Euphausiids occupy all slopes and basins of the World Ocean. They occur from the surface layer to at least 4000 m depth and from arctic to antarctic waters. There are 86 known species, several of which have geographical forms or variants. All are marine. The deeper-living species are large in size and broadly ranging, often circumglobal. Those inhabiting the upper 300 m or so tend to be restricted by particular climate zones and water types. Continental barriers are believed to have been particularly important in the evolutionary divergence of species. Euphausiids are often a dominant component of net-caught zooplankton, particularly at high latitudes such as the antarctic and the subarctic Pacific and Atlantic ocean basins. In the California Current System of the mid-latitude North Pacific, euphausiids have been found to comprise seasonally variable averages of 20 to 40% of the net-caught zooplankton biomass across a current-wide survey area (Brinton and Reid 1986).
Some species are coastal, occupying productive boundary currents (Nyctiphanes distribution). It is common for many species to co-occur, or for closely-related species to segregate, across much of an oceanographic region in which various and variable patterns of circulation and food production, often seasonal or interannual, are believed to contribute to the development and conservation of distinctive populations (E. gibba group distribution A , E. gibba group distribution B).
In the warmer seas life spans of many of the epipelagic species ~1 cm body length, probably are a matter of months. Toward higher latitudes where species' life histories are better known and toward mid-depths where body lengths reach 2-4 cm, projected life spans are estimated in years (Mauchline 1977, 1985). The epipelagic antarctic species Euphausia superba is 6 cm in length after about 6 years, and giant Thysanopoda species of the bathypelagic realm have been estimated to be ~25 years old at >12 cm length.
Euphausiids can aggregate in layers which concentrate in relation to horizontal or vertical thermal characteristics, such as the thermocline, oxycline (Saanich Inlet aggregation), or oceanic fronts. Dusk and predawn vertical migrations by layered euphausiids contribute to the migrating Deep (sonic) Scattering Layer. Three-dimensional aggregations of euphausiids, usually a daytime phenomenon, have been observed in many species. In some aggregations, euphausiids form tight balls (N. australis aggregation) or layers ( N. simplex surface aggregation), with densely packed individuals in seemingly random orientations. In other aggregations, euphausiids form schools: animals have parallel orientations with rather constant spacing between individuals (Krill schools , E. superba school under sea ice). Aggregations tend to break up at night as the euphausiids disperse for feeding.
Euphausia species are fished commercially in Japanese, Canadian, and antarctic waters. They are a high protein food for aquarium and farm animals as well as humans. They can be a delicacy, particularly when freshly caught (E. superba for lunch).