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[[File:Antarctic krill.jpg |thumb|right|Antarctic krill]]
'''Antarctic krill''' ('''''Euphausia superba''''') is a species of [[krill]] found in the [[Antarctica|Antarctic]] waters of the [[Southern Ocean]]. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic meter. It feeds directly on minute phytoplankton, thereby using the primary production energy that the phytoplankton originally derived from the sun in order to sustain their open ocean life cycle.
'''Antarctic krill''' ('''''Euphausia superba''''') is a species of [[krill]] found in the [[Antarctica|Antarctic]] waters of the [[Southern Ocean]]. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic meter. It feeds directly on minute phytoplankton, thereby using the primary production energy that the phytoplankton originally derived from the sun in order to sustain their open ocean life cycle.


It grows to a length of 6 cm, weighs up to 2 grams, and can live for up to six years. It is a key species in the Antarctic ecosystem and is, in terms of biomass, probably the most abundant animal species on the planet (approximately 500 million tonnes, corresponding to 300 to 400 trillion individuals).
It grows to a length of 6 cm, weighs up to 2 grams, and can live for up to six years. It is a key species in the Antarctic ecosystem and is, in terms of biomass, probably the most abundant animal species on the planet (approximately 500 million tonnes, corresponding to 300 to 400 trillion individuals).
In 2019, the Antarctic krill was featured on a [[Stamps of Westarctica|stamp of Westarctica]] to celebrate its role as a vital part of the Antarctic ecosystem.


==Life cycle==
==Life cycle==
The main spawning season of Antarctic krill is from January to March, both above the [[continental shelf]] and also in the upper region of deep sea oceanic areas. In the typical way of all krill, the male attaches a [[spermatophore]] to the genital opening of the female. For this purpose, the first [[pleopod]]s (legs attached to the abdomen) of the male are constructed as mating tools. Females lay 6,000–10,000 [[egg (biology)|eggs]] at one time. They are [[fertilisation|fertilised]] as they pass out of the genital opening.<ref name="RQ86">{{cite journal |author1=Robin M. Ross |author2=Langdon B. Quetin |year=1986 |title=How productive are Antarctic krill? |journal=[[BioScience]] |volume=36 |issue=4 |pages=264–269 |jstor=1310217 |doi=10.2307/1310217}}</ref>
The main spawning season of Antarctic krill is from January to March, both above the continental shelf and also in the upper region of deep sea oceanic areas. In the typical way of all krill, the male attaches a spermatophore to the genital opening of the female. For this purpose, the first pleopods (legs attached to the abdomen) of the male are constructed as mating tools. Females lay 6,000–10,000 eggs at one time. They are fertilized as they pass out of the genital opening.


According to the classical hypothesis of Marriosis De' Abrtona,<ref name="Marr62">{{cite book |author=James William Slessor Marr |year=1962 |title=The natural history and geography of the Antarctic krill (''Euphausia superba'' Dana) |series="Discovery" Reports |volume=32 |pages=33–464}}</ref> derived from the results of the expedition of the famous British research vessel [[RRS Discovery|RRS ''Discovery'']], egg development then proceeds as follows: [[gastrulation]] (development of egg into embryo) sets in during the descent of the {{convert|0.6|mm|adj=on|abbr=on}} eggs on the shelf at the bottom, in oceanic areas in depths around {{convert|2000|-|3000|m}}. The egg hatches as a [[nauplius (larva)|nauplius larva]]; once this has moulted into a metanauplius, the young animal starts migrating towards the surface in a migration known as developmental ascent.<ref>{{cite journal|author1=Irmtraut Hempel |author2=Gotthilf Hempel |year=1986 |title=Field observations on the developmental ascent of larval ''Euphausia superba'' (Crustacea) |journal=[[Polar Biology]] |volume=6 |issue=2 |pages=121–126 |doi=10.1007/BF00258263}}</ref>
At 15 mm, the juvenile krill resembles the habitus of the adults. Krill reach maturity after two to three years. Like all crustaceans, krill must moult in order to grow. Approximately every 13 to 20 days, krill shed their chitinous exoskeleton and leave it behind as exuvia.
 
The next two larval stages, termed second nauplius and metanauplius, still do not eat but are nourished by the remaining [[yolk]]. After three weeks, the young krill has finished the ascent. They can appear in enormous numbers counting 2 per litre in {{convert|60|m|abbr=on}} water depth. Growing larger, additional larval stages follow (second and third calyptopis, first to sixth furcilia). They are characterised by increasing development of the additional legs, the compound eyes and the setae (bristles). At {{convert|15|mm|abbr=on}}, the juvenile krill resembles the habitus of the adults. Krill reach maturity after two to three years. Like all [[crustacean]]s, krill must [[ecdysis|moult]] in order to grow. Approximately every 13 to 20 days, krill shed their [[chitin]]ous [[exoskeleton]] and leave it behind as [[exuvia]].
 
[[File:Kilsheadkils.jpg|thumb|The head of Antarctic krill. Observe the [[bioluminescence|bioluminescent organ]] at the [[eyestalk]] and the [[nerve]]s visible in the [[Antenna (biology)|antennae]], the [[gastric mill]], the filtering net at the [[thoracopod]]s and the rakes at the tips of the thoracopods.]]


==Food==
==Food==
The gut of ''E. superba'' can often be seen shining green through the animal's transparent skin, an indication that this species feeds predominantly on [[phytoplankton]]—especially very small [[diatom]]s (20&nbsp;[[micrometre|μm]]), which it filters from the water with a ''feeding basket''.<ref>{{cite web |url=http://www.ecoscope.com/krill/filter/index.htm |title=Antarctic krill ''Euphausia superba'' filter of thoracopods |publisher=Ecoscope.com |author=Uwe Kils}}</ref> The glass-like shells of the [[diatom]]s are cracked in the "[[gastric mill]]" and then digested in the [[hepatopancreas]]. The krill can also catch and eat [[copepod]]s, [[amphipod]]s and other small [[zooplankton]]. The gut forms a straight tube; its digestive efficiency is not very high and therefore a lot of [[carbon]] is still present in the [[feces]].
The gut of ''E. superba'' can often be seen shining green through the animal's transparent skin, an indication that this species feeds predominantly on phytoplankton—especially very small diatoms, which it filters from the water with a ''feeding basket''. The glass-like shells of the diatoms are cracked in the "gastric mill" and then digested in the hepatopancreas. The krill can also catch and eat copepods, amphipods and other small zooplankton. The gut forms a straight tube; its digestive efficiency is not very high and therefore a lot of carbon is still present in the feces.


