Crabs aredecapod crustaceans, either theBrachyura (the "true crabs") or various groups within the closely relatedAnomura (hermit crabs and allies), characterised by having a heavily armoured shell, their tail segments concealed under the body, the ability to run sideways, and the habit of hiding in rocky crevices. They do not form a single natural group orclade, but haveconvergently evolved multiple times from the ancestral decapod body plan throughcarcinisation, the process of creating this set of characteristics. As a group, they are thuspolyphyletic, meaning they have multiple evolutionary origins.

Crabs vary in size from thepea crab, a few millimeters wide, to theJapanese spider crab, with a leg span up to 4 m (13 ft). Many crabs are free-living marineomnivores; others are specialistherbivores orcarnivores, while some areparasitic. A substantial number of species areadapted to freshwater orother non-marine habitats.

Crabs make up about 20% of the marinecrustaceans that are caught or farmed for human consumption. In British cuisine,dressed crab is a traditional seafood meal, while inGoa and Mozambique,crab curry is a typical dish. Crabs feature inGreek andMalay mythology, and as the astrological signCancer. They have appeared in art in media including pottery, paintings, blouse panels, and book illustrations. Hermit crabs are often kept inaquariums and as pets. A popularmeme jokes that everything will evolve into crabs, based inaccurately on the genuine evolutionary trend within the decapods.

Crabs are not a single taxonomic group.^[1] Instead, alongside theBrachyura or true crabs, are multiple groups of theAnomura that are called crabs, including thehermit crabs,mole crabs,king crabs, andporcelain crabs.^[2] A distantly-related group ofarthropods, thehorseshoe crabs, with an armoured carapace but a quite different body plan, is a member of theChelicerata, the group that includes thespiders andscorpions.^[3]The crab body form and associated behaviour have arisen independently at different times in multiple groups ofdecapod crustaceans (boldface in tree).^[1]

Horseshoe crabs (Xiphosura) are an ancient group, known from the lateOrdovician of Canada, around 445 million years ago.^[9] Their bodies are divided into an anteriorprosoma (fused head and thorax) and a posterioropisthosoma, or abdomen. The upper surface of the prosoma is covered by a semicircularcarapace (top part of the shell), while the underside bears five pairs of walking legs and a pair of pincer-likechelicerae. The mouth is on the underside of the prosoma, between the bases of the walking legs.^[10]

Crab-like decapods span multiple groups:

• True crabs (Brachyura) are generally covered with a thickexoskeleton (jointed shell), composed primarily of highly mineralizedchitin.^[11][12] Males often have largerclaws than females.^[13]
• Porcelain crabs (Porcellanidae) are small flattened decapods that hide under rocks. They often shed limbs to escapepredators.^[14]
• Thehairy stone crab (Lomisidae) is a slow-moving crab of the Australian shore. It iscamouflaged with brown hair.^[15]
• Hermit crabs (Paguroidea) have heavily armoured crab-like claws, but in place of a carapace, they inhabit empty scavengedgastropod mollusc shells to protect their fragile bodies.^[16][17]
• King crabs (Lithodidae) live mainly in cold deep water. They resemble brachyurans but are more closely related to hermit crabs.^[18]
• Thecoconut crab (Birgus) is a large terrestrial hermit crab of islands in the Pacific and Indian oceans.^[19]
• Patagurus rex is a specialised hermit crab fromPolynesia, living at a depth of 400 metres. Its carapace is unique but enough to make it look crab-like.^[20]

Crabs vary in size from thepea crab, a few millimeters wide,^[21] to theJapanese spider crab, with a leg span up to 4 m (13 ft).^[22] Thecoconut crab is the largest terrestrialarthropod, and indeed the largest extant terrestrialinvertebrate, at up to 40 cm (16 in) long and weighing up to 4.1 kg (9 lb).^[19]

• Smallest and largest
• 
  Adultpea crab, one of the smallest species, a few millimetres across^[21]
• 
  Coconut crab, the largest terrestrialarthropod, weighing up to 4.1 kg (9 lb)
• 
  Japanese spider crab, with a leg span of as much as 4 m (13 ft)

