Antarctic krill
Five species of krill, small free-swimming crustaceans, are found in the Southern Ocean. The Antarctic krill ("Euphausia superba") is one of the most abundant animal species on earth, with a biomass of around 500 million tonnes. Each individual is long and weighs over . The swarms that form can stretch for kilometres, with up to 30,000 individuals per , turning the water red. Swarms usually remain in deep water during the day, ascending during the night to feed on plankton. Many larger animals depend on krill for their own survival. During the winter when food is scarce, adult Antarctic krill can revert to a smaller juvenile stage, using their own body as nutrition.