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Decapods - Crabs  

True crabs are decapod crustaceansand belong to a group called the Brachyura. They have a very short projecting "tail" and their small abdomens are completely hidden under the thorax. There are almost 7,000 speciesof true crabs with over 800 species freshwater.

Other animals, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice do not belong in the Brachyura - the true crabs.

All crabs have one pair of pincers (chelipeds) and four pairs of walking legs. They are the first pair of legs on a crab and are used for holding and carrying food, digging, cracking open shells and warning off would be attackers. The carapace protects the internal organs of the head, thorax and gills. The eyes are on the ends of short stalks and the mouthparts are a series of pairs of short legs, specialised to manipulate and chew food. The abdomen is small and tightly held under the body. The sexes are separate and the size of the abdomen distinguishes them.

For much more detailed information refer to the "World of Crabs" on this website.

Diversity in true crabs
Not True Crabs    
Hermit crab

Hermit crabs are more closely related to squat lobsters and porcelain crabs than they are to
the true crabs.
Most of the 800 species possess
a long, soft, spirally curved abdomen which is
covered by a discarded shell (usually from a
gastropod) carried around by the hermit crab.


Sand Bubbler Crabs
These are Sand Bubbler Crabs and they're sifting through huge amounts of sand in search of detritus.
The waste sand builds on their head and they kick off the ball before they can't see over it.



Pea crab
The pea crab is a small crab that is a parasite of oysters,
clams, mussels and other species of bivalves.
Image © Marck Hicks via Flickr

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