Malacostraca is the largest of the six classes of crustaceans, containing over 25,000 extant species, divided among 16 orders. Its members display a greater diversity of body forms than any other class of animals, and include crabs, lobsters, shrimp, krill, woodlice, scuds (Amphipoda), mantis shrimp and many other less familiar animals. They are abundant in all marine environments and have also colonised freshwater and terrestrial habitats. They are united by a common , comprising 20 body segments (rarely 21), divided into a head, thorax and abdomen.
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).[1] 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 shrimp, are carnivores.[2]
The name Malacostraca comes from the Greek roots ?a?a??? (malak?s, meaning "soft") and ?st?a???
(?strakon, meaning "shell").[3] The name is misleading, since the shell is only soft immediately after moulting, and is usually hard.[4] Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxon Entomostraca.[5]
Leptostraca such as Nebalia bipes retain the primitive condition of having seven abdominal segments.
The Class Malacostraca i
ncludes over 25,000 species,[6] and "arguably ... contains a greater diversity of body forms than any other class in the animal kingdom".[7] Its members are characterised by the presence of three tagmata ? a five-segmented head, an eight-segmented thorax and an abdomen with six segments, except in the Leptostraca, which retain the ancestral condition of seven abdominal segments.[7] This arrangement is known as the "caridoid facies", a term coined by William Thomas Calman in 1909. Each body segment bears a pair of jointed appendages, although these may be lost secondarily.[6]
Tagmata
The head bears two pairs of antennae, the first of which is biramous and the second
uniramous, and two pairs of maxillae. There is usually a pair of stalked compound eyes, although these may be sessile, reduced or lost.[2]
Up to eight thoracic segments may be fused with the head to form a cephalothorax, and up to three pairs of appendages may be modified as maxillipeds (accessory mouthparts).[7] A carapace may be absent, present, or secondarily lost, and may cover from two thoracic segments to the entire thorax and some of the abdomen.[7]
Each segment of the abdomen except the last carries a pair of pleopods. The appendages of the last segment are typically flattened into uropods, which together with the terminal telson, make up the "tail fan".[2] In Leptostraca, these appendages instead form caudal rami.[8]
Life cycle
Most malacostracans are gonochoristic (i.e., they have separate sexes), although there are a few hermaphroditic species.[2] The female genital openings are on the sixth thoracic segment, while the male genital opening is usually on the sixth thoracic segment, but is occasionally on the seventh.[9] Each of the thoracic appendages is biramous and also carries a gill.[2] The larval stages are often reduced, but where they occur, there is usually a metamorphosis between the larval and the adult form.[9]
Martin and Davis present the following classification of living malacostracans into orders,[10] to which extinct orders have been added, indicated by an obelisk (?).
Porcellio scaber and Oniscus asellus (Peracarida: Isopoda)
Cancer pagurus (Eucarida: Decapoda)
Class Malacostraca Latreille, 1802
Subclass Phyllocarida Packard, 1879
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
Mictacea Bowman et al., 1985
Amphipoda Latreille, 1816
Isopoda Latreille, 1817
Tanaidacea Dana, 1849
Cumacea Kr?yer, 1846
Superorder Eucarida Calman, 1904
Euphausiacea Dana, 1852
Amphionidacea Williamson, 1973
Decapoda Latreille, 1802
Phylogenetics
Main article: Phylogeny of Malacostraca
While
the monophyly of Malacostraca as a whole is widely supported, a number of problems make it difficult to determine the relationships between the orders of Malacostraca. These include differences in rates of evolution in different lineages, different patterns of evolution being apparent in different sources of data, including convergent evolution, and long branch attraction.[11]
Fossil record
The first malacostracans appeared in the Early Cambrian,[12] when animals belonging to the Phyllocarida appeared.[4]
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Cancer pagurus (Eucarida: Decapoda)
Class Malacostraca Latreille, 1802
Subclass Phyllocarida Packard, 1879
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
Mictacea Bowman et al., 1985
Amphipoda Latreille, 1816
Isopoda Latreille, 1817
Tanaidacea Dana, 1849
Cumacea Kr?yer, 1846
Superorder Eucarida Calman, 1904
Euphausiacea Dana, 1852
Amphionidacea Williamson, 1973
Decapoda Latreille, 1802
Phylogenetics
Main article: Phylogeny of Malacostraca
While the monophyly of Malacostraca as a whole is widely supported, a number of problems make it difficult to determine the relationships between the orders of Malacostraca. These include differences in rates of evolution in different lineages, different patterns of evolution being apparent in different sources of data, including convergent evolution, and long branch
attraction.[11]
Fossil record
The first malacostracans appeared in the Early Cambrian,[12] when animals belonging to the Phyllocarida appeared.[4]
^ abcde P. J. Hayward, M. J. Isaac, P. Makings, J. Moyse, E. Naylor & G. Smaldon (1995). "Crustaceans". In P. J. Hayward & John Stanley Ryland. Handbook of the Marine Fauna of North-West Europe. Oxford University Press. pp. 290?461. ISBN 9780198540557. http://books.google.co.uk/books?id=sZ8mdRT4jbIC&pg=PA317.
