Krill (Euphausiids) are small crustaceans that live only in the ocean. They belong to the order Euphausiacea and are found in all of the world's oceans. The word "krill" comes from the Norwegian language and means "small fry of fish," a term sometimes used for young fish.

Krill play an important role in the food chain. They eat phytoplankton and, to a lesser degree, zooplankton. Many larger animals, such as whales, seals, penguins, seabirds, squid, and fish, rely on krill as a main food source. In the Southern Ocean, the Antarctic krill species has a total weight of about 379 million tonnes (418 million tons), making it one of the species with the largest total weight. Over half of this amount is eaten by animals each year. Most krill species move up and down in the water daily, providing food for predators near the surface at night and in deeper water during the day.

Krill are fished commercially in the Southern Ocean and near Japan. Each year, about 150,000 to 200,000 tonnes (170,000 to 220,000 tons) of krill are harvested, mostly from the Scotia Sea. Krill is used for fish farming, aquarium food, sport fishing bait, and medicines. People in some countries also eat krill. In Japan, krill are called okiami. In Spain and the Philippines, they are known as camarones. In the Philippines, they are also called alamang and are used to make a salty paste called bagoong.

Krill are the main food source for baleen whales, including the blue whale.

Taxonomy

Krill are part of the large arthropod group called Crustacea. The most well-known group within Crustacea is the class Malacostraca, which includes the superorder Eucarida. This superorder contains three main groups: Euphausiacea (krill), Decapoda (shrimp, prawns, lobsters, and crabs), and Amphionidacea (planktonic organisms).

The order Euphausiacea includes two families. The more common family, Euphausiidae, has 10 genera and 85 species. The largest genus in this family is Euphausia, which includes 31 species. The less common family, Bentheuphausiidae, has only one species: Bentheuphausia amblyops. This species lives in deep ocean waters below 1,000 meters (3,300 feet) and is considered the most primitive living krill species.

Some well-known krill species in the Euphausiidae family include Antarctic krill (Euphausia superba), Pacific krill (E. pacifica), and Northern krill (Meganyctiphanes norvegica). These species are important in commercial krill fisheries.

As of 2013, scientists believe the order Euphausiacea is monophyletic, meaning all its members descended from a single ancestor. This is supported by unique physical traits, such as their naked, filamentous gills and thin thoracopods, as well as molecular studies.

Scientists have debated the classification of Euphausiacea. Early in the 19th century, the species Thysanopode tricuspide was described, and its biramous thoracopods led to grouping euphausiids with Mysidacea in the order Schizopoda. This group was later split into two separate orders. In 1904, William Thomas Calman classified Mysidacea in the superorder Peracarida and euphausiids in the superorder Eucarida. However, some scientists continued to support the Schizopoda classification into the 1930s. Later, some researchers proposed grouping Euphausiacea with Penaeidae (a family of prawns) in the Decapoda due to shared developmental traits. This debate continues because krill share some features with decapods and others with mysids.

Molecular studies have not clearly grouped Euphausiacea with other groups, possibly due to the lack of rare species like Bentheuphausia amblyops and Amphionides reynaudii. Some studies support the monophyly of Eucarida (with Mysida as a base group), while others group Euphausiacea with Mysida (Schizopoda) or with Hoplocarida.

No living fossils have been definitively assigned to Euphausiacea. Some extinct species, such as Anthracophausia, Crangopsis (now classified as Aeschronectida in Hoplocarida), and Palaeomysis, were once thought to be related to krill. Scientists estimate the last common ancestor of the Euphausiidae family (excluding Bentheuphausia amblyops) lived during the Lower Cretaceous period, about 130 million years ago, based on molecular clock methods.

Distribution

Krill are found in all oceans around the world, but many species live only in specific areas, such as near coasts. One species, Bentheuphausia amblyops, lives in deep ocean waters and is found in many parts of the world.

Thysanoessa species are found in both the Atlantic and Pacific Oceans. Euphausia pacifica lives only in the Pacific Ocean. Northern krill are found in the Atlantic Ocean, from the Mediterranean Sea to the north.

Some krill species live near coasts. Four species in the genus Nyctiphanes are very common in areas where ocean currents bring up nutrients, such as near the California, Humboldt, Benguela, and Canarias currents. Another coastal species, Euphausia crystallorophias, lives only along the Antarctic coastline.

Some krill species are found only in certain regions. Nyctiphanes capensis is found only in the Benguela Current. Euphausia mucronata is found only in the Humboldt Current. Six species of Euphausia live only in the Southern Ocean.

In the Antarctic, seven krill species are known. One is in the genus Thysanoessa (T. macrura), and six are in Euphausia. Antarctic krill (Euphausia superba) usually live in waters up to 100 meters (330 feet) deep. Ice krill (Euphausia crystallorophias) can live as deep as 4,000 meters (13,100 feet), but they most often live between 300 and 600 meters (1,000 and 2,000 feet). Krill move in large groups up and down the water column daily. These movements can reach depths of up to 400 meters. Both species live in areas south of 55° S, with E. crystallorophias mainly found south of 74° S and in ice-covered regions. Other krill species in the Southern Ocean include E. frigida, E. longirostris, E. triacantha, and E. vallentini.

