Why do crabs walk sideways?

Because that's the way their legs bend. This lets them squeeze into holes and crevices to keep out of the way of fish with sharp teeth and larger crabs that might think the crab was a tasty meal.

Seashore

One of the first animals that you are likely to find on a visit to the coast is the crab. They can be found in many different shapes and sizes, scurrying across the sand at the approach of human feet. Look for them under stones and rocks and pieces of seaweed.

All crabs have ten legs, two of which have developed into very large and powerful claws, used for grasping their food and fighting each other. Crabs are very greedy and quarrelsome. They never seem to be satisfied with what they have. Even with a juicy worm in its crusty jaws, a large crab will try and steal food from another crab. In the numerous fights, they often lose one of their claws.

Crabs have two eyes perched on stalks that stick out from the hard shell, and a pair of small feelers between them. They are very common both on the shore and in the sea, with different species living in slightly different places and ways.

Moulting

Crabs belong to a group of animals known as the Crustacea. The soft body is protected by a hard outside shell known as a exoskeleton. Unfortunately, this hard shell does not grow.

Instead, a soft shell grows inside the crab. Eventually, the crab grows too big and it has to shed its old hard shell. This process is called 'moulting' and it is a very dangerous time for the crab. The new shell is larger than the old one, but it is soft and takes time to harden.

While it remains soft, the crab tries to stay hidden so that it is not attacked by hungry fish and other animals. During the moult, the crab can grow back a lost claw, but it will be smaller than the one that was broken off.

The old discarded shell looks like a dead crab, but if you look closely you will find there are holes where the eyes should be. It is hollow inside and it will sometimes float to the surface where it may be washed ashore.

Reproduction

Female crabs often carry a clump of eggs on the underside of their body. They are cared for for several months before the eggs hatch into tiny swimming crab larvae which are released to take their chances in the sea. The larvae join the plankton, which is the masses of small plants and animals that live and grow in the surface waters of the sea.

After a few months, the larvae that have survived fall to sea bottom and turn into tiny adult crabs. In the summer hundreds can be found on rocky shores and in estuaries.

CRABS FOUND ON THE SHORE

Shore Crab Carcinus maenas

This is the crab that the visitor is most likely to see on the shore. They will be found in all sizes from tiny crabs just about visible to the naked eye to hand-sized crabs that look quite dangerous with their big claws and can injure anyone who gets his, or her, fingers in the way.

Most of the crabs are a mottled brown colour which makes them difficult to see amongst the seaweeds. However, about one in every twenty crabs is a bright green colour, and the small crabs have lots of different patterns. Orange crabs are also found.

Edible Crab Cancer pagurus

This is the red crab that fishmonger's sell for eating. The crab has a wide oval shell with a pie-crust edge. This crab will crouch under rocks, buried completely or with the top of their shell just visible. They are found low down on the shore near the sea. You are not allowed to collect them, as the crab must be allowed to grow big enough for eating.

The claws of the Edible Crab are very strong, so it can crush open a mussel or a sea urchin.

Hairy Crab   Pilumnus hirtellus

This small crab is the same colour is the Edible Crab but if you look closely, you can quickly tell the crabs apart because the Hairy Crab has hairs all over its legs and shell. Also, its two claws are always different sizes.

The Hairy Crab can be found under rocks and hiding in holes on many rocky shores.

Velvet Swimming Crab    Necora puber

This is an attractive crab with bright red eyes with a covering of very fine hair on the shell that gives it a velvety texture. Beware, if you get that close or dip your hands into a pool without looking, this crab can give you a nip that will bring tears to your eyes.

Usually only small crabs are found on the shore, but you will sometimes find one too big to lift up by a single hand. If you dare to pick it up, you must carefully grab the rear of the shell away from the claws. It looks a bit look like the Shore Crab. If you look closely you should be able to see that the rear legs are flattened so that it can swim a bit. In the Shore Crab, the rear legs are pointed like all the other legs for gripping on to rocks.

Velvet Swimming Crabs will attack and eat smaller crabs.

The Sandy Swimming Crab, Liocarcinus depurator, is a sandy or orange coloured swimming crab that is found over sandy ground, in the shallows just below low tide mark. Another swimming crab Liocarcinus vernalis can be found in shallow water and on the shore on the very low spring tides.

Masked Crab Corystes cassivelaunus

Not all crabs walk sideways. The Masked Crab burrows backwards into the sand to hide.

The two antennae between its eyes are modified into a tube to inhale sea water for it to breathe.

