Krill are considered an important trophic level connection near the bottom of the food chain, because they feed on phytoplankton and zooplankton, converting these into a form suitable for many larger animals for which krill make up the largest part of their diets. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of around 379,000,000 tonnes, making it among the species with the largest total biomass. Of this, over half is eaten by whales, seals, penguins, squid, and fish each year, and is replaced by growth and reproduction. 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.

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