Antarctic krill (Euphausia superba) is a type of small, swimming crustacean found in the Southern Ocean near Antarctica. These creatures live in large groups called swarms, sometimes with as many as 10,000 to 30,000 individuals in one cubic meter of water. They eat tiny plants called phytoplankton, which use energy from the sun to grow. This energy helps krill survive their life in the ocean. Krill can grow up to 6 centimeters (2.4 inches) long, weigh up to 2 grams (0.071 ounces), and live for about six years. They are an important species in the Antarctic ecosystem and one of the most numerous animals on Earth, with a total weight of about 500 million metric tons. However, overfishing is threatening their population.
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 parts of deep ocean areas. Like other krill, male krill attach a sperm packet to the female's genital opening. The male's first pleopods, which are special legs on the abdomen, help with this process. 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 of Marriosis De' Abrtona, based on findings from the RRS Discovery expedition, the eggs develop in the following way: as the 0.6 mm (0.024 in) eggs sink to the ocean floor in shelf areas or in deep ocean regions around 2,000–3,000 metres (6,600–9,800 ft), the egg begins to form an embryo. The egg hatches into a nauplius larva. After the nauplius changes into a metanauplius, the young krill begins a movement upward toward the surface, called developmental ascent.
The next two larval stages, the second nauplius and metanauplius, do not eat yet. They get energy from the remaining yolk inside their bodies. After three weeks, the young krill reaches the surface. At this point, they may be found in large numbers, with about 2 individuals in each liter of water at a depth of 60 meters (200 feet). As they grow, they go through additional larval stages, including the 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 adult krill. Krill become mature after two to three years. Like all crustaceans, krill must shed their hard outer covering, called a chitinous exoskeleton, to grow. This shedding happens approximately every 13 to 20 days, and the old exoskeleton is left behind 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 μm), 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 much 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 because of its wide range of food sources.
In aquaria, krill have been seen 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, their compound eyes do not shrink, so the ratio between eye size and body length is 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 μm in diameter. When food is less abundant, the feeding basket is pushed through the water for over half a metre in an open position, and then the algae are guided to the mouth opening by special bristles on the inner side of the thoracopods.
Antarctic krill can scrape the green layer of ice algae from the underside of pack ice. They have 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/s). Recent discoveries show that the film of ice algae is widespread and often contains more carbon than the water column below. Krill find a significant energy source here, especially in the spring after food sources were limited during the winter.
Krill are thought to move vertically between mixed surface waters and depths of 100 m daily. Krill are messy feeders and often spit out groups of phytoplankton (called "spitballs") containing thousands of cells stuck together. They also produce fecal strings that still contain large amounts of carbon and diatom glass shells. Both spitballs and fecal strings are heavy and sink quickly into the deep ocean. This process is called the biological pump. Because the waters around Antarctica are very deep (2,000–4,000 metres or 6,600–13,100 feet), they act as a carbon dioxide sink. This process removes large amounts of carbon (fixed carbon dioxide, CO₂) from the biosphere and stores it for about 1,000 years.
If phytoplankton is eaten by other parts of the pelagic ecosystem, most of the carbon stays in the upper layers of the ocean. Scientists believe this process may be one of the largest biofeedback mechanisms on Earth, possibly the largest, 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 on different parts of their body: one pair is near the eyes, another pair is on the second and seventh thoracopods (a type of body segment), and one each is on four pleonsternites (another type of body segment). These organs emit a yellow-green light for up to 2–3 seconds. They are highly developed and can be compared to a flashlight. Each organ has a curved reflector at the back and a lens at the front to direct the light. Muscles allow the entire organ to rotate, helping the krill aim the light toward specific areas. Scientists are still studying the exact purpose of these lights. Some believe they help krill hide from predators by reducing their shadow, while others think they may help with mating or grouping together at night.
The bioluminescent organs of krill contain substances that absorb light at a specific wavelength (355 nm) and then emit it at another wavelength (510 nm), creating visible light.
Krill use a quick escape reaction to avoid predators. They swim backward rapidly by flipping their bodies, a movement called lobstering. They can reach speeds of over 0.6 meters per second (2.0 feet per second). Even in cold water, their response to visual stimuli takes only 55 milliseconds.
