Crab

Crab

Overview
True crabs are decapod
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

 crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s of the infraorder Brachyura, which typically have a very short projecting "tail
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...

" ( = short, = tail), or where the reduced abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 is entirely hidden under the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

. Many other animals with similar names – such as hermit crab
Hermit crab
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...

s, king crab
King crab
King crabs, also called stone crabs, are a superfamily of crab-like decapod crustaceans chiefly found in cold seas. Because of their large size and the taste of their meat, many species are widely caught and sold as food, the most common being the red king crab, Paralithodes camtschaticus.King...

s, porcelain crab
Porcelain crab
Porcelain crabs are decapod crustaceans in the widespread family Porcellanidae, which superficially resemble true crabs. They are typically less than wide, and have flattened bodies as an adaptation for living in rock crevices...

s, horseshoe crab
Horseshoe crab
The Atlantic horseshoe crab, Limulus polyphemus, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America...

s and crab lice
Crab louse
Crab lice are parasitic insects notorious for infesting human pubic hair. The species may also live on other areas with hair, including the eyelashes. They feed exclusively on blood...

 – are not true crabs.


Crabs are generally covered with a thick exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

, and armed with a single pair of chelae
Chela (organ)
A chela is a pincer-like organ terminating certain limbs of some arthropods. The name comes from Greek through New Latin . The plural form is chelae. Legs bearing a chela are called chelipeds. Another name is claw because most chelae are curved and have a sharp point like a claw....

 (claws). Crabs are found in all of the world's oceans, while many crabs live in fresh water
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 and on land
Land crab
Gecarcinidae is a family of true crabs that are adapted for terrestrial existence, commonly known as land crabs. Similar to all other crabs, land crabs possess a series of gills. In addition, the part of the carapace covering the gills is inflated and equipped with blood vessels. These organs...

, particularly in tropical regions.
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Encyclopedia
True crabs are decapod
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

 crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s of the infraorder Brachyura, which typically have a very short projecting "tail
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...

" ( = short, = tail), or where the reduced abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 is entirely hidden under the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

. Many other animals with similar names – such as hermit crab
Hermit crab
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...

s, king crab
King crab
King crabs, also called stone crabs, are a superfamily of crab-like decapod crustaceans chiefly found in cold seas. Because of their large size and the taste of their meat, many species are widely caught and sold as food, the most common being the red king crab, Paralithodes camtschaticus.King...

s, porcelain crab
Porcelain crab
Porcelain crabs are decapod crustaceans in the widespread family Porcellanidae, which superficially resemble true crabs. They are typically less than wide, and have flattened bodies as an adaptation for living in rock crevices...

s, horseshoe crab
Horseshoe crab
The Atlantic horseshoe crab, Limulus polyphemus, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America...

s and crab lice
Crab louse
Crab lice are parasitic insects notorious for infesting human pubic hair. The species may also live on other areas with hair, including the eyelashes. They feed exclusively on blood...

 – are not true crabs.

Evolution



Crabs are generally covered with a thick exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

, and armed with a single pair of chelae
Chela (organ)
A chela is a pincer-like organ terminating certain limbs of some arthropods. The name comes from Greek through New Latin . The plural form is chelae. Legs bearing a chela are called chelipeds. Another name is claw because most chelae are curved and have a sharp point like a claw....

 (claws). Crabs are found in all of the world's oceans, while many crabs live in fresh water
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 and on land
Land crab
Gecarcinidae is a family of true crabs that are adapted for terrestrial existence, commonly known as land crabs. Similar to all other crabs, land crabs possess a series of gills. In addition, the part of the carapace covering the gills is inflated and equipped with blood vessels. These organs...

, particularly in tropical regions. Crabs vary in size from the pea crab
Pea crab
The New Zealand pea crab, Pinnotheres novaezelandiae, is a small, parasitic crab that lives most commonly inside New Zealand green-lipped mussels. Adult females are about the size and shape of a pea, while adult males are smaller and flatter. Adult New Zealand pea crabs are completely reliant on...

, a few millimetres wide, to the Japanese spider crab
Japanese spider crab
The , Macrocheira kaempferi, is a species of marine crab that lives in the waters around Japan. It has the largest leg span of any arthropod, reaching up to and weighing up to . It is the subject of small-scale fishery.-Description:...

, with a leg span of up to 4 metres (13.1 ft).

