View source for Krill

{{Short description|Order of crustaceans}}
{{other uses}}
{{Use dmy dates|date=August 2021}}
{{Automatic taxobox
| image = Meganyctiphanes norvegica2.jpg
| image_caption = [[Northern krill]] (''Meganyctiphanes norvegica'')
| taxon = Euphausiacea
| authority = [[James Dwight Dana|Dana]], 1852
| subdivision_ranks = Families and genera
| subdivision = ;Euphausiidae
*''[[Euphausia]]'' <small>Dana, 1852</small>
*''[[Hansarsia]]'' <small>Shaw, 2023</small>
*''[[Meganyctiphanes]]'' <small>[[Ernest William Lyons Holt|Holt]] and [[W. M. Tattersall]], 1905</small>
*''[[Nematobrachion]]'' <small>[[William Thomas Calman|Calman]], 1905</small>
*''[[Nyctiphanes]]'' <small>G. O. Sars, 1883</small>
*''[[Pseudeuphausia]]'' <small>[[Hans Jacob Hansen|Hansen]], 1910</small>
*''[[Stylocheiron]]'' <small>G. O. Sars, 1883</small>
*''[[Tessarabrachion]]'' <small>Hansen, 1911</small>
*''[[Thysanoessa]]'' <small>[[Johann Friedrich von Brandt|Brandt]], 1851</small>
*''[[Thysanopoda]]'' <small>[[Latreille]], 1831</small>
;Bentheuphausiidae
*''[[Bentheuphausia]]'' <small>G. O. Sars, 1885</small>
}}

'''Krill''' ''(Euphausiids)''<ref name="Euphausiids (Krill)">{{cite web |title=Euphausiids (Krill) |url=https://parks.canada.ca/amnc-nmca/qc/saguenay/info/plan/gestion-management#section8-0 |website=Government of Canada |publisher=Fisheries and Oceans Canada |access-date=18 April 2024 |date=2022-04-06 |quote=Many different species of euphausiids are found on Canada's east and west coasts.}}</ref> ({{singular}}: krill) are small and exclusively marine [[crustacean]]s of the [[order (biology)|order]] '''Euphausiacea''', found in all of the world's oceans.<ref>{{Cite book|url=https://academic.oup.com/book/25513/chapter-abstract/192772141?redirectedFrom=fulltext&login=false|title=Marine Plankton: A practical guide to ecology, methodology, and taxonomy |chapter=Crustacea: Euphausiacea |first=Alistair J.|last=Lindley|editor-first1=Claudia|editor-last1=Castellani|editor-first2=Martin|editor-last2=Edwards|date=16 February 2017|volume=1 |publisher=Oxford University Press|pages=0|doi=10.1093/oso/9780199233267.003.0030 |isbn=978-0-19-923326-7 |via=Silverchair}}</ref> The name "krill" comes from the [[Norwegian language|Norwegian]] word ''{{lang|no|[[:wikt:krill|krill]]}}'', meaning "small [[Fry (biology)|fry]] of fish",<ref>{{cite encyclopedia|url=http://www.etymonline.com/index.php?search=krill|title=Krill|dictionary=Online Etymology Dictionary|access-date=22 June 2010}}</ref> which is also often attributed to species of fish.

Krill are considered an important [[trophic level]] connection near the bottom of the [[food chain]]. They feed on [[phytoplankton]] and, to a lesser extent, [[zooplankton]], and are also the main source of food for many larger animals. In the [[Southern Ocean]], one species, the [[Antarctic krill]], makes up an estimated [[biomass (ecology)|biomass]] of around 379 million [[tonne]]s,<ref>{{cite journal|author1=A. Atkinson |author2=V. Siegel |author3=E.A. Pakhomov |author4=M.J. Jessopp |author5=V. Loeb |title=A re-appraisal of the total biomass and annual production of Antarctic krill |journal=[[Deep-Sea Research Part I]] |year=2009 |volume=56 |issue=5 |pages=727–740 |url=http://www.iced.ac.uk/documents/Atkinson%20et%20al,%20Deep%20Sea%20Research%20I,%202009.pdf |doi=10.1016/j.dsr.2008.12.007|bibcode=2009DSRI...56..727A }}</ref> making it among the species with the largest total biomass. Over half of this biomass is eaten by whales, [[Pinniped|seals]], penguins, seabirds, [[squid]], and fish each year. Most krill species display large [[diel vertical migration|daily vertical migrations]], providing food for predators near the surface at night and in deeper waters during the day.

Krill are fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000&nbsp;tonnes annually, mostly from the [[Scotia Sea]]. Most krill catch is used for [[aquaculture]] and [[aquarium]] feeds, as [[bait (luring substance)|bait]] in [[Recreational fishing|sport fishing]], or in the pharmaceutical industry. Krill are also used for human consumption in several countries. They are known as {{nihongo|''okiami''|オキアミ}} in Japan and as ''camarones'' in Spain and the Philippines. In the Philippines, they are also called ''alamang'' and are used to make a salty paste called ''[[bagoong]]''.

Krill are also the main food for [[baleen whale]]s, including the [[blue whale]].

== Taxonomy ==
Krill belong to the large [[arthropod]] [[subphylum]], the [[crustacean|Crustacea]]. The most familiar and largest group of crustaceans, the [[Class (biology)|class]] [[Malacostraca]], includes the [[superorder]] [[Eucarida]] comprising the three orders, Euphausiacea (krill), [[Decapoda]] (shrimp, prawns, lobsters, crabs), and the planktonic [[Amphionides|Amphionidacea]].

The order Euphausiacea comprises two [[Family (biology)|families]]. The more abundant [[Euphausiidae]] contains 10 different [[genus|genera]] with a total of 85 species. Of these, the genus ''[[Euphausia]]'' is the largest, with 31 species.<ref>{{Cite WoRMS|id=110671|title=Euphausiidae Dana, 1852|db=krill|year=2011|access-date=25 November 2011|last=Siegel|first=Volker}}</ref> The lesser-known family, the [[Bentheuphausiidae]], has only one [[species]], ''[[Bentheuphausia amblyops]]'', a [[bathypelagic]] krill living in deep waters below {{cvt|1000|m|-2}}. It is considered the most primitive extant krill species.<ref>{{cite journal |author=E. Brinton |title=The distribution of Pacific euphausiids |journal=Bull. Scripps Inst. Oceanogr. |volume=8 |issue=2 |pages=51–270 |year=1962 |url=http://escholarship.org/uc/item/6db5n157}}</ref>

Well-known species of the Euphausiidae of commercial [[Krill fishery|krill fisheries]] include [[Antarctic krill]] (''Euphausia superba''), [[Pacific krill]] (''E. pacifica'') and [[Northern krill]] (''Meganyctiphanes norvegica'').<ref name="nicol">{{cite journal |author1=S. Nicol |author2=Y. Endo |year=1999 |title=Krill fisheries: Development, management and ecosystem implications |journal=[[Aquatic Living Resources]] |volume=12 |issue=2 |pages=105–120 |doi=10.1016/S0990-7440(99)80020-5|bibcode=1999AqLR...12..105N |s2cid=84158071 |url=https://www.alr-journal.org/10.1016/S0990-7440(99)80020-5/pdf }}</ref>

