Antarctic krill is a species of krill found in the Antarctic waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic metre. It feeds directly on minute phytoplankton, thereby using the primary production energy that the phytoplankton originally derived from the sun in order to sustain their pelagic life cycle. It grows to a length of 6 centimetres , weighs up to 2 grams , and can live for up to six years. It is a key species in the Antarctic ecosystem and in terms of biomass, is one of the most abundant animal species on the planet – approximately 500 million metric tons .