Many crabs are free-living marineomnivores, feeding on a mixture ofalgae, small animals such as molluscs,polychaete worms, other crustaceans, and detritus.^[23] Others are more specialised: the mottled crabGrapsus albolineatus, for example, isherbivorous, feeding mainly on algae and preferring the more nutritious filamentous algae to leafy (foliose) algae,^[24] while the yellow moon crabAshtoret lunaris iscarnivorous.^[25] Theporcelain crabs areplankton feeders,filtering their prey from seawater using long feathery bristles on their mouthparts.^[26] The tiny soft-bodiedoyster crab is akleptoparasite ofoysters, living inside the host's shell and eating its food.^[27]

• Diverse feeding methods
• 
  Themottled lightfoot crab isherbivorous.
• 
  Theyellow moon crab iscarnivorous.
• 
  Porcelain crabs arefilter feeders, using feathery bristles on their mouthparts.

The tufted ghost crabOcypode cursor is semi-terrestrial, consuming terrestrial animals such as insects.^[23] Other species, including the pea crabs (Pinnotheridae), areparasitic, living inside hosts such asbivalve molluscs.^[21] The tree crab orCaribbean hermit crab isterrestrial as an adult,^[28] only returning to the ocean to spawn.^[29] It feeds on plants and by scavenging,^[30] and like otherhermit crabs, takes over a mollusc shell for protection, breathing air with a lung.^[28] Some 1,300 species of crabs in 8 families areadapted to freshwater.^[31] Christmas Island red crabs make anannual mass migration to the sea to lay their eggs.^[32]

• Diverse ecological niches
• 
  Theoyster crab (centre, orange-coloured) is akleptoparasite ofoysters.
• 
  TheCaribbean hermit crab is mainlyterrestrial.
• 
  Among many species offreshwater crab,Potamon potamios lives in or near rivers.
• 
  Christmas Island red crabs on theirannual migration to the sea

Most crabs are members of theBrachyura, sometimes called "true crabs", with around 7,000 species.^[33] Several other groups ofdecapod crustaceans among theAnomura, such asking crabs andporcelain crabs, have a similar appearance; all haveconvergently evolved through the process ofcarcinisation to the crab body form and way of life. Crabs are thus not a single taxonomic group orclade, but arepolyphyletic.^{[34][35][36][37]} Many crabs can run swiftly sideways ("crabwise"), though others walk forwards,^[38] and some can swim.^[39]

The carcinised body form is defined by Keiler and colleagues (2014) as having the following attributes:^[1]

• "Thecarapace is flatter than it is broad and possesses lateral margins."
• "Thesternites are fused into a wide sternalplastron which possesses a distinct emargination on its posterior margin."
• "Thepleon is flattened and strongly bent, in dorsal view completely hiding thetergites of the fourth pleonal segment, and partially or completely covers the plastron."

Crabs make up some 20% of all marinecrustaceans caught, farmed, and consumed worldwide, amounting to 1.5 milliontonnes annually. One species, the Asian blue crabPortunus trituberculatus, accounts for one-fifth of that total. Other commercially importanttaxa includePortunus pelagicus, several species in the genusChionoecetes, the blue crab (Callinectes sapidus),Charybdis spp.,Cancer pagurus, theDungeness crab (Metacarcinus magister), andScylla serrata, each of which yields more than 20,000 tonnes annually.^[40]

• Crab fisheries
• 
  Small-scale crab fishing
• 
  Commercialking crab traps, awaiting the crabbing season
• 
  Crab boat in theBering Sea
• 
  Hauling in a crab trap

InWestern Europe, much of the crab meat is from the brown crabCancer pagurus, noted for its sweet, delicate flavour. The United Kingdom hosts significant fisheries of this species, with major operations inScotland and theSouth West of England.^[41]Dressed crab is a traditional seafood meal inBritish cuisine made of the meat of the brown crab served in its own shell.^[42]

In North America, there are commercial fisheries for the blue crabCallinectes sapidus along the Atlantic coast of the United States, and in the Gulf of Mexico. The fishery was centered on theChesapeake Bay, but other places are increasing in importance.^[43]Crab cakes are traditionally made from Chesapeake Bay crabs.^[44][45]

InGoa and Mozambique,crab curry is a typical dish, flavoured with chilis, garlic, coconut, and spices.^[46]

