There are many different types of crabs, but only two broad types:

  •     Hermit Crabs
  •     True Crabs

Hermit Crabs

Hermit crabs are decapod crustaceans of the superfamily Paguroidea, distinct from the true crabs in the infra-order Brachyura.
There are many species of hermit crabs. They are known for their curious habit of making their homes in empty shells. They do this to protect their soft-skinned abdomens. To fit into the spiral of their adopted homes, the abdomen has become twisted and the number of appendages on the inside of the curve has been reduced. The first pair of legs carries nippers, the left nipper often enlarged to seal the opening of the shell when the crab withdraws inside. The second and third pair of legs are used for walking. The last two pairs of legs are reduced in order to grip the shell.

As a hermit crab grows it becomes necessary to move to larger homes, and they move to progressively larger shells. Fights often occur between crabs over ownership of the better shells. Most of the hermit crabs are scavengers, but some of them filter food particles from the water. This is done by use of their antennae.

Some facts about hermit crabs:

  •     Hermit crabs shed their exoskeleton. This is called moulting.
  •     Hermit crabs are usually nocturnal. That means they move around more at night than during the day.
  •     Hermit crabs can live up to 20 years.
  •     Hermit crabs have been heard making croaking sounds.
  •     The hermit crab's relatives are the spider and lobster.
  •     Hermit crabs live in colonies up to 100 or more.
  •     Hermit crabs will eat the exoskeleton that they shed to get extra calcium to help grow a new one.

True Crabs

The term crab is often applied to several different groups of short (nose to tail) decapod crustaceans with thick exoskeletons, but only members of the Brachyura are true crabs. True crabs are the most specialised crustaceans. The abdomen is not part of the tail; it is tucked beneath the thorax. The abdominal limbs are not used to swim; they only hold the eggs in the female or transfer sperm in the male. The fantail is no longer present. The first pair of walking legs bear nippers and the remaining four pairs of legs are used for walking. Crabs scuttle sideways so they can lengthen their stride without danger of entangling their legs. The head and thorax are covered by a shield like carapace housing gills on either side of the body. The eggs hatch into planktonic larvae that settle and are transformed into juveniles.

Brachyurans are the most advanced of the decapods in that they have the body most modified from the primitive shrimp-like decapod ancestor.

Other taxa, such as hermit crabs and king crabs are, despite superficial similarities, are not crabs at all; they belong to the Anomura and can be distinguished from true crabs by counting the legs. In Anomura, the last pair of walking legs is hidden inside the carapace, so only four pairs are visible (counting the claws), whereas uninjured true crabs always have five visible pairs.

Example   Mud crab (Genus Scylla)   Family Portunidae
Crabs of the genus Scylla are strongly associated with mangrove areas throughout the Pacific and Indian oceans and form the basis of substantial fishery and aquaculture operations. Mud crab species have an oval carapace with nine pairs of equal sized marginal teeth. The nippers are very large. The colour is green-brown. The limbs are coloured with a net-like pattern, especially the paddle-shaped last pair. This is the giant of swimming crabs, its body size being thirty centimetres. The adult crabs burrow in estuarine mud and migrate to sea to reproduce. They are predatory, but only feed on tiny prey, including small molluscs. The nippers are large and strong enough to cause a human to loss a finger.  These crabs found in many parts of the world. They are commercially fished in Australasia. Their slow growth makes them susceptible to over-exploitation.

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