Pea Crab   Pinnotheres sp.

This crab, as its name suggests, has a shell the size of a pea. Or, at least the brown male Pea Crab has. This is the crab that can be seen swimming in estuaries over mussel beds. It the most active swimmer of all the British crabs.

The female is quite different. She is almost twice the size of the male and usually yellow with a bright red blob on its soft shell. She is hardly able to crawl and cannot swim.
 

Short-legged Spider Crab Eurynome aspera

Not all the crabs look the same. The spider crabs are a family of crabs that are roughly triangular in shape and have eight long legs as well as the two claws which are not quite as powerful as the other crabs. The claws are more like tools for extracting morsels of food out of small places.
 

 

 
 
 
 
 

The small spider crab Eurynome aspera has a fascination with all sea anemones

All the spider crabs have a tendency to decorate their shells with scraps of food to disguise their appearance and fool fishes and other predators. The Short Legged Spider Crab only grows to about 25 mm in shell length.

Pisa

It is not easy to spot and is often mistaken for a clump of weed. There are two, possibly three, British species. Pisa armata is rarely found between the tides. The specimen below was discovered and photographed by Robert Jones of Trowbridge.

Pisa armata  is the other species.

Long-legged Spider Crab   Macropodia rostrata

As its name suggests this crab has very long legs. It has a very triangular shell and it looks like a Daddy Long-legs.
 

In Blanes (on the Mediterranean coast of Spain), M. rostrata is a specialist decorator that uses virtually nothing but Dictyota linearis.  (Refs.)

20 August 2001
A small badly damaged specimen of the Scorpion Spider Crab, Inachus phalangium, was my first confirmed record from the Sussex shore, although it is frequently discovered in shallow water offshore. This is a small species of long-legged crab that often associates with sea anemones. (TQ 235 047)
 

Spiny Spider Crab Maja squinado

This is a very large crab with a very spiny shell with lots of bumps and sharp bits. It grows quickly to 18 cm in length from between its two black eyes to its curved rear end.
 

It is much too big a crab to live on the shore all the year round, but in June are regular visitors at low tide on southern (English Channel) coasts, occasionally smaller specimens get washed ashore at other times of the year, or even larger ones thrown out by fishermen.

Nowadays, this crab is sometimes sold by fishmongers. Only the meat inside the claws is eaten.

Other species of crab are found on the shore and in the shallows off the west coast of Britain.

One of these is called the Furrowed Crab, Xantho incisus. When disturbed this crab stretches its two large claws out wide.

Other Crab-like Animals

There are many crab-like animals to be found on the shore. The common species are:

Hairy Porcelain Crab Porcellana platycheles

This is not a true crab, but with its flattened body clamped to the underside of a rock, it looks like one. It is small; you could hide underneath a 10 pence coin. The shell is covered in hair and the two large claws contain a fringe of hair.

If you look carefully, this animal has only eight visible legs, including the claws.

Long-clawed Porcelain Crab Pisidia longicornis
 

Common Hermit Crab Pagurus bernhardus

Pick up a sea snail crawling along the floor of rock pool and look carefully and you may spot the two tiny claws. This means that a Hermit Crab has taken up residence inside the shell.

The shell is a useful protection for the soft coiled end of its body.

The largest of the two claws is on the right one on the left when the crab is seen from the front.

Hermit Crabs are common between the tides on rocky shores. Large Hermit Crabs occupy the large shell of a whelk. A ragworm may live inside the shell as well and can be seen poking its head out of the shell when the crab feeds. Other animals like barnacles and even a sea anemone can sometimes be found living on the outside of the shell.

At least 15 species of hermit crabs are found in the seas around Britain. One species which I will call the South-claw Hermit Crab, Diogenes pugilator, has the largest claw as the left one, and is sometimes found on the sandy shallows, partly buried in the sediment, in the south-west.

Squat Lobster Galathea squamifera

This is not a true crab or a true lobster. It looks like a crab with claws outstretched like a lobster. It lives in holes and under rocks and jumps backwards like a prawn when disturbed.

It will fit easily on the palm of your hand.

Occasionally, a true European Lobster, Homarus gammarus, can be found hiding under a large boulder on the shore or in the deeper pools.

References:
Key to the Crabs and Crab-like Animals of British Inshore waters by John and Marilyn Crothers, AIDGAP Guide published by the Field Studies Council, and Richmond Publishing Company. ISBN 1 85153 155 5.
Decapod Crustacea DataBase (under construction)

Further Information:
Spanish Crabs