Crabs are decapod crustaceans, either the Brachyura (the "true crabs") or various groups within the closely related Anomura (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 or clade, but have convergently evolved multiple times from the ancestral decapod body plan through carcinisation, the process of creating this set of characteristics. As a group, they are thus polyphyletic, meaning they have multiple evolutionary origins.
Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to . Many crabs are free-living marine ; others are specialist or , while some are parasitic. A substantial number of species are freshwater crab or terrestrial crab.
Crabs make up about 20% of the marine 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 mythology and Malay mythology, and as the astrological sign Cancer. They have appeared in art in media including pottery, paintings, blouse panels, and book illustrations. Hermit crabs are often kept in and as pets. A popular Internet meme jokes that everything will evolve into crabs, based inaccurately on the genuine evolutionary trend within the decapods.
Diversity
Taxonomic range
Phylogeny
Crabs are not a single taxonomic group.
Instead, alongside the
Brachyura or true crabs, are multiple groups of the
Anomura that are called crabs, including the
,
,
, and
.
A distantly-related group of
, the
, with an armoured carapace but a quite different body plan, is a member of the
Chelicerata, the group that includes the
and
.
The crab body form and associated behaviour have arisen independently at different times in multiple groups of
Decapoda crustaceans (
boldface in tree).
Specific groups
Chelicerata
Xiphosura (Xiphosura) are an ancient group, known from the late
Ordovician of Canada, around 445 million years ago.
Their bodies are divided into an anterior
prosoma (fused head and thorax) and a posterior
opisthosoma, or abdomen. The upper surface of the prosoma is covered by a semicircular
carapace (top part of the shell), while the underside bears five pairs of walking legs and a pair of pincer-like
. The mouth is on the underside of the prosoma, between the bases of the walking legs.
Decapoda
Crab-like decapods span multiple groups:
-
Brachyura (Brachyura) are generally covered with a thick exoskeleton (jointed shell), composed primarily of highly mineralized chitin.
-
(Porcellanidae) are small flattened decapods that hide under rocks. They often shed limbs to escape Predation.
-
The hairy stone crab (Lomisidae) is a slow-moving crab of the Australian shore. It is with brown hair.
-
(Paguroidea) have heavily armoured crab-like claws, but in place of a carapace, they inhabit empty scavenged Gastropoda mollusc shells to protect their fragile bodies.
-
(Lithodidae) live mainly in cold deep water. They resemble brachyurans but are more closely related to hermit crabs.
-
The coconut crab ( Birgus) is a large terrestrial hermit crab of islands in the Pacific and Indian oceans.
-
Patagurus rex is a specialised hermit crab from Polynesia, living at a depth of 400 metres. Its carapace is unique but enough to make it look crab-like.
Size and shape
Crabs vary in size from the
pea crab, a few millimeters wide,
to the Japanese spider crab, with a leg span up to .
The
coconut crab is the largest terrestrial
arthropod, and indeed the largest extant terrestrial
invertebrate, at up to long and weighing up to .
File:Crabe petit pois (Pinnotheres pisum) dans les aquariums de mareis.jpg|Adult pea crab, one of the smallest species, a few millimetres across
Feeding methods
Many crabs are free-living marine
, feeding on a mixture of
algae, small animals such as molluscs,
polychaete worms, other crustaceans, and detritus.
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,
while the yellow moon crab
Ashtoret lunaris is
carnivorous.
The
are
plankton feeders,
filter feeder from seawater using long feathery bristles on their mouthparts.
The tiny soft-bodied
oyster crab is a
kleptoparasite of
, living inside the host's shell and eating its food.
File:Grapsus albolineatus in natural environment. Eriyadu, Maldives.jpg|The mottled lightfoot crab is herbivorous.
File:Ashtoret lunaris (cropped).jpg|The Ashtoret lunaris is carnivorous.
File:Porcellana platycheles millport (mouthparts detail).jpg| are , using feathery bristles on their mouthparts.
Ecological niches
The tufted ghost crab
Ocypode cursor is semi-terrestrial, consuming terrestrial animals such as insects.
Other species, including the pea crabs (
Pinnotheridae), are
Parasitism, living inside hosts such as
Bivalvia.
The tree crab or Caribbean hermit crab is
Terrestrial crab as an adult,
only returning to the ocean to spawn.
It feeds on plants and by scavenging,
and like other
, takes over a mollusc shell for protection, breathing air with a lung.