Anatomy and morphology

Krill are crustaceans, and like all crustaceans, they have a hard outer shell made of a material called chitin. Their bodies are divided into three main parts: the cephalothorax, which is formed by the head and thorax fused together; the abdomen, which has ten swimming appendages and a tail fan. The outer shell of most krill species is transparent.

Krill have compound eyes that are complex in structure. Some species use special pigments in their eyes to adjust to different light conditions.

Krill have two antennae and several pairs of legs attached to their thorax, called pereiopods or thoracopods. These legs vary in number depending on the species and include legs used for feeding and grooming.

Krill are likely closely related to decapods because both groups share five pairs of swimming legs called swimmerets, similar to those found in lobsters or crayfish. However, krill are not decapods because they lack true legs for walking on the ground. In decapods, there are ten functioning pereiopods, but krill have no remaining legs used for movement. Additionally, krill do not consistently have ten pereiopods.

Most adult krill are 1–2 centimeters (0.4–0.8 inches) long. A few species grow up to 6–15 centimeters (2.4–5.9 inches). The largest krill species, Thysanopoda cornuta, lives in the deep open ocean. Krill can be distinguished from true shrimp by their visible gills on the outside of their bodies.

Most krill species are bioluminescent, meaning they can produce light using special organs called photophores. This light is created through a chemical reaction involving a substance called luciferin and an enzyme called luciferase. Studies suggest that many krill species obtain their luciferin from dinoflagellates, which are part of their diet. Krill photophores have lenses and can rotate with muscles, but their exact purpose is not fully understood. Possible functions include communication, navigation, or camouflage by reducing their shadow against overhead light. The exception is Bentheuphausia amblyops, which does not have bioluminescence.

Ecology

Krill are filter feeders, meaning they use their front legs, called thoracopods, to form fine combs that help them filter food from water. In species like Euphausia spp., these filters are very small, allowing them to eat phytoplankton, especially diatoms, which are single-celled algae. Krill are mostly omnivorous, but some species eat small zooplankton and fish larvae.

Krill play a key role in aquatic food chains. They transform the food they eat into a form that larger animals can consume, as these animals cannot directly eat tiny algae. Some krill species, like northern krill, have smaller filters and actively hunt copepods and larger zooplankton.

Many animals rely on krill for food, including fish, penguins, seals, and baleen whales.

Changes in an ecosystem that reduce krill numbers can harm other species. For example, during a coccolithophore bloom in the Bering Sea in 1998, diatom levels dropped. Krill cannot eat the smaller coccolithophores, so their population (mainly E. pacifica) declined sharply. This led to fewer shearwaters and prevented salmon from spawning that season.

Certain single-celled parasites, such as those in the genus Collinia, can infect krill and harm their populations. These parasites have been found in species like Thysanoessa inermis in the Bering Sea and E. pacifica, Thysanoessa spinifera, and T. gregaria along the North American Pacific coast. Some external parasites, like Oculophryxus bicaulis from the family Dajidae, attach to krill eyestalks and feed on their blood, possibly preventing the krill from reproducing.

Climate change also threatens krill populations.

Early research suggests krill can digest microplastics smaller than 5 millimeters (0.20 inches) in diameter, breaking them down and excreting them in smaller pieces back into the environment.

Life history and behavior

The life cycle of krill is well understood, though some details may vary between species. After hatching, krill go through several larval stages: nauplius, pseudometanauplius, metanauplius, calyptopsis, and furcilia. Each stage has smaller steps. The pseudometanauplius stage is only found in species that carry their eggs in a special sac called an ovigerous sac, known as "sac-spawners." As larvae grow, they shed their hard outer covering, or exoskeleton, when it becomes too small. Smaller krill shed more often than larger ones. Yolk inside their bodies helps feed them during the metanauplius stage.

By the calyptopsis stage, krill develop a mouth and digestive system and start eating phytoplankton. At this point, their yolk is gone, and they must reach the photic zone, where sunlight supports algae growth. During the furcilia stage, new segments with swimmerets are added, starting from the front. These swimmerets only work after the next moult. The number of segments added during this stage can vary within a species based on environmental conditions. After the final furcilia stage, an immature juvenile emerges, resembling an adult. It later develops reproductive organs and becomes sexually mature.

During mating, which happens at different times depending on the species and climate, the male places a sperm sack at the female’s genital opening, called thelycum. Females can carry thousands of eggs in their ovaries, which may make up one-third of their body weight. Krill can have multiple broods in one season, with short breaks between broods.