===Phylogeny===
{{main|Phylogeny of Malacostraca}}
{{see also|Eucarida#phylogeny}}
{{cladogram
 |title=Proposed phylogeny of Euphausiacea<ref name="Maas"/>
 |caption=Phylogeny obtained from morphological data, (♠) names coined in,<ref name="Maas"/> (♣) possibly paraphyletic taxon due to ''Nematobrachion'' in.<ref name="Maas"/> (♦) clades differs from Casanova (1984),<ref name="casanova">{{cite journal |author=Bernadette Casanova |year=1984 |title=Phylogénie des Euphausiacés (Crustacés Eucarides) |language=fr |trans-title=Phylogeny of the Euphausiacea (Crustacea: Eucarida) |journal=[[Bulletin du Muséum National d'Histoire Naturelle]] |volume=4 |pages=1077–1089}}</ref> where ''Pseudoeuphausia'' is sister to ''Nyctiphanes'', ''Euphausia'' is sister to ''Thysanopoda'' and ''Nematobrachion'' is sister to ''Stylocheiron''.
 |align=right
 |cladogram=
 {{clade| style=font-size:75%;line-height:75%
  |label1=Euphausiacea
  |1={{clade
   |label1=Bentheuphausiidae
   |1=''[[Bentheuphausia amblyops|Bentheuphausia]]''
   |label2= Euphausiidae
   |2={{clade
    |1=''[[Thysanopoda]]'' (♣)
    |2=''[[Nematobrachion]]'' (♦)
    |label3=Euphausiinae
    |3={{clade
     |1=''[[Northern krill|Meganyctiphanes]]''
     |label2=Euphausiini (♠)(♦)
     |2={{clade
      |1=''[[Pseudeuphausia]]''
      |2=''[[Euphausia]]''
     }}
     |label3=Nematoscelini (♠)
     |3={{clade
      |1=''[[Nyctiphanes]]''
      |label2=Nematoscelina (♠)
      |2={{clade
       |1=''[[Nematoscelis]]''
       |2=''[[Thysanoessa]]''
       |3=''[[Tessarabrachion]]''
       |4=''[[Stylocheiron]]''
      }}
     }}
    }}
   }}
  }}
 }}
}}

{{As of|2013}}, the order Euphausiacea is believed to be [[Monophyly|monophyletic]] due to several unique conserved morphological characteristics ([[autapomorphy]]) such as its naked filamentous gills and thin thoracopods<ref name="casanova03">{{cite journal |author=Bernadette Casanova |title=Ordre des Euphausiacea Dana, 1852 |journal=[[Crustaceana]] |volume=76 |issue=9 |year=2003 |pages=1083–1121 |doi=10.1163/156854003322753439 |jstor=20105650|bibcode=2003Crust..76.1083C }}</ref> and by molecular studies.<ref>{{cite journal |author1=M. Eugenia D'Amato |author2=Gordon W. Harkins |author3=Tulio de Oliveira |author4=Peter R. Teske |author5=Mark J. Gibbons |year=2008 |title=Molecular dating and biogeography of the neritic krill ''Nyctiphanes'' |url=http://www.bioafrica.net/manuscripts/AmatoMarineBiology.pdf |journal=[[Marine Biology (journal)|Marine Biology]] |volume=155 |issue=2 |pages=243–247 |doi=10.1007/s00227-008-1005-0 |bibcode=2008MarBi.155..243D |s2cid=17750015 |access-date=4 July 2010 |archive-date=17 March 2012 |archive-url=https://web.archive.org/web/20120317023748/http://www.bioafrica.net/manuscripts/AmatoMarineBiology.pdf |url-status=usurped }}</ref><ref name="Jarman">{{cite journal |author=Simon N. Jarman |year=2001 |title=The evolutionary history of krill inferred from nuclear large subunit rDNA sequence analysis |journal=[[Biological Journal of the Linnean Society]] |volume=73 |issue=2 |pages=199–212 |doi=10.1111/j.1095-8312.2001.tb01357.x|doi-access=free }}</ref><ref>{{cite journal |author1=Xin Shen |author2=Haiqing Wang |author3=Minxiao Wang |author4=Bin Liu |year=2011 |title=The complete mitochondrial genome sequence of ''Euphausia pacifica'' (Malacostraca: Euphausiacea) reveals a novel gene order and unusual tandem repeats |journal=[[Genome (journal)|Genome]] |volume=54 |issue=11 |pages=911–922 |doi=10.1139/g11-053 |pmid=22017501}}</ref>

There have been many theories of the location of the order Euphausiacea. Since the first description of ''Thysanopode tricuspide'' by [[Henri Milne-Edwards]] in 1830, the similarity of their biramous thoracopods had led zoologists to group euphausiids and Mysidacea in the order [[Schizopoda]], which was split by [[Johan Erik Vesti Boas]] in 1883 into two separate orders.<ref name="Boas">{{cite journal |author=[[Johan Erik Vesti Boas]] |year=1883 |title=Studien über die Verwandtschaftsbeziehungen der Malacostraken |language=de |trans-title=Studies on the relationships of the Malacostraca |journal=[[Morphologisches Jahrbuch]] |volume=8 |pages=485–579}}</ref> Later, [[William Thomas Calman]] (1904) ranked the [[Mysidacea]] in the superorder [[Peracarida]] and euphausiids in the superorder [[Eucarida]], although even up to the 1930s the order Schizopoda was advocated.<ref name="casanova03"/> It was later also proposed that order Euphausiacea should be grouped with the [[Penaeidae]] (family of prawns) in the Decapoda based on developmental similarities, as noted by [[Robert Gurney]] and [[Isabella Gordon]].<ref name="Gurney">{{cite book |author=[[Robert Gurney]] |year=1942 |publisher=[[Ray Society]] |title=Larvae of Decapod Crustacea |url=http://decapoda.arthroinfo.org/pdfs/12852/12852.pdf }}</ref><ref>{{cite journal |author=[[Isabella Gordon]] |year=1955 |title=Systematic position of the Euphausiacea |journal=[[Nature (journal)|Nature]] |volume=176 |issue=4489 |pages=934 |doi=10.1038/176934a0 |bibcode=1955Natur.176..934G|s2cid=4225121 |doi-access=free }}</ref> The reason for this debate is that krill share some morphological features of decapods and others of mysids.<ref name="casanova03"/>

