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Temporal range: Cambrian–Recent PreꞒ O S D C P T J K Pg NMalacostraca_header_cell_0_0_0

Scientific classification MalacostracaMalacostraca_header_cell_0_1_0
Kingdom:Malacostraca_cell_0_2_0 AnimaliaMalacostraca_cell_0_2_1
Phylum:Malacostraca_cell_0_3_0 ArthropodaMalacostraca_cell_0_3_1
Subphylum:Malacostraca_cell_0_4_0 CrustaceaMalacostraca_cell_0_4_1
Superclass:Malacostraca_cell_0_5_0 MulticrustaceaMalacostraca_cell_0_5_1
Class:Malacostraca_cell_0_6_0 Malacostraca

Latreille, 1802Malacostraca_cell_0_6_1


Malacostraca is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Malacostraca_sentence_0

Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, woodlice, amphipods, mantis shrimp and many other, less familiar animals. Malacostraca_sentence_1

They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. Malacostraca_sentence_2

They are segmented animals, united by a common body plan comprising 20 body segments (rarely 21), and divided into a head, thorax, and abdomen. Malacostraca_sentence_3

Etymology Malacostraca_section_0

The name Malacostraca was coined by a French zoologist Pierre André Latreille in 1802. Malacostraca_sentence_4

He was curator of the arthropod collection at the National Museum of Natural History in Paris. Malacostraca_sentence_5

The name comes from the Greek roots (malakós, meaning "soft") and (óstrakon, meaning "shell"). Malacostraca_sentence_6

The name is misleading, since the shell is only soft immediately after moulting, and is usually hard. Malacostraca_sentence_7

Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxon Entomostraca. Malacostraca_sentence_8

Description Malacostraca_section_1

The class Malacostraca includes about 40,000 species, and "arguably ... contains a greater diversity of body forms than any other class in the animal kingdom". Malacostraca_sentence_9

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. Malacostraca_sentence_10

Malacostracans have abdominal appendages, a fact that differentiates them from all other major crustacean taxa except Remipedia. Malacostraca_sentence_11

Each body segment bears a pair of jointed appendages, although these may be lost secondarily. Malacostraca_sentence_12

Tagmata Malacostraca_section_2

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. Malacostraca_sentence_13

The mouthparts consist of pairs each of mandibles, maxillules (second pair of mouthparts) and maxillae. Malacostraca_sentence_14

Usually a pair of stalked compound eyes is present, although in some taxa the eyes are unstalked, reduced or lost. Malacostraca_sentence_15

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). Malacostraca_sentence_16

A carapace may be absent, present or secondarily lost, and may cover the head, part or all of the thorax and some of the abdomen. Malacostraca_sentence_17

It is variable in form and may be fused dorsally with some of the thoracic segments or occasionally be in two parts, hinged dorsally. Malacostraca_sentence_18

Typically, each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Malacostraca_sentence_19

Each endopod consist of seven articulating segments; the coxa, basis, ischium, merus, carpus, propodus and dactylus. Malacostraca_sentence_20

In decapods, the claw is formed by the articulation of the dactylus against an outgrowth of the propodus. Malacostraca_sentence_21

In some taxa, the exopods are lost and the appendages are uniramous. Malacostraca_sentence_22

There is a clear demarcation between the thorax and the six or seven-segmented abdomen. Malacostraca_sentence_23

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. Malacostraca_sentence_24

The first and second abdominal pleopods may be modified in the male to form gonopods (accessory copulatory appendages). Malacostraca_sentence_25

The appendages of the last segment are typically flattened into uropods, which together with the terminal telson, make up the "tail fan". Malacostraca_sentence_26

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. Malacostraca_sentence_27

In Leptostraca, the appendages on the telson instead form caudal rami (spine-like protrusions). Malacostraca_sentence_28

Internal anatomy Malacostraca_section_3

The digestive tract 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. Malacostraca_sentence_29

The walls of this have chitinous ridges, teeth and calcareous ossicles. Malacostraca_sentence_30

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. Malacostraca_sentence_31

The hindgut is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson. Malacostraca_sentence_32

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. Malacostraca_sentence_33

The typical respiratory pigment in malacostracans is haemocyanin. Malacostraca_sentence_34

Structures that function as kidneys are located near the base of the antennae. Malacostraca_sentence_35

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. Malacostraca_sentence_36

Sensory organs include compound eyes (often stalked), ocelli (simple eyes), statocysts and sensory bristles. Malacostraca_sentence_37

The naupliar eye is a characteristic of the nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness. Malacostraca_sentence_38

Ecology Malacostraca_section_4

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). Malacostraca_sentence_39

They are abundant in all marine ecosystems, and most species are scavengers, although some, such as the porcelain crabs, are filter feeders, and some, such as mantis shrimps, are carnivores. Malacostraca_sentence_40

