Crabs

 

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Several crab species are commonly found on Penwith’s shores. Their bodies come in different shapes and sizes. This is because crabs have evolved differently to fit in with different environmental niches.

Crabs are one of the most distinctive animals found on the shore. They have eight legs two claws, two pairs of antennae and eyes on stalks.

 

The crab’s main defence is a hard shell that covers the whole body (the exoskeleton). This hard shell is inflexible and so for the crab to grow it has to periodically shed it. This process of moulting is known as ecdysis. This is a time of danger for the crab, as its shell is now soft and requires time to harden.

 

Crab claw

 

 

 

There are crabs that do not rely on a hard carapace to protect them. Hermit crabs instead use discarded gastropod shells such as the periwinkle..

 

Look closely at crab bodies found at on the shoreline. More often than not they are not the remains of a dead crab but just the empty shell after moulting.

Movement with crabs is with a sideways gait, plus many species can also swim using their back pair of limbs. These limbs have evolved flat paddle shaped tips.

 

Flattened paddle on rear pair of legs

 

 

 

Autotomy

Crabs are often seen with legs missing, this is due to their defence strategy of purposely shedding a leg or claw to a predator. This strategy is known as autotomy.

An autotomiser muscle suddenly contracts bending the limb backwards until it breaks off at a predetermined weakened spot close to the carapace. The gaping hole becomes covered with a protective membrane. This can distract a predator long enough for the crab to escape or the predator may be happy with this morsel. The limb is gradually replaced over successive moults, this is why crabs are commonly seen with a small leg or claw.  

 

Reproduction

Often you can find two crabs clasped together. Before breeding the male has to hold onto the female until she reaches a favourable breeding condition which is after she has moulted and while her shell is still soft. This clasping can persist until the female’s shell hardens, the male protecting the female from predators while she is in this unprotected state.

 

All the crabs found in Cornwall reproduce via laying eggs. Females carry their eggs underneath their shell. Such crabs are said to be in berry.

The eggs hatch into planktonic forms zoea, nothing like the adult form.

These zoea are carried by the currents until they develop into a miniature adult form, when they settle.

 

Crab in berry.

 

 

 

Crab Species Found on Cornish Shores 

 

 

 

  Velvet Swimming Crab Necora puber

  Shore Crab Carcinus maenas

  Common Spider Crab Maja squinado

  Edible Crab Cancer pagurus

  Long Clawed Porcelain Crab Porcellana longicornis

  Hermit Crab Paguridae

  Broad Clawed Porcelain Crab Porcellana platycheles

  Furrowed Crab Xantho incisus

  Masked Crab Corystes cassivelaunus 

  Furrowed Crab Xantho pilipes

 

 

 

Shore Crab Carcinus maenas

It is a small green crab, growing up to 3 inches across the shell. The colour can be variable including browns. The carapace has five sharp serrations on either side of its eyes.

This is the most common crab by far found on local beaches. This species is an opportunist and can live in a wide variety of environments. It can even live in almost freshwater conditions, being found high up estuaries.

 

Shore Crab Carcinus maenas

 

 

 

Edible Crab Cancer pagurus

This is the Cornish crab species of most economic value to man and is heavy fished using crab pots. It is a large crab reaching over 10 pounds, but specimens found on the shore are considerably smaller.

Found under rocks on the lower shore. It can easily be picked up, as it will not try to escape. The edge of the carapace is similar to that of a crimped pasty. The pincers are black tipped and very heavy set.

 

Edible Crab Cancer pagurus

 

 

 

Common Spider Crab Maja squinado

This is one of the easiest crabs to identify, with its spiney carapace. It is highly sought after for its meat, many being caught in this area. The majority are exported to the continent, and not often seen here on the fishmongers' slab.

They can grow to a large size, the shell up to 8 inches across. Can be found at the lowest tides, particularly during the breeding season, when they congregate in large numbers. Live specimens can be found on the lower shore, but more common are the moulted carapaces thrown ashore.

 

Common Spider Crab Maja squinado

 

 

 

Velvet Swimming Crab Necora puber

This is a common crab that can be easily identified by its bright red eyes and bright blue banding on the legs. There are short dark hairs on the body hence its name velvet.

It is a swimming crab and processes flattened paddles on the rear pair of legs. The crab is very aggressive with strong pincers so caution is required when handling.

 

Flattened paddle on rear pair of legs

 

Velvet Swimming Crab Necora puber

 

 

 

Masked Crab Corystes cassivelaunus  

The masked crab is particularly distinctive, as it possesses long hairy antennae. It has an elongated shell about an inch long. The carapace has two short sharp projections on either side of the eyes. The front limbs with the pincers are longer than its body, this is even more pronounced with the males.

 

Shell of a masked crab, the dimensions of the antenna represented by the dotted lines.

 

 

 

This is a common crab but is rarely seen as it burrows in sand on the lower shore tide mark. The antenna is used as a tube to feed sea water to its gills and allows the crab’s body to be completely buried, with only the tip of the antenna above the sand. If one digs out a masked crab and then places it back on the sand, the crab will rebury, doing so while standing erect.

