|Northern krill (Meganyctiphanes norvegica)|
|Families and genera|
Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name krill comes from the Norwegian word krill, meaning "young fry of fish", which is also often attributed to other species of fish.
Krill are considered an important trophic level connection – near the bottom of the food chain – because they feed on phytoplankton and to a lesser extent zooplankton, converting these into a form suitable for many larger animals for whom krill makes up the largest part of their diet. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of around 379,000,000 tonnes, more than that of humans. Of this, over half is eaten by whales, seals, penguins, squid and fish each year, and is replaced by growth and reproduction. Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.
Krill is fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, most of this from the Scotia Sea. Most of the krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry. In Japan, Philippines and Russia, krill is also used for human consumption and is known as okiami (オキアミ?) in Japan. In the Philippines, it is known as "alamang" and it is used to make a salty paste called bagoong.
- 1 Taxonomy
- 2 Distribution
- 3 Anatomy and morphology
- 4 Ecology
- 5 Life history and behavior
- 6 Relation to humans
- 7 References
- 8 Further reading
- 9 External links
Krill belong to the large arthropod subphylum, the Crustacea. The most familiar and largest group of crustaceans, the class Malacostraca, includes the superorder Eucarida comprising the three orders, Euphausiacea (krill), Decapoda (shrimp, prawns, lobsters, crabs), and the planktonic Amphionides.
The order Euphausiacea comprises two families. The more abundant Euphausiidae contains ten different genera 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 1,000 metres (3,300 ft). It is considered the most primitive extant krill species.
|Proposed phylogeny of Euphausiacea|
|Phylogeny obtained from morphological data, (♠) names coined in, (♣) possibly paraphyletic taxon due to Nematobrachion in. (♦) clades differs from Casanova (1984), where Pseudoeuphausia is sister to Nyctiphanes, Euphausia is sister to Thysanopoda and Nematobrachion is sister to Stylocheiron.|
As of 2013[update], the order Euphausiacea is believed to be monophyletic due to several unique conserved morphological characteristics (autapomorphy) such as its naked filamentous gills and thin thoracopods and by molecular studies.
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.
No extant fossil can be unequivocally assigned to Euphausiacea. Some extinct eumalacostracan taxa have been thought to be euphausiaceans such as Anthracophausia, Crangopsis – now assigned to the Aeschronectida (Hoplocarida) – and Palaeomysis. All dating of speciation events were estimated by molecular clock methods, which placed the last common ancestor of the krill family Euphausiidae (order Euphausiacea minus Bentheuphausia amblyops) to have lived in the Lower Cretaceous about .
Krill occur worldwide in all oceans, although many individual species have endemic or neritic (i.e., coastal) distributions. Bentheuphausia amblyops, a bathypelagic species, has a cosmopolitan distribution within its deep-sea habitat.
Species of the genus Thysanoessa occur in both Atlantic and Pacific 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. They are highly abundant along the upwelling regions of the California, Humboldt, Benguela, and Canarias current systems. 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 100 m (330 ft), whereas ice krill (Euphausia crystallorophias) reach depth of 4,000 m (13,100 ft), though they commonly inhabit depths of at most 300–600 m (1,000–2,000 ft). Both are found at latitudes 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.
Anatomy and morphology
Krill are crustaceans and have a chitinous exoskeleton made up of three segments: the cephalon (head), the thorax, and the abdomen. The first two segments are fused into one segment, the cephalothorax. This outer shell of krill is transparent in most species. Krill feature intricate compound eyes; some species adapt to different lighting conditions through the use of screening pigments. They have two antennae and several pairs of thoracic legs called pereiopods or thoracopods, 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. Additionally all species have five pairs of swimming legs called pleopods or "swimmerets", very similar to those of a lobster or freshwater crayfish. Most krill are about 1–2 centimetres (0.4–0.8 in) long as adults; a few species grow to sizes on the order of 6–15 centimetres (2.4–5.9 in). The largest krill species is the bathypelagic Thysanopoda spinicauda. Krill can be easily distinguished from other crustaceans such as true shrimp by their externally visible gills.
Except for Bentheuphausia amblyops, krill are bioluminescent animals having organs called photophores 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 fluorescent tetrapyrrole 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.
Many krill are filter feeders: their frontmost appendages, the thoracopods, form very fine combs with which they can filter out their food from the water. These filters can be very fine indeed in those species (such as Euphausia spp.) that feed primarily on phytoplankton, in particular on diatoms, which are unicellular algae. Krill are mostly omnivorous, although a few species are carnivorous, preying on small zooplankton and fish larvae.