^ "malacostracan". Oxford English Dictionary. Oxford University Press. 2nd ed. 1989.
^ ab Patricia Vickers Rich, Mildred Adams Fenton, Carroll Lane Fenton & Thomas Hewitt Rich (1996). "Crustaceans". The Fossil Book: a Record of Prehistoric Life (2nd ed.). Courier Dover Publications. pp. 213?221. ISBN 9780486293714. http://books.google.co.uk/books?id=_ntSspji0LYC&pg=PA217.
^ Frederick R. Schram (1974). "Convergences between Late Paleozoic and modern caridoid Malacostraca". Systematic Zoology23 (3): 323?332. doi:10.2307/2412539. JSTOR 2412539.
Amphionides reynaudii is the sole representative of the order Amphionidacea, and is a small (less than one inch long) planktonic crustacean found throughout the world's tropical oceans, mostly in shallow waters. [more]
Amphipoda
Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. The name amphipoda refers to the different forms of appendages, unlike isopods, where all the thoracic legs are alike. Of the 7,000 species, 5,500 are classified into one suborder, Gammaridea. The remainder are divided into two or three further suborders. Amphipods range in size from 1 to 340 millimetres (0.039 to 13 in) and are mostly detritivores or scavengers. They live in almost all aquatic environments; 750 species live in caves and the order also includes terrestrial animals and sandhoppers such as Talitrus saltator. [more]
Anaspidacea
Anaspidacea is an order of crustaceans, comprising eleven genera in four families. Species in the family Anaspididae vary from being strict stygobionts (only living underground) to species living in lakes, streams and moorland pools, and are found only in Tasmania. is found in Tasmania and the south-eastern part of the Australian mainland, where they live in the burrows made by crayfish and in caves. The families Psammaspididae and Stygocarididae are both restricted to caves, but Stygocarididae has a much wider distribution than the other families, with Parastygocaris having species in New Zealand and South America as well as Australia; two other genera in the family are endemic to South America, and one, Stygocarella, is endemic to New Zealand. [more]
Bathynellacea is an order of crustaceans which live interstitially in groundwater. Some species can tolerate low salt concentrations, and at least one African species is a thermophile, living in hot springs and tolerating temperatures up to 55 ?C (131 ?F). Bathynellaceans are minute, blind, worm-like animals with short, weak legs, reaching a maximum size of 3.4 millimetres (0.13 in). They are found on every continent except Antarctica, although they are missing from some islands, including Fiji, New Caledonia and the Caribbean islands. There are two families, and Parabathynellidae; a third family, "Leptobathynellidae", is considered a synonym of Parabathynellidae. [more]
Cumacea
Cumacea is an order of small marine crustaceans, occasionally called hooded shrimp. Their unique appearance and uniform body plan makes them easy to distinguish from other crustaceans. [more]
Decapoda
The decapods or Decapoda (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with approximately 3,300 fossil species. Nearly half of these species are crabs, with the shrimp (?3000 species) and Anomura (including hermit crabs, porcelain crabs, squat lobsters: c.?2500 species), making up the bulk of the remainder. The earliest fossil decapod is the Devonian Palaeopalaemon. [more]
Euphausiacea
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world. The common name krill comes from the Norwegian word , meaning "young fry of fish", which is also often attributed to other species of fish. [more]
Isopoda
