Crab

This article is about the crustaceans. For other uses, see Crab (disambiguation).

Crabs are decapod crustaceans, in either the Brachyura (the "true crabs") or the Anomura, 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 or clade, but have convergently evolved multiple times from the ancestral decapod body plan through the process of carcinisation. As a group they are thus polyphyletic.

Cancer pagurus, the edible or brown crab (Brachyura)

Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 4 m (13 ft). Many crabs are free-living marine omnivores; others are specialist herbivores or carnivores, while some are parasitic. A substantial number of species are adapted to freshwater or other non-marine habitats.

Crabs make up about 20% of the marine crustaceans that are caught or farmed for human consumption. In British cuisine, dressed crab is a traditional seafood meal, while in Goa and Mozambique, crab curry is a typical dish. Crabs feature in Greek and Malay mythology, and as the astrological sign Cancer.

Body plan

Most crabs are members of the Brachyura, sometimes called "true crabs", with around 7,000 species.^[1] Several other groups of decapod crustaceans among the Anomura, such as king crabs and porcelain crabs, have a similar appearance; all have convergently evolved through the process of carcinisation to the crab body form and way of life.^[2][3][4][5] Many crabs can run swiftly sideways ("crabwise"), though others walk forwards,^[6] and some can swim.^[7]

The carcinised body form is defined by Keiler and colleagues (2014) as having the following attributes:^[8]

• "The carapace is flatter than it is broad and possesses lateral margins."
• "The sternites are fused into a wide sternal plastron which possesses a distinct emargination on its posterior margin."
• "The pleon is flattened and strongly bent, in dorsal view completely hiding the tergites of the fourth pleonal segment, and partially or completely covers the plastron."

Crab body plan, its adaptations illustrated by comparison with a lobster (undersides shown)

Diversity

Taxonomic range

Alongside the Brachyura or true crabs, the Anomura contains multiple groups that are called crabs, including the hermit crabs, mole crabs, king crabs, and porcelain crabs.^[9] A distantly-related group of arthropods, the horseshoe crabs, with an armoured carapace but a quite different body plan, is a member of the Chelicerata, the group that includes the spiders and scorpions.^[10] The crab body form and associated behaviour has arisen independently at different times in multiple groups of decapod crustaceans (boldface in tree).^[8]

Arthropoda

Chelicerata Xiphosura (horseshoe crabs) Middle Cambrian Mandibulata Decapoda Brachyura ("true" crabs)[11] Early Jurassic Anomura Porcellanidae (porcelain crabs)[8] Late Jurassic Munididae (squat lobsters) Parapaguridae (deep water sea anemone hermit crabs) Lomisidae (hairy stone crabs)[12] Aeglidae Hippidae (mole crabs or sand crabs) Paguroidea Lithodidae (king crabs)[13][14] Late Cenozoic Paguridae Birgus latro (coconut crab) Patagurus rex[15] (other hermit crabs) Late Cretaceous Late Triassic

Size and shape

Crabs vary in size from the pea crab, a few millimeters wide,^[16] to the Japanese spider crab, with a leg span up to 4 m (13 ft).^[17] The coconut crab is the largest terrestrial arthropod, and indeed the largest extant terrestrial invertebrate, at up to 40 cm (16 in) long and weighing up to 4.1 kg (9 lb).^[18]

• Smallest and largest
• 
  Adult pea crab, one of the smallest species, with barnacles for scale^{[clarification needed]}
• 
  Japanese spider crab, with a leg span of as much as 4 m (13 ft)
• 
  Coconut crab, the largest terrestrial arthropod, weighing up to 4.1 kg (9 lb)

Ecological niche

Many crabs are free-living marine omnivores, feeding on a mixture of algae, small animals such as molluscs, polychaete worms, other crustaceans, and detritus.^[19] Others are more specialised: the mottled crab Grapsus albolineatus, for example, is herbivorous, feeding mainly on algae and preferring the more nutritious filamentous algae to leafy (foliose) algae,^[20] while the yellow moon crab Ashtoret lunaris is carnivorous.^[21] The porcelain crabs are plankton feeders, filtering their prey from seawater using long feathery bristles on their mouthparts.^[22] The tiny soft-bodied oyster crab is a kleptoparasite of oysters, living inside the host's shell and eating its food.^[23]

The semi-terrestrial tufted ghost crab Ocypode cursor consumes terrestrial animals such as insects.^[19] Other species, including the pea crabs (Pinnotheridae), are parasitic, living inside hosts such as bivalve molluscs.^[16] The tree crab or Caribbean hermit crab is terrestrial as an adult,^[24] only returning to the ocean to spawn.^[25] It feeds on plants and by scavenging,^[26] and like other hermit crabs, takes over a mollusc shell for protection, breathing air with a lung.^[24] Some 1,300 species of crabs in 8 families are adapted to freshwater.^[27] Christmas Island red crabs make an annual mass migration to the sea to lay their eggs.^[28]

