Decapod

Decapods are crustaceans belonging to the large order Decapoda within the class Malacostraca, which includes crabs, lobsters, crayfish, shrimp, and prawns. The name derives from Ancient Greek for 'ten feet,' referring to ten of their appendages being considered legs. Nearly half of these species are crabs, with shrimp (about 3,000 species) and Anomura (including hermit crabs, king crabs, porcelain crabs, and squat lobsters, totaling around 2,500 species) making up the bulk of the remainder. The earliest fossils of the group date to the Devonian. The order is estimated to contain nearly 15,000 extant species in around 2,700 genera, with approximately 3,300 fossil species. Most decapods are scavengers. Anatomically, they can possess up to 38 appendages, arranged as one pair per body segment. Ten of these are the pereiopods, found on the last five thoracic segments, which are considered legs; in many species, one pair of these legs is enlarged into pincers called chelae, making them chelipeds. Three pairs of maxillipeds, located in front of the pereiopods, function as feeding appendages. The head bears five pairs of appendages, including mouthparts, antennae, and antennules, while the abdomen has five pairs of pleopods and one final pair of uropods, which together with the telson form the tail fan. A 2019 molecular clock analysis suggests decapods originated in the Late Ordovician around 455 million years ago, with the Dendrobranchiata (prawns) diverging first. The remaining group, Pleocyemata, later split into swimming shrimp and the crawling/walking Reptantia, which includes lobsters and crabs. High species diversification occurred during the Jurassic and Cretaceous periods, coinciding with the rise of modern coral reefs. Despite this early origin, the oldest fossils, such as *Palaeopalaemon*, date only to the Late Devonian. Classification within the order depends on gill and leg structure and larval development, yielding two suborders: Dendrobranchiata (prawns) and Pleocyemata (including true shrimp and the walking groups of Reptantia).