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Krill (Euphausiids) (: krill) are small and exclusively marine of the order Euphausiacea, found in all of the world's oceans. The name "krill" comes from the Norwegian word krill]], meaning "small fry of fish", which is also often attributed to species of fish.
Krill are considered an important trophic level connection near the bottom of the food chain. They feed on phytoplankton and, to a lesser extent, zooplankton, and are also the main source of food for many larger animals. In the Southern Ocean, one species, the Antarctic krill, makes up an estimated biomass of around 379 million , making it among the species with the largest total biomass. Over half of this biomass is eaten by whales, Pinniped, penguins, seabirds, squid, and fish each year. Most krill species display large daily vertical migrations, providing food for predators near the surface at night and in deeper waters during the day.
Krill are fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, mostly from the Scotia Sea. Most krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry. Krill are also used for human consumption in several countries. They are known as オキアミ in Japan and as camarones in Spain and the Philippines. In the Philippines, they are also called alamang and are used to make a salty paste called bagoong.
Krill are also the main food for , including the blue whale.
The order Euphausiacea comprises two families. The more abundant Euphausiidae contains 10 different genus with a total of 85 species. Of these, the genus Euphausia is the largest, with 31 species. The lesser-known family, the Bentheuphausiidae, has only one species, Bentheuphausia amblyops, a bathypelagic krill living in deep waters below . It is considered the most primitive extant krill species.
Well-known species of the Euphausiidae of commercial Krill fishery include Antarctic krill ( Euphausia superba), Pacific krill ( E. pacifica) and Northern krill ( Meganyctiphanes norvegica).
There have been many theories of the location of the order Euphausiacea. Since the first description of Thysanopode tricuspide by Henri Milne-Edwards in 1830, the similarity of their biramous thoracopods had led zoologists to group euphausiids and Mysidacea in the order Schizopoda, which was split by Johan Erik Vesti Boas in 1883 into two separate orders. Later, William Thomas Calman (1904) ranked the Mysidacea in the superorder Peracarida and euphausiids in the superorder Eucarida, although even up to the 1930s the order Schizopoda was advocated. It was later also proposed that order Euphausiacea should be grouped with the Penaeidae (family of prawns) in the Decapoda based on developmental similarities, as noted by Robert Gurney and Isabella Gordon. The reason for this debate is that krill share some morphological features of decapods and others of mysids.
Molecular studies have not unambiguously grouped them, possibly due to the paucity of key rare species such as Bentheuphausia amblyops in krill and Amphionides reynaudii in Eucarida. One study supports the monophyly of Eucarida (with basal Mysida), another groups Euphausiacea with Mysida (the Schizopoda), while yet another groups Euphausiacea with Hoplocarida.
Species of the genus Thysanoessa occur in both Atlantic Ocean and Pacific Ocean oceans. The Pacific is home to Euphausia pacifica. Northern krill occur across the Atlantic from the Mediterranean Sea northward.
Species with neritic distributions include the four species of the genus Nyctiphanes.D'Amato, M.E. et al.: "", in Marine Biology vol. 155, no. 2, pp. 243–247, August 2008. They are highly abundant along the upwelling regions of the California, Humboldt Current, Benguela Current, and Canary Current Ocean current. Another species having only neritic distribution is E. crystallorophias, which is endemic to the Antarctic coastline.
Species with endemic distributions include Nyctiphanes capensis, which occurs only in the Benguela current, E. mucronata in the Humboldt current, and the six Euphausia species native to the Southern Ocean.
In the Antarctic, seven species are known, one in genus Thysanoessa ( T. macrura) and six in Euphausia. The Antarctic krill ( Euphausia superba) commonly lives at depths reaching , whereas ice krill ( Euphausia crystallorophias) reach depth of , though they commonly inhabit depths of at most . Krill perform Diel Vertical Migrations (DVM) in large swarms, and acoustic data has shown these migrations to go up to 400 metres in depth. Both are found at south of 55° S, with E. crystallorophias dominating south of 74° S and in regions of pack ice. Other species known in the Southern Ocean are E. frigida, E. longirostris, E. triacantha and E. vallentini.
