Editing Antarctic krill (section)

==Food==
The gut of ''E. superba'' can often be seen shining green through its transparent skin. 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 |access-date=2003-05-19 |archive-date=2002-05-01 |archive-url=https://web.archive.org/web/20020501120429/http://www.ecoscope.com/krill/filter/index.htm |url-status=dead }}</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]]. Antarctic krill (''E. superba'') primarily has chitinolytic enzymes in the stomach and mid-gut to break down chitinous spines on diatoms, additional enzymes can vary due to its expansive diet.<ref>{{Cite journal|last=Buchholz|first=Friedrich|date=June 1996|title=A field study on the physiology of digestion in the Antarctic krill, Euphausia superba, with special regard to chitinolytic enzymes|url=https://www.researchgate.net/publication/268521751|journal=Journal of Plankton Research|volume=18|issue=6|pages=895–906|via=Research Gate|doi=10.1093/plankt/18.6.895|doi-access=free}}</ref>

In [[Aquarium|aquaria]], krill have been observed to [[Cannibalism|eat each other]]. When they are not fed, they shrink in size after [[Ecdysis|moulting]], which is exceptional for animals this size. 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 |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 |doi-access=free }}</ref> A krill with ample food supply would have eyes proportional to body length, compared to a starving krill that would have eyes that appeared larger than what is normal.  
[[File:Filterkrillkils.gif|thumb|Modified thoracopods that form the feeding basket of the filter apparatus move through the water to bring phytoplankton cells into the mouth.]]

===Filter feeding===
{{Main|Filter feeder}}Antarctic krill directly ingest 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 which form an efficient filtering apparatus:<ref name="Kils832">{{cite book|title=On the biology of krill ''Euphausia superba'' – Proceedings of the Seminar and Report of Krill Ecology Group|author=Uwe Kils|publisher=[[Alfred Wegener Institute for Polar and Marine Research]]|year=1983|editor=S. B. Schnack|series=Berichte zur Polarforschung|issue=Special Issue 4|pages=130–155|chapter=Swimming and feeding of Antarctic krill, ''Euphausia superba'' – some outstanding energetics and dynamics - some unique morphological details}}</ref> the six [[thoracopod]]s (legs attached to the [[thorax]]) create a "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.[[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.]]

===Ice-algae raking===
Antarctic krill can scrape off the green lawn of [[ice algae]] from the underside of [[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 |access-date=2003-05-19 |archive-date=2002-12-01 |archive-url=https://web.archive.org/web/20021201202015/http://www.ecoscope.com/icecave2.htm |url-status=dead }}</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|bibcode=1988PoBio...9..129M |s2cid=19130587 }}</ref> Krill have developed special rows of rake-like setae at the tips of their [[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). Recent discoveries have found that the film of ice algae is 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 after food sources have been limited during the winter months.

===Biological pump and carbon sequestration===
[[File:Krillspitballkils3.jpg|thumb|''In situ'' image taken with an [[ecoSCOPE]]. A green spitball 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 depths of 100&nbsp;m daily.<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|doi-access=free |bibcode=2006CBio...16..R83T }}</ref> The krill is a very untidy feeder, and it often spits out aggregates of [[phytoplankton]] (spitballs) containing thousands of cells sticking together. It also produces fecal strings that still contain significant amounts of [[carbon]] and [[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.
[[File:Pelagiczone.svg|thumb|378x378px|Layers of the Pelagic Zone which contains organisms that make up an ecosystem. Antarctic Krill  are part of this ecosystem.]]
If the phytoplankton is consumed by other components of the pelagic ecosystem, most of the carbon remains in the upper layers of the ocean. 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.