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Rocky Shores | Rocky Shore Creatures

Common Shore Crab (Carcinus maenas)

Carcinus

Carcinus has five points on each side of the carapace, with the stalked eyes in between. The carapace, which covers the body is textured and has a variety of colours and patterns. It partly depends where the animal is living as the colour can reflect the habitat. Those living amongst sand can be very pale. It may be up to 8 cm across.

Like most crabs they are carnivores and scavengers. They are very mobile and can avoid some of the environmental stresses by sheltering under large algae and stones. Carcinus is very adaptable and copes with varying salinity and temperature. In fact it is the only common crab in Europe able to osmoregulate (control the body's water content) effectively. It can osmoregulate in dilute seawater, such as the conditions found in estuaries, but in freshwater it will soon die. It osmoregulates with antennal glands, small bladder like organs at the base of the antennae. This allows the crab to penetrate far up the estuary, living in concentrations of only 0.6% salinity.

Crabs have separate sexes. The abdomen is a mere flap under the body and in males this is narrow and of 5 visible segments. The female has a wider abdomen and is made up of 7 segments. The sexes mate when the female moults and after mating the female lays up to 200,000 eggs which she the carries underneath the abdomen. A larval stage hatches called a zooea and this then lives in the plankton as a carnivore. After going through a series of moults (ecdysis) and different stages over several months, the larva settles out of the plankton on to the shore. The chances of individual survival in the plankton is minimal and hence the large number of eggs produced. It may take 4 years to reach maximum size.

This crab shows a seasonal movement; in summer it is common high on the shore moving down into deeper water to survive the cold winters. In estuarine crabs the females, which are carrying eggs, move to the entrance to the estuary as hatching larvae, released here into seawater, will have a higher chance of reaching the plankton in the open sea.

Air can be bubbled through the gills allowing respiration during emersion (out of the water). Varied colour variation is lost as crabs mature; green crabs are actively growing adults whereas reds are large mature adults moulting infrequently.

Carcinus maenas in beri

Carcinus maenas in beri (female carrying eggs)

This crab is a very important carnivore of the rocky shore as it will eat most animals especially molluscs like periwinkles, topshells and dogwhelks. Holding them in one claw it snaps off the edge of the shell to get to the softer parts. Dogwhelks found in sheltered areas (where Carcinus is most abundant) tend to have thicker shells as a possible adaptation against being eaten. Carcinus can be parasitised by a barnacle called Sacculina.

A parasitised crab

A parasitised crab. See Sacculina

The Common Shore Crab is common on all shores and because of its adaptability it can be found in almost any zone. Although abundant on rocky shores it does have some competition with other crabs. In estuaries it is the only crab species and can be very common. Found throughout the western coast of Europe, including the Baltic. In the Mediterranean the shore crab, Carcinus mediterraneus, takes over. It looks almost identical to C. maenas and some consider it to be the same species.

 

A parasite called Sacculina may be found on the underside of the abdomen in mid-summer.


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