Krill feature intricate . Some species adapt to different lighting conditions through the use of screening .
They have two antennae and several pairs of thoracic legs called or , so named because they are attached to the thorax. Their number varies among genera and species. These thoracic legs include feeding legs and grooming legs.
Krill are probably the sister clade of decapods because all species have five pairs of pleopod called "swimmerets" in common with the latter, very similar to those of a lobster or freshwater crayfish.
In spite of having ten swimmerets, otherwise known as Decapod anatomy, krill cannot be considered decapods. They lack any true ground-based legs due to all their Decapod anatomy having been converted into grooming and auxiliary feeding legs. In Decapoda, there are ten functioning Decapod anatomy, giving them their name; whereas here there are no remaining locomotive Decapod anatomy. Nor are there consistently ten Decapod anatomy at all.
Most krill are about long as adults. A few species grow to sizes on the order of . The largest krill species, Thysanopoda cornuta, lives bathypelagic. Krill can be easily distinguished from other crustaceans such as true shrimp by their externally visible .
Except for Bentheuphausia amblyops, krill are bioluminescence animals having organs called that can emit light. The light is generated by an enzyme-catalysed chemiluminescence reaction, wherein a luciferin (a kind of pigment) is activated by a luciferase enzyme. Studies indicate that the luciferin of many krill species is a fluorescence Polypyrrole similar but not identical to dinoflagellate luciferin and that the krill probably do not produce this substance themselves but acquire it as part of their diet, which contains dinoflagellates. Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles. The precise function of these organs is as yet unknown; possibilities include mating, social interaction or orientation and as a form of counter-illumination camouflage to compensate their shadow against overhead ambient light.
Krill are an important element of the aquatic food chain. Krill convert the primary production of their prey into a form suitable for consumption by larger animals that cannot feed directly on the minuscule algae. Northern krill and some other species have a relatively small filtering basket and actively hunt and larger zooplankton.
Disturbances of an ecosystem resulting in a decline in the krill population can have far-reaching effects. During a coccolithophore bloom in the Bering Sea in 1998, for instance, the diatom concentration dropped in the affected area. Krill cannot feed on the smaller coccolithophores, and consequently the krill population (mainly E. pacifica) in that region declined sharply. This in turn affected other species: the shearwater population dropped. The incident was thought to have been one reason salmon did not spawn that season.
Several single-celled of the genus Collinia can infect species of krill and devastate affected populations. Such diseases were reported for Thysanoessa inermis in the Bering Sea and also for E. pacifica, Thysanoessa spinifera, and T. gregaria off the North American Pacific coast. Some of the family Dajidae (epicaridean ) afflict krill (and also shrimp and ); one such parasite is Oculophryxus bicaulis, which was found on the krill Stylocheiron affine and S. longicorne. It attaches itself to the animal's eyestalk and sucks blood from its head; it apparently inhibits the host's reproduction, as none of the afflicted animals reached maturity.
Climate change poses another threat to krill populations.
By the calyptopsis stages differentiation has progressed far enough for them to develop a mouth and a digestive tract, and they begin to eat phytoplankton. By that time their yolk reserves are exhausted and the larvae must have reached the photic zone, the upper layers of the ocean where algae flourish. During the furcilia stages, segments with pairs of swimmerets are added, beginning at the frontmost segments. Each new pair becomes functional only at the next moult. The number of segments added during any one of the furcilia stages may vary even within one species depending on environmental conditions. After the final furcilia stage, an immature juvenile emerges in a shape similar to an adult, and subsequently develops and matures sexually.
