Aquatic gill-breathing Arthropods

Photograph by Evan Jones (Shoreham and Pevensey)
Decapoda Crustacea
Why Do Crabs Walk Sideways? Hermit Crabs.  Link to Intertidal British Crabs.
Crustacea TaxaMoulting Cycle. Brachyura (True Crabs). Shrimps and Prawns


Lobster Claw (deformed)

Deformed claw of the European Lobster

This appendage with a extra claw was found on a lobster caught by John Barker (Shoreham)


Hermit Crabs information file for children
Hermit Crabs for the younger student (NE Atlantic species only)
Intertidal Crabs: British Coast

Why Do Crabs Walk Sideways?

Because that's the way their legs bend. Muscles work in pairs. A muscle can only retract, or pull; to lengthen again it must relax and be pulled back by another 'antagonistic' muscle. The muscle blocks in crabs are attached to the inner surface of the exoskeleton, including the ten tubular legs, including the legs with claws, or chelae, as well as other appendages. Crabs do not have ball-and-socket joints but the legs pivot at numerous peg-in-socket joints that are sealed by flexible chitin, and can move in one plane (similar to our knee). Each joint moves in a different plane, and so together they allow the crab to move in all directions like our shoulder and hip joints. However, many crabs have joints in their legs moving in a restricted number of planes so that they can only move sideways. The sharp ends of each leg grip on to surfaces and can grip on to tiny irregularities (watch a Hermit Crab climbing up an almost smooth rock). In some crabs, the rear legs are shaped like paddles for limited swimming.
Many crabs like the Shore Crab or the Velvet Swmming Crab, Necora puber, (above) need to scamper sideways to avoid the legs getting tangled up with each other.

exoskeleton = external skeleton (see the above article for details of moulting, ecdysis).

Water is a very dense medium, about 830 times as dense as air, and has a viscosity about 60 times that of air. This means that marine life does not require the supporting skeletons of land dwellers; the large spider crabs will collapse out of the water. Water is more difficult to move through, and marine creatures have evolved shapes to minimise the resistance. This can be seen in the flattened bodies of many common crabs.

by Andy Horton and Jane Lilley 1997.
At least 67 species of true Brachyuran crab inhabit the seas surrounding the British Isles. They vary in size from the Giant Box Crab, Paramola cuvieri, which is a rare deep water species, to the inquiline Pea Crab, Pinnotheres spp. Crabs are measured scientifically across the carapace from between the eyes to where the rear legs join.

Hermit Crabs for the younger reader
How to tell the age of a crab (when dead) New Scientist  Research
Hyperia galba
Intertidal Crabs
Lobster's Meal Time (BMLSS Scotland)
Lobster Notes
Mantis Shrimps (British)
Spider Crab, Maja squinado
Spider Crab, Maja squinado (Shoreham)
Shore Crab
Shrimps and Prawns


Hermit Crabs (British Seas)

Hermit Crab

Hermit Crab, Pagurus bernhardus, with sea anemone, Calliactis parasitica.

Photograph by Andy Horton.
All rocky shores around the British coast are occupied by numerous marine snails, called gastropod molluscs. The Common Periwinkle, Littorina littorea, is the most widespread and usually the most numerous. They are usually black in colour, but when they are found high up on the shore they can be bleached grey by the sun. Periwinkles are about 24 mm high, from the apex (pointed end) to the aperture (opening). They inhabit groynes and rock surfaces and are also found under rocks on all zones on the shore.


Occasionally they will fall in sandy pools and will slide along leaving a furrow until they make their way onto hard surfaces on which they settle when the tide goes out, feeding on microalgae when the tide comes in. But not every snail shell moving across a pool is a live mollusc. A careful eye will spot that the snail is not moving in its normal manner. Picking up the snail, the rockpooler may be pleasantly surprised to discover two tiny orange claws sticking out of the opening: a small Hermit Crab has taken up residence in the old shell.

Young Hermit Crabs

Juvenile Hermit Crabs are common on rocky shores in all months of the year and there were many reports of these fascinating crustaceans in January 1997 when on many shores the fauna was exceptionally sparse. They were seen on the shore immediately following the period of exceptionally cold weather, but this probably reflects the human presence between the tides. Of the 15 or so species recorded regularly in British seas, the species known as the Common Hermit Crab, Pagurus bernhardus, is by far the commonest. If the major claw is on the right side nearest the apex, you can almost certainly assume that a Hermit Crab found between the tides is this species. All the others are so unusual as to be worth a mention on the web pages.


Continual fights and squabbles are the result of keeping several Hermit Crabs in the same aquarium. This can be explained by the need for the crab to protect its soft abdomen with a borrowed shell. The crab adopts a univalve (snail-like) mollusc shell, which it carries around on its back throughout its life. Fights occur when the crabs dispute the available shells.