About 850 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of crab are freshwater, terrestrial or semi-terrestrial species; they are found throughout the world's tropical and semi-tropical
Subtropics
The subtropics are the geographical and climatical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...

 regions. They were previously thought to be a monophyletic group, but are now believed to represent at least two distinct lineages
Lineage (evolution)
An evolutionary lineage is a sequence of species, that form a line of descent, each new species the direct result of speciation from an immediate ancestral species. Lineages are subsets of the evolutionary tree of life. Lineages are often determined by the techniques of molecular systematics.-...

, one in the Old World
Old World
The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....

 and one in the New World
New World
The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...

.

The earliest unambiguous crab fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s date from the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, although Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 Imocaris, known only from its carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

, may be a primitive crab. The radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...

 of crabs in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 and afterward may be linked either to the break-up of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 or to the concurrent radiation of bony fish
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...

, crabs' main predators.

Sexual dimorphism



Crabs often show marked sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

. Males often have larger claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...

s, a tendency which is particularly pronounced in the fiddler crab
Fiddler crab
A fiddler crab, sometimes known as a calling crab, may be any of approximately 100 species of semi-terrestrial marine crabs which make up the genus Uca. As members of the family Ocypodidae, fiddler crabs are most closely related to the ghost crabs of the genus Ocypode. This entire group is composed...

s of the genus Uca (Ocypodidae
Ocypodidae
Ocypodidae is a family of crabs that includes the ghost crabs and fiddler crabs. Other genera previously included in the family are now treated as members of separate families in the superfamily Ocypodoidea, such as Ucididae, Dotillidae and Macrophthalmidae....

). In fiddler crabs, males have one claw which is greatly enlarged and which is used for communication, particularly for attracting a mate. Another conspicuous difference is the form of the pleon (abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

); in most male crabs, this is narrow and triangular in form, while females have a broader, rounded abdomen. This is due to the fact that female crabs brood fertilised eggs on their pleopods.

Behaviour



Crabs typically walk sideways (a behaviour which gives us the word crabwise). This is because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs walk forwards or backwards, including raninids
Raninidae
Raninidae is a family of unusual crabs named for their frog-like appearance, taken by most scientists to be quite primitive. These animals closely resemble the mole crabs, due to parallel evolution or convergent evolution. In both groups, the claws are modified into tools for digging, and the body...

, Libinia emarginata
Portly spider crab
Libinia emarginata, the portly spider crab, common spider crab or nine-spined spider crab, is a species of stenohaline crab that lives on the Atlantic coast of North America.-Distribution:...

and Mictyris platycheles
Mictyris platycheles
Mictyris platycheles is a species of crab found on Australian mudflats. It is found on the east coast of Australia from Tasmania and Victoria to Queensland. They live in large groups and are therefore commonly called soldier crabs. Adults are across and similarly tall, and are coloured blue to...

. Some crabs, notably the Portunidae
Portunidae
Portunidae is a family of crabs which contains the swimming crabs.-Description:Portunid crabs are characterised by the flattening of the fifth pair of legs into broad paddles, which are used for swimming...

 and Matutidae
Matutidae
Matutidae is a family of crabs, sometimes called moon crabs, adapted for swimming or digging. They differ from the swimming crabs of the family Portunidae in that all five pairs of legs are flattened, rather than just the last pair, as in Portunidae...

, are also capable of swimming.

Crabs are mostly active animals with complex behaviour patterns. They can communicate by drumming or waving their pincers
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...

. Crabs tend to be aggressive towards one another and males often fight to gain access to females. On rocky seashore
Shore
A shore or shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In Physical Oceanography a shore is the wider fringe that is geologically modified by the action of the body of water past and present, while the beach is at the edge of the shore,...

s, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes.

Crabs are omnivore
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...

s, feeding primarily on algae, and taking any other food, including molluscs, worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

s, other crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, fungi, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and detritus
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...

, 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
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

.

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.

Fisheries




Crabs make up 20% of all marine crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s caught, farmed, and consumed worldwide, amounting to 1½ million tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...

s annually. One species, Portunus trituberculatus
Portunus trituberculatus
Portunus trituberculatus, the gazami crab, Japanese blue crab or horse crab, is the most widely fished species of crab in the world. It is found off the coasts of East Asia and is closely related to Portunus pelagicus.-Fishery:P...

, accounts for one fifth of that total. Other commercially important taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 include Portunus pelagicus
Portunus pelagicus
Portunus pelagicus, also known as the flower crab, blue crab, blue swimmer crab, blue manna crab or sand crab, is a large crab found in the intertidal estuaries of the Indian and Pacific Oceans and the Middle-Eastern coast of the Mediterranean Sea. The name "flower crab" is used in east Asian...

