TheDecapoda ordecapods, fromAncient Greek δεκάς (dekás), meaning "ten", and πούς (poús), meaning "foot", is a largeorder ofcrustaceans within the classMalacostraca, and includescrabs,lobsters,crayfish,shrimp, andprawns. Most decapods arescavengers. 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) andAnomura includinghermit crabs,king crabs,porcelain crabs,squat lobsters (about 2500 species) making up the bulk of the remainder.[1] The earliest fossils of the group date to theDevonian.
Decapods can have as many as 38 appendages,[2] arranged in one pair per body segment. As the name Decapoda (from theGreekδέκα,deca-, "ten", andπούς / ποδός,-pod, "foot") implies, ten of these appendages are considered legs. They are thepereiopods, found on the last five thoracic segments.[2] In many decapods, one pair of these "legs" has enlarged pincers, calledchelae, with the legs being called chelipeds. In front of the pereiopods are three pairs ofmaxillipeds that function as feeding appendages. The head has five pairs of appendages, includingmouthparts, antennae, and antennules. There are five more pairs of appendages on the abdomen. They are calledpleopods. There is one final pair calleduropods, which, with thetelson, form the tail fan.[2]
A 2019molecular clock analysis suggested decapods originated in theLate Ordovician around 455 million years ago, with theDendrobranchiata (prawns) being the first group to diverge. The remaining group, calledPleocyemata, then diverged between the swimmingshrimp groupings and the crawling/walking group calledReptantia, consisting oflobsters andcrabs. High species diversification can be traced to theJurassic andCretaceous periods, which coincides with the rise and spread of moderncoral reefs, a key habitat for the decapods.[3] Despite the inferred early origin, the oldest fossils of the group such asPalaeopalaemon only date to theLate Devonian.[4]
Thecladogram below shows the internal relationships of Decapoda, from analysis by Wolfeet al. (2019).[3]
Classification within the order Decapoda depends on the structure of thegills and legs, and the way in which thelarvae develop, giving rise to two suborders:Dendrobranchiata andPleocyemata. 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 Graveet al.[1]