Isopods are an order of peracarid crustaceans, including familiar animals such as woodlice and pill bugs. The name Isopoda derives from the Greek roots (iso-, meaning "same") and p?d?? (podos, meaning "foot"). The fossil record of isopods dates back to the Carboniferous period (in the US Pennsylvanian epoch), at least 300 million years ago. [more]
Leptostraca
Leptostraca (from the Greek words for thin and shell) is an order of small, marine crustaceans. Its members, including the well-studied Nebalia, occur throughout the world's oceans and are usually considered to be filter-feeders. It is the only extant order in the subclass Phyllocarida. They are believed to represent the most primitive members of their class, the Malacostraca, and first appear in the fossil record during the Cambrian period. [more]
Lophogastrida
Lophogastrida is an order of malacostracan crustaceans in the superorder Peracarida. They are shrimp-like animals that mostly inhabit the relatively deep pelagic waters of the oceans throughout the world. [more]
Mictacea
Mictacea is an order of crustaceans, erected for six species of small shrimp-like animals of the deep sea and anchialine caves. [more]
Mysida
Mysida is a group of small, shrimp-like crustaceans, an order in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch, or marsupium, in females. Mysids are mostly found in marine waters throughout the world, but are also important in some fresh- and brackish-water ecosystems of the Northern hemisphere. Some mysids are cultured for experimental purposes and as food source for other cultured marine organisms. [more]
Mantis shrimp or stomatopods are marine crustaceans, the members of the order Stomatopoda. They are neither shrimp nor mantids, but receive their name purely from the physical resemblance to both the terrestrial praying mantis and the shrimp. They may reach 30 centimetres (12 in) in length, although exceptional cases of up to 38 cm (15 in) have been recorded. The carapace of mantis shrimp covers only the rear part of the head and the first four segments of the thorax. Mantis shrimp appear in a variety of colors, from shades of browns to bright neon colors. Although they are common animals and among the most important predators in many shallow, tropical and sub-tropical marine habitats they are poorly understood as many species spend most of their life tucked away in burrows and holes. [more]
Thermosbaenacea is a group of crustaceans that live in thermal springs in fresh water, brackish water and anchialine habitats. They have occasionally been treated as a distinct superorder (Pancarida), but are generally considered to belong to the Peracarida. Due to their troglobitic lifestyle, thermosbaenaceans lack visual pigments and are therefore blind. [more]
At least 19 species and subspecies belong to the Order Thermosbaenacea.
More info about the Order Thermosbaenacea may be found here.
^ abcde P. J. Hayward, M. J. Isaac, P. Makings, J. Moyse, E. Naylor & G. Smaldon (1995). "Crustaceans". In P. J. Hayward & John Stanley Ryland. Handbook of the Marine Fauna of North-West Europe. Oxford University Press. pp. 290?461. ISBN 9780198540557. http://books.google.co.uk/books?id=sZ8mdRT4jbIC&pg=PA317.
^ "malacostracan". Oxford English Dictionary. Oxford University Press. 2nd ed. 1989.
^ ab Patricia Vickers Rich, Mildred Adams Fent
on, Carroll Lane Fenton & Thomas Hewitt Rich (1996). "Crustaceans". The Fossil Book: a Record of Prehistoric Life (2nd ed.). Courier Dover Publications. pp. 213?221. ISBN 9780486293714. http://books.google.co.uk/books?id=_ntSspji0LYC&pg=PA217.
^ Frederick R. Schram (1974). "Convergences between Late Paleozoic and modern caridoid Malacostraca". Systematic Zoology23 (3): 323?332. doi:10.2307/2412539. JSTOR 2412539.
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