In [[aquarium|aquaria]], krill have been observed to eat each other. When they are not fed in aquaria, they shrink in size after [[ecdysis|moulting]], which is exceptional for animals the size of krill. It is likely that this is an [[adaptation]] to the seasonality of their food supply, which is limited in the dark winter months under the ice. However, the animal's compound eyes do not shrink, and so the ratio between eye size and body length has thus been found to be a reliable indicator of starvation.<ref name="SN02">{{cite journal |author1=Hyoung-Chul Shin |author2=Stephen Nicol |url=http://www.int-res.com/abstracts/meps/v239/p157-167/ |title=Using the relationship between eye diameter and body length to detect the effects of long-term starvation on Antarctic krill ''Euphausia superba'' |journal=[[Marine Ecology Progress Series]] |volume=239 |pages=157–167 |year=2002 |doi=10.3354/meps239157|bibcode=2002MEPS..239..157S }}</ref>
In aquariums, krill have been observed to eat one another. When they are not fed in aquariums, they shrink in size after moulting, which is exceptional for animals the size of krill. It is likely that this is an adaptation to the seasonality of their food supply, which is limited in the dark winter months under the ice. However, the animal's compound eyes do not shrink, and so the ratio between eye size and body length has thus been found to be a reliable indicator of starvation.


===Filter feeding===
===Filter feeding===
{{Main|Filter feeder}}
Antarctic krill directly use the minute phytoplankton cells, which no other animal of krill size can do. This is accomplished through filter feeding, using the krill's highly developed front legs, providing for an efficient filtering apparatus: the six thoracopods (legs attached to the thorax) form a very effective "feeding basket" used to collect phytoplankton from the open water. In the finest areas the openings in this basket are extremely tiny. In lower food concentrations, the feeding basket is pushed through the water for over half a metre in an opened position, and then the algae are combed to the mouth opening with special setae (bristles) on the inner side of the thoracopods.


Antarctic krill directly use the minute [[phytoplankton]] cells, which no other animal of krill size can do. This is accomplished through [[filter feeding]], using the krill's highly developed front legs, providing for an efficient filtering apparatus:<ref name="Kils83">{{cite book |series=Berichte zur Polarforschung|author=Uwe Kils |chapter=Swimming and feeding of Antarctic krill, ''Euphausia superba'' – some outstanding energetics and dynamics - some unique morphological details |title=On the biology of krill ''Euphausia superba'' – Proceedings of the Seminar and Report of Krill Ecology Group |publisher=[[Alfred Wegener Institute for Polar and Marine Research]] |volume=Special Issue 4 |year=1983 |editor=S. B. Schnack |pages=130–155}}</ref> the six [[thoracopod]]s (legs attached to the [[thorax]]) form a very effective "feeding basket" used to collect phytoplankton from the open water. In the finest areas the openings in this basket are only 1&nbsp;μm in diameter. In lower food concentrations, the feeding basket is pushed through the water for over half a metre in an opened position, and then the algae are combed to the mouth opening with special [[setae]] (bristles) on the inner side of the thoracopods.
===Ice algae raking===
[[File:Krillicekils.jpg|thumb|Antarctic krill feeding on [[ice algae]]. The surface of the ice on the left side is coloured green by the algae.]]
Antarctic krill can scrape off the green lawn of [[ice algae]] from the underside of the [[pack ice]]. Krill have developed special rows of rake-like setae at the tips of the thoracopods, and graze the ice in a zig-zag fashion. One krill can clear an area of a square foot in about 10 minutes. It is relatively new knowledge that the film of ice algae is very well developed over vast areas, often containing much more carbon than the whole water column below. Krill find an extensive energy source here, especially in the spring.
 
===Ice-algae raking===
Antarctic krill can scrape off the green lawn of [[ice-algae]] from the underside of the [[pack ice]].<ref>{{cite web |url=http://www.ecoscope.com/icecave2.htm |title=Antarctic krill ''Euphausia superba'' in ice cave |publisher=Ecoscope.com |author1=Peter Marschall |author2=Uwe Kils }}</ref><ref name="Mar88">{{cite journal |author=Hans-Peter Marschall |title=The overwintering strategy of Antarctic krill under the pack ice of the Weddell Sea |journal=[[Polar Biology]] |volume=9 |issue=2 |pages=129–135 |year=1988 |doi=10.1007/BF00442041}}</ref> Krill have developed special rows of rake-like setae at the tips of the [[thoracopod]]s, and graze the ice in a zig-zag fashion. One krill can clear an area of a square foot in about 10&nbsp;minutes (1.5&nbsp;cm<sup>2</sup>/s). It is relatively new knowledge that the film of ice algae is very well developed over vast areas, often containing much more carbon than the whole water column below. Krill find an extensive energy source here, especially in the spring.
 
