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Crustaceans have many representatives in the
sea, and the
rocky shore is no exception. Crabs, lobster, prawns, shrimps and barnacles all
belong to this group. Despite their wide variety in form all these animals are
covered by an external skeleton which is hardened by calcium salts. Typically,
they have five pairs of jointed legs, which may be modified for various
functions, and two pairs of antennae or feelers. Most crustaceans also have an
additional cover for protection called a carapace. As crustaceans grow they have
to shed regularly their hard outer skin to allow themselves to expand before
forming a new hard skin. This is called moulting.
Hermit crab Pagarus bernhardus (Linnaeus)
The hermit crab can be found living inside discarded
shells.
The shell is used to protect its soft back end. The crab can withdraw into the
shell when threatened and block the entrance with its large right claw. As the
crab gets bigger it must find a larger shell. It takes much care and time to
select shells. Young crabs use the shells of smaller molluscs such as winkles
while the adults use the larger whelk shells.
The commensal anemone, Calliactis parasitica, frequently attaches itself to the
shells of large hermit crabs. The anemone and the crab are believed to have a
friendship in which one benefits from being with the other. The stinging
tentacles of the anemone provide protection for the crab from predators, such as
the octopus. The crab, meanwhile, is a messy feeder and the anemone feeds on
bits of food that pass over the crabs. Hermit crabs have a varied diet, preying
on other animals, scavenging on sea-bed deposits and filtering food from the
surrounding water.
 Edible Crab Cancer
pagurus (Linnaeus)
Small specimens of the edible crab may be
found on the shore, although big specimens are usually found in deeper water.
They are often found in rock crevices where they can easily defend themselves
using their massive claws.
Like the shore crab (Carcinus maenus), the edible crab has to wait until the
female has moulted before he can mate with her. In winter, male crabs are
sometimes observed holding on to female crabs waiting for them to moult. The
males will even help the female to come out of her old shell. The female carries
her eggs for seven to eight months under a flap formed by her lower body. Large
females may carry as many as two million eggs, although few survive. When the
larvae hatch out, in spring and summer, they are dispersed in the currents where
many larvae are eaten or die before settling on the sea bed. Edible crabs do not
reach maturity until they are five or six years old.
Edible crabs are scavengers but they also feed on molluscs which they crush with
their large claws. These crabs are often caught around the south coast using
baited pots.
 Velvet swimming crab
Necora puber (Linnaerus)
Velvet swimming crab
Necora puber (Linnaerus)
Small velvet swimming crabs are commonly found on shores in the south and west
of Britain. The velvet swimming crab is a very aggressive crab and when
threatened it will rise up on its legs and hold its claws apart.
This crab has a very varied diet, feeding on seaweeds, such as oarweed
(Laminaria digitata) and wracks, and on molluscs and crustaceans. Its slim claws
are, however, unable to cope with crushing some mollusc shells which can be
opened by the large claws of the edible crab (Cancer pagarus).
Like the edible crab (Cancer pagarus) these crabs often form mating pairs. The
male holds on to the female for several days before she casts off her shell and
is ready to mate.
The swimming crabs flat paddle-like back legs allow them to swim rapidly
backwards.
Shore crab Carcinus maenus (Linnaeus)
The shore crab is the most common crab found on British
shores. It is able to withstand long periods out of water and is often found
under rocks on the shore.
As the crab grows it has to regularly moult its hard shell to allow its body to
expand before forming a new shell. Male crabs have to wait until the female
moults and she is soft, before he can mate with her. The female carries the eggs
she produces for several months under a flap formed by the lower part of her
body. Female crabs have been found carrying as many as 185 000 eggs. The eggs
hatch out into larvae that are dispersed in the currents for two to three months
before settling on the sea bottom.
The crab has a hazardous life on the shore where it is at risk of being caught
by seabirds or trapped by boulders. One way in which it can save its life, if it
is caught or trapped, is by shedding limbs. Crabs are often found with missing
legs but these legs gradually grow again after successive moults.
The shore crab is a scavenger. Its prey includes worms, shrimps and molluscs.
 Common prawn Palaemon
serratus (Pennant)
A close look in rock pools is likely to reveal an abundance
of these animals. At first glance, however, their transparent bodies and ability
to change their colour markings to match their background means they are not
easy to spot. If disturbed, the prawn will shoot backwards, propelling itself
with rapid movements of its tail fan and the whole of its lower body. More
casual swimming is achieved by slowly beating five pairs of short limbs found
under its body. When feeding, the prawn moves delicately around on the last
three pairs of its legs. Its antennae wave around feeling for food. The first
two pairs of legs bear pincers and these are used to pick up pieces of food.
Prawns feed on a variety of weeds, small animals and debris.
Like other crustaceans, prawns moult as they grow. Young prawns grow rapidly and
may have to moult as many times as twice a fortnight. In order to mate the male
prawn has to find a soft female that has recently moulted. Once the female has
mated she produces as many as 4000 eggs which she carries on the short limbs
found under her body. The eggs are carried for up to four months before the
larvae hatch out. The larvae float in mid-water to develop further before
settling on the sea-bed.
Common on south and west coasts. Offshore prawns are fished commercially
The words ‘prawn’ and ‘shrimp’ lack precise definition. Prawns, however, are
often used to describe the larger species. In the case of the common prawn and
common shrimp, the common prawn has a larger rostrum (an extension of the
carapace between the eyes), and its first pair of claws are much smaller than
the shrimp’s.
 Common shrimp Crangon
crangon (Linnaeus)
These shrimps are night feeders. During the day they are
concealed under the sand. Their large claws enable them to take larger prey than
that taken by the common prawn (Palaemon serratus). Large worms, young fish and
small crustaceans may all be consumed by this shrimp. Like the common prawn, the
shrimp feeds by walking slowly over the sand, feeling for prey with its antennae.
It is, however, unlikely to be seen swimming. If threatened, it burrows by
shuffling its legs and by throwing sand over its body using its feelers, until
it is concealed.
Like the common prawn, the common shrimp can only mate after the female has
moulted. The female produces thousands of eggs which are carried on the legs and
on the limbs under the body. After a month or two they hatch out into larvae and
develop further in the sea before settling on the sea bed.
The words ‘prawn’ and ‘shrimp’ lack precise definition. Shrimps, however, are
often used to decribe the smaller species. In the case of the common prawn and
common shrimp, the rostrum of the shrimp (an extension of the carapace between
the eyes) is much smaller and the shrimp’s first pair of claws are much larger
than the prawn’s.
 Common acorn barnacle
Semibalanus balanoides (Linnaeus)
Despite their mollusc-like appearance,
barnacles are crustaceans. When they are immersed in water, their feathery,
jointed legs may be seen waving around to catch food.
The barnacle’s shell plates help it to retain moisture when it is out of the
water. The shell walls of the common acorn barnacle are quite thin. This means
it is unable to live high up the shore, because it cannot survive the extreme
temperatures and drying effects associated with exposure. Thinner walls, however,
allow it to grow quicker in size than the star acorn barnacle, Chthamalus
montagui, which lives on the high shore. In fact, the common acorn barnacle
prevents Chthamalus from growing lower down on the rocks by removing the latter
barnacle by crushing and overgrowing it.
The common acorn barnacle breeds in autumn. It has both male and female sex
organs but copulation and cross ferilization still occur. Each animal transfers
sperm to its neighbour and has a large penis for this purpose. The barnacle may
have a larger penis in proportion to its size than any other animal. The larvae
are released into the sea the following spring. After several weeks floating in
the sea the young barnacles settle on the shore.
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