Molecular studies have not unambiguously grouped them, possibly due to the paucity of key rare species such as ''Bentheuphausia amblyops'' in krill and ''Amphionides reynaudii'' in Eucarida. One study supports the monophyly of Eucarida (with basal Mysida),<ref name="Spears, T. 2005">{{cite journal |author=Trisha Spears, Ronald W. DeBry, Lawrence G. Abele & Katarzyna Chodyl |year=2005 |title=Peracarid monophyly and interordinal phylogeny inferred from nuclear small-subunit ribosomal DNA sequences (Crustacea: Malacostraca: Peracarida) |journal=[[Proceedings of the Biological Society of Washington]] |volume=118 |issue=1 |pages=117–157 |doi=10.2988/0006-324X(2005)118[117:PMAIPI]2.0.CO;2 |s2cid=85557065 |url=http://decapoda.nhm.org/pdfs/10231/10231.pdf |editor1-last=Boyko |editor1-first=Christopher B.}}</ref> another groups Euphausiacea with Mysida (the Schizopoda),<ref name="Jarman"/> while yet another groups Euphausiacea with [[Hoplocarida]].<ref>{{cite journal |author1=K. Meland |author2=E. Willassen |year=2007 |title=The disunity of "Mysidacea" (Crustacea) |journal=[[Molecular Phylogenetics and Evolution]] |volume=44 |pages=1083–1104 |doi=10.1016/j.ympev.2007.02.009 |pmid=17398121 |issue=3|bibcode=2007MolPE..44.1083M |citeseerx=10.1.1.653.5935 }}</ref>

===Timeline===
No extant fossil can be unequivocally assigned to Euphausiacea. Some extinct [[eumalacostraca]]n [[taxa]] have been thought to be euphausiaceans such as ''[[Anthracophausia]]'', ''[[Crangopsis]]''—now assigned to the [[Aeschronectida]] (Hoplocarida)<ref name="Maas">{{cite journal |author1=Andreas Maas |author2=Dieter Waloszek |year=2001 |title=Larval development of ''Euphausia superba'' Dana, 1852 and a phylogenetic analysis of the Euphausiacea |url=http://biosys-serv.biologie.uni-ulm.de/Downloadfolder/PDFs%20Team/2001_Maas&Waloszek_Euphausia.pdf |journal=[[Hydrobiologia]] |volume=448 |issue=1–3 |pages=143–169 |doi=10.1023/A:1017549321961 |bibcode=2001HyBio.448..143M |s2cid=32997380 |url-status=dead |archive-url=https://web.archive.org/web/20110718095311/http://biosys-serv.biologie.uni-ulm.de/Downloadfolder/PDFs%20Team/2001_Maas%26Waloszek_Euphausia.pdf |archive-date=18 July 2011 }}</ref>—and ''Palaeomysis''.<ref name="Schram86">{{cite book |author=[[Frederick R. Schram]] |year=1986 |title=Crustacea |isbn=978-0-19-503742-5 |publisher=[[Oxford University Press]]}}</ref> All dating of [[speciation]] events were estimated by [[molecular clock]] methods, which placed the last common ancestor of the krill family Euphausiidae (order Euphausiacea minus ''Bentheuphausia amblyops'') to have lived in the [[Early Cretaceous|Lower Cretaceous]] about {{Ma|130}}.<ref name="Jarman"/>

== Distribution ==

Krill occur worldwide in all oceans, although many individual species have [[endemism|endemic]] or [[neritic]] (''i.e.,'' coastal) distributions. ''[[Bentheuphausia amblyops]]'', a [[Bathyal zone|bathypelagic]] species, has a [[cosmopolitan distribution]] within its deep-sea habitat.<ref>{{cite journal |author1=J. J. Torres |author2=J. J. Childress |year=1985 |title=Respiration and chemical composition of the bathypelagic euphausiid ''Bentheuphausia amblyops'' |journal=[[Marine Biology (journal)|Marine Biology]] |volume=87 |issue=3 |pages=267–272 |doi=10.1007/BF00397804|bibcode=1985MarBi..87..267T |s2cid=84486097 }}</ref>

Species of the genus ''[[Thysanoessa]]'' occur in both [[Atlantic Ocean|Atlantic]] and [[Pacific Ocean|Pacific]] oceans.<ref>{{cite WoRMS |author=Volker Siegel |year=2011 |title=''Thysanoessa'' Brandt, 1851 |id=110679 |access-date=18 June 2011}}</ref> The Pacific is home to ''[[Euphausia pacifica]]''. Northern krill occur across the Atlantic from the [[Mediterranean Sea]] northward.

Species with neritic distributions include the four species of the genus ''[[Nyctiphanes]]''.<ref name="damato">D'Amato, M.E. ''et al.'': "{{usurped|1=[https://web.archive.org/web/20120317023748/http://www.bioafrica.net/manuscripts/AmatoMarineBiology.pdf Molecular dating and biogeography of the neritic krill ''Nyctiphanes'']}}", in ''Marine Biology vol. 155, no. 2'', pp.&nbsp;243–247, August 2008.</ref> They are highly abundant along the [[upwelling]] regions of the [[California Current|California]], [[Humboldt Current|Humboldt]], [[Benguela Current|Benguela]], and [[Canary Current|Canarias]] [[Ocean current|current system]]s.<ref>{{cite web |author=Volker Siegel |year=2011 |title=''Nyctiphanes'' Sars, 1883 |editor=V. Siegel |work=World Euphausiacea database |publisher=[[World Register of Marine Species]] |url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=110677 |access-date=18 June 2011}}</ref><ref name="mauchline"/><ref name="gut2005">{{cite journal |author1=Jaime Gómez-Gutiérrez |author2=Carlos J. Robinson |year=2005 |title=Embryonic, early larval development time, hatching mechanism and interbrood period of the sac-spawning euphausiid ''Nyctiphanes simplex'' Hansen |journal=[[Journal of Plankton Research]] |volume=27 |issue=3 |pages=279–295 |doi=10.1093/plankt/fbi003|doi-access=free }}</ref> Another species having only neritic distribution is ''E. crystallorophias'', which is endemic to the Antarctic coastline.<ref name="jarman2002">{{cite journal |author1=S. N. Jarman |author2=N. G. Elliott |author3=S. Nicol |author4=A. McMinn |year=2002 |title=Genetic differentiation in the Antarctic coastal krill ''Euphausia crystallorophias'' |journal=[[Heredity (journal)|Heredity]] |volume=88 |pages=280–287 |pmid=11920136 |doi=10.1038/sj.hdy.6800041 |issue=4|doi-access=free |bibcode=2002Hered..88..280J }}</ref>

Species with endemic distributions include ''[[Nyctiphanes capensis]]'', which occurs only in the Benguela current,<ref name="damato"/> ''[[Euphausia mucronata|E. mucronata]]'' in the Humboldt current,<ref name="escribano">{{cite journal |author1=R. Escribano |author2=V. Marin |author3=C. Irribarren |year=2000 |title=Distribution of ''Euphausia mucronata'' at the upwelling area of Peninsula Mejillones, northern Chile: the influence of the oxygen minimum layer |journal=[[Scientia Marina]] |volume=64 |issue=1 |pages=69–77 |url=http://scientiamarina.revistas.csic.es/index.php/scientiamarina/article/viewFile/741/758 |doi=10.3989/scimar.2000.64n169|doi-access=free |bibcode=2000ScMar..64...69E }}</ref> and the six ''Euphausia'' species native to the Southern Ocean.

