Crabs are decapod crustacean
s of the infraorder
Brachyura, which typically have a very short projecting "tail
) ( el|βραχύς
|translit=brachys = short, / = tail), usually hidden entirely under the thorax
. They live in all the world's oceans, in fresh water, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers. Many other animals with similar names – such as hermit crab
s, king crab
s, porcelain crab
s, horseshoe crab
s, stone crabs, and crab lice
– are not true crabs, but many have evolved features similar to true crabs through a process known as carcinisation
Crabs are generally covered with a thick exoskeleton
, composed primarily of highly mineralized chitin
, and armed with a pair of chelae
(claws). Crabs are found in all of the world's oceans, while many crabs live in fresh water
and on land
, particularly in tropical
regions. Crabs vary in size from the pea crab
, a few millimeters wide, to the Japanese spider crab
, with a leg span up to .
About 850 species
crabs, brackish water
crabs, freshwater crabs
, terrestrial crabs
or semiterrestrial crabs.
They are found throughout the world's tropical and semitropical
regions. They were previously thought to be a monophyletic group
, but are now believed to represent at least two distinct lineages
, one in the Old World
and one in the New World
The earliest unambiguous crab fossil
s 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. Most Jurassic crabs are only known from dorsal
(top half of the body) carapaces
, making it difficult to determine their relationships. Crabs would radiate
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.
Crabs often show marked sexual dimorphism
. Males often have larger claws
, a tendency that is particularly pronounced in the fiddler crab
s of the genus ''Uca'' (Ocypodidae
). In fiddler crabs, males have one greatly enlarged claw used for communication, particularly for attracting a mate. 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. This is because female crabs brood fertilised eggs on their pleopod
Reproduction and lifecycle
Crabs attract a mate through chemical (pheromone
s), 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 diel cycle. 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.
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.
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.
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 hormone
s. 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
s, 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.
Crabs typically walk sideways
(a behaviour which gives us the word crabwise
), because of the articulation of the legs which makes a sidelong gait more efficient.
However, some crabs walk forwards or backwards, including raninids
, ''Libinia emarginata
'' and ''Mictyris platycheles
Some crabs, notably the Portunidae
, are also capable of swimming,
especially so as their last pair of walking legs is flattened into swimming paddles.
Crabs are mostly active animals with complex behaviour patterns such as communicating by drumming or waving their pincers
. Crabs tend to be aggressive towards one another, and males often fight to gain access to females. On rocky seashores
, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes. Fiddler crab
s (genus ''Uca'') dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders.
Crabs are omnivore
s, feeding primarily on alga
e, and taking any other food, including mollusc
s, other crustacean
, 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
. However, some species are more specialised in their diets. Some eat plankton, some eat primarily shellfish such as clams, and some even catch fish.
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.
Crabs make up 20% of all marine crustacean
s caught, farmed, and consumed worldwide, amounting to 1.5 million tonne
s 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.
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 belief the crab will survive and regenerate the claws.
Crabs are prepared and eaten as a dish in many 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 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
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.
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.
, 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.
[Daniel P. Puzo (February 14, 1985]
Imitation Crab Draws Criticisms
''Los Angeles Times''
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.
Crabs are often boiled alive. In 2005, Norwegian scientists concluded that crustaceans could not feel pain. However, a study by Bob Elwood and Mirjam Appel of Queens University
, found that hermit crabs reacted to electric shocks. This may indicate that some crustaceans are able to feel and remember pain.
Brachyura contains 6,793 species in 93 families
as many as the remainder of the Decapoda
The evolution of crabs is characterised 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 uropod
s are absent, having probably evolved into small devices for holding the reduced abdomen tight against the sternum
In most decapods, the gonopore
s (sexual openings) are found on the legs. However, since crabs use the first two pairs of pleopod
s (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a slimmer 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 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.
Numbers of extant and extinct (†) species are given in brackets.
The superfamily Eocarcinoidea
, containing ''Eocarcinus
'' and ''Platykotta
'', was formerly thought to contain the oldest crabs; it is now considered part of the Anomura
* Section †Callichimaeroida
* Section Dromiacea
* Section Raninoida
* Section Cyclodorippoida
* Section Eubrachyura
** Subsection Heterotremata
** Subsection Thoracotremata
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. The Moche
people of ancient Peru
worshipped nature, especially the sea, and often depicted crabs in their art. In Greek mythology
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. The auction for the crab quota in 2019, Russia is the largest revenue auction in the world except the spectrum auctions.
The Kapsiki people
of North Cameroon use the way crabs handle objects for divination
Category:Extant Jurassic first appearances