Crab (Brachyura): biology, diversity, and human importance
Crabs are decapod crustaceans of the infraorder Brachyura with a compact carapace and two claws. They occupy marine, freshwater, and terrestrial habitats and are important ecologically and as seafood.
Overview
Crabs are crustaceans belonging to the infraorder Brachyura, a group of decapods characterized by a compact, often rounded or squarish body, a protective carapace, and a pair of prominent claws (chelae). They are closely related to other decapods such as lobsters, crayfish and shrimps. Most species are marine, but many occur in freshwater or live primarily on land. About 7,000 described species illustrate a wide range of sizes, shapes and lifestyles, from tiny pea-sized shore crabs to impressive long-legged forms like the Japanese spider crab.
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Physical characteristics and life cycle
Crabs share several common anatomical features. They possess ten limbs—eight walking legs and two claws—and a hard external skeleton made of chitin and mineral that provides protection and support (exoskeleton). The body plan is typically flattened and the abdomen is folded under the thorax. Crabs breathe using gills and, even for species that live on land, moisture or specialized structures are required to enable respiration.
- Molting: Growth requires periodic shedding of the exoskeleton and temporary vulnerability as a new shell hardens.
- Reproduction and larvae: Females carry eggs until they hatch into free-swimming larval stages (commonly called zoea and megalopa) before settling into juvenile forms.
- Behavior: Many crabs are opportunistic feeders, acting as scavengers, predators or grazers; some show complex social and reproductive behaviors.
Diversity and evolutionary notes
Crab-like forms have evolved repeatedly within decapods in a process known as "carcinization," where different lineages independently adopt a flattened, short-bodied crab shape. This convergence has produced the great variety seen today, from burrowing sand crabs to reef-dwelling species and true land crabs. Fossil and molecular evidence indicate an ancient origin for several crab groups, followed by diversification into many ecological niches.
Ecological role and human uses
Ecologically, crabs play key roles as scavengers and predators that recycle nutrients and influence community structure in intertidal and benthic ecosystems. They are also a major food source for other animals. For humans, crabs are a globally important seafood resource and the focus of fisheries and aquaculture in many regions. Beyond food, they feature in cultural traditions, regulation of fisheries, and scientific study of development and adaptation.
Distinctions and notable facts
Not all animals called "crabs" are true Brachyura; some crab-like species, such as certain hermit crabs and king crabs, belong to other decapod groups and demonstrate convergent body forms. Size extremes and special adaptations are notable: some species are adapted to freshwater or terrestrial life with modified gill structures, while the Japanese spider crab is famous for its very long legs relative to body size. Crabs also exhibit regenerative ability, able to regrow lost limbs over successive molts.
For further general reading on related groups and anatomy, see resources on lobsters, crayfish, shrimps, and more about the exoskeleton. Specific interest in very large species may lead to material on the Japanese spider crab.
Systematics
The families of crabs:
- Podotremata (de Haan, 1833)
- Superfamily Cyclodorippoidea (Ortmann, 1892)
- Cyclodorippidae (Ortmann, 1892)
- Cymonomidae (Bouvier, 1897)
- Phyllotymolinidae (Tavares, 1998)
- Superfamily Homolodromioidea (Alcock, 1900)
- Homolodromiidae (Alcock, 1900)
- Superfamily Dromioidea (de Haan, 1833)
- Sponge crabs (Dromiidae) (de Haan, 1833)
- Dynomenidae (Ortmann, 1892)
- Superfamily Homoloidea (de Haan, 1839)
- Homolidae (de Haan, 1839)
- Long-legged crabs (Latreilliidae) (Stimpson, 1858)
- Poupiniidae (Guinot, 1991)
- Superfamily Raninoidea (de Haan, 1839)
- Raninidae (de Haan, 1839)
- Symethidae (Goeke, 1981)
- Eubrachyura (de Saint Laurent, 1980)
- Heterotremata (Guinot, 1977)
