Any marine crustacean of the order Euphausiacea, members of which constitute a major component of zooplankton. Krill are small shrimps with semitransparent, often reddish tinged bodies, up to about 6 cm long. They feed chiefly on microscopic phytoplankton and thus act as primary consumers in the marine food chain; they in turn are food for a wide range of other animals, including fish, birds, and whales. Some species congregate in dense swarms, notably the Antarctic krill (Euphausia superba), which is one of the most abundant animals, with an estimated population biomass of some 200 million tonnes.