• Diverse ecological niches
• 
  The mottled lightfoot crab is herbivorous.
• 
  The yellow moon crab is carnivorous.
• 
  Porcelain crabs are filter feeders, using feathery bristles on their mouthparts.
• 
  The oyster crab is a kleptoparasite of oysters.
• 
  The Caribbean hermit crab is mainly terrestrial.
• 
  Among many species of freshwater crab, Potamon potamios lives in or near rivers.
• 
  Christmas Island red crabs on their annual migration to the sea

Interactions with humans

See also: Human uses of arthropods

Fisheries and food

Further information: Crab fisheries and Crab meat

Crabs make up some 20% of all marine crustaceans caught, farmed, and consumed worldwide, amounting to 1.5 million tonnes annually. One species, the Asian blue crab Portunus trituberculatus, accounts for one-fifth of that total. 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.^[29]

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

In Western Europe, much of the crab meat is from the brown crab Cancer pagurus, noted for its sweet, delicate flavour. The United Kingdom hosts significant fisheries of this species, with major operations in Scotland and the South West of England.^[30] Dressed crab is a traditional seafood meal in British cuisine made of the meat of the brown crab served in its own shell.^[31]

In North America, there are commercial fisheries for the blue crab Callinectes sapidus along the Atlantic coast of the United States, and in the Gulf of Mexico. The fishery was centered on the Chesapeake Bay, but other places are increasing in importance.^[32] Crab cakes are traditionally made from Chesapeake Bay crabs.^[33][34]

In Goa and Mozambique, crab curry is a typical dish, flavoured with chilis, garlic, coconut, and spices.^[35]

In Ishikawa prefecture, Japan, both the meat and the eggs of the snow crab are served as sushi in wintertime. The male is known as kano-gani, the female as kobako-gani. The short fishing season for the females makes kobako-gani sushi a rare delicacy.^[36]

• Dishes made with crab
• 
  Dressed crab with salad and Marie Rose sauce
• 
  Crab cakes with sweet potato fries, coleslaw, and tartar sauce
• 
  Goan style crab curry
• 
  Kobako-gani sushi with snow crab meat and eggs

In culture

Both the constellation Cancer and the astrological sign Cancer are named after the crab, and depicted as such.^[37] The Moche people of ancient Peru worshipped nature, especially the sea,^[38] and often depicted crabs in their art.^[39] In Greek mythology, Karkinos was a crab that came to the aid of the Lernaean Hydra as it battled the hero Heracles. One of Rudyard Kipling's Just 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 the tides.^[40] The paleontologist Richard Fortey has identified Kipling's giant crab as a horseshoe crab.^[41] In Malay mythology, ocean tides were 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".^[42]

• In different media through the centuries
• 
  The crab Karkinos attacking Herakles as he fights the Lernaean Hydra. Attic lekythos pot, "the Diosphos Painter", 500–475 BC
• 
  Pre-Columbian clay pot in the shape of a crab, Colima, Mexico, 200–500 AD
• 
  Crab by Albrecht Dürer, gouache and watercolour on paper, 1495
• 
  Crabs and Chrysanthemums, Gao Qipei, ink and light colour on paper, 18th century
• 
  The constellation of Cancer, the crab, from Urania's Mirror, c. 1825
• 
  Charger attributed to the Barbizet Studio, glazed earthenware, c. 1850
• 
  "Le Crabe" by William-Adolphe Bouguereau, oil on canvas, 1869
• 
  Illustration for "The Crab that Played with the Sea" (in Just So Stories), Rudyard Kipling, 1902
• 
  Mola appliqué blouse panel, Nulanega Island, Panama, 20th century

As pets

Hermit crabs are commonly kept as pets and used in the marine aquarium trade.^[9] A popular species is the Caribbean hermit crab, Coenobita clypeatus. They can live for 30 years in captivity, if their requirements, simulating a coastal rainforest, 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.^[43]

Hermit crabs in an aquarium

Meme

The zoologist Joanna Wolfe, writing in Scientific American, notes a popular meme which jokes that crabs are the "ultimate forms"^[44] of life as "everything will eventually evolve into a crab".^[44] She explains that the meme parodies the genuine process of carcinisation which has taken place in at least five different groups of decapods, but that the process does not apply to humans or other animals.^[44] 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.^[45]

Decapods have repeatedly evolved a crablike body form under the pressures of predation and wave action. A broad low body, an armoured carapace, and the ability to scuttle sideways into a rock crevice all work well in a marine environment. This does not mean that all animals will evolve the same adaptations.^[45]