===Biological pump and carbon sequestration===
[[File:Krillspitballkils3.jpg|thumb|''In situ'' image taken with an [[ecoSCOPE]]. A green spit ball is visible in the lower right of the image and a green fecal string in the lower left.]]
Krill are thought to undergo between one and three vertical migrations from mixed surface waters to depth each day.<ref>{{cite journal |author1=Geraint A. Tarling |author2=Magnus L. Johnson |title=Satiation gives krill that sinking feeling |journal=[[Current Biology]] |volume=16 |issue=3 |pages=83–84 |year=2006 |pmid=16461267 |doi=10.1016/j.cub.2006.01.044}}</ref> The krill is a very untidy feeder, and it often spits out aggregates of [[phytoplankton]] (spit balls) containing thousands of cells sticking together. It also produces fecal strings that still contain significant amounts of [[carbon]] and the [[glass]] shells of the [[diatom]]s. Both are heavy and sink very fast into the abyss. This process is called the [[biological pump]]. As the waters around [[Antarctica]] are very deep ({{convert|2000|-|4000|m|disp=or}}), they act as a [[carbon dioxide sink]]: this process exports large quantities of carbon (fixed [[carbon dioxide]], CO<sub>2</sub>) from the biosphere and [[Carbon Sequestration|sequesters]] it for about 1,000&nbsp;years.
 
If the phytoplankton is consumed by other components of the pelagic ecosystem, most of the carbon remains in the upper strata. There is speculation that this process is one of the largest biofeedback mechanisms of the planet, maybe the most sizable of all, driven by a gigantic biomass. Still more research is needed to quantify the Southern Ocean ecosystem.


==Biology==
==Biology==


===Bioluminescence===
===Bioluminescence===
[[File:Bioluminescencekils.jpg|thumb|Watercolour of bioluminescent krill]]
[[File:Kilsheadkils.jpg|thumb|right|The head of Antarctic krill]]
Krill are often referred to as ''light-shrimp'' because they can emit light, produced by [[bioluminescence|bioluminescent]] organs. These organs are located on various parts of the individual krill's body: one pair of organs at the [[eyestalk]] (cf. the image of the head above), another pair on the hips of the second and seventh [[thoracopod]]s, and singular organs on the four [[pleonsternite]]s. These light organs emit a yellow-green light periodically, for up to 2–3&nbsp;s. They are considered so highly developed that they can be compared with a torchlight: a concave reflector in the back of the organ and a lens in the front guide the light produced, and the whole organ can be rotated by muscles. The function of these lights is not yet fully understood; some hypotheses have suggested they serve to compensate the krill's shadow so that they are not visible to predators from below; other speculations maintain that they play a significant role in [[mating]] or [[Shoaling and schooling|schooling]] at night.
Krill are often referred to as ''light-shrimp'' because they can emit light, produced by bioluminescent organs. These organs are located on various parts of the individual krill's body: one pair of organs at the eyestalk, another pair on the hips of the second and seventh thoracopods, and singular organs on the four pleonsternites. These light organs emit a yellow-green light periodically, for up to 2–3 seconds. They are considered so highly developed that they can be compared with a torchlight: a concave reflector in the back of the organ and a lens in the front guide the light produced, and the whole organ can be rotated by muscles. The function of these lights is not yet fully understood; some hypotheses have suggested they serve to compensate the krill's shadow so that they are not visible to predators from below; other speculations maintain that they play a significant role in mating or schooling at night.
 
The krill's bioluminescent organs contain several fluorescent substances. The major component has a maximum [[fluorescence]] at an excitation of 355&nbsp;[[Nanometre|nm]] and emission of 510&nbsp;nm.<ref name="HS01">{{cite conference |author1=H. Rodger Harvey |author2=Se-Jong Ju |url=http://www.ccpo.odu.edu/Research/globec/3sciinvest/harvey.htm |title=Biochemical determination of age structure and diet history of the Antarctic krill, ''Euphausia superba,'' during austral winter |conference=Third U.S. Southern Ocean GLOBEC Science Investigator Meeting |location=Arlington |date=10–12 December 2001}}</ref>


[[File:Caridoid escape reaction.gif|thumb|Lobstering krill]]
The krill's bioluminescent organs contain several fluorescent substances. The major component has a maximum fluorescence at an excitation of 355 nanometers.


===Escape reaction===
===Escape reaction===
Krill use an [[escape reaction]] to evade [[predator]]s, swimming backwards very quickly by flipping their rear ends. This swimming pattern is also known as [[Caridoid escape reaction|lobstering]]. Krill can reach speeds of over {{convert|0.6|m/s}}.<ref name="Kils82">{{cite book|author=Uwe Kils |url=http://www.ecoscope.com/biomass3.htm |title=Swimming behaviour, swimming performance and energy balance of Antarctic krill ''Euphausia superba'' |series=BIOMASS Scientific Series |volume=3 |pages=1–122 |year=1982 |isbn= |publisher=}}</ref> The [[Induction period|trigger]] time to optical [[stimulus (physiology)|stimulus]] is, despite the low temperatures, only 55&nbsp;[[millisecond|ms]].
Krill use an escape reaction to evade predators, swimming backwards very quickly by flipping their rear ends. This swimming pattern is also known as "lobstering". Krill can reach speeds of over 0.6 meters per second.


==Geographic distribution==
==Geographic distribution==
[[File:krilldistribution.jpg|thumb|Krill distribution on a [[NASA]] [[SeaWIFS]] image&nbsp;– the main concentrations are in the [[Scotia Sea]] at the [[Antarctic Peninsula]]]]
[[File:Krilldistribution.jpg|thumb|right|Distribution of krill around Antarctica]]
Antarctic krill has a circumpolar distribution, being found throughout the [[Southern Ocean]], and as far north as the [[Antarctic Convergence]].<ref name="MSIP">{{cite web |title=''Euphausia superba'' |work=Euphausiids of the World Ocean |publisher=Marine Species Identification Portal |accessdate=May 20, 2011 |url=http://species-identification.org/species.php?species_group=euphausiids&id=43}}</ref> At the Antarctic Convergence, the cold Antarctic surface water submerges below the warmer [[subantarctic]] waters. This front runs roughly at [[55th parallel south|55° south]]; from there to the continent, the Southern Ocean covers 32 million square kilometres. This is 65 times the size of the [[North Sea]]. In the winter season, more than three-quarters of this area become covered by ice, whereas {{convert|24000000|km2}} become ice free in summer. The water temperature fluctuates at {{convert|-1.3|-|3|C|F}}.
Antarctic krill have a circumpolar distribution, being found throughout the [[Southern Ocean]], and as far north as the [[Antarctic Convergence]]. At the Antarctic Convergence, the cold Antarctic surface water submerges below the warmer subantarctic waters. This front runs roughly at 55° south; from there to the continent, the Southern Ocean covers 32 million square kilometers. This is 65 times the size of the North Sea. In the winter season, more than three-quarters of this area become covered by ice, whereas 24,000,000 km2 become ice free in summer. The water temperature fluctuates at -1.3 C (3 F).


