The superorder Peracarida is a large group of

malacostraca Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobs ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

s, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or ''marsupium'', formed from thin flattened plates (

oostegite An oostegite is a large, flexible plate-like flap extending medially from the coxae (first segments) of the pereiopods (thoracic appendages) in some female Female (symbol: ♀) is the sex of an organism that produces the large non-motile ...

s) borne on the basalmost segments of the

legs A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element c ...

. Peracarida is one of the largest crustacean taxa and includes about 12,000 species. Most members are less than in length, but the largest is probably the

giant isopod A giant isopod is any of the almost 20 species of large isopods, crustaceans distantly related to shrimp and crabs, which are decapods, in the genus ''Bathynomus''. They are abundant in the cold, deep waters of the Atlantic, Pacific, and Indian ...

(''Bathynomus giganteus'') which can reach . The earliest known perecaridian was '' Oxyuropoda ligioides'', a fossil of which has been found dating to the Late

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

(more than 360 mya) of Ireland.

Characteristics

The most obvious characteristic of the group is the marsupium in females. This brood pouch is enclosed by the large, flexible oostergites, bristly flaps which extend from the basal segments of the thoracic appendages, which form the floor of a chamber roofed by the animal's sternum. This chamber is where the eggs are brooded, development being direct in most cases. Other characteristics include the possession of a single pair of maxillipeds (rarely 2–3), of mandibles with an articulated accessory process between the molar and incisor teeth in the adults (called the ''lacinia mobilis''), and of a

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

which is often reduced in size and is not fused with the posterior thoracic somites. In some orders, the young hatch at a post-larval, prejuvenile stage called a ''

manca The manca (plural: ''mancae'') is the post-larval juvenile in some crustaceans. The manca stage is the defining characteristic of a clade called Mancoida which comprises all the member of the Peracarida except the Amphipoda. Mancae closely resembl ...

'' which lacks the last pair of legs. In the underground order Thermosbaenacea, there are no oostergites and the carapace of the female is expanded to form a dorsal marsupium.

Orders

There is some disagreement as to which orders should be included within Peracarida. Martin & Davies include the following eleven orders: * Amphipoda Latreille, 1816 *

Cumacea Cumacea is an order of small marine crustaceans of the superorder Peracarida, occasionally called hooded shrimp or comma shrimp. Their unique appearance and uniform body plan makes them easy to distinguish from other crustaceans. They live in so ...

Krøyer, 1846 *

Isopoda Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and ...

Latreille, 1817 *

Lophogastrida Lophogastrida is an order of malacostracan crustaceans in the superorder Peracarida, comprising shrimp-like animals that mostly inhabit the relatively deep pelagic waters of the oceans throughout the world. Most lophogastridan species are lon ...

Sars, 1870 * Mictacea Bowman, Garner, Hessler, Iliffe & Sanders, 1985 *

Mysida Mysida is an order of small, shrimp-like crustaceans in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch or "marsupium" in females. The fact that the larvae are reared in th ...

Haworth, 1825 *†

Pygocephalomorpha The order Pygocephalomorpha is an extinct group of peracarid crustaceans. Pygocephalomorpha were abundant from the Carboniferous until their extinction in the Permian. Families The order contains extinct five families, and seven genera ''incer ...

Beurlen, 1930 (only known from fossil remains) * Spelaeogriphacea Gordon, 1957 * Tanaidacea Dana, 1849 * Thermosbaenacea

Monod Monod is a surname, and may refer to: * Adolphe Monod (1802–1856), French Protestant churchman; brother of Frédéric Monod. * Frédéric Monod (1794–1863), French Protestant pastor. * Gabriel Monod, French historian * Jacques Monod (1910� ...

, 1927 but Ruppert, Fox and Barnes exclude Thermosbaenacea and place it in a separate order, Pancarida. They also place Lophogastrida, Mysida and Pygocephalomorpha in a single order,

Mysidacea The Mysidacea is a group of shrimp-like crustaceans in the superorder Peracarida, comprising the two extant orders Mysida Mysida is an order of small, shrimp-like crustaceans in the malacostracan superorder Peracarida. Their common name opo ...

. This arrangement is disputed by Meland and Willassen who found that

molecular data In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which succi ...

shows that the three orders are not closely related.

References

External links

* * {{Authority control Malacostraca Arthropod superorders