Crabs are generally covered with a thick exoskeleton, composed primarily of highly mineralized chitin.

Males often have larger claws,[12] a tendency that is particularly pronounced in the fiddler crabs of the genus Uca (Ocypodidae).

When fertilisation has taken place, the eggs are released onto the female's abdomen, below the tail flap, secured with a sticky material.

At the time of moulting, the crab takes in a lot of water to expand and crack open the old shell at a line of weakness along the back edge of the carapace.

The crab must then extract all of itself – including its legs, mouthparts, eyestalks, and even the lining of the front and back of the digestive tract – from the old shell.

[17]: 78–79 Crabs typically walk sideways[18] (hence the term crabwise), because of the articulation of the legs which makes a sidelong gait more efficient.

[24] Fiddler crabs (genus Uca) dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders.

For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.

[17]: 85 Crabs are known to work together to provide food and protection for their family, and during mating season to find a comfortable spot for the female to release her eggs.

[28] Crabs make up 20% of all marine crustaceans caught, farmed, and consumed worldwide, amounting to 1.5 million tonnes annually.

Other commercially important taxa include Portunus pelagicus, several species in the genus Chionoecetes, the blue crab (Callinectes sapidus), Charybdis spp., Cancer pagurus, the Dungeness crab (Metacarcinus magister), and Scylla serrata, each of which yields more than 20,000 tonnes annually.

While it is sometimes disdained among some elements of the culinaryindustry as an unacceptably low-quality substitute for real crab, this does not hinder its popularity, especially as a sushi ingredient in Japan and South Korea, and in home cooking, where cost is often a chief concern.

Whether crustaceans as a whole experience pain or not is a scientific debate that has ethical implications for crab dish preparation.

[34] In 2005 a review of the literature by the Norwegian Scientific Committee for Food Safety tentatively concluded that "it is unlikely that [lobsters] can feel pain," though they note that "there is apparently a paucity of exact knowledge on sentience in crustaceans, and more research is needed."

The report assumes that the violent reaction of lobsters to boiling water is a reflex response (i.e. does not involve conscious perception) to noxious stimuli.

[35] A European Food Safety Authority (EFSA) 2005 publication[36] stated that the largest of decapod crustaceans have complex behaviour, a pain system, considerable learning abilities and appear to have some degree of awareness.

[39][40] Most Jurassic crabs are only known from dorsal (top half of the body) carapaces, making it difficult to determine their relationships.

Brachyura is separated into several sections, with the basal Dromiacea diverging the earliest in the evolutionary history, around the Late Triassic or Early Jurassic.

Trichodactylidae (freshwater crabs) Orithyiidae Belliidae Chasmocarcinidae Retroplumidae Ethusidae Dorippidae Leucosiidae Inachidae Epialtidae (paraphyletic) Majidae / Mithracidae Corystidae Euryplacidae Matutidae Calappidae Parthenopidae Cancridae Carpiliidae Aethridae Pseudocarcinus of Menippidae Menippe of Menippidae Polybiidae Portunidae Tanaochelidae Galenidae Pilumnidae Mathildellidae Eriphiidae Oziidae Vultocinidae Trapeziidae Goneplacidae Scalopidiidae Xanthidae (paraphyletic) Panopeidae Pinnotheridae Dotillidae Percnidae Xenograpsidae Cryptochiridae Ocypodidae Glyptograpsidae Grapsidae Plagusiidae Gecarcinidae Sesarmidae Mictyridae Varunidae Macrophthalmidae The infraorder Brachyura contains approximately 7,000 species in 98 families,[44][22] as many as the remainder of the Decapoda.

The telson is no longer functional in crabs, and the uropods are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum.

As the male abdomen evolved into a slimmer shape, the gonopores have moved toward the midline, away from the legs, and onto the sternum.

It is still a subject of debate whether a monophyletic group is formed by those crabs where the female, but not male, gonopores are situated on the sternum.

[51] The Moche people of ancient Peru worshipped nature, especially the sea,[52] and often depicted crabs in their art.

[citation needed] In Malay mythology (as related by Hugh Clifford to Walter William Skeat), ocean tides are believed to be caused by water rushing in and out of a hole in the Navel of the Seas (Pusat Tasek), where "there sits a gigantic crab which twice a day gets out in order to search for food".