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 copepods 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.
Climate change poses another threat to krill populations. Several single-celled endoparasitoidic ciliates 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 ectoparasites of the family Dajidae (epicaridean isopods) afflict krill (and also shrimp and mysids); 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.
Life history and behavior
The life cycle of krill is relatively well understood, despite minor variations in detail from species to species. After krill hatch, they experience several larval stages—nauplius, pseudometanauplius, metanauplius, calyptopsis, and furcilia, each of which divides into sub-stages. The pseudometanauplius stage is exclusive to species that lay their eggs within an ovigerous sac: so-called "sac-spawners". The larvae grow and moult repeatedly as they develop, replacing their rigid exoskeleton when it becomes too small. Smaller animals moult more frequently than larger ones. Yolk reserves within their body nourish the larvae through metanauplius stage. 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 gonads and matures sexually.
During the mating season, which varies by species and climate, the male deposits a sperm sack at the female's genital opening (named thelycum). The females can carry several thousand eggs in their ovary, which may then account for as much as one third of the animal's body mass. Krill can have multiple broods in one season, with interbrood intervals lasting on the order of days.
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.
Moulting occurs whenever a specimen outgrows its rigid exoskeleton. Young animals, growing faster, moult more often than older and larger ones. The frequency of moulting varies widely by species and is, even within one species, subject to many external factors such as latitude, water temperature, and food availability. The subtropical species Nyctiphanes simplex, for instance, has an overall inter-moult period of two to seven days: larvae moult on the average every four days, while juveniles and adults do so, on average, every six days. For E. superba in the Antarctic sea, inter-moult periods ranging between 9 and 28 days depending on the temperature between −1 and 4 °C (30 and 39 °F) have been observed, and for Meganyctiphanes norvegica in the North Sea the inter-moult periods range also from 9 and 28 days but at temperatures between 2.5 and 15 °C (36.5 and 59.0 °F). E. superba is able to reduce its body size when there is not enough food available, moulting also when its exoskeleton becomes too large. Similar shrinkage has also been observed for E. pacifica, a species occurring in the Pacific Ocean from polar to temperate zones, as an adaptation to abnormally high water temperatures. Shrinkage has been postulated for other temperate-zone species of krill as well.
Some high-latitude species of krill can live for more than six years (e.g., Euphausia superba); others, such as the mid-latitude species Euphausia pacifica, live for only two years. Subtropical or tropical species' longevity is still shorter, e.g., Nyctiphanes simplex, which usually lives for only six to eight months.
Most krill are swarming animals; the sizes and densities of such swarms vary by species and region. For Euphausia superba, swarms reach 10,000 to 60,000 individuals per cubic metre. Swarming is a defensive mechanism, confusing smaller predators that would like to pick out individuals. In 2012, Gandomi and Alavi presented what appears to be a successful stochastic algorithm for modelling the behaviour of krill swarms. The algorithm is based on three main factors: " (i) movement induced by the presence of other individuals (ii) foraging activity, and (iii) random diffusion."
Krill typically follow a diurnal vertical migration. Until recently it has been assumed that they spend the day at greater depths and rise during the night toward the surface. The deeper they go, the more they reduce their activity, apparently to reduce encounters with predators and to conserve energy. Swimming activity in krill varies with stomach fullness. Satiated animals that had been feeding at the surface swim less actively and therefore sink below the mixed layer. As they sink they produce faeces which implies a role in the Antarctic carbon cycle. Krill with empty stomachs swim more actively and thus head towards the surface. 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.
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 instantaneously, 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 uropods, 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 0.8 m/s (3 ft/s). Their swimming performance has led many researchers to classify adult krill as micro-nektonic life-forms, i.e., small animals capable of individual motion against (weak) currents. Larval forms of krill are generally considered zooplankton.
Relation to humans
Krill has been harvested as a food source for humans and domesticated animals since at least the 19th century, and possibly earlier in Japan, where it was known as okiami. Large-scale fishing developed in the late 1960s and early 1970s, and now occurs only in Antarctic waters and in the seas around Japan. Historically, the largest krill fishery nations were Japan and the Soviet Union, or, after the latter's dissolution, Russia and Ukraine. The harvest peaked in 1983 with more than 528,000 tonnes in the Southern Ocean alone (of which the Soviet Union took in 93%). 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.
