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Crab
features
The crab has a segmented body, with several pairs of appendages. Of these,
five pairs serve as walking legs and two as sensory antennae. The
crab’s pincers are located on its front legs. It uses the pincers
for feeding itself and for defense. The crab’s abdomen is usually
tucked under the body. The body is covered by a shell with a waxy
coating. The carapace or outer shell is usually broad. Edible crabs and rock crabs have a broad, oval body.
Gills, located in a pair of cavities beneath the sides of the
carapace, are what the crab uses for breathing. In the case of land
crabs, the cavities become enlarged and modified and they act as
lungs for breathing air. Crabs
have compound eyes and can see well. They have a good sense of smell and
taste. Their lifespan is about three to twelve years. Like
many other crustaceans, crabs are often omnivorous and act as scavengers,
but many are predatory and some are vegetarian.
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Baby
crabs
Crabs, when newly hatched from eggs, are very different from the
parents. At the larval stage, known as the zoea, they are minute
transparent organisms with legless, rounded bodies, swimming on the
surface of the sea. The next stage is called the megalops, when they cast
off their skin several times, as they grow up. In
this stage, the body and limbs are more crablike, but the abdomen is large
and not folded up. After a further change of skin, they assume a form very
similar to that of the adult.
Some
varieties
Crabs are classified as sand crabs, land crabs, swimming crabs, shore
crabs and so on. There are different kinds of crabs, with interesting
names too. These include coconut crab, blue crab, pea crab, hermit crab,
horseshoe crab, spider crab, giant crab, porcelain crab and the
Tasmanian crab. Of these, the coconut crab is a land crab. The blue crab
is a swimming crab. It is
a tropical omnivore that migrates to the sea to release larvae. Sand crabs
have long bodies and burrow backward into sand. They filter suspended
matter from the water. The shore crabs are conspicuous at or above the
water level at the seashore. Pea crabs, tiny in size live in or on marine
animals such as oysters.
The
largest among the species is the spider crab. It has long, thin legs that
can grow up to twelve feet. They also possess complicated nervous systems.
Some
symbiotic relationships
Although crabs are not parasitic, some of them do have an
“understanding” with other animals. For example, the pea crab lives
within the shells of some varieties of mollusks, worm-tubes and
echinoderms and shares their food. Spider crabs cover their shells with
growing seaweeds and sponges, so that they remain camouflaged. Hermit crabs are a variety that is dependent on others.
Their abdomens are unprotected and they are forced to live in abandoned
shells of snails. As the crab grows it changes its abode from time
to time.
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