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36-578: See text for orders. Malacostraca is the second largest of the six classes of pancrustaceans behind insects , containing about 40,000 living species , divided among 16 orders . Its members, the malacostracans , display a great diversity of body forms and include crabs , lobsters , crayfish , shrimp , krill , prawns , woodlice , amphipods , mantis shrimp , tongue-eating lice and many other less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They are segmented animals, united by

72-424: A dorso-ventrally flattened body and a shield-like carapace and are armed with powerful, raptorial claws normally carried in a folded position. There are about 300 species, most living in tropical and subtropical seas although some live in temperate areas. They are benthic, mostly hiding in cracks and crevices or living in burrows, some emerging to forage while others are ambush predators. The Eumalocostraca contains

108-476: A common body plan comprising 20 body segments (rarely 21), and divided into a head, thorax, and abdomen. The name Malacostraca is from Ancient Greek μαλακός ( malakós )  'soft' and όστρακον ( óstrakon )  'shell'. The word was used by Aristotle , who contrasted them with oysters , in comparison with which their shells are pliable. It was applied to this taxon by French zoologist Pierre André Latreille in 1802. He

144-551: A distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distinct type of construction, which is to say a particular layout of organ systems. This said, the composition of each class is ultimately determined by the subjective judgment of taxonomists . In the first edition of his Systema Naturae (1735), Carl Linnaeus divided all three of his kingdoms of nature ( minerals , plants , and animals ) into classes. Only in

180-424: A distinct rank of biological classification having its own distinctive name – and not just called a top-level genus (genus summum) – was first introduced by French botanist Joseph Pitton de Tournefort in the classification of plants that appeared in his Eléments de botanique of 1694. Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine

216-572: A fact that differentiates them from all other major crustacean taxa except Remipedia . Each body segment bears a pair of jointed appendages , although these may be lost secondarily. The head bears two pairs of antennae , the first of which is often biramous (branching into two parts) and the second pair bear exopods (outer branches) which are often flattened into antennal scales known as scaphocerites . The mouthparts consist of pairs each of mandibles , maxillules (second pair of mouthparts) and maxillae . Except for fairy shrimps , malacostracans are

252-489: A greater diversity of body forms than any other class in the animal kingdom". Its members are characterised by the presence of three tagmata (specialized groupings of multiple segments) – a five-segmented head, an eight-segmented thorax and an abdomen with six segments and a telson , except in the Leptostraca , which retain the ancestral condition of seven abdominal segments. Malacostracans have abdominal appendages,

288-506: A lecithotrophic (non-feeding) nauplius stage. Mating behavior has been studied in the freshwater shrimp Caridina ensifera . Multiple paternity, common in the Malacostrica , also occurs in C. ensifera . Reproductive success of sires was found to correlate inversely with their genetic relatedness to the mother. This finding suggests that sperm competition and/or pre- and post-copulatory female choice occurs. Female choice may increase

324-468: A single or multiple origin. Some authors advocate placing Phyllocarida in Phyllopoda, a group used in former classification systems, which would then include branchiopods, cephalocarids and leptostracans. A molecular study by American biologists Trisha Spears and Lawrence Abele concluded that phylogenetic evidence did not support the monophyly of this grouping, and that Phyllocarida should be regarded as

360-552: A small number of species, on the seventh. The naupliar larval stages are often reduced and take place before hatching, but where they occur, a metamorphosis usually occurs between the larval and the adult forms. Primitive malacostracans have a free-swimming naupliar larval stage. Research suggests the common ancestor of Malacostraca had lost the free-living nauplius larval stage, but re-evolved it again through heterochrony in Dendrobranchiata and Euphausiacea, which both has

396-432: A subclass of Malacostraca that had diverged from the main lineage at an early date. The following cladogram is based on the 2001 phylogenetic analysis of Richter & Scholtz. Phyllocarida Hoplocarida Decapoda Amphionidacea Euphausiacea Thermosbaenacea Mysida Lophogastrida Spelaeogriphacea Mictacea Cumacea Tanaidacea Isopoda Anaspidacea Bathynellacea Leptostraca

