:en:decapod

{{Automatic taxobox

| name = Decapoda

| fossil_range = {{fossil_range|Devonian|recent}}

| image = Decapoda_montage.jpg

| image_caption = From left to right: Grapsus grapsus (Brachyura), Coconut crab (Anomura), Lysmata amboinensis (Caridea), Homarus gammarus (Astacidea).

| taxon = Decapoda

| authority = Latreille, 1802

| subdivision_ranks = Suborders

| subdivision =

Dendrobranchiata

Pleocyemata

See text for superfamilies.

}}

The Decapoda or decapods, from Ancient Greek δεκάς (dekás), meaning "ten", and πούς (poús), meaning "foot", is a large 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.{{cite journal|journal=Raffles Bulletin of Zoology |year=2009 |volume=Suppl. 21 |pages=1–109 |title=A classification of living and fossil genera of decapod crustaceans |author1=Sammy De Grave |author2=N. Dean Pentcheff |author3=Shane T. Ahyong |url=http://rmbr.nus.edu.sg/rbz/biblio/s21/s21rbz1-109.pdf |display-authors=etal |url-status=dead |archive-url=https://web.archive.org/web/20110606064728/http://rmbr.nus.edu.sg/rbz/biblio/s21/s21rbz1-109.pdf |archive-date=2011-06-06 }} 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,{{cite web|url=http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Decapoda/characters.html|website=University of Bristol: Decapoda characters|title=Decapoda characters and anatomy|access-date=11 December 2017|archive-url=https://web.archive.org/web/20131110024412/http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Decapoda/characters.html|archive-date=10 November 2013|url-status=dead}} arranged in one pair per body segment. As the name Decapoda (from the Greek {{lang|el|δέκα}}, {{lang|el|deca-}}, "ten", and {{lang|el|πούς / ποδός}}, -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.

File:Haeckel Decapoda.jpg Kunstformen der Natur, 1904]]

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.{{cite journal |last1=Wolfe |first1=Joanna M. |last2=Breinholt |first2=Jesse W. |last3=Crandall |first3=Keith A. |last4=Lemmon |first4=Alan R. |last5=Lemmon |first5=Emily Moriarty |last6=Timm |first6=Laura E. |last7=Siddall |first7=Mark E. |last8=Bracken-Grissom |first8=Heather D. |display-authors=6 | date=24 April 2019 |title=A phylogenomic framework, evolutionary timeline, and genomic resources for comparative studies of decapod crustaceans |journal=Proceedings of the Royal Society B |volume=286 |issue=1901 |doi=10.1098/rspb.2019.0079 |doi-access=free |pmc=6501934 |pmid=31014217}} Despite the inferred early origin, the oldest fossils of the group such as Palaeopalaemon only date to the Late Devonian.{{Cite journal |last1=Gueriau |first1=Pierre |last2=Rak |first2=Štěpán |last3=Broda |first3=Krzysztof |last4=Kumpan |first4=Tomáš |last5=Viktorýn |first5=Tomáš |last6=Valach |first6=Petr |last7=Zatoń |first7=Michał |last8=Charbonnier |first8=Sylvain |last9=Luque |first9=Javier |date=2020-10-25 |title=Exceptional Late Devonian arthropods document the origin of decapod crustaceans |url=http://biorxiv.org/lookup/doi/10.1101/2020.10.23.352971 |language=en |doi=10.1101/2020.10.23.352971|s2cid=226229304 }}

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".{{cite book |author=Elena Mente |year=2008 |title=Reproductive Biology of Crustaceans: Case Studies of Decapod Crustaceans |publisher=Science Publishers |isbn=978-1-57808-529-3 |page=16}} Those groups that usually walk rather than swim (Pleocyemata, excluding Stenopodidea and Caridea) form a clade called Reptantia.{{cite journal |author1=G. Scholtz |author2=S. Richter |year=1995 |title=Phylogenetic systematics of the reptantian Decapoda (Crustacea, Malacostraca) |journal=Zoological Journal of the Linnean Society |volume=113 |pages=289–328 |doi=10.1006/zjls.1995.0011 |issue=3}}

This classification to the level of superfamilies follows De Grave et al.

File:Penaeus vannamei 01.jpg, Litopenaeus vannamei (Dendrobranchiata: Penaeoidea)]]

File:Heterocarpus ensifer.jpg (Caridea: Pandaloidea)]]

File:Austropotamobius pallipes.jpg (Astacidea: Astacoidea)]]

File:Upogebia deltaura.jpg (Gebiidea: Upogebiidae)]]

File:Lobster 300.jpg, Panulirus interruptus (Achelata: Palinuridae)]]

File:Polycheles sculptus.jpg (Polychelida: Polychelidae)]]

File:CoenobitaVariabilis2.jpg, Coenobita variabilis (Anomura: Paguroidea)]]

File:Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg (Brachyura: Portunoidea)]]

Order Decapoda Latreille, 1802