Mysidacea
Habitats and geographic patterns

Mysids are originally marine animals, but a few species are known as marine relicts inhabiting fresh- and brackish-water lakes and rivers. The most typical fresh-water species is Mysis relicta, which is found in lakes of northern North America and Europe. It evolved from Mysis oculata as a result of isolation in fresh waters during glacial times (Tattersall and Tattersall, 1951). Besides Mysis relicta, there are about 30 species of mysids occurring in fresh-water lakes, rivers, and fish ponds. They include Antromysis in the Amazon basin, Antromysis and Taphromysis along the coasts of the Caribbean Sea and the Gulf of Mexico, Paramysis and Diamysis in eastern Europe, Mesopodopsis and Nanomysis in southeastern Asia, and Neomysis in northern North Pacific coastal lagoons. Most of them inhabit more or less brackish waters.

In the oceans, mysids live in various environments, such as the water column, on and just above the sea-floor, within bottom substrata, in caves, and as commensals of other animals. They occur most abundantly in shallow coastal waters, in the near-bottom layers just above the sandy or muddy bottom, rocky shores, and coral reefs, sometimes in dense swarms. Their biomass is high, but estimates of its magnitude have hardly ever been made because of practical limitations. In the water column of the open sea mysids are rather rare, although in tropical and subtropical regions, at the surface, several species, such as Siriella thompsoni, are sometimes abundant during the night. There are no epipelagic species in cold waters, including the Arctic and the Antarctic. Mesopelagic species are also rare, although their numbers are greater than those in the surface layers. On the contrary, at depths greater than 700 or 800 m, mysid biomasses exceed those in the upper layers. Murano et al. (1976) estimated the average density of mysids in a 0-1000 m deep water column at a station off middle Japan at 0.7 mg mö-3 during the day, and 0.6 mg mö-3 at night; these values represented 0.3% and 0.5% of the total biomass, respectively (day+ night distribution). Below 500 m the biomass was dominated by Eucopia and Boreomysis.