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

Crab
Temporal range: Early Jurassic–Present
Top row, left to right: Dromia personata (Dromiidae), Dungeness crab (Cancridae), Tasmanian giant crab (Menippidae); Middle row: Corystes cassivelaunus (Corystidae), Liocarcinus vernalis (Portunidae), Carpilius maculatus (Carpiliidae); Bottom row: Gecarcinus quadratus (Gecarcinidae), Grapsus grapsus (Grapsidae), Ocypode ceratophthalmus (Ocypodidae).
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
(unranked): Reptantia
Infraorder: Brachyura
Linnaeus, 1758
Sections and subsections[1]

Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tail" in Greek), which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax.[a] They live in all the world's oceans, in freshwater, and on land. They are generally covered with a thick exoskeleton. They generally have five pairs of legs, and they have pincer claws on the ends of the frontmost pair. They first appeared during the Jurassic period, around 200 million years ago.

Description

[edit]
Gecarcinus quadratus, a land crab from Central America

Crabs are generally covered with a thick exoskeleton, composed primarily of highly mineralized chitin.[4][5] Behind their pair of chelae (claws) are six walking legs and then two swimming legs. The crab breathes through gills on its underside; gills must be at least moist to work.

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).[6] Several other groups of crustaceans with similar appearances – such as king crabs and porcelain crabs – are not true crabs, but have evolved features similar to true crabs through a process known as carcinisation.[7][8][9][10]

Environment

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Crabs are found in all of the world's oceans, as well as in fresh water and on land, particularly in tropical regions. About 850 species are freshwater crabs.[11]

Sexual dimorphism

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The underside of a male (top) and a female (bottom) individual of Pachygrapsus marmoratus, showing the difference in shape of the abdomen

Crabs often show marked sexual dimorphism. Males often have larger claws,[12] a tendency that is particularly pronounced in the fiddler crabs of the genus Uca (Ocypodidae). In fiddler crabs, males have one greatly enlarged claw used for communication, particularly for attracting a mate.[13] Another conspicuous difference is the form of the pleon (abdomen); in most male crabs, this is narrow and triangular in form, while females have a broader, rounded abdomen.[14] This is because female crabs brood fertilised eggs on their pleopods.

Reproduction and life cycle

[edit]
Crab (Pachygrapsus marmoratus) on Istrian coast, Adriatic Sea

Crabs attract a mate through chemical (pheromones), visual, acoustic, or vibratory means. Pheromones are used by most fully aquatic crabs, while terrestrial and semiterrestrial crabs often use visual signals, such as fiddler crab males waving their large claws to attract females. The vast number of brachyuran crabs have internal fertilisation and mate belly-to-belly. For many aquatic species, mating takes place just after the female has moulted and is still soft. Females can store the sperm for a long time before using it to fertilise their eggs. When fertilisation has taken place, the eggs are released onto the female's abdomen, below the tail flap, secured with a sticky material. In this location, they are protected during embryonic development. Females carrying eggs are called "berried" since the eggs resemble round berries.

When development is complete, the female releases the newly hatched larvae into the water, where they are part of the plankton. The release is often timed with the tidal and light/dark diurnal cycle.[15][16] The free-swimming tiny zoea larvae can float and take advantage of water currents. They have a spine, which probably reduces the rate of predation by larger animals. The zoea of most species must find food, but some crabs provide enough yolk in the eggs that the larval stages can continue to live off the yolk.

Female crab Xantho poressa at spawning time in the Black Sea, carrying eggs under her abdomen
A Grapsus tenuicrustatus climbing up a rock in Hawaii

Each species has a particular number of zoeal stages, separated by moults, before they change into a megalopa stage, which resembles an adult crab, except for having the abdomen (tail) sticking out behind. After one more moult, the crab is a juvenile, living on the bottom rather than floating in the water. This last moult, from megalopa to juvenile, is critical, and it must take place in a habitat that is suitable for the juvenile to survive.[17]: 63–77 

Most species of terrestrial crabs must migrate down to the ocean to release their larvae; in some cases, this entails very extensive migrations. After living for a short time as larvae in the ocean, the juveniles must do this migration in reverse. In many tropical areas with land crabs, these migrations often result in considerable roadkill of migrating crabs.[17]: 113–114 

Once crabs have become juveniles, they still have to keep moulting many more times to become adults. They are covered with a hard shell, which would otherwise prevent growth. The moult cycle is coordinated by hormones. When preparing for moult, the old shell is softened and partly eroded away, while the rudimentary beginnings of a new shell form under it. 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. This is a difficult process that takes many hours, and if a crab gets stuck, it will die. After freeing itself from the old shell (now called an exuvia), the crab is extremely soft and hides until its new shell has hardened. While the new shell is still soft, the crab can expand it to make room for future growth.[17]: 78–79 

