The Decapoda or decapods (literally "ten-footed") are an
order of
crustaceans within the class
Malacostraca, and includes
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.[1] Nearly half of these species are crabs, with the shrimp (about 3,000 species) and
Anomura including
hermit crabs,
porcelain crabs,
squat lobsters (about 2500 species) making up the bulk of the remainder.[1] The earliest fossils of the group date to the
Devonian.
Decapods can have as many as 38 appendages,[2] arranged in one pair per body segment. As the name Decapoda (from the
Greekδέκα, deca-, "ten", and πούς / ποδός, -pod, "foot") implies, ten of these appendages are considered legs. They are the
pereiopods, found on the last five thoracic segments.[2] In many decapods, one pair of these "legs" has enlarged pincers, called
chelae, with the legs being called chelipeds. In front of the pereiopods are three pairs of
maxillipeds that function as feeding appendages. The head has five pairs of appendages, including
mouthparts, antennae, and antennules. There are five more pairs of appendages on the abdomen. They are called
pleopods. There is one final pair called
uropods, which, with the
telson, form the tail fan.[2]
Evolution
A 2019
molecular clock analysis suggested decapods originated in the
Late Ordovician around 455 million years ago, with the
Dendrobranchiata (prawns) being the first group to diverge. The remaining group, called
Pleocyemata, then diverged between the swimming
shrimp groupings and the crawling/walking group called
Reptantia, consisting of
lobsters and
crabs. High species diversification can be traced to the
Jurassic and
Cretaceous periods, which coincides with the rise and spread of modern
coral reefs, a key habitat for the decapods.[3] Despite the inferred early origin, the oldest fossils of the group such as Palaeopalaemon only date to the
Late Devonian.[4]
The
cladogram below shows the internal relationships of Decapoda, from analysis by Wolfe et al. (2019).[3]
Classification within the order Decapoda depends on the structure of the
gills and legs, and the way in which the
larvae develop, giving rise to two suborders:
Dendrobranchiata and
Pleocyemata. The Dendrobranchiata consist of prawns, including many species colloquially referred to as "shrimp", such as the "white shrimp", Litopenaeus setiferus. The Pleocyemata include the remaining groups, including "true shrimp".[5] Those groups that usually walk rather than swim (Pleocyemata, excluding Stenopodidea and Caridea) form a clade called Reptantia.[6]
This classification to the level of superfamilies follows De Grave et al.[1]