A decapod is a crustacean in the large order of Decapoda (from Ancient Greek δεκάς (dekás), meaning "ten", and πούς (poús), meaning "foot"), within the class Malacostraca, including crabs, lobsters, crayfish, shrimp, and prawns. Most decapods are scavengers. The order is estimated to contain nearly 15,000 extant species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp (about 3,000 species) and Anomura including hermit crabs, king crabs, porcelain crabs, squat lobsters (about 2500 species) making up the bulk of the remainder. arranged in one pair per body segment. As the name Decapoda (from the Greek , ', "ten", and , -pod, "foot") implies, ten of these appendages are considered legs. They are the pereiopods, found on the last five thoracic segments. Despite the inferred early origin, the oldest fossils of the group such as Palaeopalaemon only date to the Late Devonian.
The cladogram below shows the internal relationships of Decapoda, from analysis by Wolfe et al. (2019). Those groups that usually walk rather than swim (Pleocyemata, excluding Stenopodidea and Caridea) form a clade called Reptantia.
This classification to the level of superfamilies follows De Grave et al.