As of 2003 experimental small-scale harvesting was being carried out in other areas, for example, fishing for Euphausia pacifica off British Columbia and harvesting Meganyctiphanes norvegica, Thysanoessa raschii and Thysanoessa inermis in the Gulf of St. Lawrence. These experimental operations produce only a few hundred tonnes of krill per year. Nicol & Foster consider it unlikely that any large-scale harvesting operations in these areas will be started due to opposition from local fishing industries and conservation groups.
The 2011 Antarctic harvest had increased to 150,000–180,000 tons, growing by 40% over 2009. The increase was driven by krill's use in the production of fish-meal in the aquaculture industry and in dietary and medical products. China entered the market in 2011 and was expected to rapidly increase its participation.
Krill tastes salty and somewhat stronger than shrimp. For mass-consumption and commercially prepared products they must be peeled, because their exoskeleton contains fluorides, which are toxic in high concentrations.
- "Krill". Online Etymology Dictionary. Retrieved June 22, 2010.
- A. Atkinson, V. Siegel, E.A. Pakhomov, M.J. Jessopp & V. Loeb (2009). "A re-appraisal of the total biomass and annual production of Antarctic krill". Deep-Sea Research I 56: 727–740.
- Volker Siegel (2011). V. Siegel, ed. "Euphausiidae Dana, 1852". World Euphausiacea database. World Register of Marine Species. Retrieved November 25, 2011.
- E. Brinton (1962). "The distribution of Pacific euphausiids". Bull. Scripps Inst. Oceanogr. 8 (2): 51–270.
- S. Nicol & Y. Endo (1999). "Krill fisheries: Development, management and ecosystem implications". Aquatic Living Resources 12 (2): 105–120. doi:10.1016/S0990-7440(99)80020-5.
- Andreas Maas & Dieter Waloszek (2001). "Larval development of Euphausia superba Dana, 1852 and a phylogenetic analysis of the Euphausiacea". Hydrobiologia 448: 143–169. doi:10.1023/A:1017549321961.
- Bernadette Casanova (1984). "Phylogénie des Euphausiacés (Crustacés Eucarides)" [Phylogeny of the Euphausiacea (Crustacea: Eucarida)]. Bulletin du Muséum National d'Histoire Naturelle (in French) 4: 1077–1089.
- Bernadette Casanova (2003). "Ordre des Euphausiacea Dana, 1852". Crustaceana 76 (9): 1083–1121. doi:10.1163/156854003322753439. JSTOR 20105650.
- M. Eugenia D'Amato, Gordon W. Harkins, Tulio de Oliveira, Peter R. Teske & Mark J. Gibbons (2008). "Molecular dating and biogeography of the neritic krill Nyctiphanes". Marine Biology 155 (2): 243–247. doi:10.1007/s00227-008-1005-0.
- Simon N. Jarman (2001). "The evolutionary history of krill inferred from nuclear large subunit rDNA sequence analysis". Biological Journal of the Linnean Society 73 (2): 199–212. doi:10.1111/j.1095-8312.2001.tb01357.x.
- Xin Shen, Haiqing Wang, Minxiao Wang & Bin Liu (2011). "The complete mitochondrial genome sequence of Euphausia pacifica (Malacostraca: Euphausiacea) reveals a novel gene order and unusual tandem repeats". Genome 54 (11): 911–922. doi:10.1139/g11-053. PMID 22017501.
- Johan Erik Vesti Boas (1883). "Studien über die Verwandtschaftsbeziehungen der Malacostraken" [Studies on the relationships of the Malacostraca]. Morphologisches Jahrbuch (in German) 8: 485–579.
- Robert Gurney (1942). Larvae of Decapod Crustacea (PDF). Ray Society.
- Isabella Gordon (1955). "Systematic position of the Euphausiacea". Nature 176 (4489): 934. Bibcode:1955Natur.176..934G. doi:10.1038/176934a0.
- Trisha Spears, Ronald W. DeBry, Lawrence G. Abele & Katarzyna Chodyl (2005). Boyko, Christopher B., ed. "Peracarid monophyly and interordinal phylogeny inferred from nuclear small-subunit ribosomal DNA sequences (Crustacea: Malacostraca: Peracarida)" (PDF). Proceedings of the Biological Society of Washington 118 (1): 117–157. doi:10.2988/0006-324X(2005)118[117:PMAIPI]2.0.CO;2.
- K. Meland & E. Willassen (2007). "The disunity of "Mysidacea" (Crustacea)". Molecular Phylogenetics and Evolution 44 (3): 1083–1104. doi:10.1016/j.ympev.2007.02.009. PMID 17398121.