Krill use two spawning methods. Fifty-seven species in the genera Bentheuphausia, Euphausia, Meganyctiphanes, Thysanoessa, and Thysanopoda are "broadcast spawners," releasing fertilized eggs into the water, where they sink and develop on their own. These eggs usually hatch as nauplius 1, though some hatch as metanauplius or calyptopsis stages. The remaining 29 species are "sac spawners," carrying eggs until they hatch as metanauplii, though some species may hatch as nauplius or pseudometanauplius stages.

Moulting happens when krill outgrow their exoskeleton. Young krill, growing faster, moult more often than older ones. Moulting frequency varies by species and depends on factors like latitude, water temperature, and food availability. For example, the subtropical species Nyctiphanes simplex moults every 2 to 7 days, while Euphausia superba in the Antarctic moults every 9 to 28 days, depending on water temperature. If food is scarce, E. superba can shrink its body size and moult even when its exoskeleton is too large. Similar shrinkage has been observed in E. pacifica during high temperatures.

Some high-latitude krill species, like E. superba, live over six years, while mid-latitude species like E. pacifica live only two years. Subtropical or tropical species, such as Nyctiphanes simplex, usually live six to eight months.

Most krill form large swarms, with sizes and densities varying by species and region. E. superba swarms can have 10,000 to 60,000 individuals per cubic meter. Swarming helps confuse predators. In 2012, researchers developed a model to explain krill swarm behavior, based on three factors: movement from other krill, foraging, and random movement.

Krill typically swim deeper during the day and rise to the surface at night. They reduce activity at greater depths to avoid predators and save energy. Sated krill swim less and sink, producing feces that contribute to the Antarctic carbon cycle. Krill with empty stomachs swim more actively to reach the surface.

Vertical migration may occur two to three times daily. Some species form surface swarms during the day for feeding and reproduction, even though this increases their risk of being eaten.

Studies using Artemia salina suggest that krill migrations could create water movement that affects ocean mixing. Dense swarms attract predators, and when disturbed, krill scatter. Some individuals may moult instantly, leaving their exoskeleton behind as a decoy.

Krill normally swim at 5–10 cm/s (2–3 body lengths per second), using swimmer

Biogeochemical cycles

The Antarctic krill is a key species in the Southern Ocean, where it helps move nutrients through the environment and supports the food web. It is important because it provides food for penguins, baleen whales, and blue whales.

Human uses

Krill have been used as a food source for humans and animals since at least the 19th century, and possibly earlier in Japan, where it was called okiami. Large-scale fishing began in the late 1960s and early 1970s and now happens only in Antarctic waters and around Japan. Historically, Japan and the Soviet Union were the largest krill fishing countries. After the Soviet Union ended, Russia and Ukraine became the main fishing nations. The highest krill harvest occurred in 1983, with about 528,000 tonnes (582,000 tons) in the Southern Ocean, of which the Soviet Union caught 93%. Today, the harvest is carefully managed to prevent overfishing.

In 1993, krill fishing decreased because Russia stopped participating, and the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) set limits to ensure krill populations remain healthy. In 2011, the commission decided not to change these limits. The annual Antarctic krill catch has stabilized at about 100,000 tonnes (110,000 tons), which is about one-fiftieth of the allowed limit. High costs and legal challenges have limited fishing. The Japanese fishery reached about 70,000 tonnes (77,000 tons) and has not grown further.

Krill are found worldwide, but fishing in the Southern Ocean is preferred because krill there are easier to catch and more plentiful. Antarctic krill are considered a "clean product" because the waters are pristine. In 2018, many krill fishing companies announced they would stop operating in areas near the Antarctic Peninsula, including zones around penguin breeding sites.

Although Antarctic krill may total up to 400 million tonnes (440,000,000 tons), human fishing has increased by 39% since 2010, reaching 294,000 tonnes (324,000 tons) in 2014. Major countries involved in krill fishing include Norway (56% of the 2014 catch), the Republic of Korea (19%), and China (18%).

Krill are rich in protein and omega-3 fatty acids, which are being developed for human food, dietary supplements, livestock feed, and pet food. Krill has a salty taste and a stronger fish flavor than shrimp. For commercial use, krill must be peeled to remove the hard outer shell.

Antarctic krill store fat during summer and use these reserves in winter when food is scarce. Research has found that krill meal could be a sustainable source of marine fats. In 2011, the U.S. Food and Drug Administration approved a krill oil product as safe for human consumption.

Krill (and other small shrimp-like creatures, such as Acetes spp.) are widely eaten in Southeast Asia. They are fermented with shells intact and ground into a paste called shrimp paste. This paste is used in cooking, paired with rice, or added to dishes for flavor. The liquid from the fermentation process is also used to make fish sauce.

Bio-inspired robotics

Krill are fast swimmers in a specific range of water movement conditions, where there are few solutions for robots that move underwater without people controlling them. They have inspired scientists to study how they move and to create better designs for underwater robots.