Мизиды: Мелкие ракообразные с сумкой для вынашивания потомства.

Small, shrimp like crustacean

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. 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.

Some species are benthic (living on the seabed) and others pelagic (living in mid water), but most are found close to, crawling on or burrowing into the mud or sand. Most marine species are benthic by day but leave the seabed at night to become planktonic. Locomotion is mostly by swimming, the pleopods being used for this purpose. Some mysids live among algae and seagrasses, some are solitary while many form dense swarms. Mysids form an important part of the diet of such fish as shad and flounder. The species Mysidium integrum has a mutualistic relationship with longfin damselfish, the shrimp providing nutrients for the algae farms the fish feed on and the fish providing protection from predators. The majority of Mysida are omnivores, feeding on algae, detritus, and zooplankton. Scavenging and cannibalism are also common, with the adults sometimes preying on their young once they emerge from the marsupium. The age at which mysids reach sexual maturity depends on water temperature and food availability. The young are released soon afterwards, and although their numbers are usually low, the short reproductive cycle of mysid adults means a new brood can be produced every four to seven days.

Some species of mysids are easy to culture on a large scale in the laboratory as they are highly adaptive, and can tolerate a wide range of conditions. Despite low fecundity, these species have a short reproductive cycle which means they can quickly reproduce in vast numbers. In flow through systems, juvenile mysids are continuously separated from the adult brood stock in order to reduce mortality due to cannibalism. Their high protein and fat content also makes them a good alternative to live enriched Artemia when feeding juveniles (especially those that are difficult to maintain such as young seahorses) and other small fauna.