Krill (order Euphausiacea)

Marine invertebrateCrustaceanKeystone species

Overview

Krill (order Euphausiacea) are small, shrimp-like crustaceans that drift and swim in the open ocean, often gathering in swarms so dense and vast that they colour the water and can be seen from the air. The reference shown here is Antarctic krill (Euphausia superba), one of the most abundant animals on the planet by total mass.

Though individually small, krill are a keystone of ocean ecosystems: by converting tiny plankton into food eaten by huge numbers of larger animals, they sit at the heart of polar and other marine food webs.

Note: there are many krill species; details below focus on Antarctic krill as a well-studied example. Treat figures as general and verify against authoritative marine sources.

Habitat & Range

Krill live in oceans around the world, from the surface to deep water, with some of the largest concentrations in the cold, productive seas around Antarctica. They often migrate up and down through the water column over the course of a day, rising toward the surface and sinking again.

Diet

Most krill are filter feeders that strain tiny phytoplankton (microscopic algae) and other small particles from the water using comb-like legs, with some also taking zooplankton. Around Antarctica, krill also graze algae that grow on the underside of sea ice, which makes them tied to the seasonal ice.

Behavior

Krill are famous for forming enormous swarms, which may help with feeding and offer some safety in numbers despite making them an obvious target. Many krill are bioluminescent, producing light from special organs. Like other crustaceans they moult to grow, and Antarctic krill can even shrink between seasons when food is scarce — an unusual survival strategy.

Human Interaction & Conservation

Krill underpin the diets of whales, seals, penguins, seabirds, fish, and squid, so their abundance ripples through whole ecosystems. They are also harvested by people, mainly for aquaculture feed and dietary-oil supplements, which makes careful, well-managed fisheries important — especially as polar seas change. Consult authoritative sources for current status.

More photos of the krill