Invertebrates
Antarctic Krill
Euphausia superba · also called antarctic krill
Antarctic krill is a finger-long, semi-transparent crustacean that lives in the cold water around Antarctica in almost unimaginable numbers — a total mass of several hundred million tonnes, making it one of the most abundant animals on Earth. Nearly every large predator in the Southern Ocean, from the blue whale down, is feeding, directly or indirectly, on this one species.
Overview
Antarctic krill lives in the Southern Ocean, mostly in the upper few hundred metres, with the densest populations in the Atlantic sector around the Scotia Sea. Adults are about 6 cm long and up to 2 g, translucent with a reddish tint from their food, visible internal organs, and rows of tiny light organs.
The centre of the food web
Krill grazes phytoplankton directly, filtering it from the water with a fine basket of front legs, and is in turn eaten directly by baleen whales, crabeater seals, penguins, seabirds, squid, and fish. That short path means the numbers of some of the planet’s largest animals rise and fall with the krill supply.
Tied to the ice
Young krill shelter and feed on algae growing under winter sea ice, so the extent and timing of the ice each year strongly affects how many survive. When food runs short, adult krill can shrink between moults and revert toward juvenile features to save energy, regrowing when conditions improve.
Status
The species has not been assessed on the IUCN Red List, and its biomass is still enormous, but surveys show it has declined and shifted southward in the warming waters near the Antarctic Peninsula. A commercial fishery supplying aquaculture feed and omega-3 oil operates under international catch limits set well below the estimated stock.
Antarctic Krill: common questions
Why is Antarctic krill so important?
Because an unusually large share of one of the planet's major ecosystems depends directly on it. In most food webs, energy passes through many small and medium steps between the microscopic plants at the base and the large animals at the top. In the Southern Ocean, krill short-circuits that — it eats phytoplankton directly and is in turn eaten directly by whales, seals, penguins, seabirds, fish, and squid. This means the population size of some of the largest and most iconic animals on Earth, including blue and humpback whales, is closely tied to how much krill the ocean produces each year, and disturbances to krill ripple quickly through everything that eats it.
How many Antarctic krill are there?
Estimates put the total mass of Antarctic krill somewhere in the range of a few hundred million tonnes, which by weight makes it a contender for the most abundant wild animal on the planet. Counting individuals is harder because they cluster in dense, patchy swarms rather than spreading evenly, but a large swarm can hold tens of thousands of krill per cubic metre and stretch for kilometres. Because the population naturally rises and falls with sea ice and ocean conditions from year to year, any single figure is a rough average of a number that swings widely.
Is Antarctic krill endangered or being overfished?
The species as a whole is not endangered, and its biomass remains vast. There is a commercial fishery, mainly supplying farmed-fish feed, pet food, and omega-3 oil supplements, but it is managed by an international body, CCAMLR, which sets precautionary catch limits that are a small fraction of the estimated stock. The larger concern is climate change. Warming water and shrinking winter sea ice in the Atlantic sector appear to have reduced krill numbers there and pushed the population further south, and because so many predators feed in specific areas, local depletion, whether from fishing or environmental change, can matter even when the global total looks healthy.
What is the difference between krill and shrimp?
They are both crustaceans and look broadly similar, but they belong to different groups. Krill are euphausiids, a distinct order of shrimp-like animals that live in open water throughout their lives, keep their feathery gills exposed on the outside of the body, and often carry light organs. True shrimp are decapods, the same broad group as crabs and lobsters, and many live on or near the bottom. Antarctic krill in particular is a purely oceanic, swarming, plankton-eating animal, whereas shrimp include a huge variety of lifestyles. In everyday terms krill are small, but Antarctic krill is actually one of the larger krill species.