Krill: Characteristics, Behaviour and a Food Source

Home | Category: Crustaceans (Crabs, Lobsters and Shrimp)


northern krill
Krill are shrimp-like crustaceans that form huge masses in the opens sea. They feed while swimming, trapping food particles in hairlike structures, and are the primary food sources for a number of marine animals including baleen whales, dolphins, penguins and other seabirds. Krill have the unusual distinction of being one of the few creature that can reverse molt into smaller shells when food is scarce.

According to some studies krill numbers in the southern seas and Antarctica in 2004 were a fifth of what they were in 1975. This may be a direct effect of global warming. Declines of seabirds such as murres and auklets has been linked to declines in krill populations. Tiny Cassin’s auklets, a relative of puffins, have traditionally fed their young krill after they hatched, but in recent years the krill have shown up late, causing the young birds to starve. The late appearance of the krill is blamed on climate change changes that have caused weak winds that in turn prevent upwelling of nutrient-rich waters from deep in the ocean, depriving the krill of food. The absence of krill has also led to a collapse of rockfish population which in turn have led to a decline in populations of murres that feed on them.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal

Antarctic Krill

Antarctic krill
Antarctic krill (Scientific name: Euphausia superba) live in the Southern and Indian Antarctic Oceans and waters off Antarctica from the Antarctic Continental Shelf break to the Antarctic Polar Frontal Zone. Among the places with particularly high concentrations of them are in the Antarctic Coastal Current, near the Antarctic Continent, near Prydz Bay, at northern and western coastal regions of the Antarctic Continent, and the areas where the Antarctic Coastal Current interacts with the Antarctic Circumpolar Current.[Source: Rachel Gierak, Animal Diversity Web (ADW)]

Antarctic krill inhabit the open ocean in polar, marine environments at depths from zero to 600 meters (0 to 1970 feet) at an average depth of 150 meters (492 feet) and usually not deeper than 350 meters (1150 feet). Only a few times have they been observed at 600 meter. Typically they are found in deeper waters during winter months. Larvae begin life near the sea floor and ascend toward the surface as development progresses. Males and females reach sexual maturity at around age 2.8 to 3 years. Their lifespan in the wild is two to seven years. Some have lived to be nine.

Antarctic krill have not been evaluated by the International Union for Conservation of Nature (IUCN) Red List. With the Convention on International Trade in Endangered Species (CITES) they have no special status. Although krill are currently plentiful and in no particular danger, there are concerns they might be in the future. If something were to happen to them it could be a catastrophe because so many marine animals and entire food chains depnd on them.

Krill Characteristics

Antarctic krill have an average mass of two grams (0.07 ounces). They range in length from 5.0 to 6.5 centimeters (2 to 2.5 inches). According to Animal Diversity Web: The body is pink and slightly opaque, with a hard, calcified exoskeleton (also known as a carapace) divided into a cephalothorax (head and thorax fused) and an abdomen. [Source: Rachel Gierak, Animal Diversity Web (ADW)]

krill anatomy

These animals are similar in appearance to shrimp. Antarctic krill have six pairs of thoracic appendages and a pair of antennae. A tail is formed by fusion of the final appendages. There are luminous organs called photophores located near the mouthparts, at the genitals (located on the cephalothorax), and at the base of the abdominal pleopods (which are the forked limbs these animals use for swimming). These photophores produce a blue light. The gills are located ventrally, under the carapace.

Females larger than males and shaped differently. Males have a more elongated bodies, slightly larger eyes, longer antennae and slightly shorter rostrums. Mature males have modified endopods called petasmae as the first pair of pleopods. These are used during mating to transfer spermatophores. Females have a tri-lobed structure on the ventral posterior part of their body called the thelycum. In unmated females, this structure is often a bright red color. Spermatophores (small white vesicles), can occasionally be seen attached to it in mated females.