- Frederick R. Schram (1986). Crustacea. Oxford University Press. ISBN 0-19-503742-1.
- J. J. Torres & J. J. Childress (1985). "Respiration and chemical composition of the bathypelagic euphausiid Bentheuphausia amblyops". Marine Biology 87 (3): 267–272. doi:10.1007/BF00397804.
- Volker Siegel (2011). "Thysanoessa Brandt, 1851". World Register of Marine Species. Retrieved June 18, 2011.
- J. Mauchline & L. R. Fisher (1969). The Biology of Euphausiids. Advances in Marine Biology 7. Academic Press. ISBN 978-7-7708-3615-2.
- Jaime Gómez-Gutiérrez & Carlos J. Robinson (2005). "Embryonic, early larval development time, hatching mechanism and interbrood period of the sac-spawning euphausiid Nyctiphanes simplex Hansen". Journal of Plankton Research 27 (3): 279–295. doi:10.1093/plankt/fbi003.
- S. N. Jarman, N. G. Elliott, S. Nicol & A. McMinn (2002). "Genetic differentiation in the Antarctic coastal krill Euphausia crystallorophias". Heredity 88 (4): 280–287. doi:10.1038/sj.hdy.6800041. PMID 11920136.
- R. Escribano, V. Marin & C. Irribarren (2000). "Distribution of Euphausia mucronata at the upwelling area of Peninsula Mejillones, northern Chile: the influence of the oxygen minimum layer". Scientia Marina 64 (1): 69–77. doi:10.3989/scimar.2000.64n169.
- P. Brueggeman. "Euphausia crystallorophias". Underwater Field Guide to Ross Island & McMurdo Sound, Antarctica. University of California, San Diego.
- "Krill, Euphausia superba". MarineBio.org. Retrieved February 25, 2009.
- J. A. Kirkwood (1984). "A Guide to the Euphausiacea of the Southern Ocean". ANARE Research Notes 1: 1–45.
- A. Sala, M. Azzali & A. Russo (2002). "Krill of the Ross Sea: distribution, abundance and demography of Euphausia superba and Euphausia crystallorophias during the Italian Antarctic Expedition (January–February 2000)". Scientia Marina 66 (2): 123–133. doi:10.3989/scimar.2002.66n2123.
- G. W. Hosie, M. Fukuchi & S. Kawaguchi (2003). "Development of the Southern Ocean Continuous Plankton Recorder survey". Progress in Oceanography 58 (2–4): 263–283. doi:10.1016/j.pocean.2003.08.007.
- E. Gaten. "Meganyctiphanes norvegica". University of Leicester. Retrieved February 25, 2009.
- E. Brinton (1953). "Thysanopoda spinicauda, a new bathypelagic giant euphausiid crustacean, with comparative notes on T. cornuta and T. egregia". Journal of the Washington Academy of Sciences 43: 408–412.
- "Euphausiacea". Tasmanian Aquaculture & Fisheries Institute. Retrieved June 6, 2010.
- O. Shimomura (1995). "The roles of the two highly unstable components F and P involved in the bioluminescence of euphausiid shrimps". Journal of Bioluminescence and Chemiluminescence 10 (2): 91–101. doi:10.1002/bio.1170100205. PMID 7676855.
- J. C. Dunlap, J. W. Hastings & O. Shimomura (1980). "Crossreactivity between the light-emitting systems of distantly related organisms: novel type of light-emitting compound". Proceedings of the National Academy of Sciences 77 (3): 1394–1397. doi:10.1073/pnas.77.3.1394. JSTOR 8463. PMC 348501. PMID 16592787.
- P. J. Herring, E. A. Widder (2001). "Bioluminescence in Plankton and Nekton". In J. H. Steele, S. A. Thorpe & K. K. Turekian. Encyclopedia of Ocean Science 1. Academic Press, San Diego. pp. 308–317. ISBN 0-12-227430-X.
- S. M. Lindsay & M. I. Latz (1999). "Experimental evidence for luminescent countershading by some euphausiid crustaceans". American Society of Limnology and Oceanography (ASLO) Aquatic Sciences Meeting. Santa Fe.
- Sönke Johnsen (2005). "The Red and the Black: bioluminescence and the color of animals in the deep sea" (PDF). Integrative and Comparative Biology 4 (2): 234–246. doi:10.1093/icb/45.2.234.