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432-419: A taxonomy of the flowering plants up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades . Where formal ranks have been assigned, the ranks have been reduced to a very much lower level, e.g. class Equisitopsida for the land plants, with the major divisions within the class assigned to subclasses and superorders. The class was considered the highest level of

468-402: Is haemocyanin . Structures that function as kidneys are located near the base of the antennae. A brain exists in the form of ganglia close to the antennae, there are ganglia in each segment and a collection of major ganglia below the oesophagus. Sensory organs include compound eyes (often stalked), ocelli (simple eyes), statocysts and sensory bristles . The naupliar eye is a characteristic of

504-410: Is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson. Like other crustaceans, malacostracans have an open circulatory system in which the heart pumps blood into the hemocoel (body cavity) where it supplies the needs of the organs for oxygen and nutrients before diffusing back to the heart. The typical respiratory pigment in malacostracans

540-526: 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 , porcelain crabs , squat lobsters (about 2500 species) making up the bulk of the remainder. The earliest fossils of the group date to the Devonian . Decapods can have as many as 38 appendages, arranged in one pair per body segment. As

576-426: Is straight and the foregut consists of a short oesophagus and a two-chambered stomach, the first part of which contains a gizzard-like "gastric mill" for grinding food. The walls of this have chitinous ridges, teeth and calcareous ossicles. The fine particles and soluble material are then moved into the midgut where chemical processing and absorption takes place in one or more pairs of large digestive caeca. The hindgut

612-459: Is the only extant order of Phyllocarida, the other two orders, Archaeostraca and Hoplostraca being extinct. Leptostracans are thought to be the most primitive of the malacostracans and date back to the Cambrian period. They range in length from 1 to 4 cm (0.4 to 1.6 in), most being suspension feeders though some are carnivores or scavengers. They have a two part carapace which encloses

648-825: The Cambrian , when animals belonging to the Phyllocarida appeared. The following classification of living malacostracans is based on An Updated Classification of the Recent Crustacea (2001) by the American marine biologists Joel W. Martin , curator of crustaceans at the Natural History Museum of Los Angeles County , and George E. Davies. Extinct orders have been added to this and are indicated by an obelisk (†). Class Malacostraca Latreille , 1802 Class (biology) The class as

684-568: The nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness. Malacostracans live in a wide range of marine and freshwater habitats, and three orders have terrestrial members: Amphipoda ( Talitridae ), Isopoda (Oniscidea, the woodlice ) and Decapoda ( terrestrial hermit crabs , crabs of the families Ocypodidae , Gecarcinidae , and Grapsidae , and terrestrial crayfish ). They are abundant in all marine ecosystems, and most species are scavengers , although some, such as

720-418: The porcelain crabs , are filter feeders , and some, such as mantis shrimps, are carnivores . Most species of malacostracans have distinct sexes (a phenomenon known as gonochorism ), although a few species exhibit hermaphroditism . The female genital openings or gonopores are located on the sixth thoracic segment or its appendages, while the male gonopores are on the eighth segment or its appendages, or in

756-552: The abdomen. They are called pleopods . There is one final pair called uropods , which, with the telson , form the tail fan. 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

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792-460: The animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide a convenient "artificial key" according to his Systema Sexuale , largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed

828-538: 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 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

864-482: The claw is formed by the articulation of the dactylus against an outgrowth of the propodus. In some taxa, the exopods are lost and the appendages are uniramous. There is a clear demarcation between the thorax and the six or seven-segmented abdomen. In most taxa, each abdominal segment except the last carries a pair of biramous pleopods used for swimming, burrowing, gas exchange, creating a current or brooding eggs. The first and second abdominal pleopods may be modified in