The waters of the Southern Ocean form a system of currents. Whenever there is a [[Antarctic Circumpolar Current|West Wind Drift]], the surface strata travels around Antarctica in an easterly direction. Near the continent, the [[Antarctic Coastal Current|East Wind Drift]] runs counterclockwise. At the front between both, large [[eddy (fluid dynamics)|eddies]] develop, for example, in the [[Weddell Sea]]. The krill swarms swim with these water masses, to establish one single stock all around Antarctica, with gene exchange over the whole area. Currently, there is little knowledge of the precise migration patterns since individual krill cannot yet be tagged to track their movements. The largest shoals are visible from space and can be tracked by satellite.<ref>Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. {{ISBN|978-0-565-09243-6}}.</ref> One swarm covered an area of {{convert|450|km2|mi2|abbr=off|sp=us}} of ocean, to a depth of {{convert|200|m|ft|abbr=off|sp=us}} and was estimated to contain over 2 million tons of krill.<ref name="Hoare, Ben 2009 p. 107">Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. {{ISBN|978-0-565-09243-6}}</ref> Recent research suggests that krill do not simply drift passively in these currents but actually modify them.<ref name="Hoare, Ben 2009 p. 107"/> By moving vertically through the ocean on a 12-hour cycle, the swarms play a major part in mixing deeper, nutrient-rich water with nutrient-poor water at the surface.<ref name="Hoare, Ben 2009 p. 107"/>
The waters of the Southern Ocean form a system of currents. Whenever there is a West Wind Drift, the surface strata travels around Antarctica in an easterly direction. Near the continent, the East Wind Drift runs counterclockwise. At the front between both, large eddies develop, for example, in the [[Weddell Sea]]. The krill swarms swim with these water masses, to establish one single stock all around Antarctica, with gene exchange over the whole area. Currently, there is little knowledge of the precise migration patterns since individual krill cannot yet be tagged to track their movements. The largest shoals are visible from space and can be tracked by satellite. One swarm covered an area of 450 km2 of ocean, to a depth of 200 m and was estimated to contain over 2 million tons of krill. Recent research suggests that krill do not simply drift passively in these currents but actually modify them.


==Ecology==
==Ecology==
Antarctic krill is the [[keystone species]] of the [[Antarctic]] ecosystem beyond the coastal shelf,<ref>{{cite book |editors=Guido di Prisco, Cinzia Verde |series=Adaptation and Evolution in Marine Environments |volume=1 |title=The Impacts of Global Change on Biodiversity |publisher=Springer Science & Business Media |year=2012 |isbn=9783642273513 |chapter=Sea-ice interactions with polar fish: focus on the Antarctic silverfish life history |authors=Mario Vacchi, Philippe Koubbi, Laura Ghigliotti & Eva Pisano |pages=51–73<!-- at p. 63 --> |doi=10.1007/978-3-642-27352-0_4}}</ref> and provides an important food source for [[whale]]s, [[Seal (mammal)|seals]], [[leopard seal]]s, [[fur seal]]s, [[crabeater seal]]s, [[squid]], [[Notothenioidei|icefish]], [[penguin]]s, [[albatross]]es and many other species of [[bird]]s. Crabeater seals have even developed special teeth as an adaptation to catch this abundant food source: its unusual multilobed teeth enable this species to sieve krill from the water. Its dentition looks like a perfect strainer, but how it operates in detail is still unknown. Crabeaters are the most abundant seal in the world; 98% of their diet is made up of '' E. superba''. These seals consume over 63 million [[tonne]]s of krill each year.<ref name="Bon95">{{cite book |author=B. Bonner |chapter=Birds and Mammals – Antarctic Seals |pages=202–222 |editor=R. Buckley |title=Antarctica |publisher=[[Pergamon Press]] |year=1995 |isbn=0-08-028881-2}}</ref> [[Leopard seal]]s have developed similar teeth (45% krill in diet). All seals consume 63–130 million tonnes, all whales 34–43 million tonnes, birds 15–20 million tonnes, squid 30–100 million tonnes, and fish 10–20 million tonnes, adding up to 152–313 million tonnes of krill consumption each year.<ref name="MH89">{{cite book |author1=D. G. M. Miller |author2=I. Hampton |year=1989 |title=Biology and ecology of the Antarctic krill (''Euphausia superba'' Dana): a review |series=BIOMASS Scientific Series |volume=9 |pages=1–66 |isbn=0-948277-09-2 |publisher=[[Scientific Committee on Antarctic Research]]}}</ref>
Antarctic krill are the keystone species of the [[Antarctica|Antarctic]] ecosystem beyond the coastal shelf, and provides an important food source for whales, [[leopard seal]]s, fur seals, [[crabeater seal]]s, [[glacial squid]], [[Crocodile icefish|icefish]], penguins, albatrosses and many other species of birds.
 
Crabeater seals have even developed special teeth as an adaptation to catch this abundant food source: its unusual multilobed teeth enable this species to sieve krill from the water. Its dentition looks like a perfect strainer, but how it operates in detail is still unknown. Crabeaters are the most abundant seal in the world; 98% of their diet is made up of '' E. superba''. These seals consume over 63 million tonnes of krill each year. [[Leopard seal]]s have developed similar teeth and consume approximately 45% krill in their diets. All seals combined consume 63–130 million tonnes, all whales 34–43 million tonnes, birds 15–20 million tonnes, squid 30–100 million tonnes, and fish 10–20 million tonnes, adding up to 152–313 million tonnes of krill consumption each year.