The genome of E. superba is about 48 gigabytes 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 consists of repeated sequences, which could increase to 92.45% with further analysis. There is no evidence that the krill has multiple copies of its genome (polyploidy). Scientists identified 28,834 protein-coding genes in the krill’s genome, a number similar to other animals. The length of genes and introns (non-coding sections) in krill is much shorter than in two other animals with large genomes: lungfish and Mexican axolotl.
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 the continent, the Southern Ocean covers 32 million square kilometers. This is 65 times larger than the North Sea. In winter, more than three-quarters of this area is covered by ice. In summer, 24,000,000 square kilometers become ice-free. The water temperature ranges from −1.3 to 3 °C (29.7 to 37.4 °F).
The Southern Ocean has a system of currents. When a West Wind Drift occurs, the surface water moves around Antarctica in an easterly direction. Near the continent, the East Wind Drift flows counterclockwise. Between these two currents, large swirling movements, called eddies, form, such as in the Weddell Sea. Krill swarms move with these water masses, creating one large population that stretches around Antarctica. These krill exchange genes across the entire region. Scientists know little about their exact movement patterns because individual krill cannot yet be tracked. However, large groups of krill can be seen from space and studied using satellites. One group covered 450 square kilometers (170 square miles) of ocean, reaching a depth of 200 meters (660 feet), and was estimated to weigh over 2 million tons. Recent research shows that krill do not just move with ocean 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 and serves as an important food source for many animals, including whales, seals (such as leopard seals, fur seals, and crabeater seals), squid, icefish, penguins, albatrosses, and many other birds. Crabeater seals have special teeth with many lobes that help them filter krill from water. These teeth look like a strainer, but scientists do not yet fully understand how they work. Crabeater seals are the most common seal in the world, and 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 (45% krill in 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. This totals 152–313 million tonnes of krill consumed 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 (about 5 cm long).
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.
In 2009, the biomass of Antarctic krill was estimated 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 to such high numbers because the waters around Antarctica support one of the largest plankton communities in the world. Phytoplankton, microscopic plants that use sunlight to create food, are abundant in these waters. When deep water rises to the surface, it brings nutrients from across the world’s oceans into the sunlit zone, where they support life.
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, this rate can reach 30–50 grams per square meter. These values are not extremely high compared to very productive areas like the North Sea or upwelling regions, but the area where this happens is enormous, even larger than other major producers like rainforests. Additionally, during the Antarctic summer, long hours of daylight help sustain this process. These factors make plankton and krill essential to Earth’s ecosystems.
A possible decline in Antarctic krill biomass may be linked to the reduction of pack ice caused by global warming. Krill, especially during early development, rely on pack ice structures to survive. The ice provides shelter-like spaces where krill can hide from predators. When pack ice levels are low, krill populations may be replaced by salps, barrel-shaped filter feeders that also eat plankton. Salps typically live in areas with lower productivity and lower latitudes. Warmer sea temperatures allow salps to move into areas where krill numbers have declined, 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 rising carbon dioxide levels. Krill have an exoskeleton made of carbonate, which dissolves in acidic water. Studies have shown that increased carbon dioxide can disrupt the development of krill eggs and prevent juvenile krill from hatching, leading to reduced hatching success in many regions. However, the full effects of ocean acidification on krill life cycles are still unclear, and scientists are concerned it could impact krill distribution, abundance, and survival.
Antarctic krill fishing occurs at a rate of about 100,000 tonnes per year. The main countries involved in krill fishing are South Korea, Norway, Japan, and Poland. Krill is used as animal feed and fish bait. Krill fishing is challenging because nets must have very fine mesh to catch krill, but this creates high drag and can push krill away. Fine mesh also clogs quickly. Bringing krill onboard is difficult because the organisms compress and lose liquids when the net is hauled out of the water. Experiments have tested pumping krill through tubes while still in water, and new net designs are being developed. Krill must be processed quickly after being caught because it deteriorates within hours. Its high protein and vitamin content makes krill suitable for both human consumption and animal feed.
Fishing krill, and potentially overfishing it, is a growing concern.
A study published in 2025 found high diversity of viral RNA in Antarctic krill, much of which does not match known viruses. The most common viral RNAs were identified as belonging to Penaeus vannamei picornavirus (PvPV) and covert mortality nodavirus (CMNV). The PvPV version found in krill can harm farmed shrimp. The CMNV found in krill infects fish that eat krill in nature and in laboratory settings. Using krill as aquaculture feed may carry a risk of spreading these viruses.