Antarctic krill (Euphausia superba) is a type of small, swimming crustacean found in the Southern Ocean near Antarctica. These animals 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 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 a key species in the Antarctic ecosystem and one of the most numerous animals on Earth, with a total weight of about 500 million metric tons (550 million short tons; 490 million long 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 all krill, the male attaches a spermatophore to the female’s genital opening. To do this, the male’s first pleopods (special legs on the abdomen) are used as mating tools. Each female lays between 6,000 and 10,000 eggs at one time. These eggs are fertilized as they leave the female’s body.
According to the classical hypothesis of Marriosis De’ Abrtona, based on research from the British ship RRS Discovery, the eggs develop in this way: After being released, the 0.6 mm (0.024 in) eggs begin to sink. During this descent, gastrulation (the process where the egg becomes an embryo) starts. In deep ocean areas, this happens at depths of about 2,000–3,000 metres (6,600–9,800 ft). The egg hatches into a nauplius larva. After this larva molts into a metanauplius, the young krill 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 three weeks, the young krill reaches the surface. At this point, they may be found in very large numbers, with as many as 2 per litre of water at 60 m (200 ft) depth. As they grow, they go through more larval stages, including second and third calyptopis, and first to sixth furcilia. During these stages, their legs, compound eyes, and setae (bristles) develop. When they reach 15 mm (0.59 in) in length, the juvenile krill looks similar to adult krill. Krill become sexually mature after two to three years. Like all crustaceans, krill must molt to grow. Every 13 to 20 days, krill shed their chitinous exoskeleton, leaving it behind as exuvia.
Food
The gut of E. superba is often visible as a green area through its clear skin. This species mainly eats phytoplankton, especially tiny diatoms (20 μm), which it filters from water using a feeding basket. The glass-like shells of diatoms are broken in the gastric mill and then digested in the hepatopancreas. Krill can also eat copepods, amphipods, and other small zooplankton. The gut forms a straight tube; its digestion is not very efficient, so much carbon remains in the feces. Antarctic krill (E. superba) mainly uses chitinolytic enzymes in the stomach and mid-gut to break down chitinous spines on diatoms. Additional enzymes vary because of its wide diet.
In aquariums, krill have been seen eating each other. When not fed, they shrink in size after moulting, which is unusual for animals of this size. This is likely an adaptation to the limited food supply during dark winter months under ice. However, their compound eyes do not shrink, so the ratio of eye size to body length is a reliable sign of starvation. A krill with enough food has eyes proportional to its body length, while a starving krill has eyes that appear larger than normal.
Antarctic krill directly eats tiny phytoplankton cells, which no other krill-sized animal can do. This is done through filter feeding using highly developed front legs that form an efficient filtering apparatus. The six thoracopods (legs attached to the thorax) create a "feeding basket" to collect phytoplankton from water. In areas with very fine particles, the openings in the basket are only 1 μm in diameter. In areas with less food, the feeding basket moves through water for over half a meter while open, and algae are guided to the mouth by special bristles on the inner side of the thoracopods.
Antarctic krill can scrape green 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). Studies show that ice algae films cover large areas and often contain more carbon than the water column below. Krill use this as a major energy source, especially in spring after winter food shortages.
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 (spitballs) containing thousands of cells. They also produce fecal strings that still have significant carbon and diatom glass shells. Both are heavy and sink quickly to the ocean floor. This process is called the biological pump. Because Antarctic waters 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 eaten by other parts of the pelagic ecosystem, most carbon stays in the ocean's upper layers. Scientists believe this process may be one of the largest biofeedback mechanisms on Earth, driven by a massive biomass. More research is needed to understand the Southern Ocean ecosystem fully.
Biology
Krill are sometimes called light-shrimp because they can produce light using special organs called bioluminescent organs. These organs are found in different areas of the krill's body: two organs near the eyes, two more on the second and seventh thoracopods (a type of leg), and one each on four parts of the krill's tail. These organs glow with a yellow-green light for up to 2–3 seconds. The organs are highly developed and function like a flashlight. They have a mirror-like part at the back and a lens at the front to direct the light. Muscles in the organ can rotate it, allowing the light to be aimed at specific areas. Scientists are still studying the purpose of these lights. Some think they help krill hide 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 exposed to certain types of light. The main substance glows most strongly when excited by light at 355 nm and emits light at 510 nm.
