| Mysida | |
|---|---|
| Hemimysis anomala (Mysidae) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Crustacea |
| Class: | Malacostraca |
| Subclass: | Eumalacostraca |
| Superorder: | Peracarida |
| Order: | Mysida A. H. Haworth, 1825 |
| Families | |
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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, but are also important in some fresh- and brackish-water ecosystems of the Northern hemisphere. Some mysids are cultured for experimental purposes and as food source for other cultured marine organisms.
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The majority of species are 5–25 millimetres (0.2–1.0 in) long, and vary in colour from pale, almost transparent, through to bright orange or brown. Unlike true shrimps, but as with other orders of Peracarida, embryos are carried in a brood pouch, or marsupium, which is located in the thoracic segments between the legs.[1] They differ from other species within the superorder Peracarida by featuring statocysts in their uropods (located at the last abdominal segment). These are clearly seen as circular vesicles and together with the pouch are often used as a diagnostic feature of the group. Other features include stalked compound eyes, and a carapace that covers the head and thoracic segments.[1]
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.[1] The order Mysida is extensive and currently includes approximately 160 genera, containing more than 1000 species.[2]
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.[3][4][5][6][7] While the previous grouping had good morphological support, molecular studies do not corroborate the monophyly of this group.[8] Moreover the unity of Mysida itself has been challenged, with a suggestion to remove two of the four families, Lepidomysidae and Stygiomysidae, to form the order Stygiomysida.[8]
Mysids have a cosmopolitan distribution and are found in both marine and freshwater environments, benthic and pelagic areas.[1] Most mysids are free-living but a few species, mostly in the tribe Heteromysini, are commensal and are associated with sea anemones and hermit crabs.[2] Several taxa have also been described from different groundwater habitats and caves.[2]
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.[1] Pelagic species are filter feeders while benthic species, common for the tribe Erythropini, have been observed feeding on small particles which they collect by grooming their body surface and legs.[2] The first pair of legs in the thorax can also function as accessory feeding limbs.[1]
The size of a mysid brood generally correlates to body length and environmental factors such as density and food availability.[9] Mating usually takes place at night and lasts only a few minutes.[1] The length of time until mysids reach sexual maturity depends on water temperature and food availability.[9][10][11] For the species Mysidopsis bahia, this is normally 12 to 20 days.[11] 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 4 to 7 days.[1][11][12]
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.[1][12] 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.[13] In flow-through systems, juvenile mysids are continuously separated from the adult brood stock, 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 unsaturated fatty acids (HUFA) to increase the nutritional value.[9]
Culturing mysids are thought to provide an ideal food source for many marine organisms. They are often fed to cephalopods, fish larvae and in commercial shrimp farms due to their small size and low costs.[9][14][15][16] 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.[15][16]
Their sensitivity to water quality also makes them suitable for bioassays. Mysidopsis bahia and Mysidopsis almyra are used frequently to test for pesticides and other toxicants, with M. bahia found to be more sensitive during moulting periods.[17]