In the Antarctic, seven species are known,<ref>{{cite web |author=P. Brueggeman |url=http://www.peterbrueggeman.com/nsf/fguide/arthropoda10.html |title=''Euphausia crystallorophias'' |work=Underwater Field Guide to Ross Island & McMurdo Sound, Antarctica |publisher=[[University of California, San Diego]]}}</ref> one in genus ''Thysanoessa'' (''[[Thysanoessa macrura|T. macrura]]'') and six in ''Euphausia''. The [[Antarctic krill]] (''Euphausia superba'') commonly lives at depths reaching {{cvt|100|m|-1}},<ref>{{cite web |url=http://marinebio.org/species.asp?id=518 |title=Krill, ''Euphausia superba'' |publisher=[[MarineBio.org]] |access-date=25 February 2009}}</ref> whereas ice krill (''[[Euphausia crystallorophias]]'') reach depth of {{cvt|4000|m|-2}}, though they commonly inhabit depths of at most {{cvt|300|-|600|m|-2}}.<ref>{{cite journal |author=J. A. Kirkwood |title=A Guide to the Euphausiacea of the Southern Ocean |journal=ANARE Research Notes |year=1984 |volume=1 |pages=1–45}}</ref> Krill perform Diel Vertical Migrations (DVM) in large swarms, and acoustic data has shown these migrations to go up to 400 metres in depth.<ref>{{cite journal |last1=Bianchi |first1=Daniele |last2=Mislan |first2=K.A.S. |title=Global patterns of diel vertical migration times and velocities from acoustic data |journal=Limnology and Oceanography |date=January 2016 |volume=61 |issue=1 |pages=353–364 |doi=10.1002/lno.10219|doi-access=free |bibcode=2016LimOc..61..353B }}</ref> Both are found at [[latitude]]s south of [[55th parallel south|55°&nbsp;S]], with ''E. crystallorophias'' dominating south of [[74th parallel south|74°&nbsp;S]]<ref>{{cite journal |author1=A. Sala |author2=M. Azzali |author3=A. Russo |url=http://www.icm.csic.es/scimar/download.php/Cd/c6d5ca7c8572ce508582edcd1793cf93/IdArt/3031 |title=Krill of the Ross Sea: distribution, abundance and demography of ''Euphausia superba'' and ''Euphausia crystallorophias'' during the Italian Antarctic Expedition (January–February 2000) |journal=[[Scientia Marina]] |volume=66 |issue=2 |pages=123–133 |year=2002 |doi=10.3989/scimar.2002.66n2123|doi-access=free |bibcode=2002ScMar..66..123S }}</ref> and in regions of [[pack ice]]. Other species known in the [[Southern Ocean]] are ''[[Euphausia frigida|E. frigida]]'', ''[[Euphausia longirostris|E. longirostris]]'', ''[[Euphausia triacantha|E. triacantha]]'' and ''[[Euphausia vallentini|E. vallentini]]''.<ref>{{cite journal |author1=G. W. Hosie |author2=M. Fukuchi |author3=S. Kawaguchi |url=http://www.noc.soton.ac.uk/CLIVAR/organization/southern/expertgroup/Hosie%20et%20al%20P&O%202003.pdf |title=Development of the Southern Ocean Continuous Plankton Recorder survey |journal=[[Progress in Oceanography]] |volume=58 |pages=263–283 |year=2003 |doi=10.1016/j.pocean.2003.08.007 |issue=2–4 |bibcode=2003PrOce..58..263H }}{{dead link|date=December 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>{{Clear}}

== Anatomy and morphology ==
[[File:Krillanatomykils.jpg|thumb|Krill anatomy explained, using ''[[Euphausia superba]]'' as a model]]
[[File:Euphausia gills.jpg|thumb|The [[gill]]s of krill are externally visible]]

Krill are [[crustaceans]] and, like all crustaceans, they have a [[chitin]]ous [[exoskeleton]]. They have anatomy similar to a standard [[decapod anatomy|decapod]] with their bodies made up of three [[Tagma (biology)|parts]]: the cephalothorax is composed of the [[cephalon|head]] and the [[thorax]], which are fused, and the [[Decapod anatomy#Abdomen / pleon|abdomen]], which bears the ten swimming appendages, and the [[Decapod anatomy#Telson|tail fan]]. This outer shell of krill is transparent in most species.

Krill feature intricate [[compound eye]]s. Some species adapt to different lighting conditions through the use of screening [[pigment]]s.<ref>{{cite web |author=E. Gaten |url=http://www.le.ac.uk/biology/gat/northernkrill.html |title=''Meganyctiphanes norvegica'' |access-date=25 February 2009 |publisher=[[University of Leicester]] |url-status=dead |archive-url=https://web.archive.org/web/20090701132500/http://www.le.ac.uk/biology/gat/northernkrill.html |archive-date=1 July 2009 }}</ref>

They have two [[antenna (biology)|antennae]] and several pairs of thoracic legs called [[pereiopod]]s or [[thoracopod]]s, so named because they are attached to the thorax. Their number varies among genera and species. These thoracic legs include feeding legs and grooming legs.

Krill are probably the sister clade of decapods because all species have five pairs of [[pleopod|swimming legs]] called "swimmerets" in common with the latter, very similar to those of a [[lobster]] or [[freshwater crayfish]].

In spite of having ten swimmerets, otherwise known as [[Decapod anatomy|pleopods]], krill cannot be considered decapods. They lack any true ground-based legs due to all their [[Decapod anatomy|pereiopods]] having been converted into grooming and auxiliary feeding legs. In [[Decapoda]], there are ten functioning [[Decapod anatomy|pereiopods]], giving them their name; whereas here there are no remaining locomotive [[Decapod anatomy|pereiopods]]. Nor are there consistently ten [[Decapod anatomy|pereiopods]] at all.

