Brachyura

Latreille, 1802

True Crabs

Family Guides

16

Brachyura is an infraorder of decapod crustaceans comprising the true crabs, distinguished by a highly reduced tucked beneath the . The group contains approximately 7,000 extant in 98 , making it one of the most diverse decapod groups. Members exhibit carcinisation— toward a flattened, robust body form with a crab-like appearance. They occupy marine, freshwater, and terrestrial across all continents and oceans, with significant economic importance as human food sources.

Stenorhynchus seticornis by (c) Pauline Walsh Jacobson, some rights reserved (CC BY), uploaded by Pauline Walsh Jacobson. Used under a CC-BY license.Arenaeus by (c) James Kindt, some rights reserved (CC BY), uploaded by James Kindt. Used under a CC-BY license.Achelous spinimanus by (c) Sam Kieschnick, some rights reserved (CC BY), uploaded by Sam Kieschnick. Used under a CC-BY license.

Pronunciation

How to pronounce Brachyura: //brækˈjʊərə//

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Identification

True crabs are recognized by the complete reduction and folding of the beneath the (pleon), with uropods absent or modified into small holding devices rather than functional appendages. The is typically broad and flattened. The first pair of pereiopods is modified into chelae (claws), followed by four pairs of legs; some lineages have the fifth pair modified as swimming paddles. is common: males typically possess a narrow, triangular abdomen, while females have a broader, rounded abdomen adapted for .

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Habitat

Marine environments from intertidal zones to deep sea; freshwater rivers, streams, and lakes; terrestrial in tropical and subtropical regions. Many occupy specific microhabitats including coral reefs, rocky shores, sandy beaches, mudflats, seagrass beds, mangroves, caves, and burrows in sediment or wood.

Distribution

Global distribution across all oceans from polar regions to tropics; present on all continents except Antarctica for terrestrial and freshwater . Approximately 850 species are freshwater crabs, with significant radiations in tropical Asia, Africa, and the Neotropics.

Diet

, with diets varying by and . Marine and coastal species commonly consume , detritus, molluscs, polychaetes, other crustaceans, and carrion. Some lineages are specialized: Pinnotheridae associate with bivalves as commensals or ; some deep-sea species rely on chemosynthetic food sources. Mixed plant-animal diets often support optimal growth.

Life Cycle

Development includes , planktonic larval, and benthic stages. Females fertilized eggs on pleopods beneath the ; eggs hatch into zoea larvae that drift in plankton, undergoing multiple . Zoeae molt into megalopa stage, resembling miniature with an extended abdomen. Final molt produces juvenile crab with abdomen folded beneath . Terrestrial typically release larvae into marine or estuarine waters. Sexual maturity requires multiple post-juvenile molts; molting frequency decreases with age.

Behavior

Most exhibit sideways due to leg , though some walk forward or backward. Many communicate through visual displays (claw waving), acoustic or vibratory signals, or chemical . Males frequently fight for access to females or shelter. Burrowing is common in intertidal and terrestrial species. Some portunid crabs swim using flattened rear legs as paddles. Complex social behaviors include cooperative care and mate attraction rituals.

Ecological Role

Significant , scavengers, and herbivores in benthic . Bioturbators in sedimentary through burrowing and feeding activities. Some serve as engineers, modifying habitat structure. Prey for fish, birds, marine mammals, and other . Symbiotic associations include cleaning (documented in some tropical species) and commensalism with echinoderms, molluscs, and cnidarians.

Human Relevance

Major global fishery resource: 20% of all marine crustaceans caught for human consumption, totaling 1.5 million tonnes annually. Key commercial include Portunus trituberculatus, Callinectes sapidus, Chionoecetes spp., Cancer pagurus, Metacarcinus magister, and Scylla serrata. Some fisheries practice claw-only harvest with live release. Significant in aquaculture, particularly mud crab farming in Southeast Asia. Cultural significance includes zodiac symbol (Cancer), namesake of Crab Nebula, and roles in mythology and literature.

Similar Taxa

  • AnomuraHermit crabs, king crabs, and porcelain crabs have convergently evolved crab-like body plans through carcinisation, but retain a long typically carried in gastropod shells (hermit crabs) or asymmetrical pleon; positions and larval development differ.
  • PolychelidaSpiny lobsters and slipper lobsters share decapod characteristics but have elongated and lack the full carcinised body form of true crabs.

Sources and further reading