Mysidacea

(invertebrate zoology) An order of free-swimming Crustacea included in the division Pericarida; adult consists of 19 somites, each bearing one pair of functionally modified, biramous appendages, and the carapace envelops most of the thorax and is fused dorsally with up to four of the anterior thoracic segments.

An order of free-swimming, shrimplike crustaceans belonging to the class Eumalacostraca; commonly known as opossum shrimps. They occur in vast numbers in coastal and oceanic regions of the world.

The Mysidacea are divided into suborders Lophogastrida and the Mysida. The former contains only 38 species ascribed to six genera; they live predominantly in the deep sea and range in adult body length from 0.6 to 13 in. (17 to 350 mm), except the seven species in the genus Paralophogaster which are 0.2–0.8 in. (6–20 mm) in length.

The suborder Mysida contains some 800 species ascribed to four families: the primitive Petalophthalmidae and the advanced Mysidae, Lepidomysidae, and Stygiomysidae. Except for the deep-sea Petalophthalmidae and cave-dwelling Lepidomysidae (some 30 species), all species, in about 130 genera, have statocysts in the uropods (abdominal appendages; see illustration), a feature peculiar to mysids; and have adult body lengths of about 0.6 in. (15 mm) or less. Most mysidans are distributed in shallow coastal and shelf waters, a few have invaded fresh waters, and others live in the surface layers of the oceans or in the deep sea to depths as great as 23,650 ft (7210 m).

A member of the Mysida, showing the marsupium and the paired uropods with a statocyst in the inner one.

The young are carried within a marsupium formed by transparent concave plates attached to the insides of the posterior thoracic legs. These plates have short, strong setae and interlock ventrally to form a closed chamber, the marsupium. The eggs, which are fertilized during laying, are laid directly into the marsupium; develop to miniature adults; and emerge to swim freely in the sea. Mysids, like all crustaceans, increase their body size by molting.

Most species of mysids form aggregations. These are of different types and for different purposes. The functions of these aggregations, except for breeding, are not always clear, although protection of the population from predators is important. These aggregations result in many coastal mysids occurring at high densities, especially in estuarine or sandy beach habitats. Swarms of mysids in coastal waters are exploited commercially in tropical and subtropical regions of the world. See also Crustacea; Eumalacostraca.

The noun has one meaning:

Meaning #1: opossum shrimp
  Synonym: order Mysidacea

Mysidacea
Gnathophausia zoea
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
(unranked):	Mysidacea
Orders & Families
Lophogastrida G. O. Sars, 1870 Eucopiidae Lophogastridae Mysida A. H. Haworth, 1825 Mysidae Lepidomysidae Petalophthalmidae Gnathophausiidae

The Mysidacea is a group of small, shrimp-like creatures comprising the two related orders Mysida and Lophogastrida. They are sometimes referred to as opossum shrimps though that name is also used for individual species (e.g. Neomysis americana).

Note that despite their name, and their superficial resemblance to shrimp, they are only quite distantly related to the true shrimps, which are classified in the order Decapoda. The characteristics of the Mysidacea include the following:

• They have a well developed carapace that covers most of the thorax, but it is never fused with more than four of the thoracic segments
• Their pereiopods are biramous (i.e. separated into two branches), except sometimes the last pair, which may be reduced
• Their pleopods are reduced. In males they may be modified
• They usually have a statocyst in the endopods of their uropods
• Their eyes are on stalks

The name "opossum shrimp" is derived from the females' brood pouch, formed by plates attached to the front legs. The larvae are carried in the pouch until they are able to move by themselves. Although they most commonly occur at sea, they are abundant in certain bodies of fresh water, such as the Great Lakes; some are occasionally found in caves. Wherever mysids occur, whether in salt or fresh water, they are often very abundant and form an important part of the normal diet of many fishes. In many Asian countries, mysids are a common part of local cuisine, and in certain regions, such as the Gulf of Thailand, they support extensive fisheries. Mysids are also used as a bioassay for toxins in seawater. ^[1]

Systematics

Although the traditional grouping of Mysida and Lophogastrida into Mysidacea has had strong morphological support, recent molecular studies question the monophyly of this group.^[2][3]

References

1. ^ Encarta Reference Library Premium 2005 DVD. Article - Opossum Shrimp.
2. ^ Spears T et al. (2005). Peracarid monophyly and interordinal phylogeny inferred from nuclear small-subunit ribosomal DNA sequences (Crustacea: Malacostraca: Peracarida). Proceedings of the biological society of Washington, 118, 117-157.
3. ^ Poore, G. C. (2005). Peracarida: monophyly, relationships and evolutionary success. Nauplius 13, 1-27.

External links

Wikispecies has information related to: Lophogastrida

Wikispecies has information related to: Mysida

• Mysidacea fact sheet - Guide to the marine zooplankton of south eastern Australia
• Gary Anderson: Peracarida Taxa and Literature
• Tim Deprez's website on Mysida
• Mysid, Americamysis bahia, Photo and Information at MBL Aquaculture
• Kenneth Meland's Mysidacea page (crustacea.net)

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