They feed directly on very small phytoplankton, so they can use the energy that the phytoplankton originally got from the sun in order to sustain their life in the open ocean. They grow to a length of 6 cm, weigh up to 2 g, and can live for up to six years. They are a key species in the Antarctic ecosystem and are, in terms of biomass, probably the most successful animal species on the planet (approximately 500 million tonnes).