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41-455: A crab is a type of crustacean. Crab or crabs may also refer to: Crab 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 . They live in all the world's oceans, in freshwater , and on land . They are generally covered with

82-573: 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 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

123-423: 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

164-411: A digesting fluid is secreted into the space between them. However, this fluid remains inactive until the upper part of the new cuticle has been formed. Then, by crawling movements, the organism pushes forward in the old integumentary shell , which splits down the back allowing the animal to emerge. Often, this initial crack is caused by a combination of movement and increase in pressure of hemolymph within

205-493: 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

246-492: A long-lived insect; this can make it difficult to identify an individual if it has recently undergone ecdysis. Ecdysis allows damaged tissue and missing limbs to be regenerated or substantially re-formed. Complete regeneration may require a series of moults, the stump becoming a little larger with each moult until the limb is a normal, or near normal, size. The term ecdysis comes from Ancient Greek ἐκδύω ( ekduo ) 'to take off, strip off'. In preparation for ecdysis,

287-419: 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

328-486: 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. 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

369-480: 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. 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 pleopods . Crabs attract

410-570: 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. Crabs are generally covered with a thick exoskeleton , composed primarily of highly mineralized chitin . 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

451-426: Is achieved by transfer of body fluids from soft parts before the new skin hardens. A spider with a small abdomen may be undernourished but more probably has recently undergone ecdysis. Some arthropods, especially large insects with tracheal respiration, expand their new exoskeleton by swallowing or otherwise taking in air. The maturation of the structure and colouration of the new exoskeleton might take days or weeks in

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492-403: 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. A European Food Safety Authority (EFSA) 2005 publication stated that

533-441: 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 . 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

574-592: 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

615-400: Is described as teneral , a callow ; it is "fresh", pale and soft-bodied. Within one or two hours, the cuticle hardens and darkens following a tanning process analogous to the production of leather . During this short phase the animal expands, since growth is otherwise constrained by the rigidity of the exoskeleton. Growth of the limbs and other parts normally covered by the hard exoskeleton

656-411: Is often a chief concern. 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. 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. Advocates for Animals,

697-792: 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. For

738-513: 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

779-495: The Mygalomorphae are very long-lived, sometimes 20 years or more; they moult annually even after they mature. Spiders stop feeding at some time before moulting, usually for several days. The physiological processes of releasing the old exoskeleton from the tissues beneath typically cause various colour changes, such as darkening. If the old exoskeleton is not too thick it may be possible to see new structures, such as setae , from

820-642: The pea crab , a few millimeters wide, to the Japanese spider crab , with a leg span up to 4 m (13 ft). 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 . 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 . Crabs often show marked sexual dimorphism . Males often have larger claws ,

861-470: 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

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902-1084: The Cretaceous and represented the dominant group of decapods by the end of the period. 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. Dendrobranchiata (prawns) [REDACTED] Stenopodidea (boxer shrimp) [REDACTED] Procarididea Caridea ("true" shrimp) [REDACTED] Achelata (spiny lobsters and slipper lobsters) [REDACTED] Polychelida (benthic crustaceans) Astacidea (lobsters and crayfish) [REDACTED] Axiidea (mud shrimp, ghost shrimp, and burrowing shrimp) Gebiidea (mud lobsters and mud shrimp) [REDACTED] Anomura (hermit crabs and allies) [REDACTED] Brachyura ("true" crabs) [REDACTED] Brachyura

943-410: The arthropod becomes inactive for a period of time, undergoing apolysis or separation of the old exoskeleton from the underlying epidermal cells. For most organisms, the resting period is a stage of preparation during which the secretion of fluid from the moulting glands of the epidermal layer and the loosening of the underpart of the cuticle occurs. Once the old cuticle has separated from the epidermis,

984-481: The body, forcing an expansion across its exoskeleton , leading to an eventual crack that allows for certain organisms such as spiders to extricate themselves. While the old cuticle is being digested, the new layer is secreted. All cuticular structures are shed at ecdysis, including the inner parts of the exoskeleton, which includes terminal linings of the alimentary tract and of the tracheae if they are present. Each stage of development between moults for insects in