- Superfamily Dorippoidea (MacLeay, 1838)
- Dorippidae (MacLeay, 1838)
- Orithyiidae (Dana, 1853)
- Superfamily Calappoidea (Milne Edwards, 1837)
- Boxer crabs (Calappidae) (Milne Edwards, 1837)
- Hepatidae (Stimpson, 1871)
- Superfamily Leucosioidea (Samouelle, 1819)
- Spherical crabs (Leucosiidae) (Samouelle, 1819)
- Matutidae (de Haan, 1841)
- Superfamily Majoidea (Samouelle, 1819)
- Epialtidae (MacLeay, 1838)
- Inachidae (MacLeay, 1838)
- Inachoididae (Dana, 1851)
- Triangular crabs, spider crabs, spider crabs, decorator crabs (Majidae) (Samouelle, 1819)
- Mithracidae (Balss, 1929)
- Pisidae (Dana, 1851)
- Tychidae (Dana, 1851)
- Superfamily Hymenosomatoidea (MacLeay, 1838)
- False spider crabs (Hymenosomatidae) (MacLeay, 1838)
- Superfamily Parthenopoidea (MacLeay, 1838)
- Aethridae (Dana, 1851)
- Dairidae (Ng & Rodriguez, 1986)
- Daldorfiidae (Ng & Rodriguez, 1986)
- Parthenopidae (MacLeay, 1838)
- Superfamily Retroplumoidea (Gill, 1894)
- Retroplumidae (Gill, 1894)
- Superfamily Cancroidea (Latreille, 1802)
- Atelecyclidae (Ortmann, 1893)
- Edible crabs, rock crabs (Cancridae) (Latreille, 1802)
- Cheiragonidae (Ortmann, 1893)
- Corystidae (Samouelle, 1819)
- Pirimelidae (Alcock, 1899)
- Thiidae (Dana, 1852)
- Superfamily Portunoidea (Rafinesque, 1815)
- Carcinidae (MacLeay, 1838)
- Catopidae (Borradaile, 1902)
- Deep-sea crabs (Geryonidae) (Colosi, 1923)
- Macropipidae (Stephenson & Campbell, 1960)
- Primelidae (Alcock, 1899)
- Swimming crabs (Portunidae) (Rafinesque, 1815)
- Thiidae (Dana, 1852)
- Superfamily Bythograeoidea (Williams, 1980)
- Bythograeidae (Williams, 1980)
- Superfamily Xanthoidea (MacLeay, 1838)
- Carpiliidae (Ortmann, 1893)
- Eumedonidae (Dana, 1853)
- Goneplacidae (MacLeay, 1838)
- Hexapodidae (Miers, 1886)
- Menippidae (Ortmann, 1893)
- Panopeidae (Ortmann, 1893)
- Woolly crabs (Pilumnidae) (Samouelle, 1819)
- Platyxanthidae (Guinot, 1977)
- Pseudorhombilidae (Alcock, 1900)
- Coral crabs (Trapeziidae) (Miers, 1886)
- Round crabs (Xanthidae) (MacLeay, 1838)
- Superfamily Bellioidea (Dana, 1852)
- Belliidae (Dana, 1852)
- Superfamily Potamoidea (Ortmann, 1896)
- Deckeniidae (Ortmann, 1897)
- Platythelphusidae (Colosi, 1920)
- Potamidae (Ortmann, 1896)
- Potamonautidae (Bott, 1970)
- Sudanonautes (Bott, 1955).
- Superfamily Pseudothelphusoidea (Ortmann, 1893)
- Pseudothelphusidae (Ortmann, 1893)
- Superfamily Gecarcinucoidea (Rathbun, 1904)
- Gecarcinucidae (Rathbun, 1904)
- Parathelphusidae (Alcock, 1910)
- Superfamily Cryptochiroidea (Paulson, 1875)
- Gall crabs (Cryptochiridae) (Paulson, 1875)
- Thoracotremata (Guinot, 1875)
- Superfamily Pinnotheroidea (de Haan, 1833)
- Pinnotheridae (de Haan, 1833)
- Superfamily Ocypodoidea (Rafinesque, 1815)
- Camptandriidae (Stimpson, 1858)
- Dotillidae (Stimpson, 1858)
- Heloeciidae (H. Milne-Edwards, 1852)
- Macrophthalmidae (Dana, 1851)
- Soldier crabs (Mictyridae) (Dana, 1851)
- Ocypodidae (Rafinesque, 1815)
- Palicidae (Bouvier, 1898)
- Ucididae (Števčić, 2005)
- Xenophthalmidae (Stimpson, 1858)
- Superfamily Grapsoidea (MacLeay, 1838)
- Land crabs (Gecarcinidae) (MacLeay, 1838)
- Glyptograpsidae (Schubart, Cuesta & Felder, 2002)
- Square crabs (Grapsidae) (MacLeay, 1838)
- Plagusiidae (Dana, 1851)
- Sesarmidae (Dana, 1851)
- Varunidae (Milne Edwards, 1853)
Questions and answers
Q: What are crabs?
A: Crabs are a type of decapods that have eight walking legs and two grasping claws.
Q: What is their order within the decapods called?
A: Their order within the decapods is called Brachyura.
Q: Are crabs successful?
A: Yes, crabs are an extremely successful group found all over the world.
Q: What is the size of the smallest crab?
A: The smallest crabs are the size of a pea.
Q: Do all crabs live in sea-water?
A: No, there are some crabs that live in fresh water and some that live on land.
Q: What is the size of the largest crab?
A: The largest crab, the Japanese spider crab, grows to a leg span of 4 meters.
Q: How many species of crabs are known?
A: About 7,000 species of crabs are known.
Related articles
Author
AlegsaOnline.com Crab (Brachyura): biology, diversity, and human importance Leandro Alegsa
URL: https://en.alegsaonline.com/art/23908
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