Mating and Moulting

Like all decapod (ten-legged) crustaceans, Hermit Crabs mate, and afterwards the female carries the eggs underneath her coiled abdomen for several months when she is are said to be 'in berry'. In the Common Hermit Crab, Pagurus bernhardus, the eggs hatch into larvae in the first two months of the year. At this hazardous stage they will undergo moults like all crustaceans.

Planktonic Stages

The first planktonic stages are called zoea. Later when they develop claws (chelae) they are termed megalopa, before they settle on the sea floor and and search around for a gastropod shell to inhabit. Hermit Crabs are widespread on different demersal habitats, and the choice of shell depends on what is available. Pheasant Shells, Tricolia pullus, and Little Netted Dogwhelk Shells, Hinia incrassata, are a popular early choice on mid-Sussex shores. These shells can be less than 10 mm high, and as Hermit Crabs continue to moult and grow like other crabs, they need to embark on a constant quest for new and larger shells to inhabit.

Gastropod Shells

In most cases they will use empty shells of dead snails, but squabbles for possession of occupied shells occur frequently in the wild. A Hermit Crab dispossessed of its home by a more aggressive crab of the same species is vulnerable to attack by its many enemies. It is fun to discover exactly what species of gastropod mollusc the inhabited shell used to belong to. In Sussex, the shells of the Periwinkle, as well as the Grey Topshell, Gibbula cineraria, and the Common Netted Dogwhelk, Hinia reticulata, are often occupied. Common Hermit Crabs will eventually grow large enough to occupy the commodius shelter of the Common Whelk, Buccinum undatum, This is the largest gastropod found in the shallow seas around the British Isles.

Commensal Associations

One of the most fascinating aspects of Hermit Crab behaviour is their association with other animals. Acorn Barnacles, tubeworms and hydroids will frequently settle on the outside of the shell. And one species of hydroid, Hydractinia echinata, is found on their shells and nowhere else.

Small Hermit Crab with a shell adorned with the hydroid, Hydractinia echinata, although the furry appearance of the hydroid is not very clear in this photograph.

A ragworm, Nereis fucata, even resides in the spiral end of the larger whelk shells, poking its head out to feed on the fragments of the crab's meal. However, the most spectacular of the commensal relationships is the sharing of the shell with a species of sea anemone. One anemone species that is found on large Hermit Crabs in the south and west only is the brownish Calliactis parasitica (pic). The sea anemone is the active partner in the relationship with this species of Hermit Crab. Another species of Hermit Crab called Pagurus prideauxi shares a mutual relationship with the Cloak Anemone, Adamsia palliata.


Hermit Crabs are omnivorous scavengers, picking up scraps from the surface of rocks and shredding larger items. They also filter feed, extracting living plankton from the sea on fine setae and other appendages.


Hermit Crabs are classified in the infraorder Anomura of the order Decapoda in the subphylum Crustacea of the phylum Arthropoda. They are not true crabs like the Brachyura and can be differentiated by the reduced rear pair of legs, which can be seen when the Hermit Crab changes shells.


Edible Crab Video
EMail for Shorewatch Reports of shells occupied by Hermit Crabs
Hermit Crab  Pagurus bernhardus Database
Shorewatch Scheme

Crustacean Photograph Portfolio 
(Marine Wildlife of the North-east Atlantic Ocean Group)

Crab Photographs
Crustacean EBiota of Cetaceans (notes only)
Crustacea-L  List
Decapod Crustacea DataBase (under construction)
Lepeophtheirus nordmanni (copepod)
Lobster's Meal Time (BMLSS Scotland)
Mysid Culture
Publications (International)

Prawns and Shrimps

Bilge Bugs (Sea Slater); a wood louse (Isopoda)
Crustacean Society

Stomatopods (Lurker's Guide) (External)

Spanish Crabs

Taxa of Crustacea
          Crustaceans are subdivided into the following groups: 

                   CLASS                                   ORDER

          1.  Branchiopoda    Cladocera - waterfleas, daphnia. 
                                              Anostraca - fairy shrimps including Artemia.
          2.  Branchiura  -  parasitic louse  e.g. Argulus
          3.  Cirripedia  -   barnacles. 
          4.  Ostracoda  -  small mussel shrimps 
          5.  Copepoda  -  planktonts 
          6.  Malacostraca .         Stomatopoda -  mantis shrimps 
                                Mysidacea -     opossum (mysid) shrimps 
                                              Cumacea -       small sand-dwellers 
                                              Tanaidacea -    small benthic dwellers 
                                              Isopoda -         wood louse types 
                                              Amphipoda -    small flattened types 'sandhoppers' 
                                              Euphausiacea -  krill 
                                Decapoda -      prawns, crabs, lobsters etc. 

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