, several species in the genus Chionoecetes
Chionoecetes
Chionoecetes is a genus of crabs that live in the northern Pacific and Atlantic Oceans.Other names for crabs in this genus include "queen crab" and "spider crab" – they are known by different names in different areas of the world....

, the blue crab (Callinectes sapidus), Charybdis spp.
Charybdis (genus)
Charybdis is a genus of swimming crabs in the family Portunidae; "Charybdis" is Greek for whirlpool.-Species:The genus Charybdis contains the following species:Subgenus Charybdis De Haan, 1833*Charybdis acuta Charybdis is a genus of swimming crabs in the family Portunidae; "Charybdis" is Greek for...

, Cancer pagurus
Edible crab
Cancer pagurus, commonly known as the edible crab or brown crab, is a species of crab found in the North Sea, North Atlantic Ocean and perhaps in the Mediterranean Sea. It is a robust crab of a reddish-brown colour, having an oval carapace with a characteristic "pie crust" edge and black tips to...

, the Dungeness crab
Dungeness crab
The Dungeness crab, Metacarcinus magister , is a species of crab that inhabits eelgrass beds and water bottoms on the west coast of North America. It typically grows to across the carapace and is a popular seafood...

 (Metacarcinus magister) and Scylla serrata
Scylla serrata
Scylla serrata is an economically important crab species found in the estuaries and mangroves of Africa, Australia and Asia. In their most common form, the shell colour varies from a deep, mottled green to very dark brown...

, each of which yields more than 20,000 tonnes annually.

Cookery




Crabs are prepared and eaten as a dish in several different ways all over the world. Some species are eaten whole, including the shell, such as soft-shell crab; with other species just the claws and/or legs are eaten. The latter is particularly common for larger crabs, such as the snow crab. Mostly in East Asia
East Asia
East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms...

n cultures, the roe
Roe
Roe or hard roe is the fully ripe internal egg masses in the ovaries, or the released external egg masses of fish and certain marine animals, such as shrimp, scallop and sea urchins...

 of the female crab is also eaten, which usually appears orange or yellow in color in fertile crabs.

In some regions spices improve the culinary experience. In Asia, masala
Masala
Masala or massala is a term used in South Asian cuisines to mostly describe a mixture of spices. A masala can either be a combination of dried spices, or a paste made from a mixture of spices and other ingredients—often garlic, ginger, onions and chilli paste...

 crab and chilli crab
Chilli crab
Chili crab is a seafood dish originating from Singapore. It was created in 1950 by Singapore chef, Cher Yam Tian with her husband, Lim Choon Ngee. The couple ran Palm Beach Seafood Restaurant on Upper East Coast Road Mud crabs are commonly used and are stir-fried in a semi-thick, sweet and...

 are examples of heavily spiced dishes. In the Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

 region, blue crab is often eaten with Old Bay Seasoning
Old Bay Seasoning
Old Bay Seasoning is a blend of herbs and spices that is currently marketed in the United States by McCormick & Company, and produced in Maryland. It is produced in the Chesapeake Bay area where it was developed by German immigrant Gustav Brunn in the 1940s, and where the seasoning is very popular...

.

For the British dish Cromer crab, the crab meat
Crab meat
Crab meat or Crabmeat is the meat found within a crab. It is used in many cuisines across the world, prized for its soft, delicate taste. Brown crab , blue crabs , blue swimming crabs , red swimming crabs are among the most commercially available species of crabmeat globally.-Grades:In Western...

 is extracted and placed inside the hard shell. One American way to prepare crab meat is by extracting it and adding a flour mix, creating a crab cake
Crab cake
A crab cake is an American dish composed of crab meat and various other ingredients, such as bread crumbs, milk, mayonnaise, eggs, yellow onions, and seasonings. Occasionally other ingredients such as red or green peppers or pink radishes are added, at which point the cake is then sautéed, baked,...

.

Crabs are also used in bisque
Bisque (food)
Bisque is a smooth, creamy, highly-seasoned soup of French origin, classically based on a strained broth of crustaceans. It can be made from lobster, crab, shrimp or crayfish...

, a global dish of French origin.

