Antarctic krill (Euphausia superba) is a type of small, swimming crustacean that lives in the Southern Ocean near Antarctica. It lives in large groups called swarms, sometimes with as many as 10,000 to 30,000 krill in one cubic meter of water. It eats tiny phytoplankton, using the energy from the sun that phytoplankton originally get to support its life in the open ocean. Antarctic krill grows to about 6 centimeters (2.4 inches) long, weighs up to 2 grams (0.071 ounces), and can live for up to six years. It plays a key role in the Antarctic ecosystem and has a very large total weight, making it one of the most numerous animal species on Earth. Its total weight is about 500 million metric tons (550 million short tons; 490 million long tons). However, E. superba is being overfished.
Life cycle
The main time when Antarctic krill reproduce is from January to March. This happens both above the continental shelf and in the upper part of deep ocean areas. Like other krill, the male uses special legs on its abdomen, called first pleopods, to attach a spermatophore to the female's genital opening. Each female lays between 6,000 and 10,000 eggs at once. These eggs are fertilized as they leave the female's body.
According to the classical hypothesis from the RRS Discovery expedition, the eggs begin to develop into embryos as they sink to the ocean floor. On the continental shelf, this happens as the 0.6 mm (0.024 in) eggs descend. In deep ocean areas, the eggs sink to depths of about 2,000–3,000 metres (6,600–9,800 ft). The egg hatches into a nauplius larva. After this larva moults into a metanauplius, it begins to move upward toward the ocean surface in a process called developmental ascent.
The next two larval stages, called second nauplius and metanauplius, do not eat. They get their nourishment from the remaining yolk inside the egg. After about three weeks, the young krill reaches the surface. At this point, they may be found in very large numbers, with 2 individuals per litre of water at 60 m (200 ft) depth. As they grow, they go through additional larval stages, including second and third calyptopis, and first to sixth furcilia. These stages are marked by the development of more legs, compound eyes, and bristles. When the krill reaches 15 mm (0.59 in) in size, it looks similar to an adult. Krill become sexually mature after two to three years. Like all crustaceans, krill must shed their hard outer covering, or exoskeleton, to grow. They do this every 13 to 20 days, leaving behind the old exoskeleton as exuvia.
Food
The gut of E. superba can often be seen shining green through its transparent skin. This species mainly eats phytoplankton, especially very small diatoms (20 micrometers), which it filters from the water using a feeding basket. The glass-like shells of the diatoms are broken in the gastric mill and then digested in the hepatopancreas. The krill can also catch and eat copepods, amphipods, and other small zooplankton. The gut forms a straight tube; its digestive efficiency is not very high, so a lot of carbon remains in the feces. Antarctic krill (E. superba) primarily has chitinolytic enzymes in the stomach and mid-gut to break down chitinous spines on diatoms. Additional enzymes can vary due to its wide range of food sources.
In aquariums, krill have been observed eating each other. When they are not fed, they shrink in size after moulting, which is unusual for animals of this size. This is likely an adaptation to the seasonal changes in their food supply, which is limited during the dark winter months under the ice. However, the animal's compound eyes do not shrink, so the ratio between eye size and body length has been found to be a reliable indicator of starvation. A krill with enough food would have eyes proportional to its body length, while a starving krill would have eyes that appear larger than normal.
Antarctic krill directly ingest tiny phytoplankton cells, which no other animal of krill size can do. This is achieved through filter feeding, using the krill's highly developed front legs to form an efficient filtering apparatus. The six thoracopods (legs attached to the thorax) create a "feeding basket" to collect phytoplankton from the open water. In the finest areas, the openings in this basket are only 1 micrometer in diameter. In lower food concentrations, the feeding basket is pushed through the water for over half a meter while open, and then the algae are moved to the mouth opening using special bristles on the inner side of the thoracopods.
Antarctic krill can scrape off the green layer of ice algae from the underside of pack ice. Krill have developed special rows of rake-like bristles at the tips of their thoracopods and graze the ice in a zig-zag pattern. One krill can clear an area the size of a square foot in about 10 minutes (1.5 cm per second). Recent discoveries show that the film of ice algae is widespread and often contains more carbon than the water column below. Krill find a large energy source here, especially in the spring after food sources were limited during winter.