Krill Behavior and Feeding

Antarctic krill are a schooling species, with schools primarily moving horizontally in the water column, along with currents. Schools may be extremely large, with an average length of 100 meters, but may extend to 100 kilometers, with an average thickness of 15 meters. Densities may be as high 1,0000-100,000 krill per cubic meter, with lower density schools measuring 1 to 100 krill per cubic meter. Schooling in groups of similar body size enables these animals to avoid any one individual being singled out by a predator. [Source: Rachel Gierak, Animal Diversity Web (ADW)]

Antarctic krill feeding
In the schools, individual krill swim in the same direction, evenly spaced from each other. All individuals in a given school are approximately the same size. An individual will gauge its size in relation to the rest of the school and join or leave as appropriate. Individuals at the front of a school use rheotactic cues, such as turning to face oncoming currents, to communicate while swimming.

Vision helps individuals to maintain schools and during feeding. Mechanoreception and olfaction may also play a role in schooling behavior. Chemoreceptors are used to detect amino acids (even at very low levels), which indicate the presence of food sources, and pheromones likely play a role in mating.

Usually,Antarctic krill feed by using their thoracic endopodites to create a watertight feeding basket, which encloses a pocket of food and water. Water is then filtered out laterally by compression-filtration through setae. Phytoplankton remains caught in the feeding basket as water is filtered out, and is brushed forward by the setae into the mouth for ingestion. Antarctic krill are primarily planktivores, but occasionally eat other krill or molted exoskeletons. They are considered the dominant herbivore of the Southern Ocean.

During the winter, Antarctic krill rely heavily on ice algae as a food source. They are filter feeders, but do not feed continually, relying on chemical cues to indicate the presence of food particles.

Krill Reproduction

Antarctic krill are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). They engage in seasonal breeding and internal reproduction in which sperm from the male parent fertilizes an egg from the female parent. Antarctic krill breed once yearly — during the summer months. The fish are polygynandrous (promiscuous), with both males and females having multiple partners. The average gestation period is 10 days. There is no parental involvement in the raising of offspring..[Source: Rachel Gierak, Animal Diversity Web (ADW)]

According to Animal Diversity Web: Mating involves five phases: chase, probe, embrace, flex, and push. First, a male (sometimes more than one at a time) pursues a gravid female. Then, one male probes a female with his petasma (specialized structures found on the first pair pleopods). Male and female then embrace each other, abdomen to abdomen. Spermatophores are transferred as the male flexes his body around the female, forming a T-shaped pair. Hooks on his petasma aid in spermatophore transfer. Rapid spinning occurs during flexing, lasting about 5 seconds, which aids in pushing spermatophores into her thelycum. After flexing, the pair continue to swim together as the male pushes his rostrum and antennae against the female's ventral surface. Finally, the pair detach and swim away from each other.

All adult female Antarctic krill develop a brood during a reproductive season, with eggs produced periodically and released in several spawning events. Up to four oocytes may undergo vitellogenesis (yolk production) per female. Warmer temperatures may increase spawning and molting activities in females. Females lay their eggs in deep waters, between December and March. Eggs begin development on the sea floor, but it is not known where in the water column they are laid. Eggs sink for approximately 10 days before hatching and entering the larval stages.

Role of kill in the biological pump

Krill Development

Krill undergo several larval stages, known as nauplius, metanauplius, calyptopus, and furcilia; molts occur between (and sometimes within) each stage, with each larval stage lasting from 8-15 days. Once eggs have been laid, they sink for about 10 days, as deep as a few hundred to 2,000 meters deep. There, they hatch as nauplii, which have only one eye and no body segments or limb buds. [Source: Rachel Gierak, Animal Diversity Web (ADW)]

Nauplii ascend and enter a metanauplius stage, where limb development begins. As larvae continue to rise, they develop into calyptopes; these reach the surface and begin to feed. After three additional molts, larvae become known as furcilia. The furcilia stage is marked by the development of movable compound eyes, which project from the edge of the carapace. Furcilia develop into juveniles, reaching lengths of 4-10 millimeters by early winter, with growth slowing down by late March. Juveniles begin to develop gonads during their second year (spring/summer) and begin to spawn at two years of age.

