Writing -- Ireland's Wildlife: Shore Crab

The following article is one of a weekly series on Ireland's wildlife commissionedfor the popular national general interest magazine, Ireland's Own. It appeared in the 15 August 2003 issue.

All sorts of weird and wonderful creatures frequent the pools left behind by the receding tide. As children many of us will have whiled away hours at the seaside exploring these miniature havens for marine wildlife.

One of the stars of those early seashore expeditions was inevitably the shore crab. Its curious sideways gait is unmistakeable as it scuttles for cover beneath rocks or seaweed at the first sign of disturbance. Shore crabs are one of the more active and common seashore animals, and are a firm favourite with children exploring the seaside.

The shore crab, or European green crab as it is sometimes called, is the most common of Ireland's crab species. Turn over a rock or piece of seaweed anywhere around the Irish coast and you'll often find a crab lurking beneath. They even make themselves at home on man-made piers or jetties.

It is a medium sized crab with a "shell", or carapace, that is broader than it is long. The front of the carapace is serrated, with three teeth between the prominent stalked eyes and five teeth to either side. On the shore you will find them in all sizes from tiny specimens only just visible to the naked eye, right through to the largest adults. The crabs generally attain a size of about 6 cm (2.5 inches) from the back to the front of the carapace and 10cm (4inches) wide (across the carapace).

Like all crustaceans crabs have to moult as they grow. Their hard inflexible external skeleton (exoskeleton) must be shed and the new one beneath allowed to harden. Crabs are particularly vulnerable during this moult and tend to stay well hidden until their new exoskeleton is well hardened.

Adult shore crabs are usually mottled brown colour, although around 5% of animals are bright green and orange specimens are sometimes found. Juveniles exhibit a mixture of colours on the carapace, including, but not limited to, yellows, reds, greens, browns and black.

Shore crabs are members of a group of crustaceans known as decapods - which includes crabs, prawns and lobsters. In common with other animals in the group they have ten jointed legs. The front pair are powerful claws, adapted for catching and crushing the crab's prey: worms, small molluscs, small crabs, algae, carrion and almost anything else it can catch and subdue.

Larger specimens have to be handled with caution, as the claws - quite capable of snapping a pencil in two - can give careless fingers a nasty nip.

Crabs have a curved tail, sometimes called the "purse" that is held underneath the body. In females this is larger, and is used as a holding pouch for her eggs until they are about to hatch. They are then released into the water, to hatch as free swimming crab larvae.

The larvae join the plankton - a living soup of tiny plants and animals that exists in the surface layers of the sea. Many will fall prey to other animals during their time drifting in the plankton, but after a few months those that survive will drop to the sea bed or be washed up on the shore as tiny crabs.

Shore crabs can tolerate brackish water, and can be found some way up estuaries. Large numbers of small crabs can be found on the muddy sand and mudflats of estuaries during the summer months. In winter, however, colder weather reduces the crabs' ability to tolerate freshwater, and they retreat to the saltier environs of the coast.