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 this pouch and are not free-swimming characterises the order. The mysid's head bears a pair of stalked eyes and two pairs of antennae. The thorax consists of eight segments each bearing branching limbs, the whole concealed beneath a protective carapace and the abdomen has six segments and usually further small limbs.
Mysids are found throughout the world in both shallow and deep marine waters where they can be benthic or pelagic, but they are also important in some fresh water and brackish ecosystems. Many benthic species make daily vertical migrations into higher parts of the water column. Mysids are filter feeders, omnivores that feed on algae, detritus and zooplankton. Some mysids are cultured in laboratories for experimental purposes and are used as a food source for other cultured marine organisms. They are sensitive to water pollution, so are sometimes used as bioindicators to monitor water quality.
The head of a mysid bears two pairs of antennae and a pair of large, stalked eyes. The head and first segment (or sometimes the first three segments) of the thorax are fused to form the cephalothorax. The eight thoracic segments are covered by the carapace which is attached only to the first three. The first two thoracic segments bear maxillipeds which are used to filter plankton and organic particulate from the water. The other six pairs of thoracic appendages are biramous (branching) limbs known as pereopods, and are used for swimming, as well as for wafting water towards the maxillipeds for feeding. Unlike true shrimps (Caridea), females have a marsupium beneath the thorax. This brood pouch is enclosed by the large, flexible oostegites, bristly flaps which extend from the basal segments of the pereopods and 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.[2]
The abdomen has six segments, the first five of which bear pleopods, although these may be absent or vestigial in females. The fourth pleopod is longer than the others in males and has a specialized reproductory function.[2]
The majority of species are 5–25 mm (0.2–1.0 in) long, and vary in colour from pale and transparent, through to bright orange or brown. They differ from other species within the superorder Peracarida by featuring statocysts on their uropods (located on the last abdominal segment). These help the animal orient itself in the water and are clearly seen as circular vesicles: together with the pouch the statocysts are often used as features that distinguish mysids from other shrimp-like organisms.[3]
Mysids have a cosmopolitan distribution and are found in both marine and freshwater environments, the deep sea, estuaries, shallow coastal waters, lakes, rivers and underground waters. They are primarily marine and fewer than ten percent are found in freshwater. There are about 72 freshwater species in total, being predominantly found in the Palearctic and Neotropical realms. These non-marine mysids occur in four distinct types of habitats; some are estuarine species; some were isolated in the Ponto-Caspian Basin where Paramysis has since radiated enormously (23 species); some are glacial relicts and some are subterranean Tethyan relicts.[4]