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These filters can be very fine indeed in those species (such as Euphausia spp.) that feed primarily on phytoplankton, in particular on diatoms, which are unicellular algae.
Krill are mostly omnivorous, Krill are an important element of the aquatic food chain.
The most familiar and largest group of crustaceans, the class Malacostraca, includes the superorder Eucarida comprising the three orders, Euphausiacea (krill), Decapoda (shrimp, prawns, lobsters, crabs), and the planktonic Amphionidacea. The more abundant Euphausiidae contains 10 different genera with a total of 85 species.
Of these, the genus Euphausia is the largest, with 31 species.
The lesser known family, the Bentheuphausiidae, has only one species, Bentheuphausia amblyops, a bathypelagic krill living in deep waters below 1,000 m (3,300 ft).
It is considered the most primitive extant krill species.
Krill are fished commercially in the Southern Ocean and in the waters around Japan.
Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.Krill are crustaceans and have a chitinous exoskeleton made up of three tagmata: the cephalon (head), the pereion (fused to the cephalon to form a cephalothorax), and the pleon.This outer shell of krill is transparent in most species.Bentheuphausia amblyops, a bathypelagic species, has a cosmopolitan distribution within its deep-sea habitat. Northern krill occur across the Atlantic from the Mediterranean Sea northward.Species with neritic distributions include the four species of the genus Nyctiphanes.One study supports the monophyly of Eucarida (with basal Mysida), No extant fossil can be unequivocally assigned to Euphausiacea.