Decapods may have branched off 455 million years ago
Decapods—crabs, lobsters, crayfish, shrimp, and prawns—are ten-footed malacostracan crustaceans, mostly scavengers. Molecular-clock work published in 2019 pushes their origin near 455 million years back, into Late Ordovician seas, with prawn-like Dendrobranchiata splitting early.
Decapoda, Greek for 'ten feet', is an order within the Malacostraca covering crabs, lobsters, crayfish, shrimp and prawns, most of them scavengers. Nearly 15,000 living species in about 2,700 genera are known, plus some 3,300 fossil species. Crabs make up close to half, shrimp account for about 3,000, and the Anomura, including hermit crabs, king crabs, porcelain crabs and squat lobsters, add roughly 2,500.
Each body segment can carry a pair of appendages, up to 38 in all. The ten legs behind the order's name are the pereiopods on the last five thoracic segments, and in many species one pair ends in pincers, or chelae. Ahead of them sit three pairs of maxillipeds used for feeding; the head bears five pairs including antennae, antennules and mouthparts, the abdomen has five pairs of pleopods, and a last pair of uropods joins the telson to form the tail fan.
A 2019 molecular clock study dated the group's origin to about 455 million years ago in the Late Ordovician, with prawns, the Dendrobranchiata, branching off first. The rest, the Pleocyemata, divided into swimming shrimp lines and the walking Reptantia of lobsters and crabs. Yet the oldest known fossils, such as Palaeopalaemon, go back only to the Late Devonian.
Species numbers surged in the Jurassic and Cretaceous alongside the spread of modern coral reefs, still a key decapod habitat. The two suborders are told apart by gill and leg structure and by larval development, and the commercially fished white shrimp, Litopenaeus setiferus, is in fact a dendrobranchiate prawn. The enigmatic Glypheidea lack enough DNA data for firm placement but probably sit beside Polychelida within Reptantia.
Source: Decapod
