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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 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. | 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 | 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== | ==Food== | ||
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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: 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 | ===Ice algae raking=== | ||
Antarctic krill can scrape off the green lawn of [[ice | 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== | ==Biology== | ||
===Bioluminescence=== | ===Bioluminescence=== | ||
[[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 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. | 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. | ||
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==Geographic distribution== | ==Geographic distribution== | ||
[[File:Krilldistribution.jpg|thumb|right|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). | 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 | 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 are the keystone species of the [[Antarctica|Antarctic]] ecosystem beyond the coastal shelf, and provides an important food source for whales, [[leopard seal]]s, | 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. | 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 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. | 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=== | ||
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===Decline with shrinking pack ice=== | ===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 | 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=== | ===Ocean acidification=== | ||
Another challenge for Antarctic krill, as well as many calcifying organisms (corals, bivalve mussels, snails etc.), is the | 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=== | ||
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==Future visions and ocean engineering== | ==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 | 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. | ||
[[Category:Animals of Westarctica]] | [[Category:Animals of Westarctica]] | ||
[[Category: Featured Articles]] | |||