Crab
Polyphyletic decapod crustaceans including true crabs and crab-like anomurans
Crabs are decapod crustaceans, either the Brachyura (the "true crabs") or various groups within the closely related Anomura (hermit crabs and allies), characterised by having a heavily armoured shell, their tail segments concealed under the body, the ability to run sideways, and the habit of hiding in rocky crevices. They do not form a single natural group or clade, but have convergently evolved multiple times from the ancestral decapod body plan through carcinisation. As a group, they are thus polyphyletic, meaning they have multiple evolutionary origins.
Quick Facts
- Taxonomic group
- Infraorder Brachyura
- Dietary role
- Omnivorous scavengers and predators
- Locomotion
- Most species walk sideways; some swim or climb
- Shell composition
- Calcium carbonate exoskeleton
- Habitat range
- Marine, freshwater, and terrestrial environments
Lore & Background
Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 3.8 m. Many crabs are free-living marine omnivores; others are specialist herbivores or carnivores, while some are parasitic. A substantial number of species are adapted to freshwater or other non-marine habitats. Crabs make up about 20% of the marine crustaceans that are caught or farmed for human consumption. In British cuisine, dressed crab is a traditional seafood meal, while in Goa and Mozambique, crab curry is a typical dish.
Crabs feature in Greek and Malay mythology, and as the astrological sign Cancer. They have appeared in art in media including pottery, paintings, blouse panels, and book illustrations. Hermit crabs are often kept in aquariums and as pets. A popular meme jokes that everything will evolve into crabs, based inaccurately on the genuine evolutionary trend within the decapods.
Reader's Guide
Crabs are not a single taxonomic group. Instead, alongside the Brachyura or true crabs, are multiple groups of the Anomura that are called crabs, including the hermit crabs, mole crabs, king crabs, and porcelain crabs. A distantly-related group of arthropods, the horseshoe crabs, with an armoured carapace but a quite different body plan, is a member of the Chelicerata. The crab body form and associated behaviour have arisen independently at different times in multiple groups of decapod crustaceans.
True crabs (Brachyura) are generally covered with a thick exoskeleton composed primarily of highly mineralized chitin. Porcelain crabs (Porcellanidae) are small flattened decapods that hide under rocks. The hairy stone crab (Lomisidae) is a slow-moving crab of the Australian shore. Hermit crabs (Paguroidea) have heavily armoured crab-like claws, but inhabit empty scavenged gastropod mollusc shells.
King crabs (Lithodidae) live mainly in cold deep water. The coconut crab (Birgus) is a large terrestrial hermit crab. Patagurus rex is a specialised hermit crab from Polynesia. Many crabs can run swiftly sideways, though others walk forwards, and some can swim.
Did You Know?
- The coconut crab is the largest terrestrial arthropod, up to 1 m long and weighing up to 4.1 kg.
- Some 1,300 species of crabs in 8 families are adapted to freshwater.
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