The size step between krill and its prey is unusually large: generally it takes three or four steps from the 20&nbsp;μm small [[phytoplankton]] cells to a krill-sized organism (via small [[copepod]]s, large copepods, [[mysid]]s to 5&nbsp;cm [[fish]]).<ref name="KK79"/> ''E. superba'' lives only in the Southern Ocean. In the North Atlantic, ''[[Meganyctiphanes norvegica]]'' and in the Pacific, ''[[Euphausia pacifica]]'' are the dominant species.
The size step between krill and its prey is unusually large: generally it takes three or four steps from the small phytoplankton cells to a krill-sized organism (via small copepods, large copepods, mysids to 5 cm fish. ''E. superba'' lives only in the [[Southern Ocean]]. In the North Atlantic, ''Meganyctiphanes norvegica'' and in the Pacific, ''Euphausia pacifica'' are the dominant species.


===Biomass and production===
===Biomass and production===
The [[Biomass (ecology)|biomass]] of Antarctic krill is estimated to be 125 to 725 million [[tonne]]s.<ref name="FAO05">{{cite web |publisher=[[Food and Agriculture Organization]] |url=http://www.fao.org/figis/servlet/species?fid=3393 |title=Species Fact Sheet ''Euphausia superba'' |accessdate=June 16, 2005}}</ref> The reason Antarctic krill are able to build up such a high biomass and production is that the waters around the icy Antarctic continent harbour one of the largest [[plankton]] assemblages in the world, possibly ''the'' largest. The ocean is filled with [[phytoplankton]]; as the water rises from the depths to the light-flooded surface, it brings [[nutrient]]s from all of the world's oceans back into the [[photic zone]] where they are once again available to living organisms.
The biomass of Antarctic krill is estimated to be 125 to 725 million tonnes. The reason Antarctic krill are able to build up such a high biomass and production is that the waters around the icy Antarctic continent harbor one of the largest plankton assemblages in the world, possibly ''the'' largest. The ocean is filled with phytoplankton; as the water rises from the depths to the light-flooded surface, it brings nutrients from all of the world's oceans back into the photic zone where they are once again available to living organisms.


Thus [[primary production]]—the conversion of sunlight into organic biomass, the foundation of the food chain—has an annual carbon fixation of 1–2&nbsp;g/m<sup>2</sup> in the open ocean. Close to the ice it can reach 30–50&nbsp;g/m<sup>2</sup>. These values are not outstandingly high, compared to very productive areas like the [[North Sea]] or [[upwelling]] regions, but the area over which it takes place is enormous, even compared to other large primary producers such as [[rainforest]]s. In addition, during the Austral summer there are many hours of daylight to fuel the process. All of these factors make the plankton and the krill a critical part of the planet's ecocycle.
Thus primary production—the conversion of sunlight into organic biomass, the foundation of the food chain—has an annual carbon fixation of 1–2 g/m in the open ocean. Close to the ice it can reach 30–50 g/m. These values are not outstandingly high, compared to very productive areas like the North Sea or upwelling regions, but the area over which it takes place is enormous, even compared to other large primary producers such as rainforests. In addition, during the Austral summer there are many hours of daylight to fuel the process. All of these factors make the plankton and the krill a critical part of the planet's ecocycle.


===Decline with shrinking pack ice===
===Decline with shrinking pack ice===
[[File:Krillicekils.gif|thumb|Temperature and pack ice area over time, after data compiled by Loeb ''et al.'' 1997.<ref name="L+97">{{cite journal |author1=V. Loeb |author2=V. Siegel |author3=O. Holm-Hansen |author4=R. Hewitt |author5=W. Fraser |author6=W. Trivelpiece |author7=S. Trivelpiece |year=1997 |title=Effects of sea-ice extent and krill or salp dominance on the Antarctic food web |journal=[[Nature (journal)|Nature]] |volume=387 |issue=6636 |pages=897–900 |doi=10.1038/43174 |url=http://www.magazine.noaa.gov/stories/pdfs/loeb.nature.paper.1997.pdf |format=[[Portable Document Format|PDF]] |bibcode=1997Natur.387..897L}}</ref> The scale for the ice is inverted to demonstrate the correlation; the horizontal line is the freezing point—the oblique line the average of the temperature.]]
A possible decline in Antarctic krill biomass may have been caused by the reduction of the [[pack ice]] zone due to [[global warming]]. Antarctic krill, especially in the early stages of development, seem to need the pack ice structures in order to have a fair chance of survival. The pack ice provides natural cave-like features which the krill uses to evade their predators. In the years of low pack ice conditions the krill tend to give way to salps, a barrel-shaped free-floating filter feeder that also grazes on plankton.
A possible decline in Antarctic krill biomass may have been caused by the reduction of the [[pack ice]] zone due to [[global warming]].<ref name="Gr05">{{cite journal |author=Liza Gross |year=2005 |title=As the Antarctic ice pack recedes, a fragile ecosystem hangs in the balance |journal=[[PLoS Biology]] |volume=3 |issue=4 |page=e127 |doi=10.1371/journal.pbio.0030127 |pmid=15819605 |pmc=1074811}}</ref> Antarctic krill, especially in the early stages of development, seem to need the pack ice structures in order to have a fair chance of survival. The pack ice provides natural cave-like features which the krill uses to evade their predators. In the years of low pack ice conditions the krill tend to give way to [[salp]]s,<ref name="A+04">{{cite journal |author1=Angus Atkinson |author2=Volker Siegel |author3=Evgeny Pakhomov |author4=Peter Rothery |year=2004 |title=Long-term decline in krill stock and increase in salps within the Southern Ocean |journal=[[Nature (journal)|Nature]] |volume=432 |issue=7013 |pages=100–103 |doi=10.1038/nature02996 |pmid=15525989 |bibcode=2004Natur.432..100A}}</ref> a barrel-shaped free-floating [[filter feeder]] that also grazes on plankton.


