Decapoda (crabs, crayfish, lobsters, shrimps; subphylum Crustacea, class Malacostraca) Order of crustaceans, most of which are marine, but some of which inhabit freshwater or terrestrial habitats, or are amphibious. The first three pairs of thoracic appendages are modified; and the remaining five pairs, from which the order derives its name (from the Greek deka, ‘ten’, and pod-, ‘foot’) are legs. The first pair of legs is usually large, heavy, and chelate (the ‘chelipeds’): these legs are used in prey capture, defence, and sexual signalling. The head and thorax are fused dorsally, lateral expansions of the carapace enclosing the gills in branchial chambers. Gills may be dendritic, filamentous, or lamellar, and ventilating currents are passed over them by means of gill bailers. The blood contains haemocyanin and flows through the lamellae of the gills. Land crabs have a reduced gill number, and so conserve water. Most decapods are predacious or scavenging (the prey including echinoderms, bivalves, polychaetes, and other crustaceans), but most freshwater and terrestrial decapods are herbivorous. There is considerable sexual dimorphism and the complex courtship displays have visual, acoustic, and pheromonal components. Fertilization is internal in the true crabs (infra-order Brachyura) but occurs at the moment of egg-laying in most others. Decapods are very diverse in form, habit, and modes of locomotion, and are very important both in the marine ecosystem and to humans, for food and by-products. The most successful members of the order are the true crabs, which have evolved a specialized, short body form, the abdomen fitting tightly under the cephalothorax, which permits rapid locomotion (1.6 m/s in Ocypode, a genus of ghost crab). Decapoda is the largest crustacean order, comprising some 8500 species, or about one-third of all known crustacean species.