Mysida

Mysida
Hemimysis anomala (Mysidae)
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Subclass:	Eumalacostraca
Superorder:	Peracarida
Order:	Mysida Haworth, 1825[1]
Families
Mysidae Petalophthalmidae

Mysida is a group of small, shrimp-like crustaceans, an order in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch, or marsupium, in females. Mysids are mostly found in marine waters throughout the world where they can be benthic or pelagic, but are also important in some fresh- and brackish-water ecosystems in the Northern Hemisphere. Some mysids are cultured for experimental purposes and as a food source for other cultured marine organisms.

Description

The head of mysids bear 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 pereopods used for swimming, as well as for wafting water towards the maxillipeds for feeding. Unlike true shrimps, females have a marsupium beneath the thorax. This brood pouch is enclosed by the large, flexible oostergites, 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, almost 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 and together with the pouch are often used as a diagnostic feature of the group.^[3]

Distribution

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, about 72 species in total, being predominantly found in the palearctic and neoptropical zones. 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]

Systematics

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 superorder Eucarida is their lack of free-swimming larvae.^[3] The order Mysida is extensive and currently includes approximately 160 genera, containing more than 1000 species.^[5]

Traditionally, Mysida were united with another, externally similar group of pelagic crustaceans, the Lophogastrida, into a broader order Mysidacea, but that classification is currently generally abandoned.^{[1][6][7][8][9]} While the previous grouping had good morphological support, molecular studies do not corroborate the monophyly of this group.^[10] Previously Mysida included two other families, Lepidomysidae and Stygiomysidae, but these have now been placed in a separate order, Stygiomysida.^[10]

Classification

Behaviour

There are both benthic and pelagic species of mysid, but most are found close to, crawling on or burrowing into the substrate. Most marine species are benthic by day but leave the seabed at night to become planktonic. Locomotion is mostly by swimmiming, the pleopods being used for this purpose. Some 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.^[2] In general, they are free-living, but a few species, mostly in the subfamily Heteromysinae, are commensal and are associated with sea anemones and hermit crabs.^[5] Several taxa have also been described from different groundwater habitats and caves.^[5] Mysis relicta and its close relatives inhabit cold, deep lakes and have a diurnal cycle of vertical migrations.^[13]

The majority of Mysida are omnivores, feeding on algae, detritus, and zooplankton. Scavenging and cannibalism are also common, with the adults preying on their young once they emerge from the marsupium.^[3] The pelagic and most other species are filter 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 by setae on the second maxillae. Larger planktonic animals can be caught in a trap composed of the endopods of the thoracic appendages.^[2] Some benthic species, especially members of the subfamily Erythropinae, have been observed feeding on small particles which they collect by grooming the surfaces of their bodies and legs.^[5]

Mysids are ghonochoristic, with individuals being either male or female, and fertilisation is external. The gonads are in the thorax and are tubular in shape. Males have two gonopores in the eighth thoracic segment and a pair of long penises. The female gonopores are in the sixth thoracic segment and the oostergites are attached to the first to seventh pereopods to form a brood pouch. During mating, 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 being direct with the young hatching as miniature adults.^[2] The size of a mysid brood generally correlates to body length and environmental factors such as density and food availability.^[14] Mating usually takes place at night and lasts only a few minutes.^[3] The length of time until mysids reach sexual maturity depends on water temperature and food availability.^[14][15][16] For the species Mysidopsis bahia, this is normally 12 to 20 days.^[16] The young are released soon after, 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.^[3][16][17]

Mysid culture

Mysids are good candidates for large-scale culture, as they are highly adaptive, and can occur in a wide range of habitats, and despite their low fecundity, having a short reproductive cycle means they can quickly reproduce in vast numbers.^[3][17] They can be cultured in static or flow-through systems, the latter shown to be able to carry a higher stocking density than a static system.^[18] In flow-through systems, juvenile mysids are continuously separated from the adult brood stock, to reduce mortality due to cannibalism.^[14]

Artemia (brine shrimp) juveniles (incubated for 24 hours) are the most common food in mysid cultures, sometimes enriched with highly unsaturated fatty acids to increase the nutritional value.^[14]

Culturing mysids are thought to provide an ideal food source for many marine organisms. They are often fed to cephalopods, fish larvae, and commercial farmed shrimp due to their small size and low cost.^{[14][19][20][21]} 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 seahorses) and other small fauna.^[20][21]

Their sensitivity to water quality also makes them suitable for bioassays. Americamysis bahia and Americamysis almyra are used frequently to test for pesticides and other toxicants, with A. bahia found to be more sensitive during moulting periods.^[22]

External links

• Data related to Mysida at Wikispecies

References