Animals That Prey on Antarctic Krill

Antarctic krill serve as prey to many marine mammals, invertebrates, fishes, and birds. The only anti-predatory adaptation of these krill is their schooling behavior. A disruption to the school may cause mass molting, which can act as a distraction to predators. Krill may also avoid predators by remaining in deep, cold water below the surface. [Source: Rachel Gierak, Animal Diversity Web (ADW)]

Among the animals that are known to prey on krill are whales, Leopard seals, Weddell seals, Crabeater seals, Baleen whales, Ross seals, Fur seals, Squid, Callianira antarctica, Bony fishes, Antarctic silverfish, Bald notothen, Albatrosses, Petrels, Adelie penguins, Chinstrap penguins, and Gentoo penguins.

Antarctic krill play an important role as a primary food source for many animals in the Southern Ocean. They may be parasitized by several organisms, in particular by protozoans in the genus Ephelota. Infected krill become more opaque and whitish in color and are affected with tumors and molting problems, in which parts of their exoskeletons remain attached.

Role of Antarctic krill in biogeochemical cycles

Krill, the Antarctic and Climate Change

Helen Scales wrote in National Geographic: The cold, nutrient-rich waters off the coast of the Antarctic Peninsula support a plethora of living things, including thousands of seals and penguins and the tiny, shrimplike crustaceans called krill they depend on. Urgent efforts are under way to formally protect the region’s delicate — and threatened — web of life. During the past 50 years, ice shelves along the Antarctic Peninsula have calved into the sea, often in tabular icebergs — a loss of more than 11,000 square miles of ice. Warming seas accelerate the breakup of ice that seals and birds need for rearing young. [Source:Helen Scales, National Geographic, October 19, 2021]

This icy world is imperiled: The Antarctic Peninsula is one of the fastest warming places on the planet. Air temperatures during a heat wave in February 2020 reached a record 64.94°F at Argentina’s Esperanza Base, toward the northern tip of the peninsula. (Summer temperatures normally aren’t more than a few degrees above freezing.) As air temperature rises, sea ice around the peninsula recedes, and in 2016 it dwindled to its least amount since satellite monitoring of changes in the ice began in the 1970s.

That’s a problem, because freezing sea-water shelters pinkie-finger–size crustaceans — Antarctic krill — that are the key to the web of life in the Southern Ocean. Teeming shoals of krill feed great gatherings of other animals. Minke whales and humpbacks come to scoop up mouthfuls. Squid, fish, and penguins eat krill too. Many of those krill feeders in turn are hunted by top-level predators — leopard seals from below, skuas and giant petrels from above. Take away krill, and the ecosystem unravels. It’s unclear what quantity of krill has been lost to warming conditions.

Harvesting Krill for Human Use

Krill is rich in omega 3 fatty acids. Norwegian and Canadian companies are already marketing krill health pills. The crustaceans are also harvested for special enzymes that can be use by surgeons to clean wounds, and also to clean contact lenses. There is some discussion of major human harvesting of krill. As it stand now only a about 100,000 tons is taken each year but some predict that figure could rise to a several million tons in the not to distant future, especially as demand for protein sources increases. Already there are concerns about the overfishing of krill. According to some studies krill numbers in Antarctica in 2004 were one fifth what they were in 1975.

There have been attempts to use Antarctic krill for human consumption, mainly as domesticated animal and aquaculture feed. Krill products have pharmaceutical and industrial uses. Chitin has showed promise in lowering cholesterol levels, and the lipid composition of Antarctic krill may be useful as a nutritional source of fatty acids. The lipids of Antarctic krill are more stable than those of some fishes consumed by humans. Krill digestive proteases can also be injected into humans to reduce pressure on nerve roots between vertebral discs. [Source: Rachel Gierak, Animal Diversity Web (ADW)]

But Antarctic krill are essential to the diet of many animals in the Antarctic and the Southern Oceans. There are worries that krill fisheries are exploited too much to, say, feed farmed fish, the decrease of Antarctic krill as a food source could threaten many animal species living in the region. Measures being taken to protect Antarctic krill include preventing krill fisheries from expanding, taking regular biomass surveys, and strengthening and funding programs dedicated to monitoring the Antarctic ecosystem.