Life cycle Malacostraca_section_5

Most species of malacostracans have distinct sexes (a phenomenon known as gonochorism), although a few species exhibit hermaphroditism. Malacostraca_sentence_41

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 a small number of species, on the seventh. Malacostraca_sentence_42

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. Malacostraca_sentence_43

Primitive malacostracans have a free-swimming naupliar larval stage. Malacostraca_sentence_44

Mating Malacostraca_section_6

Mating behavior has been studied in the freshwater shrimp Caridina ensifera. Malacostraca_sentence_45

Multiple paternity, common in the Malacostrica, also occurs in C. ensifera. Malacostraca_sentence_46

Reproductive success of sires was found to correlate inversely with their genetic relatedness to the mother. Malacostraca_sentence_47

This finding suggests that sperm competition and/or pre- and post-copulatory female choice occurs. Malacostraca_sentence_48

Female choice may increase the fitness of progeny by avoiding inbreeding that can lead to expression of homozygous deleterious recessive mutations. Malacostraca_sentence_49

Phylogenetics Malacostraca_section_7

Main article: Phylogeny of Malacostraca Malacostraca_sentence_50

The monophyly of Malacostraca is widely accepted. Malacostraca_sentence_51

This is supported by several common morphological traits which are present throughout the group and is confirmed by molecular studies. Malacostraca_sentence_52

However, a number of problems make it difficult to determine the relationships between the orders of Malacostraca. Malacostraca_sentence_53

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. Malacostraca_sentence_54

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 a single or multiple origin. Malacostraca_sentence_55

Some authors advocate placing Phyllocarida in Phyllopoda, a group used in former classification systems, which would then include branchiopods, cephalocarids and leptostracans. Malacostraca_sentence_56

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 a subclass of Malacostraca that had diverged from the main lineage at an early date. Malacostraca_sentence_57

Subclass Phyllocarida Malacostraca_section_8

Leptostraca is the only extant order of Phyllocarida, the other two orders, Archaeostraca and Hoplostraca being extinct. Malacostraca_sentence_58

Leptostracans are thought to be the most primitive of the malacostracans and date back to the Cambrian period. Malacostraca_sentence_59

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. Malacostraca_sentence_60

They have a two part carapace which encloses the head, the whole thorax and part of the abdomen and are the only malacostracans with seven abdominal segments. Malacostraca_sentence_61

Three families are known with several genera and about twenty species. Malacostraca_sentence_62

They are found worldwide from the intertidal zone to the deep ocean, all but one species being benthic (living on the seabed). Malacostraca_sentence_63

Subclass Hoplocarida Malacostraca_section_9

Stomatopoda is the only extant order of Hoplocarida, the other two orders, Aeschronectida and Archaeostomatopoda being extinct. Malacostraca_sentence_64

Stomatopodans, commonly known as mantis shrimps, range in length from 5 to 36 cm (2 to 14 in) and are predators. Malacostraca_sentence_65

They have a dorso-ventrally flattened body and a shield-like carapace and are armed with powerful, raptorial claws normally carried in a folded position. Malacostraca_sentence_66

There are about 300 species, most living in tropical and subtropical seas although some live in temperate areas. Malacostraca_sentence_67

They are benthic, mostly hiding in cracks and crevices or living in burrows, some emerging to forage while others are ambush predators. Malacostraca_sentence_68

Subclass Eumalacostraca Malacostraca_section_10

The Eumalocostraca contains the vast majority of the approximately 40,000 living species of malacostracans and consists of three superorders, Syncarida, Peracarida and Eucarida. Malacostraca_sentence_69

Syncaridans are mostly small and found in freshwater and subterranean habitats. Malacostraca_sentence_70

Peracaridans are characterised by having a marsupium in which they brood their young. Malacostraca_sentence_71

They are found in marine, freshwater and terrestrial habitats and include Amphipoda, Cumacea, Isopoda and Mysida. Malacostraca_sentence_72

Eucarida includes lobsters, crabs, shrimps, prawns and krill. Malacostraca_sentence_73

Fossil record Malacostraca_section_11

The first malacostracans appeared sometime in the Cambrian, when animals belonging to the Phyllocarida appeared. Malacostraca_sentence_74

Classification Malacostraca_section_12

See also: List of prehistoric malacostracans Malacostraca_sentence_75

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. Malacostraca_sentence_76

Extinct orders have been added to this and are indicated by an obelisk (†). Malacostraca_sentence_77

Class Malacostraca Latreille, 1802 Malacostraca_sentence_78




  • Subclass Hoplocarida Calman, 1904Malacostraca_item_2_5



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