an order of crustaceans of the subclass Malocostraca. The body length of most species is from 0.5 to 2.5 cm; deep-water species may reach 18 cm. These crustaceans resemble small shrimp but differ from them in that their carapace freely covers the thorax and fuses only with the front somites. The thoracic limbs are two-jointed, the abdominal limbs are partially reduced, and the posterior limbs are well developed. The Mysidaceae have an organ of equilibrium, or statocyst. The organisms develop directly, without metamorphosis, in the brood pouch of the mother.
There are more than 600 species of Mysidaceae, found primarily in seas from shallow depths to depths of 7 km. They are also found in brackish waters, lakes, and rivers. In the USSR there are approximately 100 species, which serve as food for the Baltic herring (Clupea harengus membras) and various other herring, fish of the genera Sprattus and Clupeonella, pike perch, cod, carp, and other commercial fishes. The Mysidaceae have been acclimatized to many bodies of water, thus increasing their numbers.