Behaviour

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Carpilius convexus consuming Heterocentrotus trigonarius in Hawaii

Crabs typically walk sideways[18] (hence the term crabwise), because of the articulation of the legs which makes a sidelong gait more efficient.[19] Some crabs walk forward or backward, including raninids,[20] Libinia emarginata[21] and Mictyris platycheles.[18] Some crabs, like the Portunidae and Matutidae, are also capable of swimming,[22] the Portunidae especially so as their last pair of walking legs are flattened into swimming paddles.[17]: 96 

Crabs are mostly active animals with complex behaviour patterns such as communicating by drumming or waving their pincers. Crabs tend to be aggressive toward one another, and males often fight to gain access to females.[23] On rocky seashores, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes.[24] Fiddler crabs (genus Uca) dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders.[17]: 28–29, 99 

Crabs are omnivores, feeding primarily on algae,[25] and taking any other food, including molluscs, worms, other crustaceans, fungi, bacteria, and detritus, depending on their availability and the crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness.[26][27] Some species are more specialised in their diets, based in plankton, clams or fish.[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]

Human consumption

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Fisheries

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A short video on catching and exporting shellfish in Wales.
Main article: Crab fisheries

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

In some crab species, meat is harvested by manually twisting and pulling off one or both claws and returning the live crab to the water in the knowledge that the crab may survive and regenerate the claws.[30][31][32]

Cookery

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Wikibooks Cookbook has a recipe/module on
See also: Crab meat and List of crab dishes
Photo of cooked crab in bowl of soup
Crab masala from Karnataka, India

Crabs are prepared and eaten as a dish in many different ways all over the world. Some species can be eaten whole, including the shell, as soft-shell crabs; with other species, just the claws or legs are eaten. The latter is particularly common for larger crabs, such as the snow crab. In many cultures, the roe of the female crab is also eaten, which usually appears orange or yellow in fertile crabs. This is popular in Southeast Asian cultures, some Mediterranean and Northern European cultures, and on the East, Chesapeake, and Gulf Coasts of the United States.

In some regions, spices improve the culinary experience. In Southeast Asia and the Indosphere, masala crab and chilli crab are examples of heavily spiced dishes. In the Chesapeake Bay region, blue crab is often steamed with Old Bay Seasoning. Alaskan king crab or snow crab legs are usually simply boiled and served with garlic or lemon butter.

Sushi with crab meat and eggs

For the British dish dressed crab, the crab meat is extracted and placed inside the hard shell. One American way to prepare crab meat is by extracting it and adding varying amounts of binders, such as egg white, cracker meal, mayonnaise, or mustard, creating a crab cake. Crabs can also be made into a bisque, a global dish of French origin which in its authentic form includes in the broth the pulverized shells of the shellfish from which it is made.

Imitation crab, also called surimi, is made from minced fish meat that is crafted and colored to resemble crab meat. While it is sometimes disdained among some elements of the culinary industry 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.[33] Indeed, surimi is an important source of protein in most East and Southeast Asian cultures, appearing in staple ingredients such as fish balls and fish cake.

Pain

[edit]

Whether crustaceans as a whole experience pain or not is a scientific debate that has ethical implications for crab dish preparation. Crabs are very often boiled alive as part of the cooking process.

This section is an excerpt from Pain in crustaceans § Opinions.[edit]

Advocates for Animals, a Scottish animal welfare group, stated in 2005 that "scientific evidence ... strongly suggests that there is a potential for decapod crustaceans and cephalopods to experience pain and suffering". This is primarily due to "The likelihood that decapod crustaceans can feel pain [which] is supported by the fact that they have been shown to have opioid receptors and to respond to opioids (analgesics such as morphine) in a similar way to vertebrates." Similarities between decapod and vertebrate stress systems and behavioral responses to noxious stimuli were given as additional evidence for the capacity of decapods to experience pain.[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." This conclusion is based on the lobster's simple nervous system. 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. Based on this evidence, they placed all decapod crustaceans into the same category of research-animal protection as vertebrates.

In 2021, following a review,[37] the United Kingdom officially recognized decapod crustaceans and cephalopods as sentient beings capable of experiencing pain.[38]

Evolution

[edit]
Reconstruction of Eocarcinus, the earliest known crab

The earliest unambiguous crab fossils date from the Early Jurassic, with the oldest being Eocarcinus from the early Pliensbachian of Britain, which likely represents a stem-group lineage, as it lacks several key morphological features that define modern crabs.[39][40] Most Jurassic crabs are only known from dorsal (top half of the body) carapaces, making it difficult to determine their relationships.[41] Crabs radiated in the Late Jurassic, corresponding with an increase in reef habitats, though they would decline at the end of the Jurassic as the result of the decline of reef ecosystems. Crabs increased in diversity through the Cretaceous and represented the dominant group of decapods by the end of the period.[42]

