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Crabs are decapod of the infraorder Brachyura (from Ancient Greek βραχύς ( brakhús), meaning "short", and οὐρά ( ourá), meaning "tail") which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. Their exoskeleton is often Sclerotization. They generally have Arthropod leg, and they have "pincers" or "" on the ends of the frontmost pair, scientifically termed the chelae. They are present in all the world's oceans, Freshwater crab, and Terrestrial crab, often hiding themselves in small crevices or burrowing into sediment. Crabs are , feeding on a variety of food, including a significant proportion of Algae eater, as well as Detritivore and other . Crab meat by humans as food, with over 1.5 million tonnes Crab fisheries annually.
True crabs first appeared in the fossil record during the Jurassic period, around 200 million years ago, achieving great diversity by the Cretaceous period; around 7,000 extant species in 96 families are known. A number of other crustacean groups converged on the body plan of crabs, sometimes closely resembling them; this phenomenon is called carcinisation.
Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to . Several other groups of crustaceans with similar appearances – such as and – are not true crabs, but have evolved features similar to true crabs through a process known as carcinisation.
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 tide and light/dark diel cycle. The free-swimming tiny Zoea 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 Egg 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 Ecdysis, 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 Animal migration 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 . 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, , 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 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 shore, where nearly all caves and crevices are occupied, crabs may also fight over hiding holes. (2025). 9781842367155, Miles Kelly Publishing. ISBN 9781842367155 (genus Uca) dig burrows in sand or mud, which they use for resting, hiding, and mating, and to defend against intruders.
Crabs are , feeding primarily on , and taking any other food, including , , other , 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. Some species are more specialised in their diets, based in plankton, clams or fish. Some crabs, such as members of Pinnotheridae, live on or inside animals such as Bivalvia, either being Commensalism (benign) or Parasitism on their host.
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.
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.
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:
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:
In most decapods, the (sexual openings) are found on the legs. Since crabs use their first two pairs of (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. 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 Monophyly is formed by those crabs where the female, but not male, gonopores are situated on the sternum.
Recent studies have found the following superfamilies and families to not be monophyletic, but rather paraphyletic or polyphyletic:
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.
In some regions, spices improve the culinary experience. In Southeast Asia and the Indosphere, spice mix 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.
Crab stick, 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.
The Kapsiki people of North Cameroon use the way crabs handle objects for divination.
The term crab mentality is derived from a type of detrimental social behavior observed in crabs.
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