Most krill are about {{convert|1|-|2|cm|1}} long as adults. A few species grow to sizes on the order of {{convert|6|-|15|cm|1}}. The largest krill species, ''Thysanopoda cornuta'', lives [[bathypelagic|deep in the open ocean]].<ref>{{cite journal |author=E. Brinton |title=''Thysanopoda spinicauda'', a new bathypelagic giant euphausiid crustacean, with comparative notes on ''T. cornuta'' and ''T. egregia'' |journal=[[Journal of the Washington Academy of Sciences]] |volume=43 |pages=408–412 |year=1953}}</ref> Krill can be easily distinguished from other crustaceans such as true [[shrimp]] by their externally visible [[gill]]s.<ref name="tafi2008">{{cite web |publisher=Tasmanian Aquaculture & Fisheries Institute |url=http://www.tafi.org.au/zooplankton/imagekey/malacostraca/euphausiacea/ |title=Euphausiacea |access-date=6 June 2010 |url-status=dead |archive-url=https://web.archive.org/web/20090930075356/http://www.tafi.org.au/zooplankton/imagekey/malacostraca/euphausiacea/ |archive-date=30 September 2009 }}</ref>

Except for ''[[Bentheuphausia amblyops]]'', krill are [[bioluminescence|bioluminescent]] animals having organs called [[photophore]]s that can emit light. The light is generated by an [[enzyme]]-catalysed [[chemiluminescence]] reaction, wherein a [[luciferin]] (a kind of pigment) is activated by a [[luciferase]] enzyme. Studies indicate that the luciferin of many krill species is a [[fluorescence|fluorescent]] [[Polypyrrole|tetrapyrrole]] similar but not identical to [[dinoflagellate]] luciferin<ref>{{cite journal |author=O. Shimomura |pmid=7676855 |title=The roles of the two highly unstable components F and P involved in the bioluminescence of euphausiid shrimps |journal=Journal of Bioluminescence and Chemiluminescence |volume=10 |issue=2 |pages=91–101 |year=1995 |doi=10.1002/bio.1170100205|doi-access= }}</ref> and that the krill probably do not produce this substance themselves but acquire it as part of their diet, which contains dinoflagellates.<ref>{{cite journal |author1=J. C. Dunlap |author2=J. W. Hastings |author3=O. Shimomura |year=1980 |title=Crossreactivity between the light-emitting systems of distantly related organisms: novel type of light-emitting compound |journal=[[Proceedings of the National Academy of Sciences]] |volume=77 |issue=3 |pages=1394–1397 |doi=10.1073/pnas.77.3.1394 |pmid=16592787 |jstor=8463 |pmc=348501|bibcode=1980PNAS...77.1394D |doi-access=free }}</ref> Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles.<ref>{{cite book |author1=P. J. Herring |author2=E. A. Widder |chapter-url=http://www.isbc.unibo.it/Files/BC_PlanktonNekton.htm |chapter=Bioluminescence in Plankton and Nekton |editor1=J. H. Steele |editor2=S. A. Thorpe |editor3=K. K. Turekian |title=Encyclopedia of Ocean Science |volume=1 |pages=[https://archive.org/details/encyclopediaofoc0000unse/page/308 308–317] |publisher=[[Academic Press]], San Diego |year=2001 |isbn=978-0-12-227430-5 |url-access=registration |url=https://archive.org/details/encyclopediaofoc0000unse/page/308 }}</ref> The precise function of these organs is as yet unknown; possibilities include mating, social interaction or orientation and as a form of counter-illumination camouflage to compensate their shadow against overhead ambient light.<ref>{{cite conference|author1=S. M. Lindsay |author2=M. I. Latz |title=Experimental evidence for luminescent countershading by some euphausiid crustaceans |conference=American Society of Limnology and Oceanography (ASLO) Aquatic Sciences Meeting |location=Santa Fe |year=1999}}</ref><ref>{{cite journal |author=Sönke Johnsen |title=The Red and the Black: bioluminescence and the color of animals in the deep sea |journal=[[Integrative and Comparative Biology]] |volume=4 |issue=2 |pages=234–246 |year=2005 |url=http://www.biology.duke.edu/johnsenlab/pdfs/pubs/blcolor.pdf |doi=10.1093/icb/45.2.234 |pmid=21676767 |s2cid=247718 |url-status=dead |archive-url=https://web.archive.org/web/20051002164113/http://www.biology.duke.edu/johnsenlab/pdfs/pubs/blcolor.pdf |archive-date=2 October 2005 |doi-access=free }}</ref>

{{clear}}

== Ecology ==
[[File:Processes in the biological pump.webp|thumb|upright=2.0|right| {{center|'''Processes in the biological pump'''}} Phytoplankton convert CO<sub>2</sub>, which has dissolved from the atmosphere into the surface oceans (90 Gt yr−1) into particulate organic carbon (POC) during primary production (~ 50 Gt C yr−1). Phytoplankton are then consumed by krill and small zooplankton grazers, which in turn are preyed upon by higher trophic levels. Any unconsumed phytoplankton form aggregates, and along with zooplankton faecal pellets, sink rapidly and are exported out of the mixed layer (< 12 Gt C yr−1 14). Krill, zooplankton and microbes intercept phytoplankton in the surface ocean and sinking detrital particles at depth, consuming and respiring this POC to CO<sub>2</sub> (dissolved inorganic carbon, DIC), such that only a small proportion of surface-produced carbon sinks to the deep ocean (i.e., depths > 1000 m). As krill and smaller zooplankton feed, they also physically fragment particles into small, slower- or non-sinking pieces (via sloppy feeding, coprorhexy if fragmenting faeces), retarding POC export. This releases dissolved organic carbon (DOC) either directly from cells or indirectly via bacterial solubilisation (yellow circle around DOC). Bacteria can then remineralise the DOC to DIC (CO<sub>2</sub>, microbial gardening). Diel vertically migrating krill, smaller zooplankton and fish can actively transport carbon to depth by consuming POC in the surface layer at night, and metabolising it at their daytime, mesopelagic residence depths. Depending on species life history, active transport may occur on a seasonal basis as well. Numbers given are carbon fluxes (Gt C yr−1) in white boxes and carbon masses (Gt C) in dark boxes.<ref name=Cavan2019/>]]
{{see also|Carbon sequestration|biological pump}}

===Feeding===
Many krill are [[filter feeding|filter feeders]]:<ref name="mauchline"/> their frontmost [[appendage]]s, the thoracopods, form very fine combs with which they can filter out their food from the water. These filters can be very fine in species (such as ''Euphausia'' spp.) that feed primarily on [[phytoplankton]], in particular on [[diatom]]s, which are unicellular [[algae]]. Krill are mostly [[omnivorous]],<ref name="cripps">{{cite journal |author1=G. C. Cripps |author2=A. Atkinson |year=2000 |title=Fatty acid composition as an indicator of carnivory in Antarctic krill, ''Euphausia superba'' |journal=Canadian Journal of Fisheries and Aquatic Sciences |volume=57 |issue=S3 |pages=31–37 |doi=10.1139/f00-167|bibcode=2000CJFAS..57S..31C }}</ref> although a few species are [[carnivorous]], preying on small [[zooplankton]] and fish [[larvae]].<ref name="saether">{{cite journal |author1=Olav Saether |author2=Trond Erling Ellingsen |author3=Viggo Mohr |year=1986 |title=Lipids of North Atlantic krill |journal=[[Journal of Lipid Research]] |volume=27 |pages=274–285 |pmid=3734626 |url=http://www.jlr.org/content/27/3/274.full.pdf |issue=3}}</ref>