The crab infraorder Brachyura belongs to the group Reptantia, which consists of the walking/crawling decapods (lobsters and crabs). Brachyura is the sister clade to the infraorder Anomura, which contains the hermit crabs and relatives. The cladogram below shows Brachyura's placement within the larger order Decapoda, from analysis by Wolfe et al., 2019.[43]


 Decapoda 
     

Dendrobranchiata (prawns)

 Pleocyemata 

Stenopodidea (boxer shrimp)

Procarididea

Caridea ("true" shrimp)

 

 Reptantia 

Achelata (spiny lobsters and slipper lobsters)

Polychelida (benthic crustaceans)

Astacidea (lobsters and crayfish)

Axiidea (mud shrimp, ghost shrimp, and burrowing shrimp)

Gebiidea (mud lobsters and mud shrimp)

Anomura (hermit crabs and allies)

Brachyura ("true" crabs)

(crawling / 
walking 
decapods)
 
 


Brachyura is separated into several sections, with the basal Dromiacea diverging the earliest in the evolutionary history, around the Late Triassic or Early Jurassic. The group consisting of Raninoida and Cyclodorippoida split off next, during the Jurassic period. The remaining clade Eubrachyura then divided during the Cretaceous period into Heterotremata and Thoracotremata. A summary of the high-level internal relationships within Brachyura can be shown in the cladogram below: [44] [43]

Brachyura

There is a no consensus on the relationships of the subsequent superfamilies and families. The proposed cladogram below is from analysis by Tsang et al, 2014:[44]

Brachyura
Dromiacea
Dromioidea

Dromiidae (may be paraphyletic)

Dynomenidae

Homoloidea

Homolidae (paraphyletic)

Latreilliidae

Eubrachyura
Heterotremata
Thoracotremata

Classification

[edit]

The infraorder Brachyura contains approximately 7,000 species in 98 families,[44][22] as many as the remainder of the Decapoda.[45] The evolution of crabs is characterized by an increasingly robust body, and a reduction in the abdomen. Although many other groups have undergone similar processes, carcinisation is most advanced in crabs. 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.

In most decapods, the gonopores (sexual openings) are found on the legs. Since crabs use their first two pairs of pleopods (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a slimmer shape, the gonopores have moved toward the midline, away from the legs, and onto the sternum.[46] A similar change occurred, independently, with the female gonopores. The movement of the female gonopore to the sternum defines the clade Eubrachyura, and the later change in the position of the male gonopore defines the Thoracotremata. 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.[45]

Superfamilies

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Numbers of extant and extinct (†) species are given in brackets.[1] The superfamily Eocarcinoidea, containing Eocarcinus and Platykotta, was formerly thought to contain the oldest crabs; it is now considered part of the Anomura.[47]

Examples of different crab sections
Dromia personata (Dromiacea: Dromiidae)
Ranina ranina (Raninoida: Raninidae)
Corystes cassivelaunus (Heterotremata: Corystidae)
Ocypode quadrata (Thoracotremata: Ocypodidae)
Goneplax rhomboides

Recent studies have found the following superfamilies and families to not be monophyletic, but rather paraphyletic or polyphyletic:[44][43][50][49]

Cultural influences

[edit]
A crab divination pot in Kapsiki, North Cameroon.

Both the constellation Cancer and the astrological sign Cancer are named after the crab, and depicted as a crab. William Parsons, 3rd Earl of Rosse drew the Crab Nebula in 1848 and noticed its similarity to the animal; the Crab Pulsar lies at the centre of the nebula.[51] The Moche people of ancient Peru worshipped nature, especially the sea,[52] and often depicted crabs in their art.[53] In Greek mythology, Karkinos was a crab that came to the aid of the Lernaean Hydra as it battled 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.[54] The auction for the crab quota in 2019, Russia is the largest revenue auction in the world except the spectrum auctions.[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".[55]: 7–8 

The Kapsiki people of North Cameroon use the way crabs handle objects for divination.[citation needed]

The term crab mentality is derived from a type of detrimental social behavior observed in crabs.

Explanatory notes

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  1. ^ Greek: βραχύς, romanized: brachys = short,[2] οὐρά / Greek: oura = tail[3]

References

[edit]
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  2. ^ Henry George Liddell; Robert Scott. "βραχύς". A Greek–English Lexicon. Perseus Digital Library. Retrieved May 24, 2010.
  3. ^ Henry George Liddell; Robert Scott. "οὐρά". A Greek–English Lexicon. Perseus Digital Library. Retrieved May 24, 2010.
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  5. ^ P. Chen; A.Y. Lin; J. McKittrick; M.A. Meyers (May 2008). "Structure and mechanical properties of crab exoskeletons". Acta Biomaterialia. 4 (3): 587–596. doi:10.1016/j.actbio.2007.12.010. PMID 18299257.
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