The peracarid crustacean orders Mysida, Lophogastrida and Stygiomysida were formerly grouped under the name "Mysidacea". Although the treatment in three separate orders is now accepted, we have chosen to present mysids, lophogastrids and stygiomysids in the same web interface. The higher classification (orders, families, subfamilies) follows Meland & Willassen (2007), and the fossil order of Pygocephalomorpha is also included in the database.
Mysids, lophogastrids and stygiomysids are shrimplike in appearance, the majority of species being between 5 mm and 25 mm in length. Due to the presence of oostegites forming a ventral female marsupium, mysids are often referred to as opossum shrimp. The "Mysidacea" have earlier been allied with decapods, euphasiids, stomatopods, and even nebaliaceans (see Tattersall & Tattersall, 1951, for taxonomic history), but are readily distinguished from other shrimplike crustaceans, and also the remaining Peracarida, by the presence of a statocyst in the proximal part of the endopods of the uropods. Since this balance organ is missing in the more "primitive" mysid families (Lophogastridae, Gnathophausiidae, Eucopiidae and Petalophthalmidae) some confusion can arise, but they are still easily distinguished from euphasiids and carideans by the presence of a well developed brood pouch in mature females.
Mysids have a worldwide distribution. Most species are free living and marine, the majority of species inhabiting coastal and open sea waters. A few species are adapted to continental fresh water, and these are also included in the database. Some species, as seen in the tribe Heteromysini, have also taken on a commensal habit, often described in association with sponges, sea anemones, corals, and hermit crabs. Several taxa have also been described from different groundwater habitats, marine and anchialine caves. In addition to having pelagic forms, mysid species are often found associated with the benthic boundary layer (the hyperbenthos, sensu Mees & Jones, 1997), burrowing, or hovering immediately above or resting on the sediment surface. The majority of Mysidacea are filter feeders and generally regarded as omnivores, feeding on algae, detritus, and zooplankton (Mauchline, 1980). Pelagic forms filter particles during swimming while benthic species, common for the tribe Erythropini, are often seen feeding on small particles which they collect by grooming their carapace and legs (Fosså, 1986). In other species tendencies towards a strict carnivorous habit are observed, and then often seen as scavengers, feeding on carcasses of polychaetes, copepods, amphipods and even other mysids, but true predators on zooplankton are also described (Tattersall & Tattersall, 1951).
The order Mysida (2 families, 176 genera, 1169 species) contains the largest number of species across the most diversity of habitats, with species found in subterranean, fresh, brackish, coastal, and surface to deep-sea habitats. For a review of the freshwater mysids, see : Porter, M.L., K. Meland, & W. Price. 2008. Global diversity of mysids (Crustacea-Mysida) in freshwater. Hydrobiologia 595: 213-218. In comparison, the Lophogastrida (3 families, 9 genera, 53 species) are mainly meso- to bathypelagic while the Stygiomysida (2 families, 2 genera, 16 species) are found primarily in subterranean waters.
The taxonomy in the database is edited and updated by volunteer experts, assisted by the database management team. Currently there are 2,030 name records, of which 1,291 are valid names, the rest invalid synonyms, nomina dubia, etc. The editors aim to resolve all conflicting or out of date records promptly when noticed, and we hope you will tell us about any duplications or other confusions you find. Taxonomists are encouraged to send us a copy of new publications so that we can see the details of new and changed nomenclature. If you happen to disagree with any of our editing decisions, please help us by making a reasoned case for any alternative you suggest. Fossil names are also included.