900-547: 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

936-402: The fitness of progeny by avoiding inbreeding that can lead to expression of homozygous deleterious recessive mutations. The monophyly of Malacostraca is widely accepted. This is supported by several common morphological traits which are present throughout the group and is confirmed by molecular studies. However, a number of problems make it difficult to determine the relationships between

972-580: The head, the whole thorax and part of the abdomen and are the only malacostracans with seven abdominal segments. Three families are known with several genera and about twenty species. They are found worldwide from the intertidal zone to the deep ocean, all but one species being benthic (living on the seabed). Stomatopoda is the only extant order of Hoplocarida , the other two orders, Aeschronectida and Archaeostomatopoda being extinct. Stomatopodans, commonly known as mantis shrimps, range in length from 5 to 36 cm (2 to 14 in) and are predators. They have

1008-631: The internal relationships of Decapoda, from analysis by Wolfe et al. (2019). Dendrobranchiata (prawns) [REDACTED] Stenopodidea (boxer shrimp) [REDACTED] Procarididea Caridea ("true" shrimp) [REDACTED] Achelata (spiny lobsters and slipper lobsters) [REDACTED] Polychelida (benthic crustaceans) Astacidea (lobsters and crayfish) [REDACTED] Axiidea (mud shrimp, ghost shrimp, and burrowing shrimp) Gebiidea (mud lobsters and mud shrimp) [REDACTED] Anomura (hermit crabs and allies) [REDACTED] Brachyura ("true" crabs) [REDACTED] In

1044-484: The male to form gonopods (accessory copulatory appendages). The appendages of the last segment are typically flattened into uropods , which together with the terminal telson, make up the "tail fan". It is the sudden flexion of this tail fan that provides the thrust for the rapid escape response of these crustaceans and the tail fan is also used in steering. In Leptostraca, the appendages on the telson instead form caudal rami (spine-like protrusions). The digestive tract

1080-634: The name Decapoda (from the Greek δέκα , 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. 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

1116-420: The only extant arthropods with compound eyes placed on moveable stalks, although in some taxa the eyes are unstalked, reduced or lost. Up to three thoracic segments may be fused with the head to form a cephalothorax ; the associated appendages turn forward and are modified as maxillipeds (accessory mouthparts ). A carapace may be absent, present or secondarily lost , and may cover the head, part or all of

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1152-408: The orders of Malacostraca. These include differences in mutation rates in different lineages , different patterns of evolution being apparent in different sources of data, including convergent evolution , and long branch attraction . There is less agreement on the status of the subclass Phyllocarida with its single extant order, Leptostraca, depending on whether foliaceous (leaf-like) limbs have

1188-490: The taxonomic hierarchy until George Cuvier 's embranchements , first called Phyla by Ernst Haeckel , were introduced in the early nineteenth century. Decapoda Dendrobranchiata Pleocyemata See text for superfamilies. The Decapoda or decapods ( lit.   ' 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

1224-452: The thorax and some of the abdomen. It is variable in form and may be fused dorsally with some of the thoracic segments or occasionally be in two parts, hinged dorsally. Typically, each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Each endopod consist of seven articulating segments; the coxa, basis, ischium, merus, carpus, propodus and dactylus. In decapods ,

1260-557: The vast majority of the approximately 40,000 living species of malacostracans and consists of three superorders, Syncarida , Peracarida and Eucarida . Syncaridans are mostly small and found in freshwater and subterranean habitats. Peracaridans are characterised by having a marsupium in which they brood their young. They are found in marine, freshwater and terrestrial habitats and include Amphipoda , Cumacea , Isopoda and Mysida . Eucarida includes lobsters, crabs, shrimps, prawns and krill. The first malacostracans appeared sometime in

1296-646: Was curator of the arthropod collection at the National Museum of Natural History in Paris. This scientific name is misleading, since the shell is soft only immediately after moulting , and is usually hard. Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxon Entomostraca . The class Malacostraca includes about 40,000 species, and "arguably ... contains

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