===Ocean acidification===
===Ocean acidification===
Another challenge for Antarctic krill, as well as many calcifying organisms (corals, bivalve mussels, snails etc.), is the [[Ocean acidification|Acidification of the oceans]] caused by increasing levels of carbon dioxide.<ref name="ACECRC">{{cite book |authors=Australian Antarctic Climate and Ecosystems, Cooperative Research Centre  |lastauthoramp=yes |title=Position analysis: CO<sub>2</sub> emissions and climate change: OCEAN impacts and adaptation issues |issn=1835-7911 |year=2008}}</ref> Krill exoskeleton contains carbonate, which is susceptible to dissolution under low [[pH]] conditions. It has already been shown that increased carbon dioxide can disrupt the development of krill eggs and even prevent the juvenile krill from hatching, leading to future geographically widespread decreases in krill hatching success.<ref>{{cite journal |author1=So Kawaguchi |author2=Haruko Kurihara |author3=Robert King |author4=Lillian Hale |author5=Thomas Berli |author6=James P. Robinson |author7=Akio Ishida |author8=Masahide Wakita |author9=Patti Virtue |author10=Stephen Nicol |author11=Atsushi Ishimatsu |year=2011 |title=Will krill fare well under Southern Ocean acidification? |journal=[[Biology Letters]] |volume=7 |issue=2 |pages=288–291 |doi=10.1098/rsbl.2010.0777 |url=http://www.webpages.uidaho.edu/envs501/downloads/Kawaguchi%20et%20al.%202010.pdf |format=[[Portable Document Format|PDF]]}}</ref><ref>{{cite journal |author1=So Kawaguchi |author2=Akio Ishida |author3=Robert King |author4=Ben Raymond |author5=N. Waller |author6=A. Constable |author7=Stephen Nicol |author8=Masahide Wakita |author9=Atsushi Ishimatsu |year=2013 |title=Risk maps for Antarctic krill under projected Southern Ocean acidification |journal=[[Nature Climate Change]] |volume=3 |issue=9 |pages=843–847 |url=https://www.researchgate.net/profile/Atsushi_Ishimatsu/publication/249009598_Risk_maps_for_Antarctic_krill_under_projected_Southern_Ocean_acidification/links/540f9c9f0cf2f2b29a3de215.pdf |format=[[Portable Document Format|PDF]]|bibcode=2013NatCC...3..843K |doi=10.1038/nclimate1937 }}</ref> The further effects of ocean acidification on the krill life cycle however remains unclear but scientists fear that it could significantly impact on its distribution, abundance and survival.<ref>{{cite news |title=Swiss marine researcher moving in for the krill |author=Jill Rowbotham |publisher=[[The Australian]] |date=September 24, 2008 |url=http://www.theaustralian.news.com.au/story/0,25197,24392216-27703,00.html}}</ref><ref>{{cite journal |author1=James C. Orr |author2=Victoria J. Fabry |author3=Olivier Aumont |author4=Laurent Bopp |author5=Scott C. Doney |year=2005 |title=Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms |journal=[[Nature (journal)|Nature]] |volume=437 |issue=7059 |pages=681–686 |doi=10.1038/nature04095 |pmid=16193043 |bibcode=2005Natur.437..681O|display-authors=etal}}</ref>
Another challenge for Antarctic krill, as well as many calcifying organisms (corals, bivalve mussels, snails etc.), is the acidification of the oceans caused by increasing levels of carbon dioxide. The further effects of ocean acidification on the krill life cycle however remains unclear but scientists fear that it could significantly impact on its distribution, abundance and survival.


===Fisheries===
===Fisheries===
{{Main|Krill fishery}}
The fishery of Antarctic krill is on the order of 100,000 tonnes per year. The major catching nations are South Korea, Norway, Japan and Poland. The products are used as animal food and fish bait. Krill fisheries are difficult to operate in two important respects. First, a krill net needs to have very fine meshes, producing a very high drag, which generates a bow wave that deflects the krill to the sides. Second, fine meshes tend to clog very fast.
[[File:krillcatch.gif|thumb|Annual world catch of ''E. superba'', compiled from [[Food and Agriculture Organization|FAO]] data.<ref name="FAO05"/>]]
The fishery of Antarctic krill is on the order of 100,000&nbsp;tonnes per year. The major catching nations are [[South Korea]], [[Norway]], [[Japan]] and [[Poland]].<ref>[http://www.ccamlr.org/pu/e/e_pubs/sb/sb-vol20.pdf CCAMLR Statistical Bulletin vol. 20 (1998-2007)] {{webarchive |url=https://web.archive.org/web/20090225124314/http://www.ccamlr.org/pu/e/e_pubs/sb/sb-vol20.pdf |date=February 25, 2009 }}, CCAMLR, Hobart, Australia, 2008. URL last accessed July 3, 2008.</ref> The products are used as animal food and fish bait. Krill fisheries are difficult to operate in two important respects. First, a krill net needs to have very fine meshes, producing a very high [[Drag (physics)|drag]], which generates a [[bow wave]] that deflects the krill to the sides. Second, fine meshes tend to clog very fast.


Yet another problem is bringing the krill catch on board. When the full net is hauled out of the water, the organisms compress each other, resulting in great loss of the krill's liquids. Experiments have been carried out to pump krill, while still in water, through a large tube on board. Special krill nets also are currently under development. The processing of the krill must be very rapid since the catch deteriorates within several hours. Its high protein and vitamin content makes krill quite suitable for both direct human consumption and the animal-feed industry.<ref name="E+00">{{Cite book |author1=Inigo Everson |author2=David J. Agnew |author3=Denzil G. M. Miller |chapter=Krill fisheries and the future |pages=345–348 |editor=Inigo Everson |title=Krill: Biology, Ecology and Fisheries |location=Oxford |publisher=[[Blackwell Science]] |series=Fish and aquatic resources series |year=2000 |isbn=978-0-632-05565-4}}</ref>
Yet another problem is bringing the krill catch on board. When the full net is hauled out of the water, the organisms compress each other, resulting in great loss of the krill's liquids. Experiments have been carried out to pump krill, while still in water, through a large tube on board. Special krill nets also are currently under development. The processing of the krill must be very rapid since the catch deteriorates within several hours. Its high protein and vitamin content makes krill quite suitable for both direct human consumption and the animal-feed industry.


