Invertebrates

Krill

Euphausiacea · also called krill, Antarctic krill

Least Concern ↓ filter feederomnivore Ocean Polar & tundra

Krill are finger-joint-sized crustaceans that hold up an ocean. They graze the microscopic plants of the sea, pack that energy into an oily body, and gather in swarms so vast they can be seen from space and turn the water red — and nearly everything large in the Southern Ocean, from penguins to blue whales, eats them.

Overview

Krill are the order Euphausiacea, about 85 shrimp-like species in every ocean, but overwhelmingly concentrated as Antarctic krill (Euphausia superba) in the Southern Ocean. They are filter feeders, grazing phytoplankton from the water and scraping ice algae from the underside of the sea ice, with some zooplankton and detritus too. By biomass — roughly 300–500 million tonnes of Antarctic krill — the single species is among the most abundant animals on Earth.

Swarms, glow, and shrinking

Krill form swarms of tens of thousands per cubic metre stretching for kilometres. They glow blue-green from light organs on the eyestalks and belly, matching the light from above so predators below cannot see their silhouette. To survive the dark polar winter, Antarctic krill drop their metabolism, live off reserves, and actually shrink, moulting smaller and regrowing in spring. They are long-lived for their size — 6 to 10 years.

The hinge of the ecosystem

Baleen whales, crabeater and leopard seals, every Antarctic penguin, many seabirds, and much of the fish and squid depend on krill directly or at one remove. A good or bad krill year ripples through everything, and the recovery of the great whales after whaling depends on krill staying abundant.

Status

Least Concern and still hugely abundant, but with a worrying decreasing trend — surveys suggest a southward contraction and decline in the southwest Atlantic as sea ice shrinks — and the fishery (several hundred thousand tonnes a year for aquaculture feed and omega-3 oil) is concentrated right on top of major penguin and whale feeding grounds.

Krill: common questions

Is krill a single animal or a group?

Krill is the common name for an entire order of crustaceans, the Euphausiacea, which contains about 85 species. They are all small, shrimp-like, mostly open-water animals with feathery filtering limbs and visible external gills. When people say krill without qualification they usually mean Antarctic krill, Euphausia superba, which is the largest, most abundant, and most studied species and the target of the krill fishery, but the North Pacific krill, northern krill of the Atlantic, and several others are also ecologically important. The word krill comes from Norwegian and means roughly small fry or young fish.

Why is krill so important?

Because it is the main link between the microscopic plants of the ocean and its large animals, especially in the Southern Ocean. Krill graze phytoplankton and ice algae, concentrate that energy into a rich, oily body, and gather in huge swarms, which makes them an efficient, reliable food source for predators too big to feed on plankton directly. Antarctic baleen whales, crabeater seals, leopard seals, all the Antarctic penguin species, many flying seabirds, and numerous fish and squid depend on krill. A blue whale can eat several tonnes of krill in a day. Because so much sits on top of this one link, changes in krill abundance affect the entire Antarctic ecosystem, and the ability of whale populations to recover from whaling depends on krill staying plentiful.

Is the krill fishery a problem?

The overall Antarctic krill stock is large and the total catch, a few hundred thousand tonnes a year, is a small fraction of it, so the fishery is not overfishing krill in a global sense. The concern is more specific. The fishery has become concentrated in a small region of the southwest Atlantic near the Antarctic Peninsula, in the same waters and often the same season that penguins, seals, and whales are feeding and raising young, so intensive local fishing can compete directly with wildlife even when the wider stock is fine. This is happening against a background of shrinking winter sea ice, which krill larvae depend on, and evidence of a long-term southward shift in the population. Management by the body known as CCAMLR is trying to spread the catch out and set precautionary limits.

What is krill used for?

Most of the commercial catch goes into aquaculture, as feed for farmed fish such as salmon, where krill meal improves growth and gives farmed salmon their pink colour, and as feed for farmed shrimp and for pets and ornamental fish. A growing share is processed into krill oil, sold as a human dietary supplement for its omega-3 fatty acids and the antioxidant astaxanthin, marketed as an alternative to fish oil. Small amounts are eaten directly by people, mainly in Japan, Russia, and the Philippines, and krill are also used as bait and in scientific research. Historically some countries explored krill as a direct human food source but the shells, the high fluoride content in the shell, and rapid spoilage limited that.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 10, 2026. Last updated September 10, 2026.

Quick facts

Scientific name: Euphausiacea
Diet: Mainly phytoplankton, single-celled algae grazed from the water and, in Antarctic krill, scraped from the underside of sea ice with the front limbs, supplemented by small zooplankton such as copepods and by detritus, so krill are the step that turns microscopic plants into food for large animals
One of the most abundant animals alive: Antarctic krill alone are estimated at around 300 to 500 million tonnes of biomass, comparable to or greater than the total weight of all humans, which makes the single species one of the most successful animals on the planet by sheer mass
Swarms visible from space: Krill gather in dense swarms that can hold tens of thousands of individuals per cubic metre and stretch for kilometres, sometimes turning the sea reddish, and the largest swarms have been detected from satellites and by the ships and predators that feed on them
They glow: Krill have light-producing organs called photophores on the eyestalks and underside that emit a blue-green glow, used to communicate within the swarm and probably to match the dim light from above so a predator looking up from below cannot see the krill's silhouette
They can shrink when starved: Antarctic krill can survive the long, dark, food-poor polar winter by lowering their metabolism, living off body reserves, and actually shrinking in body length, moulting into a smaller size and then regrowing when food returns in spring, so an old krill is not always a large krill
A slow-growing, long-lived crustacean: For an animal so small, Antarctic krill live a long time, up to about six to ten years, growing slowly to around 6 cm, whereas most shrimp-sized crustaceans live only a year or two
The hinge of the Southern Ocean: Baleen whales, crabeater and other seals, all the Antarctic penguins, many seabirds, squid, and fish such as icefish depend on krill directly or at one remove, so a good or bad krill year ripples through the entire ecosystem, and the recovery of great whales after whaling depends on krill remaining abundant
Range: All oceans, with about 85 species worldwide, but by far the greatest biomass in the Southern Ocean around Antarctica, where Antarctic krill occur in a broad band of cold water south of the Antarctic Convergence, from the surface to several hundred metres deep and around the edge of the sea ice, with other important species in the North Pacific, the North Atlantic, and upwelling zones off North and South America.
Conservation status: Least Concern ↓ ≈ Antarctic krill are not currently endangered and remain enormously abundant, but there is concern about the trend, since some surveys suggest the krill population has contracted southward and declined in the southwest Atlantic sector over recent decades as sea ice has shrunk, and the commercial fishery, which takes several hundred thousand tonnes a year for aquaculture feed, fishmeal, and omega-3 supplements, is concentrated in a small area close to major penguin and whale feeding grounds, raising the risk of local depletion even while the overall stock looks healthy in the wild