| Mysida | |
|---|---|
 | |
| Hemimysis anomala (Mysidae) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Malacostraca |
| Superorder: | Peracarida |
| Order: | Mysida Boas, 1883[1] |
| Families | |
Mysida is anorder of small,shrimp-likecrustaceans in themalacostracansuperorderPeracarida. Theircommon nameopossum shrimps stems from the presence of abrood pouch or "marsupium" in females. The fact that thelarvae are reared in this pouch and are notfree-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 protectivecarapace 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 bebenthic orpelagic, but they are also important in somefresh water andbrackishecosystems. Many benthic species makedaily vertical migrations into higher parts of thewater column. Mysids arefilter feeders,omnivores that feed onalgae,detritus andzooplankton. 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 waterpollution, so are sometimes used asbioindicators to monitorwater 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 thecephalothorax. The eight thoracic segments are covered by thecarapace which is attached only to the first three. The first two thoracic segments bearmaxillipeds which are used to filter plankton and organic particulate from the water. The other six pairs of thoracic appendages arebiramous (branching) limbs known aspereopods, and are used for swimming, as well as for wafting water towards the maxillipeds for feeding. Unlike true shrimps (Caridea), females have amarsupium 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'ssternum. 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 bearpleopods, 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 superorderPeracarida by featuringstatocysts on theiruropods (located on the last abdominal segment). These help the animal orient itself in the water and are clearly seen as circularvesicles: together with the pouch the statocysts are often used as features that distinguish mysids from other shrimp-like organisms.[3]
Mysids have acosmopolitan 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 thePalearctic andNeotropical realms. These non-marine mysids occur in four distinct types of habitats; some are estuarine species; some were isolated in thePonto-Caspian Basin whereParamysis has since radiated enormously (23 species); some are glacialrelicts and some are subterraneanTethyan relicts.[4]
Some species arebenthic (living on the seabed) and otherspelagic (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 becomeplanktonic. Locomotion is mostly by swimming, the pleopods being used for this purpose. Some mysids live amongalgae andseagrasses, some are solitary while many form dense swarms. Mysids form an important part of the diet of such fish asshad andflounder.[2] In general, they are free-living, but a few species, mostly in the subfamilyHeteromysinae, arecommensal and are associated withsea anemones andhermit crabs.[5] Severaltaxa have also been described from different freshwater habitats and caves.[5]Mysis relicta and its close relatives inhabit cold, deep lakes and have a diurnal cycle of vertical migrations.[6] The speciesMysidium integrum has amutualistic relationship withlongfin damselfish, the shrimp providing nutrients for the algae farms the fish feed on and the fish providing protection from predators.[7][8]
The majority of Mysida areomnivores, feeding on algae,detritus, andzooplankton.Scavenging andcannibalism are also common, with the adults sometimes preying on their young once they emerge from the marsupium.[3] The pelagic and most other species arefilter feeders, creating a feeding current with the exopods of their pereopods. This wafts food particles into a ventral food groove along which they are passed before being filtered bysetae (bristles) on the second maxillae. Larger planktonic prey can be caught in a trap composed of the endopods of the thoracic appendages.[2] Some benthic species, especially members of the subfamilyErythropinae, have been observed feeding on small particles which they collected by grooming the surfaces of their bodies and legs.[5]
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Individual mysids are either male or female, and fertilisation is external. Thegonads are in the thorax and are tubular in shape. Males have twogonopores in the eighth thoracic segment and a pair of long penises. The female gonopores are in the sixth thoracic segment and the oostegites are attached to the first to seventh pereopods to form a brood pouch.[2] Mating usually takes place at night and lasts only a few minutes.[3] During the process, the male inserts his penises into the marsupium and releases sperm. This stimulates the female and the eggs are usually released into the marsupium within an hour. Here they are fertilised and retained, development of the embryos in the brood pouch beingdirect with the young hatching from the eggs as miniature adults.[2] The size of a mysid brood generally correlates with body length and environmental factors such as density and food availability.[9] The age at which mysids reachsexual maturity depends on water temperature and food availability.[9][10][11] For the speciesMysidopsis bahia, this is normally at 12 to 20 days.[11] The young are released soon afterwards, and although their numbers are usually low, the shortreproductive cycle of mysid adults means a new brood can be produced every four to seven days.[3][11][12]
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 lowfecundity, these species have a short reproductive cycle which means they can quickly reproduce in vast numbers.[3][12] They can be cultured in static or flow-through systems, the latter having been shown to be able to maintain a higher stocking density than a static system.[13] In flow-through systems, juvenile mysids are continuously separated from the adult brood stock in order to reduce mortality due to cannibalism.[9]Artemia (brine shrimp) juveniles (incubated for 24 hours) are the most common food in mysid cultures, sometimes enriched with highly unsaturatedfatty acids to increase their nutritional value.[9]
Cultured mysids are thought to provide an ideal food source for many marine organisms. They are often fed tocephalopods, fish larvae, and commercialfarmed shrimp due to their small size and low cost.[9][14][15][16] Their high protein and fat content also makes them a good alternative to live enrichedArtemia when feeding juveniles (especially those that are difficult to maintain such as youngseahorses) and other small fauna.[15][16]
Their sensitivity to water quality also makes them suitable forbioassays.Americamysis bahia andAmericamysis almyra are frequently used to test forpesticides and other toxic substances, withA. bahia found to be more sensitive during the periods when it ismoulting.[17]
The Mysida belong to the superorder Peracarida, which means "near to shrimps". Although in many respects mysids appear similar to some shrimps, the main characteristic separating them from the superorderEucarida is their lack of free-swimminglarvae.[3] The order Mysida is extensive and currently includes approximately 160genera, containing more than 1000 species.[5]
Traditionally, Mysida were united with another, externally similar group of pelagic crustaceans, theLophogastrida, into a broader orderMysidacea, but that classification is generally abandoned at present.[1][18][19][20][21]While the previous grouping had goodmorphological support,molecular studies do not corroborate themonophyly of this group.[22] Previously Mysida included two other families,Lepidomysidae andStygiomysidae, but these have now been placed in a separate order,Stygiomysida.[22]
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Data related toMysida at Wikispecies
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