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Wikipedia / Decapod

A decapod is a crustacean in the large order of Decapoda (from Ancient Greek δέκά déka 'ten' + πούς poús '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. The earliest fossils of the group date to the Devonian.

Anatomy

Decapods can have as many as 38 appendages, 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. 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.

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. 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).

In the cladogram above, the clade Glypheidea is excluded due to lack of sufficient DNA evidence, but is likely the sister clade to Polychelida, within Reptantia.

Classification

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". 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.

Order Decapoda Latreille, 1802

Suborder Dendrobranchiata Bate, 1888

Suborder Pleocyemata Burkenroad, 1963

Infraorder Caridea Dana, 1852

Infraorder Astacidea Latreille, 1802

Infraorder Glypheidea Winckler, 1882

Infraorder Anomura MacLeay, 1838

Infraorder Brachyura Linnaeus, 1758

Section Dromiacea De Haan, 1833

Section Eubrachyura de Saint Laurent, 1980

Subsection Heterotremata Guinot, 1977

Subsection Thoracotremata Guinot, 1977

See also

References