 

Hermit Crab Paguridae

Hermit crabs are not true crabs but are more closely related to the lobsters. These crabs have no protective hard carapace on their hind parts. Instead they use a discarded gastropod shell for their protection and home.

 

Hermit crabs are not true crabs but are more closely related to the lobsters. These crabs have no protective hard carapace on their hind parts. Instead they use a discarded gastropod shell for their protection and home.

Using an others shell has proved to be an effective strategy, as hermit crabs are very common on Cornish shores. However hermit crabs have had to considerably evolve to fit their shells. As the body follows the shell’s coil it has become asymmetrical. One claw is closer to the shell’s aperture, this claw the right one has become larger and is used to close off the shell’s aperture when the hermit crab retreats into the shell. The hind body has become flexible losing a carapace allowing it to coil within the shell.

There are sacrifices with such a life strategy. A heavy shell plus two pairs of legs having being adapted to grip the shell, leaves only two pairs for walking thus reducing speed and agility.

As the hermit crab grows, its shell needs to be replaced with a larger one. This is a time of danger as the hermit crab has to leave its protection. Selection of a shell takes time, hermit crabs are picky, closely investigating each until a suitable one is found.

 

Hermit Crab Moving

Hermit Crab Shell Overturned 

 

 

 

Associations

Hermit crabs have a variety of associations with other marine creatures. To what degree this arrangement advantages each animal involved depends on the specific association.

There is the sulphur sponge Suberites domuncula which grows on mollusc shells inhabited by hermit crabs. The sponge may eventually eat away the shell leaving the hermit crab completely dependant on the sponge for protection. This is a mutualistic or symbiotic association, with both sides gaining. The sessile sponge is now mobile being carried by the crab. The crab is camouflaged plus there is the additional benefit of the sponge’s unpleasant taste and smell which deters predators.

A ragworm Nereis fucata commonly lives in the shell with the hermit crab. In this case the ragworm benefits, as when the hermit crab finds food, the worm also feeds, plus it has the protection of the shell. It is unclear how the hermit benefits from this association but the relationship is not detrimental. Such a relationship is commensal. Another common commensal association is that with the hydroid Hydractinia echinata. The hydroid appears as furry growths on the hermit crab’s shell home.

There is a parasitic association of that with the Peltogaster paguri a parasitic barnacle. This association benefits one partner but is detrimental to another. The barnacle is rarely seen as it lives on the abdomens of hermit crabs situated within the shells.

The most well known associations are those between hermit crabs and sea anemones. Unfortunately the majority of the hermit crabs seen on the shore do not carry sea anemones, it is usually the more mature adults, which may be seen at the lowest tides in the sublittoral zone but are mainly in shallow water.

Both the parties benefit from these associations. The hermit crab is protected by the stinging tentacles of the sea anemone. The sea anemone derives feeding opportunities by catching fragments of food produced as the hermit crab eats, and mobility. The common hermit crab is often associated with the sea anemone Calliactis parasitica, the name is a misnomer as it is not parasitic. Pagurus prideauxi lives in association with Adamsia palliate.  

The most common species in Cornwall is the Common Hermit Crab Eupagurus bernhardus. They are commonly seen moving in rock pools, and at first glance will be taken as a mollusc. With closer observation, their gait gives them away. They can reach up to four inches in length, at this stage only a large whelk shell will suffice. Large specimens are not normally found on the shore as they tend to move into shallow water.

A not so common species, Pagurus prideauxi, also occurs in Cornwall. Not often seen on the shore, as it is a sublittoral species, living on sandy bottoms. It does not grow as large as the common hermit crab, only reaching two and a half inches in length.

 

Broad Clawed Porcelain Crab  Porcellana platycheles  

A common small rounded flatish crab under half an inch across.

The crab is hairy and can be easily identified by its relatively large broad flattened claws.

These hairs trap mud and sediment providing camourflage.

 

Broad Clawed Porcelain Crab  Porcellana

 

 

 

Long Clawed Porcelain Crab  Porcellana longicornis

A common small crab under a half an inch across.

It is usually deep brown, with a round shell and claws much longer than the rest of its body.

 

Long Clawed Porcelain Crab  Porcellana

 

 

 

Furrowed Crab Xantho incisus

A relatively common crab in this part of Cornwall. The first thing you notice when you find this crab, is that it stretches out it claws. These outstretched large claws make the crab appear to have instantly grown in size.

As the name implies the carapace is furrowed with eight teeth like projections on its edge. The shore life guides that I have come across show and describe it as brown in colouring with black tipped claws.

Those I have found come in a range of colouring and markings. Many with wholly brown claws, some with patterned shells.

 

Furrowed Crab Xantho incisus with patterned shell

 

Furrowed Crab Xantho incisus as described in Field Guides.

 

 

 

Furrowed Crab Xantho pilipes

A second furrowed crab commonly found in this part of Cornwall. The field guides give it a description of a brown carapace with a darker brown tipped claws. This is not the case in the most of the finds I have made, as shown by that pictured here.

Xantho pilipes will only reach half the size of Xantho incisus. The carapace is furrowed with eight teeth like projections on its edge. It can be distinguished from Xantho incisus by bristles on the legs.

 

Furrowed Crab Xantho pilipes with patterned shell

 

 

 

 

 

 

 

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