- G. C. Cripps & A. Atkinson (2000). "Fatty acid composition as an indicator of carnivory in Antarctic krill, Euphausia superba". Canadian Journal of Fisheries and Aquatic Sciences 57 (S3): 31–37. doi:10.1139/f00-167.
- Olav Saether, Trond Erling Ellingsen & Viggo Mohr (1986). "Lipids of North Atlantic krill" (PDF). Journal of Lipid Research 27 (3): 274–285. PMID 3734626.
- M. J. Schramm (October 10, 2007). "Tiny Krill: Giants in Marine Food Chain". NOAA National Marine Sanctuary Program. Retrieved June 4, 2010.
- J. Weier (1999). "Changing currents color the Bering Sea a new shade of blue". NOAA Earth Observatory. Retrieved June 15, 2005.
- R. D. Brodeur, G. H. Kruse, P. A. Livingston, G. Walters, J. Ianelli, G. L. Swartzman, M. Stepanenko & T. Wyllie-Echeverria (1998). Draft Report of the FOCI International Workshop on Recent Conditions in the Bering Sea. NOAA. pp. 22–26.
- Rusty Dornin (July 6, 1997). "Antarctic krill populations decreasing". CNN. Retrieved June 18, 2011.
- J. Roach (17 July 2003). "Scientists discover mystery krill killer". National Geographic News.
- J. Gómez-Gutiérrez, W. T. Peterson, A. de Robertis, R. D. Brodeur (2003). "Mass mortality of krill caused by parasitoid ciliates". Science 301 (5631): 339. doi:10.1126/science.1085164. PMID 12869754.
- J. D. Shields & J. Gómez-Gutiérrez (1996). "Oculophryxus bicaulis, a new genus and species of dajid isopod parasitic on the euphausiid Stylocheiron affine Hansen". International Journal for Parasitology 26 (3): 261–268. doi:10.1016/0020-7519(95)00126-3.
- M. D. Knight (1984). "Variation in larval morphogenesis within the Southern California Bight population of Euphausia pacifica from Winter through Summer, 1977–1978". CalCOFI Report XXV.
- "Euphausia superba". Species factsheet. Food and Agriculture Organization. Retrieved June 4, 2010.
- R. M. Ross & L. B. Quetin (1986). "How productive are Antarctic krill?". BioScience 36 (4): 264–269. doi:10.2307/1310217. JSTOR 1310217.
- Janine Cuzin-Roudy (2000). "Seasonal reproduction, multiple spawning, and fecundity in northern krill, Meganyctiphanes norvegica, and Antarctic krill, Euphausia superba". Canadian Journal of Fisheries and Aquatic Sciences 57 (S3): 6–15. doi:10.1139/f00-165.
- J. Gómez-Gutiérrez (2002). "Hatching mechanism and delayed hatching of the eggs of three broadcast spawning euphausiid species under laboratory conditions". Journal of Plankton Research 24 (12): 1265–1276. doi:10.1093/plankt/24.12.1265.
- E. Brinton, M. D. Ohman, A. W. Townsend, M. D. Knight & A. L. Bridgeman (2000). Euphausiids of the World Ocean. World Biodiversity Database CD-ROM Series, Springer Verlag. ISBN 3-540-14673-3.
- F. Buchholz (2003). "Experiments on the physiology of Southern and Northern krill, Euphausia superba and Meganyctiphanes norvegica, with emphasis on moult and growth – a review". Marine and Freshwater Behaviour and Physiology 36 (4): 229–247. doi:10.1080/10236240310001623376.
- H.-C. Shin & S. Nicol (2002). "Using the relationship between eye diameter and body length to detect the effects of long-term starvation on Antarctic krill Euphausia superba". Marine Ecology Progress Series 239: 157–167. doi:10.3354/meps239157.
- B. Marinovic, & M. Mangel (1999). "Krill can shrink as an ecological adaptation to temporarily unfavourable environments". Ecology Letters 2: 338–343.
- J. G. Gómez (1995). "Distribution patterns, abundance and population dynamics of the euphausiidsNyctiphanes simplex and Euphausia eximia off the west coast of Baja California, Mexico" (PDF). Marine Ecology Progress Series 119: 63–76. doi:10.3354/meps119063.
- U. Kils & P. Marshall (1995). "Der Krill, wie er schwimmt und frisst – neue Einsichten mit neuen Methoden ("The Antarctic krill – how it swims and feeds – new insights with new methods")". In I. Hempel & G. Hempel. Biologie der Polarmeere – Erlebnisse und Ergebnisse (Biology of the Polar Oceans Experiences and Results). Fischer Verlag. pp. 201–210. ISBN 3-334-60950-2.