Chinese Exploitation of Antarctic Krill

China has sent ships to the Antarctic the harvest swarms of krill as protein source for the fish farming industry. Environmentalist are worried because they are targeting shoreline areas where seals and penguins rely on krill as one their primary food sources.

Conservationists are warning of a potential disaster in Antarctica as China seeks to exploit the world’s last untapped ocean. Chinese trawlers are preparing to sail for the Southern Ocean to harvest krill, the shrimp-like creatures on which the continent’s colonies of penguins and seals survive. [Source: Frank Pope, Times of London July 18 2010]

Two ships were sent this year to fish the swarms of krill that converge off the coast of Antarctica, which also sustains the blue whales that cruise in deeper waters. China is planning a new expedition as part of a 5-year exploration program to investigate the potential of krill to provide protein and omega-3 oils to fuel its booming fish-farming industry.

Depletion of the food could have disastrous consequences. Marine scientists complain that overfishing had consumed some 95 percent of large fish in many of the world’s seas and pushed some species to the brink of collapse. Although the Chinese krill catches have so far been small — at about 115,000 tonnes a year from a stock that scientists estimate could support a harvest of 3.5 million tonnes — they disguise the wide dispersal of the krill.

Gerry Leape of the Pew Environment Trust says: “The problem is one of prey depletion for land based krill predators. As they are forced to move farther offshore to feed, this could impact on their reproductive success.” The inefficiency in farming and food and the risks to the environment are exhibited once more.

Krill Fishing

The waters around the Antarctic Peninsula supply the largest industrial krill fishery in the Southern Ocean; Helen Scales wrote in National Geographic: The western coast of the Antarctic Peninsula is the hatchery for the krill that sustain many species, including migratory whales. But fishing vessels from Chile, China, South Korea, Norway, and Ukraine extract thousands of tons for products such as health supplements and fish food, and global demand for krill keeps growing. Under a new proposal, krill fishing could continue in designated areas but would be prohibited in general protection zones. Such areas bolster the entire fishery by giving species a refuge in which to feed and breed. [Source: Helen Scales, National Geographic, October 19, 2021]

Factory ships extract more than 800 tons of krill a day. Krill are pumped up continuously from nets that may remain submerged for several weeks at a time. The crustaceans are processed on board to make products rich in omega-3 fatty acids, such as fish meal used in livestock feeds and krill oil that’s added to nutritional supplements for humans and pets. Climate change threats and industrial fishing are closely entwined, Lynch says. “As the sea ice declines, the krill fishing boats are able to move in.”

Regulating Krill Fishing

The Commission for the Conservation of Antarctic Marine Living Resources, an international body formed in 1982 to conserve Antarctic marine life in response to increasing commercial interest in krill fishing, has set an annual krill quota for waters surrounding the Antarctic Peninsula of 171,000 tons — less than one percent of the estimated standing stock, as fisheries managers refer to the total biomass. Helen Scales wrote in National Geographic: Overall, experts say, that should be an ecologically sustainable fishery, with a caveat: Krill fishing must be targeted. “For the penguins whose krill supply has dried up, it matters absolutely nothing to them that the krill that was taken was a small percentage of all the krill available in that region,” Lynch says. [Source: Helen Scales, National Geographic, October 19, 2021]

“If you explore the fishing patterns in the last 10 or 15 years, they have been consistently going to the same spots,” says César Cárdenas, of the Chilean Antarctic Institute. He’s working on plans for the protected area. Fishing fleets favor the richest areas for krill, where whales and penguins go to feed. A 2020 analysis of more than 30 years of monitoring data indicated that when local krill catch rates are high, penguins do poorly according to a suite of measures including the weight of their fledglings and their breeding success.

Restricting krill fishing in certain parts of the protected area can help ensure the krill populations stay robust in places where parent penguins forage, so they don’t have to compete with fishing boats in securing food for their young. With the scientific basis of the Antarctic Peninsula MPA in place, the next step lies largely within the political sphere — reaching consensus among all members of the commission. Given the importance of krill fishing, vigorous discussions are likely to lie ahead — especially if negotiations for the Ross Sea Region Marine Protected Area, which came into play four years ago after protracted wrangling, are any indication.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023

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