Krill are an important element of the aquatic [[food chain]]. Krill convert the [[primary production]] of their prey into a form suitable for consumption by larger animals that cannot feed directly on the minuscule algae. Northern krill and some other species have a relatively small filtering basket and actively hunt [[copepod]]s and larger zooplankton.<ref name="saether"/>

===Predation===
Many animals feed on krill, ranging from smaller animals like fish or penguins to larger ones like [[pinniped|seals]] and [[baleen whale]]s.<ref name="noaa_krill">{{cite web |author=M. J. Schramm |url=http://sanctuaries.noaa.gov/news/features/1007_krill.html |title=Tiny Krill: Giants in Marine Food Chain |publisher=NOAA National Marine Sanctuary Program |date=10 October 2007 |access-date=4 June 2010}}</ref>

Disturbances of an [[ecosystem]] resulting in a decline in the krill population can have far-reaching effects. During a [[coccolithophore]] bloom in the [[Bering Sea]] in 1998,<ref>{{cite web |author=J. Weier |url=http://earthobservatory.nasa.gov/Features/Coccoliths/bering_sea.php |title=Changing currents color the Bering Sea a new shade of blue |publisher=[[National Oceanic and Atmospheric Administration|NOAA]] Earth Observatory |year=1999 |access-date=15 June 2005}}</ref> for instance, the diatom concentration dropped in the affected area. Krill cannot feed on the smaller coccolithophores, and consequently the krill population (mainly ''E. pacifica'') in that region declined sharply. This in turn affected other species: the [[shearwater]] population dropped. The incident was thought to have been one reason [[salmon]] did not spawn that season.<ref>{{cite book |author1=R. D. Brodeur |author2=G. H. Kruse |author3=P. A. Livingston |author4=G. Walters |author5=J. Ianelli |author6=G. L. Swartzman |author7=M. Stepanenko |author8=T. Wyllie-Echeverria |title=Draft Report of the FOCI International Workshop on Recent Conditions in the Bering Sea |pages=22–26 |publisher=[[National Oceanic and Atmospheric Administration|NOAA]] |year=1998}}</ref>

Several single-celled [[endoparasitoid]]ic [[ciliate]]s of the genus ''[[Collinia]]'' can infect species of krill and devastate affected populations. Such diseases were reported for ''[[Thysanoessa]] inermis'' in the Bering Sea and also for ''E. pacifica'', ''Thysanoessa spinifera'', and ''T. gregaria'' off the North American Pacific coast.<ref>{{cite news |author=J. Roach |url=http://news.nationalgeographic.com/news/2003/07/0717_030717_krillkiller.html |archive-url=https://web.archive.org/web/20030724074903/http://news.nationalgeographic.com/news/2003/07/0717_030717_krillkiller.html |url-status=dead |archive-date=24 July 2003 |title=Scientists discover mystery krill killer |publisher=[[National Geographic News]] |date=17 July 2003}}</ref><ref>{{cite journal |author1=J. Gómez-Gutiérrez |author2=W. T. Peterson |author3=A. de Robertis |author4=R. D. Brodeur |title=Mass mortality of krill caused by parasitoid ciliates |journal=[[Science (journal)|Science]] |volume=301 |issue=5631 |page=339 |year=2003 |pmid=12869754 |doi=10.1126/science.1085164 |s2cid=28471713 }}</ref> Some [[ectoparasite]]s of the family [[Dajidae]] (epicaridean [[isopod]]s) afflict krill (and also shrimp and [[mysid]]s); one such parasite is ''[[Oculophryxus]] bicaulis'', which was found on the krill ''Stylocheiron affine'' and ''S. longicorne''. It attaches itself to the animal's eyestalk and sucks blood from its head; it apparently inhibits the host's reproduction, as none of the afflicted animals reached maturity.<ref>{{cite journal |author1=J. D. Shields |author2=J. Gómez-Gutiérrez |doi=10.1016/0020-7519(95)00126-3 |title=''Oculophryxus bicaulis'', a new genus and species of dajid isopod parasitic on the euphausiid ''Stylocheiron affine'' Hansen |journal=[[International Journal for Parasitology]] |volume=26 |issue=3 |pages=261–268 |year=1996|pmid=8786215 }}</ref>

[[Climate change]] poses another threat to krill populations.<ref>{{cite news |author=Rusty Dornin |url=http://www.cnn.com/EARTH/9707/06/krill.kill/ |title=Antarctic krill populations decreasing |publisher=[[CNN]] |date=6 July 1997 |access-date=18 June 2011}}</ref>

===Plastics===
Preliminary research indicates krill can digest [[microplastic]]s under {{cvt|5|mm}} in diameter, breaking them down and excreting them back into the environment in smaller form.<ref>{{cite journal|doi=10.1038/s41467-018-03465-9|pmid=29520086|pmc=5843626|title=Turning microplastics into nanoplastics through digestive fragmentation by Antarctic krill|journal=Nature Communications|volume=9|issue=1|pages=1001|year=2018|last1=Dawson|first1=Amanda L|last2=Kawaguchi|first2=So|last3=King|first3=Catherine K|last4=Townsend|first4=Kathy A|last5=King|first5=Robert|last6=Huston|first6=Wilhelmina M|last7=Bengtson Nash|first7=Susan M|bibcode=2018NatCo...9.1001D}}</ref>

{{clear}}

== Life history and behavior ==
[[File:Nauplius Hatching.jpg|thumb|A [[nauplius (larva)|nauplius]] of ''[[Euphausia pacifica]]'' hatching, emerging backwards from the egg]]

The life cycle of krill is relatively well understood, despite minor variations in detail from species to species.<ref name="Gurney"/><ref name="mauchline">{{cite book |author1=J. Mauchline |author2=L. R. Fisher |year=1969 |title=The Biology of Euphausiids |series=Advances in Marine Biology |volume=7 |publisher=[[Academic Press]] |isbn=978-7-7708-3615-2 }}</ref> After krill hatch, they experience several larval stages—''[[nauplius (larva)|nauplius]]'', ''[[pseudometanauplius]]'', ''[[metanauplius]]'', ''[[calyptopsis]]'', and ''[[furcilia]]'', each of which divides into sub-stages. The pseudometanauplius stage is exclusive to species that lay their eggs within an ovigerous sac: so-called "sac-spawners". The larvae grow and [[ecdysis|moult]] repeatedly as they develop, replacing their rigid exoskeleton when it becomes too small. Smaller animals moult more frequently than larger ones. [[Yolk]] reserves within their body nourish the larvae through metanauplius stage.

