Malacostraca - WikiHQ

Malacostraca is the second largest of the six classes of pancrustaceans after 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, spiny lobsters, crayfish, shrimp, krill, prawns, isopods, amphipods, mantis shrimp, 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 a common body plan comprising 20 body segments (rarely 21), and divided into a head, thorax, and abdomen.

Etymology

The name Malacostraca is . 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 was curator of the arthropod collection at the National Museum of Natural History in Paris.

This scientific name is a misnomer, 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.

Description

thumb|right|[[Leptostraca such as Nebalia bipes retain the primitive condition of having seven abdominal segments.]]

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

Tagmata

The head bears two pairs of antennae, the first of which is often biramous (branching into two parts). The second pair of antennae bears exopods (outer branches) which are often flattened into antennal scales known as scaphocerites. 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).

Internal anatomy

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. 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 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 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 the nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness. 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. 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.

Mating

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.

Phylogenetics

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

The next cladogram follows a 2023 revision of pancrustacean phylogeny based on molecular data. The cladogram shows traditional paraphyletic groups and presents new monophyletic clades.

The presence of two clades syncarids, with Euphausiacea being closer to one of them (Anaspidacea) than to Decapoda, renders the classically recognized Eucarida paraphyletic. Syncarida is also polyphyletic which Bathynellacea and Anaspidacea are not sister groups to each other. Together, syncarids and eucarids are grouped in Syneucarida. Eumalacostraca was found paraphyletic as it classically excludes Hoplocarida and only includes peracarids, syncarids and eucarids. Thus, Malacostraca is divided into three clades: Phyllocarida, Stomatocarida, and Peracarida.

Fossil record

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

Classification

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 (†).

thumb|[[Peacock mantis shrimp|Odontodactylus scyllarus (Hoplocarida: Stomatopoda)]]

thumb|[[Porcellio scaber and Oniscus asellus (Peracarida: Isopoda)]]

thumb|[[Cancer pagurus (Eucarida: Decapoda)]]

Class Malacostraca Latreille, 1802

Subclass Phyllocarida Packard, 1879

:* † Archaeostraca Claus 1888

:* † Hoplostraca Schram, 1973

:* Leptostraca Claus, 1880

Subclass Hoplocarida Calman, 1904

:* † Aeschronectida Schram, 1969

:* † Archaeostomatopoda Schram, 1969

:* Stomatopoda Latreille, 1817

Subclass Eumalacostraca Grobben, 1892
Superorder Syncarida Packard, 1885
† Palaeocaridacea Brooks, 1979
Bathynellacea Chappuis, 1915
Anaspidacea Calman, 1904
Superorder Peracarida Calman, 1904
Spelaeogriphacea Gordon, 1957
Thermosbaenacea Monod, 1927
Lophogastrida Sars, 1870
Mysida Haworth, 1825
Stygiomysida Tchindonova, 1981
Mictacea Bowman et al., 1985
Amphipoda Latreille, 1816
Isopoda Latreille, 1817
Tanaidacea Dana, 1849
Cumacea Krøyer, 1846
Superorder Eucarida Calman, 1904
† Angustidontida Gueriau, Charbonnier & Clément, 2014
Euphausiacea Dana, 1852
Decapoda Latreille, 1802

References

External links

Malacostraca, Tree of Life Web Project
Introduction to the Malacostraca, University of California, Berkeley
Malacostraca, The Paleobiology Database
Malacostraca image key - Guide to the marine zooplankton of south eastern Australia, Tasmanian Aquaculture and Fisheries Institute