crab
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What is a crab?
A crab is a short-tailed member of the crustacean order Decapoda, especially the brachyurans, or true crabs. They are found in all oceans, in fresh water, and on land. Crabs have a broad carapace (upper body shield) and their tails are curled under the thorax, or midsection. The first pair of legs is modified into chelae, or pincers.
Crabs play an important role as predators in aquatic ecosystems. Most crabs live in the sea, but land crabs are abundant in tropical countries. As a rule, crabs breathe by gills, which are lodged in a pair of cavities beneath the sides of the carapace.
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To which animal group do crabs belong?
Crabs belong to the subphylum Crustacea, a group of invertebrate animals within the phylum Arthropoda. Crabs are short-tailed members of the order Decapoda and include true crabs (Brachyura) and other forms like hermit crabs (Anomura). Arthropoda is the largest phylum in the animal kingdom. Crustaceans are mostly marine arthropods, but some, like crayfish, inhabit fresh water, and pill bugs live on land. There are roughly 45,000 species of crustaceans, including crabs, lobsters, shrimp, and woodlice.
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What are some main body parts of a crab?
Crabs, members of the decapod crustacean order, have bodies divided into three sections: the head, thorax, and abdomen. The head and thorax are fused, forming a cephalothorax covered by a carapace, which extends over the gills.
Key body parts include:
* Antennae: The first two pairs are sensory organs for touch, balance, and olfaction.
* Mandibles and Maxillae: The remaining head appendages function as food manipulators for crushing and chewing.
* Pereiopods: Thoracic appendages used as walking legs.
* Chelipeds: The pair of legs that bears large chelae (claws).
* Pleopods: Abdominal limbs used for swimming.
* Uropods: The last pair of abdominal limbs, which, along with the telson, form a tail fan.6 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Where do crabs usually live?
Crabs, as crustaceans, exhibit a wide distribution in diverse aquatic and terrestrial environments. The majority of crab species reside in marine environments; however, numerous crabs also inhabit freshwater habitats, and some are land-dwelling.
Most crabs breathe through gills, though land crabs have developed lung-like cavities for breathing air. They are typically omnivorous scavengers, while some are predators or vegetarians.
Examples of crab habitats:
* Marine crabs Most crabs live in the sea. Dungeness crabs live on the Pacific coast from Alaska to lower California.
* Freshwater crabs The river crab of southern Europe lives in freshwater.
* Land crabs Land crabs are abundant in tropical countries. Some land crabs live in fields, swamps, and mangrove thickets.
* Hermit crabs Hermit crabs live in empty shells discarded by gastropod mollusks.
* Other Some crabs live in coral, sponge, or the empty tubes formed by polychaete tubeworms.6 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do crabs use their claws?
Crabs use their claws, or chelae, for a variety of purposes, primarily related to feeding, defense, and communication.
- Feeding: Crabs use their claws to collect and handle food. Fiddler crabs, for example, use their small claws to scoop up sand, sift it for organic material, and then discard the remaining mineral material.
- Defense and Aggression: Crabs use their claws to compete for resources like food, shelter, and mates. They may engage in fights, ranging from ritualized displays to serious combat, using their claws as weapons.
- Communication: Male crabs, such as fiddler crabs, use their enlarged claws to attract females through waving and sound signals. These signals convey the sender's identity and intentions. Some species also display color-coded eyespots on their claws during posturing.
- Protection: Hermit crabs use claws as a defense by leaving only their claws exposed when they twist their bodies into shells for protection.
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What are some interesting behaviors or adaptations that help crabs survive?
Crabs have developed fascinating behaviors and adaptations to thrive in diverse environments. Many crabs utilize camouflage, blending with their surroundings to evade predators. Some species, like hermit crabs, find empty seashells for protection due to their soft exoskeletons, while decorator crabs disguise themselves with algae and sponges.
To deal with environmental challenges, crabs osmoregulate, maintaining internal balance of water and salts, especially when salinity changes. Fiddler crabs exhibit territorial behavior, with males displaying brightly colored claws and performing rhythmic dances to attract females. For locomotion, most crabs walk or crawl, but some, like the Portunidae family, swim adeptly with paddle-shaped legs. When threatened, eucaridan crabs use their tail fan for a sudden backward escape.
9 Britannica Sources- reproductive behaviour: Crustaceans
- osmoregulation
- 14 Questions About Aquatic Animals Answered: Which animals live inside empty seashells?