By the calyptopsis stages [[cellular differentiation|differentiation]] has progressed far enough for them to develop a mouth and a digestive tract, and they begin to eat phytoplankton. By that time their yolk reserves are exhausted and the larvae must have reached the [[photic zone]], the upper layers of the ocean where algae flourish. During the furcilia stages, segments with pairs of swimmerets are added, beginning at the frontmost segments. Each new pair becomes functional only at the next moult. The number of segments added during any one of the furcilia stages may vary even within one species depending on environmental conditions.<ref>{{cite journal |author=M. D. Knight |url=http://calcofi.org/publications/calcofireports/v25/Vol_25_Knight.pdf |title=Variation in larval morphogenesis within the Southern California Bight population of ''Euphausia pacifica'' from Winter through Summer, 1977–1978 |journal=CalCOFI Report |volume=XXV |year=1984 |access-date=5 November 2017 |archive-date=3 August 2019 |archive-url=https://web.archive.org/web/20190803044000/http://www.calcofi.org/publications/calcofireports/v25/Vol_25_Knight.pdf |url-status=dead }}</ref> After the final furcilia stage, an immature juvenile emerges in a shape similar to an adult, and subsequently develops [[gonad]]s and matures sexually.<ref name="fao_factsheet">{{cite web |publisher=[[Food and Agriculture Organization]] |url=http://www.fao.org/fishery/species/3393/en |work=Species factsheet |title=''Euphausia superba'' |access-date=4 June 2010}}</ref>

===Reproduction===
[[File:Nematoscelis difficilis female.jpg|thumb|left|The head of a female krill of the sac-spawning species ''[[Nematoscelis difficilis]]'' with her brood sac. The eggs have a diameter of {{convert|0.3|–|0.4|mm}}]]

During the mating season, which varies by species and climate, the male deposits a [[spermatophore|sperm sack]] at the female's genital opening (named ''thelycum''). The females can carry several thousand eggs in their [[ovary]], which may then account for as much as one third of the animal's body mass.<ref>{{cite journal |author1=R. M. Ross |author2=L. B. Quetin |title=How productive are Antarctic krill? |journal=[[BioScience]] |volume=36 |issue=4 |pages=264–269 |year=1986 |doi=10.2307/1310217 |jstor=1310217}}</ref> Krill can have multiple broods in one season, with interbrood intervals lasting on the order of days.<ref name="gut2005"/><ref name="cuzin">{{cite journal |author=Janine Cuzin-Roudy |year=2000 |title=Seasonal reproduction, multiple spawning, and fecundity in northern krill, ''Meganyctiphanes norvegica'', and Antarctic krill, ''Euphausia superba'' |journal=[[Canadian Journal of Fisheries and Aquatic Sciences]] |volume=57 |issue=S3 |pages=6–15 |doi=10.1139/f00-165|bibcode=2000CJFAS..57S...6C }}</ref>

Krill employ two types of spawning mechanism.<ref name="gut2005"/> The 57 species of the genera ''Bentheuphausia'', ''Euphausia'', ''Meganyctiphanes'', ''Thysanoessa'', and ''Thysanopoda'' are "broadcast spawners": the female releases the fertilised eggs into the water, where they usually sink, disperse, and are on their own. These species generally hatch in the nauplius 1 stage, but have recently been discovered to hatch sometimes as metanauplius or even as calyptopis stages.<ref>{{cite journal |author=J. Gómez-Gutiérrez |title=Hatching mechanism and delayed hatching of the eggs of three broadcast spawning euphausiid species under laboratory conditions |journal=[[Journal of Plankton Research]] |volume=24 |issue=12 |pages=1265–1276 |year=2002 |doi=10.1093/plankt/24.12.1265|doi-access=free }}</ref> The remaining 29 species of the other genera are "sac spawners", where the female carries the eggs with her, attached to the rearmost pairs of thoracopods until they hatch as metanauplii, although some species like ''Nematoscelis difficilis'' may hatch as nauplius or pseudometanauplius.<ref>{{cite book |author1=E. Brinton |author2=M. D. Ohman |author3=A. W. Townsend |author4=M. D. Knight |author5=A. L. Bridgeman |url=http://species-identification.org/species.php?species_group=euphausiids&menuentry=inleiding |title=Euphausiids of the World Ocean |publisher=World Biodiversity Database CD-ROM Series, [[Springer Verlag]] |year=2000 |isbn=978-3-540-14673-5 |access-date=4 December 2009 |archive-date=26 February 2012 |archive-url=https://web.archive.org/web/20120226200322/http://species-identification.org/species.php?species_group=euphausiids&menuentry=inleiding |url-status=dead }}</ref>

===Moulting===
Moulting occurs whenever a specimen outgrows its rigid exoskeleton. Young animals, growing faster, moult more often than older and larger ones. The frequency of moulting varies widely by species and is, even within one species, subject to many external factors such as latitude, water temperature, and food availability. The subtropical species ''Nyctiphanes simplex'', for instance, has an overall inter-moult period of two to seven days: larvae moult on the average every four days, while juveniles and adults do so, on average, every six days. For ''E. superba'' in the Antarctic sea, inter-moult periods ranging between 9 and 28 days depending on the temperature between {{cvt|−1|and|4|C}} have been observed, and for ''Meganyctiphanes norvegica'' in the [[North Sea]] the inter-moult periods range also from 9 and 28 days but at temperatures between {{cvt|2.5|and|15|C}}.<ref>{{cite journal |author=F. Buchholz |title=Experiments on the physiology of Southern and Northern krill, ''Euphausia superba'' and ''Meganyctiphanes norvegica'', with emphasis on moult and growth&nbsp;– a review |journal=[[Marine and Freshwater Behaviour and Physiology]] |volume=36 |issue=4 |pages=229–247 |year=2003 |doi=10.1080/10236240310001623376|bibcode=2003MFBP...36..229B |s2cid=85121989 }}</ref> ''E. superba'' is able to reduce its body size when there is not enough food available, moulting also when its exoskeleton becomes too large.<ref>{{cite journal |author1=H.-C. Shin |author2=S. 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> Similar shrinkage has also been observed for ''E. pacifica'', a species occurring in the Pacific Ocean from polar to temperate zones, as an adaptation to abnormally high water temperatures. Shrinkage has been postulated for other temperate-zone species of krill as well.<ref>{{cite journal |author1=B. Marinovic |author2=M. Mangel |url=http://people.ucsc.edu/~msmangel/MM.pdf |title=Krill can shrink as an ecological adaptation to temporarily unfavourable environments |journal=[[Ecology Letters]] |volume=2 |pages=338–343 |year=1999}}</ref>{{Clear}}

===Lifespan===
Some high-latitude species of krill can live for more than six years (e.g., ''Euphausia superba''); others, such as the mid-latitude species ''Euphausia pacifica'', live for only two years.<ref name="nicol"/> Subtropical or tropical species' longevity is still shorter, e.g., ''Nyctiphanes simplex'', which usually lives for only six to eight months.<ref>{{cite journal|doi=10.3354/meps119063 |author=J. G. Gómez |title=Distribution patterns, abundance and population dynamics of the euphausiids''Nyctiphanes simplex'' and ''Euphausia eximia'' off the west coast of Baja California, Mexico |journal=[[Marine Ecology Progress Series]] |volume=119 |pages=63–76 |year=1995 |url=https://www.int-res.com/articles/meps/119/m119p063.pdf |bibcode=1995MEPS..119...63G |doi-access=free }}</ref>