==Future visions and ocean engineering==
==Future visions and ocean engineering==
{{Main|Iron fertilization}}
Despite the lack of knowledge available about the whole Antarctic ecosystem, large scale experiments involving krill are already being performed to increase carbon sequestration: in vast areas of the [[Southern Ocean]] there are plenty of nutrients, but still, the phytoplankton does not grow much. These areas are termed HNLC (high nutrient, low chlorophyll). The phenomenon is called the "Antarctic Paradox", and occurs because iron is missing. Relatively small injections of iron from research vessels trigger very large blooms, covering many miles. The hope is that such large scale exercises will draw down carbon dioxide as compensation for the burning of fossil fuels.
Despite the lack of knowledge available about the whole Antarctic ecosystem, large scale experiments involving krill are already being performed to increase [[carbon sequestration]]: in vast areas of the Southern Ocean there are plenty of nutrients, but still, the phytoplankton does not grow much. These areas are termed [[HNLC]] (high nutrient, low chlorophyll). The phenomenon is called the [[Antarctic Paradox]], and occurs because [[iron]] is missing.<ref>{{cite web|url=http://www.palomar.edu/oceanography/iron.htm |title=The Iron Hypothesis |author=Caroline Dopyera |date=October 1996 |deadurl=yes |archiveurl=https://web.archive.org/web/20050306011126/http://www.palomar.edu/oceanography/iron.htm |archivedate=2005-03-06 |df= }}</ref> Relatively small injections of iron from research vessels trigger very large blooms, covering many miles. The hope is that such large scale exercises will draw down [[carbon dioxide]] as compensation for the burning of [[fossil fuel]]s.
 
[[Category:Animals of Westarctica]]
[[Category: Featured Articles]]

Latest revision as of 07:30, 1 December 2019

Antarctic krill

Antarctic krill (Euphausia superba) is a species of krill found in the Antarctic waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic meter. It feeds directly on minute phytoplankton, thereby using the primary production energy that the phytoplankton originally derived from the sun in order to sustain their open ocean life cycle.

It grows to a length of 6 cm, weighs up to 2 grams, and can live for up to six years. It is a key species in the Antarctic ecosystem and is, in terms of biomass, probably the most abundant animal species on the planet (approximately 500 million tonnes, corresponding to 300 to 400 trillion individuals).

In 2019, the Antarctic krill was featured on a stamp of Westarctica to celebrate its role as a vital part of the Antarctic ecosystem.

Life cycle

The main spawning season of Antarctic krill is from January to March, both above the continental shelf and also in the upper region of deep sea oceanic areas. In the typical way of all krill, the male attaches a spermatophore to the genital opening of the female. For this purpose, the first pleopods (legs attached to the abdomen) of the male are constructed as mating tools. Females lay 6,000–10,000 eggs at one time. They are fertilized as they pass out of the genital opening.

At 15 mm, the juvenile krill resembles the habitus of the adults. Krill reach maturity after two to three years. Like all crustaceans, krill must moult in order to grow. Approximately every 13 to 20 days, krill shed their chitinous exoskeleton and leave it behind as exuvia.

Food

The gut of E. superba can often be seen shining green through the animal's transparent skin, an indication that this species feeds predominantly on phytoplankton—especially very small diatoms, which it filters from the water with a feeding basket. The glass-like shells of the diatoms are cracked in the "gastric mill" and then digested in the hepatopancreas. The krill can also catch and eat copepods, amphipods and other small zooplankton. The gut forms a straight tube; its digestive efficiency is not very high and therefore a lot of carbon is still present in the feces.

In aquariums, krill have been observed to eat one another. When they are not fed in aquariums, they shrink in size after moulting, which is exceptional for animals the size of krill. It is likely that this is an adaptation to the seasonality of their food supply, which is limited in the dark winter months under the ice. However, the animal's compound eyes do not shrink, and so the ratio between eye size and body length has thus been found to be a reliable indicator of starvation.

Filter feeding

Antarctic krill directly use the minute phytoplankton cells, which no other animal of krill size can do. This is accomplished through filter feeding, using the krill's highly developed front legs, providing for an efficient filtering apparatus: the six thoracopods (legs attached to the thorax) form a very effective "feeding basket" used to collect phytoplankton from the open water. In the finest areas the openings in this basket are extremely tiny. In lower food concentrations, the feeding basket is pushed through the water for over half a metre in an opened position, and then the algae are combed to the mouth opening with special setae (bristles) on the inner side of the thoracopods.

Ice algae raking

Antarctic krill can scrape off the green lawn of ice algae from the underside of the pack ice. Krill have developed special rows of rake-like setae at the tips of the thoracopods, and graze the ice in a zig-zag fashion. One krill can clear an area of a square foot in about 10 minutes. It is relatively new knowledge that the film of ice algae is very well developed over vast areas, often containing much more carbon than the whole water column below. Krill find an extensive energy source here, especially in the spring.

Biology

Bioluminescence

The head of Antarctic krill

Krill are often referred to as light-shrimp because they can emit light, produced by bioluminescent organs. These organs are located on various parts of the individual krill's body: one pair of organs at the eyestalk, another pair on the hips of the second and seventh thoracopods, and singular organs on the four pleonsternites. These light organs emit a yellow-green light periodically, for up to 2–3 seconds. They are considered so highly developed that they can be compared with a torchlight: a concave reflector in the back of the organ and a lens in the front guide the light produced, and the whole organ can be rotated by muscles. The function of these lights is not yet fully understood; some hypotheses have suggested they serve to compensate the krill's shadow so that they are not visible to predators from below; other speculations maintain that they play a significant role in mating or schooling at night.