Krill are thought to move vertically between mixed surface waters and depths of 100 meters daily. Krill are messy feeders and often spit out clumps of phytoplankton (called "spitballs") containing thousands of cells. They also produce fecal strings that still contain significant amounts of carbon and diatom shells. Both are heavy and sink quickly to the deep ocean. This process is called the biological pump. Since the waters around Antarctica are very deep (2,000–4,000 meters), they act as a carbon dioxide sink, removing large amounts of carbon (fixed carbon dioxide, CO₂) from the biosphere and storing it for about 1,000 years.
If phytoplankton is consumed by other parts of the pelagic ecosystem, most of the carbon remains in the upper layers of the ocean. Some scientists suggest this process may be one of the largest biofeedback mechanisms on Earth, driven by a massive biomass. More research is needed to fully understand the Southern Ocean ecosystem.
Biology
Krill are sometimes called light-shrimp because they produce light using special organs called bioluminescent organs. These organs are found in different areas of the krill's body: one pair is near the eyes, another pair is on the second and seventh thoracopods (sections of the body), and one each is on the four pleonsternites (sections near the tail). These organs glow with a yellow-green light for up to 2–3 seconds. They are very advanced, with a curved reflector at the back of the organ and a lens at the front that help direct the light. Muscles allow the entire organ to rotate, which helps aim the light to specific areas. Scientists do not fully understand the purpose of these lights. Some think they help hide krill from predators by reducing their shadow, while others believe they help with finding mates or staying together in groups at night.
The bioluminescent organs of krill contain substances that glow when light hits them. The main substance glows most strongly when excited by light at 355 nm and emits light at 510 nm.
Krill escape predators by quickly swimming backward, a movement called lobstering. They can swim at speeds faster than 0.6 meters per second (2.0 feet per second). Even in cold water, krill react to visual stimuli in just 55 milliseconds.
The genome of E. superba is about 48 gigabases in size, making it one of the largest genomes in the animal kingdom and the largest fully assembled genome known. Approximately 70% of its DNA is repetitive, and this percentage could increase to 92.45% with further analysis, which is the highest known for any genome. There is no evidence that the krill has multiple sets of chromosomes. Scientists identified 28,834 protein-coding genes in the Antarctic krill genome, a number similar to other animals. The genes and introns (sections between genes) in Antarctic krill are much shorter than those in lungfishes and Mexican axolotls, which also have large genomes.
Geographic distribution
Antarctic krill is found all around the Southern Ocean and as far north as the Antarctic Convergence. At the Antarctic Convergence, cold water from Antarctica sinks below warmer water from the subantarctic region. This boundary is near 55° south. From there to Antarctica, the Southern Ocean covers 32 million square kilometers, which is 65 times the size of the North Sea. In winter, more than three-quarters of this area is covered by ice, while 24,000,000 square kilometers become ice-free in summer. The water temperature ranges from −1.3 to 3 °C.
The Southern Ocean has a system of currents. When the West Wind Drift occurs, surface water moves around Antarctica in an easterly direction. Near the continent, the East Wind Drift flows counterclockwise. Between these two currents, large swirling water patterns form, such as in the Weddell Sea. Krill swarms move with these currents, forming a single population that spans all of Antarctica. Genetic exchange happens across the entire area. Scientists know little about the exact migration paths because individual krill cannot yet be tracked. The largest krill groups can be seen from space and monitored by satellites. One swarm covered 450 square kilometers of ocean, reaching a depth of 200 meters, and was estimated to weigh over 2 million tons. Recent research shows that krill do not just move with currents but also influence them. By moving up and down in the ocean every 12 hours, krill help mix deep, nutrient-rich water with surface water that has fewer nutrients.