Krill employ two types of spawning mechanism. The 57 species of the genera Bentheuphausia, Euphausia, Meganyctiphanes, Thysanoessa, and Thysanopoda are "broadcast spawners": the female releases the fertilised eggs into the water, where they usually sink, disperse, and are on their own. These species generally hatch in the nauplius 1 stage, but have recently been discovered to hatch sometimes as metanauplius or even as calyptopis stages. The remaining 29 species of the other genera are "sac spawners", where the female carries the eggs with her, attached to the rearmost pairs of thoracopods until they hatch as metanauplii, although some species like Nematoscelis difficilis may hatch as nauplius or pseudometanauplius. (2025). 9783540146735, World Biodiversity Database CD-ROM Series, Springer Verlag. . ISBN 9783540146735
Vertical migration may be a 2–3 times daily occurrence. Some species (e.g., Euphausia superba, E. pacifica, E. hanseni, Pseudeuphausia latifrons, and Thysanoessa spinifera) form surface swarms during the day for feeding and reproductive purposes even though such behaviour is dangerous because it makes them extremely vulnerable to predators.
Experimental studies using Artemia salina as a model suggest that the vertical migrations of krill several hundreds of metres, in groups tens of metres deep, could collectively create enough downward jets of water to have a significant effect on ocean mixing.
Dense swarms can elicit a feeding frenzy among fish, birds and mammal predators, especially near the surface. When disturbed, a swarm scatters, and some individuals have even been observed to moult instantly, leaving the exuvia behind as a decoy.
Krill normally swim at a pace of 5–10 cm/s (2–3 body lengths per second), using their swimmerets for propulsion. Their larger migrations are subject to ocean currents. When in danger, they show an escape reaction called lobstering—flicking their caudal structures, the telson and the , they move backwards through the water relatively quickly, achieving speeds in the range of 10 to 27 body lengths per second, which for large krill such as E. superba means around . Their swimming performance has led many researchers to classify adult krill as nekton life-forms, i.e., small animals capable of individual motion against (weak) currents. Larval forms of krill are generally considered zooplankton.
In 1993, two events caused a decline in krill fishing: Russia exited the industry; and the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) defined maximum catch quotas for a sustainable exploitation of Antarctic krill. After an October 2011 review, the Commission decided not to change the quota.
The annual Antarctic catch stabilised at around 100,000 tonnes, which is roughly one fiftieth of the CCAMLR catch quota. The main limiting factor was probably high costs along with political and legal issues. The Japanese fishery saturated at some 70,000 tonnes.
Although krill are found worldwide, fishing in Southern Oceans are preferred because the krill are more "catchable" and abundant in these regions. Particularly in Antarctic seas which are considered as , they are considered a "clean product".
In 2018 it was announced that almost every krill fishing company operating in Antarctica will abandon operations in huge areas around the Antarctic Peninsula from 2020, including "buffer zones" around breeding colonies of penguins.
Krill is a rich source of protein and omega-3 fatty acids which are under development in the early 21st century as human food, dietary supplements as oil capsules, livestock food, and pet food. Krill tastes salty with a somewhat stronger fish flavor than shrimp. For mass consumption and commercially prepared products, they must be peeled to remove the inedible exoskeleton.
In 2011, the US Food and Drug Administration published a letter of no objection for a manufactured krill oil product to be generally recognized as safe (GRAS) for human consumption.
Krill (and other planktonic shrimp, notably Acetes spp.) are most widely consumed in Southeast Asia, where it is fermented (with the shells intact) and usually ground finely to make shrimp paste. It can be stir-fried and eaten paired with white rice or used to add umami flavors to a wide variety of traditional dishes. The liquid from the fermentation process is also harvested as fish sauce.
Human consumption
Although the total biomass of Antarctic krill may be as abundant as 400 million tonnes, the human impact on this keystone species is growing, with a 39% increase in total fishing yield to 294,000 tonnes over 2010–2014. Major countries involved in krill harvesting are Norway (56% of total catch in 2014), the Republic of Korea (19%), and China (18%).
Bio-inspired robotics
Krill are agile swimmers in the intermediate Reynolds number regime, in which there are not many solutions for uncrewed underwater robotics, and have inspired robotic platforms to both study their locomotion as well as find design solutions for underwater robots.
See also
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