Difference between revisions of "Antarctic krill"
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Westarctica (talk | contribs) (Created page with "'''Antarctic krill''' ('''''Euphausia superba''''') is a species of krill found in the Antarctic waters of the Southern Ocean. It is a small, swimming c...") |
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==Life cycle== | ==Life cycle== | ||
The main spawning season of Antarctic krill is from January to March, both above the | 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== | ==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 | 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 | 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=== | ||
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 | |||
===Ice-algae raking=== | ===Ice-algae raking=== | ||
Antarctic krill can scrape off the green lawn of [[ice-algae]] from the underside of the [[pack 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=== | ||
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. | ||
[[Category:Animals of Westarctica]] |