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| ==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. |
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| 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. |
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| 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]].
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| [[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.]]
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| ==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 [[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. |
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| 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. |
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| ===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. |
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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:<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 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. | |
| [[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.]]
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| ===Ice-algae raking=== | | ===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 minutes (1.5 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. | | 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. |
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| ===Biological pump and carbon sequestration===
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| [[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.]]
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| 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 years.
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| 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.
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| ==Biology== | | ==Biology== |
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| ===Bioluminescence=== | | ===Bioluminescence=== |
| [[File:Bioluminescencekils.jpg|thumb|Watercolour of bioluminescent krill]]
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| 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 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 [[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 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. |
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| {{Main|Iron fertilization}} | | {{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.<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. | | 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. |
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| | [[Category:Animals of Westarctica]] |