- R. Piper (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Press. ISBN 0-313-33922-8.
- Gandomi, A.H.; Alavi, A.H. (2012). "Krill Herd Algorithm: A New Bio-Inspired Optimization Algorithm". Communications in Nonlinear Science and Numerical Simulation 17 (12): 4831. doi:10.1016/j.cnsns.2012.05.010.
- J. S. Jaffe, M. D. Ohmann & A. de Robertis (1999). "Sonar estimates of daytime activity levels of Euphausia pacifica in Saanich Inlet". Canadian Journal of Fisheries and Aquatic Sciences 56 (11): 2000–2010. doi:10.1139/cjfas-56-11-2000.
- Geraint A. Tarling & Magnus L. Johnson (2006). "Satiation gives krill that sinking feeling". Current Biology 16 (3): 83–84. doi:10.1016/j.cub.2006.01.044. PMID 16461267.
- Dan Howard (2001). "Krill". In Herman A. Karl, John L. Chin, Edward Ueber, Peter H. Stauffer & James W. Hendley II. Beyond the Golden Gate – Oceanography, Geology, Biology, and Environmental Issues in the Gulf of the Farallones. United States Geological Survey. pp. 133–140. Circular 1198. Retrieved October 8, 2011.
- D. Howard. "Krill in Cordell Bank National Marine Sanctuary". National Oceanic and Atmospheric Administration. Retrieved June 15, 2005.
- David A. Demer & Stéphane G. Conti (2005). "New target-strength model indicates more krill in the Southern Ocean". ICES Journal of Marine Science 62 (1): 25–32. doi:10.1016/j.icesjms.2004.07.027.
- U. Kils (1982). "Swimming behavior, swimming performance and energy balance of Antarctic krill Euphausia superba". BIOMASS Scientific Series 3, BIOMASS Research Series: 1–122.
- Schiermeier, Quirin (September 2, 2010). "Ecologists fear Antarctic krill crisis". Nature 467 (15): 15. doi:10.1038/467015a. Retrieved 9 December 2011.
- "Harvested species: krill (Eupausia superba)". Convention for the Conservation of Antarctic Marine Living Resources. Retrieved June 20, 2005.
- Minturn J. Wright (1987). "The Ownership of Antarctica, its Living and Mineral Resources". Journal of Law and the Environment 4 (2): 49–78.
- S. Nicol & J. Foster (2003). "Recent trends in the fishery for Antarctic krill". Aquatic Living Resources 16: 42–45. doi:10.1016/S0990-7440(03)00004-4.
- K. Haberman (26 February 1997). "Answers to miscellaneous questions about krill". NASA. Retrieved September 6, 2007.
- Boden, Brian P.; Johnson, Martin W.; Brinton, Edward: Euphausiacea (Crustacea) of the North Pacific. Bulletin of the Scripps Institution of Oceanography. Volume 6 Number 8, 1955.
- Brinton, Edward: Euphausiids of Southeast Asian waters. Naga Report volume 4, part 5. La Jolla: University of California, Scripps Institution of Oceanography, 1975.
- Conway, D. V. P.; White, R. G.; Hugues-Dit-Ciles, J.; Galienne, C. P.; Robins, D. B.: Guide to the coastal and surface zooplankton of the South-Western Indian Ocean, Order Euphausiacea, Occasional Publication of the Marine Biological Association of the United Kingdom No. 15, Plymouth, UK, 2003.
- Everson, I. (ed.): Krill: biology, ecology and fisheries. Oxford, Blackwell Science; 2000. ISBN 0-632-05565-0.
- Mauchline, J.: Euphausiacea: Adults, Conseil International pour l'Exploration de la Mer, 1971. Identification sheets for adult krill with many line drawings. PDF file, 2 Mb.
- Mauchline, J.: Euphausiacea: Larvae, Conseil International pour l'Exploration de la Mer, 1971. Identification sheets for larval stages of krill with many line drawings. PDF file, 3 Mb.
- Tett, P.: The biology of Euphausiids, lecture notes from a 2003 course in Marine Biology from Napier University.
- Tett, P.: Bioluminescence, lecture notes from the 1999/2000 edition of that same course.
- Webcam of Krill Aquarium at Australian Antarctic Division
- The dictionary definition of krill at Wiktionary
- 'Antarctic Energies' animation by Lisa Roberts