Introduction to Mysids (Opossum shrimps)
Shrimp-like crustaceans with a carapace covering most of the thorax, though it is not attached to the last 4 thoracic segments. Some species have a distinct rostrum. Antenna 2 has a prominent scale, as in decapod shrimps, that is important for species identification. They have 2 pairs of maxillipeds, i.e., thoracic limbs that have become specialized for food handling. There are no "gnathopods" (clawed legs), but 6 pairs of almost identical thoracic legs, often with swimming setae on the outer branch. The 6 pairs of pleopods (abdominal appendages) are reduced, except ♂ pleopod 4, which is long and modified for sperm transfer. They have large, stalked compound eyes, and distinct statocysts are located in base of inner branch of uropods. Uropods and telson form a "tail-fan" as in decapod shrimps. The shape and size of telson is important for species identification. The ♀ brood pouch, formed by thin lamellae (oostegites) from the bases of the last two pairs of thoracic legs, shows that they are peracarids, like amphipods and isopods. Most species are shorter than 25 mm.
Only one mysid species has invaded brackish waters of the Nordic countries, the Ponto-Caspian Hemimysis anomala (Sars, 1907). In addition there are a few exclusively freshwater species that may be found at very low salinities of coastal lagoons and estuaries of the Baltic, e.g., Limnomysis benedeni Czerniavsky, 1882, and Paramysis spp. All of these are Ponto-Caspian species that have been intentionally transferred among different water-bodies within the former Soviet Union.