InIshikawa prefecture, Japan, both the meat and the eggs of thesnow crab are served assushi in wintertime. The male is known askano-gani, the female askobako-gani. The short fishing season for the females makeskobako-gani sushi a rare delicacy.^[47]

• Dishes made with crab
• 
  Dressed crab with salad and Marie Rose sauce
• 
  Crab cakes with sweet potato fries, coleslaw, and tartar sauce
• 
  Goan stylecrab curry
• 
  Kobako-ganisushi withsnow crab meat and eggs

Both theconstellationCancer and theastrological signCancer are named after the crab, and depicted as such.^[48] InGreek mythology,Karkinos was a crab that came to the aid of theLernaean Hydra as it battled the heroHeracles.^[49] The crab is at best a secondary character in the myth, and sometimes omitted altogether. This has been explained by the suggestion that it was introduced into the myth byMesopotamian influence, asastrologers tried to associate theLabours of Hercules with the twelvesigns of the Zodiac.^[50][51]

• Mythology and astrology
• 
  The crabKarkinos attackingHerakles as he fights theLernaean Hydra. Atticlekythos pot, "theDiosphos Painter", 500–475 BC
• 
  Theconstellation ofCancer, the crab, fromUrania's Mirror, c. 1825

Crabs have appeared in art since ancient times in many different media. TheMoche people of ancientPeru worshipped nature, especially the sea,^[52] and often depicted crabs in their art.^[53] Among the many later representations, the German artistAlbrecht Dürer made a meticulous painting of the crabEriphia verrucosa in 1495; since the species lives on theAdriatic coast, he likely painted the animal when he visitedVenice.^[54] Quite a different artistic portrayal is the monumental crabtable setting charger created by the Barbizet Studio that made glazed earthenware pottery between 1850 and 1890.^[55] In China, Gao Qipei (1672–1734) paintedCrabs and Chrysanthemums in ink and light watercolour on paper.^[56]In Panama, theGuna people of theSan Blas Islands makeMolaappliqué blouse panels decorated with motifs such as crabs from the waters of the Caribbean.^[57]

• In different media through the centuries
• 
  Crab byAlbrecht Dürer,gouache andwatercolour on paper, Germany, 1495
• 
  Crabs and Chrysanthemums,Gao Qipei, ink and light colour on paper, China, 18th century
• 
  Charger attributed to the Barbizet Studio, glazedearthenware, c. 1850
• 
  Molaappliqué blouse panel, Nulanega Island, Panama, 20th century

One ofRudyard Kipling'sJust So Stories, "The Crab that Played with the Sea", tells the story of a gigantic crab who made the waters of the sea go up and down like thetides.^[58] The paleontologistRichard Fortey has identified Kipling's giant crab as ahorseshoe crab.^[59]InMalay mythology, ocean tides were believed to be caused by water rushing in and out of a hole in theNavel of the Seas (Pusat Tasek), where "there sits a gigantic crab which twice a day gets out in order to search for food".^[60]

Hermit crabs are commonlykept as pets and used in themarine aquarium trade.^[2] A popular species is the Caribbean hermit crab,Coenobita clypeatus. They can live for 30 years in captivity if their requirements, including simulating a coastalrainforest, are met. The size of tank must be substantial. There must be a substrate of sand and coconut fibre that they can dig in to facilitate moulting. The temperature and humidity of the air must be controlled. A pool of fresh water and a pool of correctly formulated salt water are both necessary.^[61]

The zoologist Joanna Wolfe, writing inScientific American, notes a popularmeme which jokes that crabs are the "ultimate forms"^[62] of life as "everything will eventually evolve into a crab".^[62] Sara Kiley Watson, writing inPopular Science, comments that the joke "comes from an actual truth", that decapods span multiple crab-like groups, including the true crabs but not limited to them.^[63]Wolfe explains that the memeparodies the genuine process ofcarcinisation which has taken place in at least five different groups of decapods, but that the process does not apply to humans or other animals.^[62] The evolutionary palaeobiologist Matthew Wills comments that all the crabs are decapods, and the evolutionary pressures apply in a marine environment where defence, living in crevices, and being wave-swept favour armoured protection, a broad compact body, and the ability to scuttle sideways.^[64]

• Decapods
• Convergent evolution
• Arthropod common names

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