The krill's bioluminescent organs contain several fluorescent substances. The major component has a maximum fluorescence at an excitation of 355 nanometers.

Escape reaction

Krill use an escape reaction to evade predators, swimming backwards very quickly by flipping their rear ends. This swimming pattern is also known as "lobstering". Krill can reach speeds of over 0.6 meters per second.

Geographic distribution

Distribution of krill around Antarctica

Antarctic krill have a circumpolar distribution, being found throughout the Southern Ocean, and as far north as the Antarctic Convergence. At the Antarctic Convergence, the cold Antarctic surface water submerges below the warmer subantarctic waters. This front runs roughly at 55° south; from there to the continent, the Southern Ocean covers 32 million square kilometers. This is 65 times the size of the North Sea. In the winter season, more than three-quarters of this area become covered by ice, whereas 24,000,000 km2 become ice free in summer. The water temperature fluctuates at -1.3 C (3 F).

The waters of the Southern Ocean form a system of currents. Whenever there is a West Wind Drift, the surface strata travels around Antarctica in an easterly direction. Near the continent, the East Wind Drift runs counterclockwise. At the front between both, large eddies develop, for example, in the Weddell Sea. The krill swarms swim with these water masses, to establish one single stock all around Antarctica, with gene exchange over the whole area. Currently, there is little knowledge of the precise migration patterns since individual krill cannot yet be tagged to track their movements. The largest shoals are visible from space and can be tracked by satellite. One swarm covered an area of 450 km2 of ocean, to a depth of 200 m and was estimated to contain over 2 million tons of krill. Recent research suggests that krill do not simply drift passively in these currents but actually modify them.

Ecology

Antarctic krill are the keystone species of the Antarctic ecosystem beyond the coastal shelf, and provides an important food source for whales, leopard seals, fur seals, crabeater seals, glacial squid, icefish, penguins, albatrosses and many other species of birds.

Crabeater seals have even developed special teeth as an adaptation to catch this abundant food source: its unusual multilobed teeth enable this species to sieve krill from the water. Its dentition looks like a perfect strainer, but how it operates in detail is still unknown. Crabeaters are the most abundant seal in the world; 98% of their diet is made up of E. superba. These seals consume over 63 million tonnes of krill each year. Leopard seals have developed similar teeth and consume approximately 45% krill in their diets. All seals combined consume 63–130 million tonnes, all whales 34–43 million tonnes, birds 15–20 million tonnes, squid 30–100 million tonnes, and fish 10–20 million tonnes, adding up to 152–313 million tonnes of krill consumption each year.

The size step between krill and its prey is unusually large: generally it takes three or four steps from the small phytoplankton cells to a krill-sized organism (via small copepods, large copepods, mysids to 5 cm fish. E. superba lives only in the Southern Ocean. In the North Atlantic, Meganyctiphanes norvegica and in the Pacific, Euphausia pacifica are the dominant species.

Biomass and production

The biomass of Antarctic krill is estimated to be 125 to 725 million tonnes. The reason Antarctic krill are able to build up such a high biomass and production is that the waters around the icy Antarctic continent harbor one of the largest plankton assemblages in the world, possibly the largest. The ocean is filled with phytoplankton; as the water rises from the depths to the light-flooded surface, it brings nutrients from all of the world's oceans back into the photic zone where they are once again available to living organisms.

Thus primary production—the conversion of sunlight into organic biomass, the foundation of the food chain—has an annual carbon fixation of 1–2 g/m in the open ocean. Close to the ice it can reach 30–50 g/m. These values are not outstandingly high, compared to very productive areas like the North Sea or upwelling regions, but the area over which it takes place is enormous, even compared to other large primary producers such as rainforests. In addition, during the Austral summer there are many hours of daylight to fuel the process. All of these factors make the plankton and the krill a critical part of the planet's ecocycle.

Decline with shrinking pack ice

A possible decline in Antarctic krill biomass may have been caused by the reduction of the pack ice zone due to global warming. Antarctic krill, especially in the early stages of development, seem to need the pack ice structures in order to have a fair chance of survival. The pack ice provides natural cave-like features which the krill uses to evade their predators. In the years of low pack ice conditions the krill tend to give way to salps, a barrel-shaped free-floating filter feeder that also grazes on plankton.

Ocean acidification

Another challenge for Antarctic krill, as well as many calcifying organisms (corals, bivalve mussels, snails etc.), is the acidification of the oceans caused by increasing levels of carbon dioxide. The further effects of ocean acidification on the krill life cycle however remains unclear but scientists fear that it could significantly impact on its distribution, abundance and survival.

Fisheries

The fishery of Antarctic krill is on the order of 100,000 tonnes per year. The major catching nations are South Korea, Norway, Japan and Poland. The products are used as animal food and fish bait. Krill fisheries are difficult to operate in two important respects. First, a krill net needs to have very fine meshes, producing a very high drag, which generates a bow wave that deflects the krill to the sides. Second, fine meshes tend to clog very fast.

Yet another problem is bringing the krill catch on board. When the full net is hauled out of the water, the organisms compress each other, resulting in great loss of the krill's liquids. Experiments have been carried out to pump krill, while still in water, through a large tube on board. Special krill nets also are currently under development. The processing of the krill must be very rapid since the catch deteriorates within several hours. Its high protein and vitamin content makes krill quite suitable for both direct human consumption and the animal-feed industry.

Future visions and ocean engineering

Despite the lack of knowledge available about the whole Antarctic ecosystem, large scale experiments involving krill are already being performed to increase carbon sequestration: in vast areas of the Southern Ocean there are plenty of nutrients, but still, the phytoplankton does not grow much. These areas are termed HNLC (high nutrient, low chlorophyll). The phenomenon is called the "Antarctic Paradox", and occurs because iron is missing. Relatively small injections of iron from research vessels trigger very large blooms, covering many miles. The hope is that such large scale exercises will draw down carbon dioxide as compensation for the burning of fossil fuels.