crab is considered an invasive species, due to its ability to withstand a very wide range of environmental conditions, and its wide and varied appetite. They are a very large threat to the British shellfish harvesting bodies, as they devour nearly any mollusk they have access to. The shore crab's diet includes worms, mollusks, carcasses, detritus, and anything else it can get its claws on.
The edible crab, or Cancer pagurus, is a heavyset reddish-brown crab, easily identifiable by the “piecrust” edge of its carapace, as well as the dark black tips of each claw. As the name suggests, the harvest of this crab species makes up the bulk of the crab fishery of Western Europe. This crab abounds in all areas of Britain's shore. The edible crab's diet is mainly smaller crab species and mollusks. This crab is an active predator, and can be seen digging deeply to find buried bivalves.
The common spider crab, or Maja squinado, is a very large migratory crab, distinctive for its red, spiny shell. They, too, are fished for their meat, but not with the volume that the edible crab is harvested. The common spider crab can be seen eating sea urchins and sea cucumbers in the summer, and seaweeds and molluscs in the winter. This varied diet results from their migratory nature.
The velvet swimming crab, or Necora puber, is a fast-swimming blue crab found all along the shoreline. Its mainly blue carapace is dotted with small red prominences. The crab's eyes are a brilliant red, giving it another name: the devil crab. The velvet swimming crab is not important commercially. The velvet swimming crab's diet is anything that it can attack, often other crustaceans. It also scavenges for food.
The masked crab, or Corystes cassivelaunus, is a small burrowing crab. It gets its name from the vague shape of its carapace: a human face. The crab has two stiff antennae, that it uses as a breathing tube when it burrows deeply into marine sand. This species is also commercially unimportant. Its diet mainly consists of marine worms and bivalves.
These are the most commonly found crabs on the British shorelines. Some are more important to humans than others, but all serve important roles (positive or negative) in their ecosystems.