Pain



Live crabs are often boiled. In 2005, Norwegian scientists concluded that lobster
Lobster
Clawed lobsters comprise a family of large marine crustaceans. Highly prized as seafood, lobsters are economically important, and are often one of the most profitable commodities in coastal areas they populate.Though several groups of crustaceans are known as lobsters, the clawed lobsters are most...

s cannot feel pain. However, later research suggests that crustaceans are indeed able to feel and remember pain.

Classification


The infraorder Brachyura contains 6,793 species in 93 families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

, as many as the remainder of the Decapoda
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

. The evolution of crabs is characterised by an increasingly robust body, and a reduction in the abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

. Although many other groups have undergone similar processes, carcinisation
Carcinisation
In evolutionary biology, carcinisation is a hypothesised process whereby a crustacean evolves into a crab-like form from a non-crab-like form. The term was introduced by L. A...

 is most advanced in crabs. The telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...

 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
Sternum (arthropod)
The sternum is the ventral portion of a segment of an arthropod thorax or abdomen.In insects, the sterna are usually single, large sclerites, and external...

.

In most decapods, the gonopore
Gonopore
A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores...

s (sexual openings) are found on the legs. However, since crabs use the first two pairs of pleopods (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a narrower shape, the gonopores have moved towards the midline, away from the legs, and onto the sternum. A similar change occurred, independently, with the female gonopores. The movement of the female gonopore to the sternum defines the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Eubrachyura
Eubrachyura
Eubrachyura is a group of crustaceans comprising the more advanced crabs. It is divided into two subsections, based on the position of the genital openings in the two sexes. In the Heterotremata, the openings are on the legs in the males, but on the sternum in females, while in the Thoracotremata,...

, and the later change in the position of the male gonopore defines the Thoracotremata
Thoracotremata
Thoracotremata is a clade of crabs, comprising those crabs in which the genital openings are on the sternum, rather than on the legs in both sexes. It comprises 17 families in four superfamilies .-Superfamilies and...

. It is still a subject of debate whether those crabs where the female, but not male, gonopores are situated on the sternum, form a monophyletic group
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

.

Superfamilies






Numbers of extant and extinct (†) species are given in brackets. The superfamily Eocarcinoidea
Eocarcinoidea
Eocarcinoidea is a family of fossil decapod crustaceans. Formerly thought to be the earliest true crabs, they are now thought to be the oldest members of the Anomura. Two species are included, each in its own family: Eocarcinus praecursor and Platykotta akaina ....

, containing Eocarcinus
Eocarcinus
Eocarcinus praecursor is a Jurassic species of decapod crustacean, sufficiently distinct from its relatives to be placed in its own family...

and Platykotta
Platykotta
Platykotta akaina is a species of decapod crustacean from the Triassic of the United Arab Emirates. It is the oldest known fossil from the infraorder Anomura, and is most closely related to Eocarcinus praecursor.-Stratigraphy:The holotype of P...

, was formerly thought to contain the oldest crabs; it is now considered part of the Anomura
Anomura
Anomura is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans includes the word crab, all true crabs are in the sister group to the Anomura, the Brachyura .-Description:The name Anomala reflects the unusual variety of forms in this group;...

.
  • Section Dromiacea
    Dromiacea
    Dromiacea is a group of crabs, ranked as a section. It contains 240 extant and nearly 300 extinct species. Where they are considered to form a monophyletic group, Dromiacea and two other groups of crabs, namely the Raninoida and Cyclodorippoidea, may be collected together into the Podotremata, each...

    • Dakoticancroidea
      Dakoticancroidea
      Dakoticancroidea is a superfamily of fossil crabs, containing six species in five genera, divided into two families. The family Dakoticancridae is only known from North American rocks of Late Cretaceous age, while the single species in the Ibericancridae was found in Spain.Dakoticancridae Rathbun,...

       (6†)
    • Dromioidea
      Dromioidea
      Dromioidea is a superfamily of crabs mostly found in Madagascar. The earliest fossils attributable to the Dromioidea date from the Danian....

       (147, 85†)
    • Glaessneropsoidea
      Glaessneropsoidea
      Glaessneropsoidea is a superfamily of fossil crabs. They are found in rocks from Late Jurassic age to Late Cretaceous. The 45 species in the superfamily are divided among 11 genera in four families:Family Glaessneropsidae Patrulius, 1959...

       (45†)
    • Homolodromioidea (24, 107†)
    • Homoloidea
      Homoloidea
      Homoloidea is a superfamily of dromiacean crabs. The close relation between Homoloidea and Dromioidea is primarily established through ultrastructural characteristics of the sperm ....