Ecology
Antarctic krill is a key species in the Antarctic ecosystem beyond the coastal shelf. It serves as an important food source for whales, seals (such as leopard seals, fur seals, and crabeater seals), squid, icefish, penguins, albatrosses, and many other bird species. Crabeater seals have special teeth that help them eat krill. Their teeth have many lobes, which act like a strainer to filter krill from the water. Scientists do not yet fully understand how these teeth work in detail. Crabeater seals are the most numerous seal species in the world, and about 98% of their diet consists of E. superba. These seals eat over 63 million tonnes of krill each year. Leopard seals also have similar teeth, and krill makes up about 45% of their diet. All seals together consume 63–130 million tonnes of krill annually, whales consume 34–43 million tonnes, birds 15–20 million tonnes, squid 30–100 million tonnes, and fish 10–20 million tonnes. Combined, these animals consume 152–313 million tonnes of krill each year.
The size difference between krill and its prey is unusually large. Typically, it takes three or four steps in the food chain to go from tiny phytoplankton cells (20 micrometers in size) to krill-sized organisms. This includes small copepods, large copepods, mysids, and small fish.
E. superba lives only in the Southern Ocean. In the North Atlantic, the dominant krill species is Meganyctiphanes norvegica, and in the Pacific, it is Euphausia pacifica.
The total biomass of Antarctic krill was estimated in 2009 to be 0.05 gigatons of carbon (Gt C), which is similar to the total biomass of humans (0.06 Gt C). Antarctic krill can grow in large numbers because the waters around Antarctica support one of the largest plankton communities in the world. Phytoplankton, microscopic plants that use sunlight to grow, are abundant in these waters. When deep water rises to the surface, it brings nutrients from across the world back into the sunlit zone, where they are used by living organisms.
Primary production—the process of converting sunlight into organic matter, which forms the base of the food chain—has an annual carbon fixation rate of 1–2 grams per square meter in the open ocean. Near ice-covered areas, this rate can reach 30–50 grams per square meter. While these numbers are not extremely high compared to very productive areas like the North Sea or upwelling zones, the area where this process occurs is enormous, even larger than other major producers like rainforests. Additionally, during the Antarctic summer, long hours of daylight help increase production. These factors make plankton and krill vital to the planet's ecosystems.
A possible decrease in Antarctic krill populations may be linked to the loss of sea ice caused by global warming. Krill, especially in their early life stages, rely on sea ice structures to survive. The ice creates natural shelters that krill use to avoid predators. When sea ice levels are low, krill populations may decline, and salps—a type of barrel-shaped filter feeder—may increase in number. Salps typically live in areas with lower productivity and lower latitudes. Rising sea temperatures allow salps to move into regions where krill populations are declining, potentially competing with krill for food.
Another challenge for Antarctic krill and other calcifying organisms (such as corals, mussels, and snails) is ocean acidification caused by increasing carbon dioxide levels. Krill have exoskeletons made of carbonate, which can dissolve in acidic water. Studies have shown that higher carbon dioxide levels can harm krill egg development and prevent juvenile krill from hatching, which could reduce krill populations in the future. However, the full effects of ocean acidification on krill life cycles are not yet fully understood, and scientists are concerned about potential impacts on krill distribution, abundance, and survival.
The Antarctic krill fishery harvests about 100,000 tonnes of krill annually. The main fishing countries are South Korea, Norway, Japan, and Poland. Krill is used as animal feed and fish bait. Krill fishing is challenging for two reasons. First, fishing nets must have very fine mesh, which creates high drag and causes krill to be pushed away from the net. Second, fine mesh nets clog quickly. Another challenge is bringing krill on board. When the net is pulled from the water, krill compress and lose body fluids. Experiments have tested pumping krill through a tube while still in water, and new net designs are being developed. Krill must be processed quickly because the catch deteriorates within hours. Krill is rich in protein and vitamins, making it suitable for human consumption and animal feed.
Fishing krill, and potentially overfishing it, is a growing concern.
A study published in 2025 found a wide variety of viral RNA in Antarctic krill, most of which do not match known viruses. The most common RNAs were from Penaeus vannamei picornavirus (PvPV) and covert mortality nodavirus (CMNV). The PvPV found in krill can harm farmed shrimp, and CMNV can infect fish that eat krill in nature and in laboratories. Using krill as aquaculture feed may carry a risk of spreading these viruses.