1025-466: The crab may 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

1066-693: The crab species. For many crabs, a mixed diet of plant and animal matter results in the fastest growth and greatest fitness . Some species are more specialised in their diets, based in plankton, clams or 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 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

1107-416: The egg sac, and the spiderling that emerges broadly resembles the adult. The number of moults varies, both between species and sexes, but generally will be between five times and nine times before the spider reaches maturity. Not surprisingly, since males are generally smaller than females, the males of many species mature faster and do not undergo ecdysis as many times as the females before maturing. Members of

1148-426: 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,

1189-478: The entire process, either dangling from a drop line, or fastening their claws into webbed fibres attached to a suitable base. The discarded, dried exoskeleton typically remains hanging where it was abandoned once the spider has left. To open the old exoskeleton, the spider generally contracts its abdomen ( opisthosoma ) to supply enough fluid to pump into the prosoma with sufficient pressure to crack it open along its lines of weakness. The carapace lifts off from

1230-436: 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

1271-464: The front, like a helmet, as its surrounding skin ruptures, but it remains attached at the back. Now the spider works its limbs free and typically winds up dangling by a new thread of silk attached to its own exuviae, which in turn hang from the original silk attachment. At this point the spider is a callow; it is teneral and vulnerable. As it dangles, its exoskeleton hardens and takes shape. The process may take minutes in small spiders, or some hours in

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1312-454: 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 knowledge that

1353-530: The high-level internal relationships within Brachyura can be shown in the cladogram below: Dromiacea Raninoida Cyclodorippoida Heterotremata Thoracotremata 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: Dromiidae (may be paraphyletic) Dynomenidae Homolidae (paraphyletic) Ecdysis After moulting, an arthropod

1394-514: 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. The earliest unambiguous crab fossils date from the Early Jurassic , with the oldest being Eocarcinus from the early Pliensbachian of Britain, which likely represents

1435-436: 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 (hence

1476-401: The new epicuticle is secreted. This prevents the new procuticle from getting digested as it is laid down. The lower regions of the old cuticle, the endocuticle and mesocuticle , are then digested by the enzymes and subsequently absorbed. The exocuticle and epicuticle resist digestion and are hence shed at ecdysis. Spiders generally change their skin for the first time while still inside

1517-470: The outside. However, contact between the nerves and the old exoskeleton is maintained until a very late stage in the process. The new, teneral exoskeleton has to accommodate a larger frame than the previous instar , while the spider has had to fit into the previous exoskeleton until it has been shed. This means the spider does not fill out the new exoskeleton completely, so it commonly appears somewhat wrinkled. Most species of spiders hang from silk during

1558-443: The separation of the cuticle from the underlying epidermal cells (apolysis) and ends with the shedding of the old cuticle (ecdysis). In many species it is initiated by an increase in the hormone ecdysone . This hormone causes: After apolysis the insect is known as a pharate . Moulting fluid is then secreted into the exuvial space between the old cuticle and the epidermis, this contains inactive enzymes which are activated only after

1599-469: 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

1640-484: The taxon Endopterygota is called an instar , or stadium, and each stage between moults of insects in the Exopterygota is called a nymph: there may be up to 15 nymphal stages. Endopterygota tend to have only four or five instars. Endopterygotes have more alternatives to moulting, such as expansion of the cuticle and collapse of air sacs to allow growth of internal organs. The process of moulting in insects begins with

1681-1147: The term crabwise ), because of the articulation of the legs which makes a sidelong gait more efficient. Some crabs walk forward or backward, including raninids , Libinia emarginata and Mictyris platycheles . Some crabs, like the Portunidae and Matutidae , are also capable of swimming, the Portunidae especially so as their last pair of walking legs are 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 toward 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 crabs (genus Uca ) dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders. Crabs are omnivores , feeding primarily on algae , and taking any other food, including molluscs , worms , other crustaceans , fungi , bacteria , and detritus , depending on their availability and

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