       (73, 49†)
  • Section Raninoida (46, 196†)
  • Section Cyclodorippoida (99, 27†)
  • Section Eubrachyura
    Eubrachyura
    Eubrachyura is a group of crustaceans comprising the more advanced crabs. It is divided into two subsections, based on the position of the genital openings in the two sexes. In the Heterotremata, the openings are on the legs in the males, but on the sternum in females, while in the Thoracotremata,...

    • Subsection Heterotremata
      Heterotremata
      Heterotremata is a clade of crabs, comprising those crabs in which the genital openings are on the sternum in females, but on the legs in males. It comprises 68 families in 28...

    • Aethroidea (37, 44†)
    • Bellioidea (7)
    • Bythograeoidea (14)
    • Calappoidea
      Calappoidea
      Calappoidea is a superfamily of crabs comprising the two families Calappidae and Matutidae. The earliest fossils attributable to the Callpoidea date from the Aptian....

       (101, 71†)
    • Cancroidea
      Cancroidea
      Cancroidea is a superfamily of crabs, comprising the families Atelecyclidae and Cancridae. Two families previously included in this superfamily have since been separated off into new superfamilies – Cheiragonidae into Cheiragonoidea, Corystidae into Corystoidea – while two others have been...

       (57, 81†)
    • Carpilioidea (4, 104†)
    • Cheiragonoidea (3, 13†)
    • Corystoidea (10, 5†)
    • Componocancroidea (1†)
    • Dairoidea
      Dairoidea
      Dairoidea is a superfamily of crabs, comprising two families which each contain a single genus: Dairidae and Dacryopilumnidae  .-Species:Dairidae*Daira americana Stimpson, 1860...

       (4, 8†)
    • Dorippoidea
      Dorippoidea
      Dorippoidea is a superfamily of crabs. The earliest fossils attributable to the Dorippoidea date from the Late Cretaceous....

       (101, 73†)
    • Eriphioidea
      Eriphioidea
      Eriphioidea is a superfamily of crabs, containing the six families Dairoididae, Eriphiidae, Hypothalassiidae, Menippidae, Oziidae and Platyxanthidae. They are united by a number of characters, including a marked difference in size between the left and right claws, where the larger one has a...

       (67, 14†)
    • Gecarcinucoidea
      Gecarcinucoidea
      Gecarcinucoidea is a superfamily of freshwater crabs. Its members have been grouped into families in various ways, with some authors recognising families such as "Deckeniidae", "Sundathelphusidae", but only two families are currently recognised: Gecarcinucidae and Parathelphusidae....

       (349)
    • Goneplacoidea
      Goneplacoidea
      Goneplacoidea is a superfamily of crabs containing 10 extant families, and two families known only from fossils....

       (182, 94†)
    • Hexapodoidea (21, 25†)
    • Leucosioidea
      Leucosioidea
      Leucosioidea is a superfamily of crabs containing the two families Leucosiidae and Iphiculidae....

       (488, 113†)
    • Majoidea
      Majoidea
      Majoidea is a superfamily of crabs. It comprises six families:*Epialtidae*Hymenosomatidae*Inachidae*Inachoididae*Majidae*OregoniidaeThe families "Pisidae" and "Tychidae" are now treated as the subfamilies Pisinae and Tychinae of the family Epialtidae, and "Mithracidae" is now treated as the...

       (980, 89†)
    • Orithyioidea (1)
    • Palicoidea
      Palicoidea
      Palicoidea is a superfamily of crabs, comprising the two families Crossotonotidae and Palicidae. Together, they contain 13 genera, including two genera in the Palicidae known only from fossils. The two families were previously treated as two subfamilies in a Palicidae of wider...

       (63, 6†)
    • Parthenopoidea
      Parthenopoidea
      Parthenopidae is a family of crabs, placed in its own superfamily, Parthenopoidea. It comprises nearly 40 genera, divided into two subfamilies, with three genera incertae sedis:Daldorfiinae Ng & Rodríguez, 1986*Daldorfia Rathbun, 1904...

       (144, 36†)
    • Pilumnoidea
      Pilumnoidea
      Pilumnoidea is a superfamily of crabs, whose members were previously included in the Xanthoidea. The three families are unified by the free articulation of all the segments of the male crab's abdomen and by the form of the gonopods...

       (405, 47†)
    • Portunoidea
      Portunoidea
      Portunoidea is a superfamily of crabs that includes the family Portunidae, the swimming crabs. Which other crab families are also placed here is a matter of some contention, and may be revised following molecular phylogenetic analyses.-Description:...