Some 1,300 species of crabs in 8 families are
Freshwater crab.
Christmas Island red crabs make an
Animal migration to the sea to lay their eggs.
File:Zaops ostreum.jpg|The oyster crab (centre, orange-coloured) is a kleptoparasite of .
File:Caribbean hermit crab.JPG|The Caribbean hermit crab is mainly Terrestrial crab.
File:Freshwater crab (Potamon potamios) Nazilli.jpg|Among many species of freshwater crab, Potamon potamios lives in or near rivers.
File:Christmas Island Crabs on annual migration.JPG|Christmas Island red crabs on their Animal migration to the sea
Similarity of body plan through carcinisation
Most crabs are members of the
Brachyura, sometimes called "true crabs", with around 7,000 species.
Several other groups of
decapod crustaceans among the
Anomura, such as
and
, have a similar appearance; all have convergently evolved through the process of
carcinisation to the crab body form and way of life. Crabs are thus not a single taxonomic group or
clade, but are
polyphyletic.
Many crabs can run swiftly sideways (""), though others walk forwards,
and some can swim.
The carcinised body form is defined by Keiler and colleagues (2014) as having the following attributes:
-
"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 Tergum of the fourth pleonal segment, and partially or completely covers the plastron."
Interactions with humans
Fisheries and food
Crabs make up some 20% of all marine
caught, farmed, and consumed worldwide, amounting to 1.5 million
annually. One species, the Asian blue crab
Portunus trituberculatus, accounts for one-fifth of that total. Other commercially important
taxon include
Portunus pelagicus, several species in the genus
Chionoecetes, the blue crab (
Callinectes sapidus),
Charybdis spp.,
Edible crab, the
Dungeness crab (
Metacarcinus magister), and
Scylla serrata, each of which yields more than 20,000 tonnes annually.
File:Untitled - panoramio - karlee ladyk (11).jpg|Small-scale crab fishing
File:King crab pots.jpg|Commercial king crab traps, awaiting the crabbing season
File:Crab fishing boat.png|Crab boat in the Bering Sea
File:Catching crabs.jpg|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.
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.
In Goa and Mozambique, crab curry is a typical dish, flavoured with chilis, garlic, coconut, and spices.
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.
File:Crab meat in shell with salad and Marie Rose sauce.jpg|Dressed crab with salad and Marie Rose sauce
File:Bartley's.jpg| with sweet potato fries, coleslaw, and tartar sauce
File:Caril_de_caranguejo.jpg| style crab curry
File:Kobako crab sushi (32301286552).jpg| 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.
In
Greek mythology,
Karkinos was a crab that came to the aid of the
Lernaean Hydra as it battled the hero
Heracles.
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 by
influence, as
Astrology tried to associate the Labours of Hercules with the twelve signs of the Zodiac.
File:Lernaean Hydra Louvre CA598 n2.jpg|The crab Karkinos attacking Heracles as he fights the Lernaean Hydra. Attic lekythos pot, "the Diosphos Painter", 500–475 BC
File:Sidney Hall - Urania's Mirror - Cancer.jpg|The constellation of Cancer, the crab, from Urania's Mirror, c. 1825
Crabs have appeared in art since ancient times in many different media. The Moche people of ancient Peru worshipped nature, especially the sea,
File:Albrecht Dürer 108.jpg|Crab by Albrecht Dürer, gouache and watercolour on paper, Germany, 1495
File:Brooklyn Museum - Crabs and Chrysanthemums - Gao Qipei.jpg| Crabs and Chrysanthemums, Gao Qipei, ink and light colour on paper, China, 18th century
File:Charger attributed to the Barbizet Studio, c. 1850, Cincinnati Art Museum.jpg|Charger attributed to the Barbizet Studio, glazed earthenware, c. 1850
File:Crab in Mola (blouse panel), Cuna (Kuna) Indians, Honolulu Museum of Art, 3740.1 (cropped).JPG| Mola appliqué blouse panel, Nulanega Island, Panama, 20th century
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 .
As pets
are commonly kept as pets and used in the
marine aquarium trade.
A popular species is the Caribbean hermit crab,
Coenobita clypeatus. They can live for 30 years in captivity if their requirements, including 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.
Meme
The zoologist Joanna Wolfe, writing in
Scientific American, notes a popular
meme which jokes that crabs are the "ultimate forms"
of life as "everything will eventually evolve into a crab".
Sara Kiley Watson, writing in
Popular 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.
Wolfe 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.
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.