- feeding behaviour: Types of food procurement
- coloration: Background matching
- crustacean: Exoskeleton
- crab: Distribution and variety
- biosphere: Salinity
- malacostracan: Locomotion
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
crab, any short-tailed member of the crustacean order Decapoda (phylum Arthropoda)—especially the brachyurans (infraorder Brachyura), or true crabs, but also other forms such as the anomurans (suborder Anomura), which include the hermit crabs. Decapods occur in all oceans, in fresh water, and on land; about 10,000 species have been described.
Unlike those of other decapods (e.g., shrimp, lobster, crayfish), crabs’ tails are curled under the thorax, or midsection. The carapace (upper body shield) is usually broad. The first pair of legs is modified into chelae, or pincers.
Distribution and variety
Most crabs live in the sea; even the land crabs, which are abundant in tropical countries, usually visit the sea occasionally and pass through their early stages in it. The river crab of southern Europe (the Lenten crab, Potamon fluviatile) is an example of the freshwater crabs abundant in most of the warmer regions of the world. As a rule, crabs breathe by gills, which are lodged in a pair of cavities beneath the sides of the carapace, but in the true land crabs the cavities become enlarged and modified so as to act as lungs for breathing air.
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Walking or crawling is the usual mode of locomotion, and the familiar sidelong gait in the common shore crab is characteristic of most members of the group. The crabs of the family Portunidae, as well as some others, swim with great dexterity by means of their flattened paddle-shaped legs.
Like many other crustaceans, crabs are often omnivorous and act as scavengers, but many are predatory and some are vegetarian.
Though no crab is truly parasitic, some live commensally with other animals. One example is the little pea crab (Pinnotheridae), which lives within the shells of mussels and a variety of other mollusks, worm-tubes, and echinoderms and shares its hosts’ food; another example is the coral-gall crab (Hapalocarcinidae), which irritates the growing tips of certain corals so that they grow to enclose the female in a stony prison. Many of the sluggish spider crabs (Majidae) cover their shells with growing seaweeds, zoophytes, and sponges, which afford them a very effective disguise.
The giant crab of Japan (Macrocheira kaempferi) and the Tasmanian crab (Pseudocarcinus gigas) are two of the largest known crustaceans. The former may span nearly 4 metres (12 feet) from tip to tip of its outstretched legs. The Tasmanian crab, which may weigh well over 9 kg (20 pounds), has much shorter, stouter claws; the major one may be 43 cm (17 inches) long; the body, or carapace, of a very large specimen may measure 46 cm (18 inches) across. At the other extreme are tiny crabs measuring in adulthood scarcely more than a centimetre or two in length.
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Better-known anomuran crabs are the hermit crabs, which live in empty shells discarded by gastropod mollusks. As the crab grows, it must find a larger shell to occupy. If the supply of empty shells of appropriate size is limited, competition for shells among hermit crabs can be severe. In tropical countries hermit crabs of the family Coenobitidae live on land, often at considerable distances from the sea, to which they must return to release their larvae. The large robber, or coconut, crab (another anomuran) of the Indo-Pacific islands (Birgus latro) has given up the habit of carrying a portable dwelling, and the upper surface of its abdomen has become covered by shelly plates.
As in most crustaceans, the young of nearly all crabs, when newly hatched from eggs, are very different from the parents. The larval stage, known as the zoea, is a minute transparent organism with a legless, rounded body, that swims and feeds in the plankton. After casting off its skin several times, the enlarging crab passes into a stage known as the megalopa, in which the body and limbs are more crablike, but the abdomen is large and not folded up under the thorax. After a further molt the animal assumes a form very similar to that of the adult. There are a few crabs, especially those living in fresh water, that do not pass through a series of free-living larval stages but instead leave the eggshell as miniature adults.
Economic importance
Many crabs are eaten by humans. The most important and valuable are the edible crab of the British and European coasts (Cancer pagurus) and, in North America, the blue crab (Callinectes sapidus) of the Atlantic coast and the Dungeness crab (Cancer magister) of the Pacific coast. In the Indo-Pacific region the swimming crabs, Scylla and Portunus, related to the American blue crab, are among the most important sources of seafood. Commercially valuable anomurans are the lithodid (literally “stone”) crabs, of which the so-called king crab (Paralithodes camtschatica) found off Japan and in the Bering Sea and Alaskan waters is the most important.