       (455, 200†)
    • Potamoidea
      Potamoidea
      Potamoidea is a superfamily of freshwater crabs, comprising the two families Potamidae and Potamonautidae. Two previously recognised families, Deckeniidae and Platythelphusidae, are now treated as parts of the family Potamonautidae....

       (662, 8†)
    • Pseudothelphusoidea (276)
    • Pseudozioidea
      Pseudozioidea
      Pseudozioidea is a superfamily of crabs, formerly treated in the Eriphioidea, Carpilioidea, Xanthoidea, Pilumnoidea and Goneplacoidea. A number of fossils from the Eocene onwards are known from the family Pseudoziidae...

       (22, 6†)
    • Retroplumoidea (10, 27†)
    • Trapezioidea
      Trapezioidea
      Trapezioidea is a superfamily of crabs. Its members live symbiotically with corals, and have a fossil record stretching back to the Eocene ....

       (58, 10†)
    • Trichodactyloidea (50)
    • Xanthoidea
      Xanthoidea
      Xanthoidea is a superfamily of crabs, comprising the three families Xanthidae, Panopeidae and Pseudorhombilidae. Formerly, a number of other families were included in Xanthoidea, but many of these have since been removed to other superfamilies. These include Carpilioidea, Eriphioidea, Hexapodoidea,...

       (736, 134†)
    • Subsection Thoracotremata
      Thoracotremata
      Thoracotremata is a clade of crabs, comprising those crabs in which the genital openings are on the sternum, rather than on the legs in both sexes. It comprises 17 families in four superfamilies .-Superfamilies and...

    • Cryptochiroidea (46)
    • Grapsoidea
      Grapsoidea
      Grapsoidea is a superfamily of crabs. These crabs are well known and contain many taxa which are terrestrial , semi-terrestrial or limnic...

       (493, 28†)
    • Ocypodoidea
      Ocypodoidea
      Ocypodoidea is a superfamily of crabs, named after the genus Ocypode. It contains over 300 extant species in the following eight families:* Camptandriidae Stimpson, 1858* Dotillidae Stimpson, 1858* Heloeciidae H. Milne-Edwards, 1852...

       (304, 14†)
    • Pinnotheroidea (304, 13†)

Cultural influences


Both the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Cancer
Cancer (constellation)
Cancer is one of the twelve constellations of the zodiac. Its name is Latin for crab and it is commonly represented as such. Its symbol is . Cancer is small and its stars are faint...

 and the astrological sign
Astrological sign
Astrological signs represent twelve equal segments or divisions of the zodiac. According to astrology, celestial phenomena reflect or govern human activity on the principle of "as above, so below", so that the twelve signs are held to represent twelve basic personality types or characteristic modes...

 Cancer
Cancer (astrology)
Cancer is the fourth astrological sign in the Zodiac. It is considered a water sign and one of four cardinal signs. Cancer is ruled by the Moon. Individuals born when the Sun is in this sign are considered Cancerian individuals...

 are named after the crab, and depicted as a crab. William Parsons, 3rd Earl of Rosse
William Parsons, 3rd Earl of Rosse
William Parsons, 3rd Earl of Rosse, Knight of the Order of St Patrick was an Irish astronomer who had several telescopes built. His 72-inch telescope "Leviathan", built 1845, was the world's largest telescope until the early 20th century.-Life:He was born in Yorkshire, England, in the city of...

 drew the Crab Nebula in 1848 and noticed its similarity to the animal; the Crab pulsar
Crab Pulsar
The Crab Pulsar is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054...

 lies at the centre of the nebula.

The Moche
Moche
'The Moche civilization flourished in northern Peru from about 100 AD to 800 AD, during the Regional Development Epoch. While this issue is the subject of some debate, many scholars contend that the Moche were not politically organized as a monolithic empire or state...

 people of ancient Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

 worshipped nature, especially the sea, and often depicted crabs in their art. In Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

, Karkinos was a crab that came to the aid of the Lernaean Hydra
Lernaean Hydra
In Greek mythology, the Lernaean Hydra was an ancient nameless serpent-like chthonic water beast, with reptilian traits, that possessed many heads — the poets mention more heads than the vase-painters could paint, and for each head cut off it grew two more — and poisonous breath so virulent even...

 as it battled Heracles
Heracles
Heracles ,born Alcaeus or Alcides , was a divine hero in Greek mythology, the son of Zeus and Alcmene, foster son of Amphitryon and great-grandson of Perseus...

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External links