Krill use a quick escape method to avoid predators. They swim backward rapidly by flipping their tails, a movement called lobstering. Krill can swim at speeds faster than 0.6 meters per second (2.0 feet per second). Even in cold water, their reaction time to light is very fast, taking 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 discovered so far. Approximately 70% of its DNA consists of repeated sequences, and this percentage may increase to 92.45% after 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 genes that code for proteins in the Antarctic krill genome, a number similar to other animals. The lengths of genes and the spaces between them (introns) in Antarctic krill are much shorter than those in lungfish and Mexican axolotl, two other species with 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, and from there to Antarctica, the Southern Ocean covers 32 million square kilometers. This area is 65 times larger than the North Sea. In winter, more than three-quarters of this region is covered by ice, while 24,000,000 square kilometers become ice-free in summer. Water temperatures in this area range from −1.3 to 3 °C (29.7 to 37.4 °F).
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 currents, large eddies form, such as in the Weddell Sea. Krill swarms move with these water masses, creating a single population that spans the entire area around Antarctica. Scientists have limited knowledge of krill migration patterns because individual krill cannot yet be tracked with tags. Large 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 contain over 2 million tons of krill. Recent research shows that krill do not simply drift with currents but actively influence them. By moving up and down in the ocean every 12 hours, krill swarms help mix deep, nutrient-rich water with nutrient-poor surface water.
Ecology
Antarctic krill is a key species in the Antarctic ecosystem beyond the coastal shelf. It serves as a vital food source for many animals, including 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 with many bumps that help them filter krill from the water. These teeth look like a strainer, but scientists do not yet fully understand how they work. Crabeater seals are the most numerous seal species 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, and krill makes up 45% of their diet. All seals together consume 63–130 million tonnes of krill annually, whales consume 34–43 million tonnes, birds consume 15–20 million tonnes, squid consume 30–100 million tonnes, and fish consume 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 (about 20 micrometers in size) to krill-sized organisms. This involves small copepods, large copepods, mysids, and small fish (about 5 cm in length).
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). The high biomass of krill is due to the large plankton communities in the waters around Antarctica. These waters are rich in phytoplankton, which thrive when nutrient-rich water rises from the ocean depths to the surface, where sunlight is available. This process supports primary production—the conversion of sunlight into organic matter, which forms the base of the food chain. In the open ocean, primary production averages 1–2 grams of carbon per square meter annually. Near ice-covered areas, this can reach 30–50 grams of carbon per square meter. While these values are not the highest in the world, the vast area over which this occurs makes krill and plankton critical to the planet's ecosystems. During the Antarctic summer, long daylight hours further support this process.
A possible decline in Antarctic krill biomass may be linked to the reduction of pack ice caused by global warming. Krill, especially in early life stages, rely on pack ice structures to survive. The ice provides shelter where krill can hide from predators. When pack ice decreases, krill may be replaced by salps, which are barrel-shaped, free-floating filter feeders that also eat plankton. Salps typically live in lower-productivity areas with lower latitudes, but rising sea temperatures allow them to move into regions where krill populations are declining, potentially competing with krill for food.
Another challenge for krill and other calcifying organisms (such as corals, bivalve mussels, and snails) is ocean acidification, caused by rising carbon dioxide levels. Krill have an exoskeleton made of carbonate, which dissolves in low-pH conditions. Studies show that increased carbon dioxide can disrupt krill egg development and prevent juvenile krill from hatching, leading to reduced hatching success in the future. However, the full effects of ocean acidification on krill life cycles remain unclear, and scientists are concerned it could harm krill populations.
The Antarctic krill fishery harvests about 100,000 tonnes of krill annually. Major fishing nations include South Korea, Norway, Japan, and Poland. Krill is used for animal feed and as fish bait. Krill fisheries face challenges, such as the need for fine mesh nets, which create high drag and cause krill to avoid the nets. Fine mesh nets also clog quickly. Another issue is transporting krill on board ships. When nets are pulled from the water, krill compress, causing significant loss of bodily fluids. 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, as it deteriorates within hours. Its high protein and vitamin content make it valuable for both human consumption and animal feed.
Fishing krill, and the risk of overfishing, is a growing concern.
A study published in 2025 found high diversity of viral RNA in Antarctic krill, most of which do not match known viruses. The most common RNAs were identified as belonging to Penaeus vannamei picornavirus (PvPV) and covert mortality nodavirus (CMNV). The version of PvPV found in krill is harmful to farmed P. vannamei 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.