===Swarming===
[[File:Krill swarm.jpg|thumb|A krill swarm]]

Most krill are [[swarm]]ing animals; the sizes and densities of such swarms vary by species and region. For ''Euphausia superba'', swarms reach 10,000 to 60,000 individuals per cubic metre.<ref>{{cite book|author1=U. Kils |author2=P. Marshall |chapter=Der Krill, wie er schwimmt und frisst – neue Einsichten mit neuen Methoden ("''The Antarctic krill – how it swims and feeds – new insights with new methods''") |editor1=I. Hempel |editor2=G. Hempel |title=Biologie der Polarmeere&nbsp;– Erlebnisse und Ergebnisse (''Biology of the Polar Oceans Experiences and Results'') |publisher=[[Fischer Verlag]]|year=1995 |pages=201–210 |isbn=978-3-334-60950-7}}</ref><ref>{{cite book |author=R. Piper |title=Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals |url=https://archive.org/details/extraordinaryani0000pipe |url-access=registration |publisher=[[Greenwood Press (publisher)|Greenwood Press]] |year=2007 |isbn=978-0-313-33922-6}}</ref> Swarming is a defensive mechanism, confusing smaller predators that would like to pick out individuals. In 2012, Gandomi and Alavi presented what appears to be a [[Swarm intelligence#Krill herd algorithm|successful stochastic algorithm]] for modelling the behaviour of krill swarms. The algorithm is based on three main factors: " (i) movement induced by the presence of other individuals (ii) foraging activity, and (iii) random diffusion."<ref name=kha2012>{{cite journal |first1=A.H. |last1= Gandomi |first2=A.H. |last2=Alavi |title= Krill Herd: A New Bio-Inspired Optimization Algorithm |journal= Communications in Nonlinear Science and Numerical Simulation |doi=10.1016/j.cnsns.2012.05.010|year=2012 |volume=17 |issue=12 |pages=4831–4845 |bibcode=2012CNSNS..17.4831G }}</ref>

===Vertical migration===
[[File:Pleopods euphausia superba.jpg|right|thumb|Beating [[pleopod]]s of a swimming [[Antarctic krill]]]]

Krill typically follow a [[diurnality|diurnal]] [[Diel vertical migration|vertical migration]]. It has been assumed that they spend the day at greater depths and rise during the night toward the surface. The deeper they go, the more they reduce their activity,<ref>{{cite journal |author1=J. S. Jaffe |author2=M. D. Ohmann |author3=A. de Robertis |url=http://jaffeweb.ucsd.edu/files/pubs/Sonar%20estimates%20of%20daytime%20activity%20levels%20of%20Euphausia%20pacifica%20in%20Saanich%20Inlet.pdf |title=Sonar estimates of daytime activity levels of ''Euphausia pacifica'' in Saanich Inlet |journal=[[Canadian Journal of Fisheries and Aquatic Sciences]] |volume=56 |pages=2000–2010 |year=1999 |doi=10.1139/cjfas-56-11-2000 |issue=11 |s2cid=228567512 |url-status=dead |archive-url=https://web.archive.org/web/20110720075623/http://jaffeweb.ucsd.edu/files/pubs/Sonar%20estimates%20of%20daytime%20activity%20levels%20of%20Euphausia%20pacifica%20in%20Saanich%20Inlet.pdf |archive-date=20 July 2011 }}</ref> apparently to reduce encounters with predators and to conserve energy. Swimming activity in krill varies with stomach fullness. Sated animals that had been feeding at the surface swim less actively and therefore sink below the mixed layer.<ref>{{cite journal|author1=Geraint A. Tarling |author2=Magnus L. Johnson |year=2006 |title=Satiation gives krill that sinking feeling |journal=[[Current Biology]] |volume=16 |issue=3 |pages=83–84 |doi=10.1016/j.cub.2006.01.044 |pmid=16461267|doi-access=free |bibcode=2006CBio...16..R83T }}</ref> As they sink they produce [[feces]] which employs a role in the Antarctic [[carbon cycle]]. Krill with empty stomachs swim more actively and thus head towards the surface.

Vertical migration may be a 2–3 times daily occurrence. Some species (e.g., ''Euphausia superba'', ''E. pacifica'', ''E. hanseni'', ''Pseudeuphausia latifrons'', and ''Thysanoessa spinifera'') form surface swarms during the day for feeding and reproductive purposes even though such behaviour is dangerous because it makes them extremely vulnerable to predators.<ref name="howard">{{cite book |author=Dan Howard |chapter=Krill|chapter-url=http://pubs.usgs.gov/circ/c1198/chapters/133-140_Krill.pdf |pages=133–140 |editor1=Herman A. Karl |editor2=John L. Chin |editor3=Edward Ueber |editor4=Peter H. Stauffer |editor5=James W. Hendley II |url=http://pubs.usgs.gov/circ/c1198/ |title=Beyond the Golden Gate – Oceanography, Geology, Biology, and Environmental Issues in the Gulf of the Farallones |publisher=[[United States Geological Survey]] |id=Circular 1198 |year=2001 |access-date=8 October 2011}}</ref>

Experimental studies using ''[[Artemia salina]]'' as a model suggest that the vertical migrations of krill several hundreds of metres, in groups tens of metres deep, could collectively create enough downward jets of water to have a significant effect on ocean mixing.<ref>{{Cite journal|last=Wishart|first=Skye|date=July–August 2018|title=The krill effect|url=https://www.nzgeo.com/stories/the-krill-effect/|journal=New Zealand Geographic|issue=152|pages=24}}</ref>

Dense swarms can elicit a [[feeding frenzy]] among fish, birds and mammal predators, especially near the surface. When disturbed, a swarm scatters, and some individuals have even been observed to moult instantly, leaving the [[exuvia]] behind as a decoy.<ref>{{cite web |author=D. Howard |url=http://oceanexplorer.noaa.gov/explorations/02quest/background/krill/krill.html|title=Krill in Cordell Bank National Marine Sanctuary |publisher=[[National Oceanic and Atmospheric Administration]]|access-date=15 June 2005}}</ref>

Krill normally swim at a pace of 5–10&nbsp;cm/s (2–3 body lengths per second),<ref>{{cite journal |journal=ICES Journal of Marine Science |year=2005 |volume=62 |issue=1 |pages=25–32 |doi=10.1016/j.icesjms.2004.07.027 |title=New target-strength model indicates more krill in the Southern Ocean |author1=David A. Demer |author2=Stéphane G. Conti |doi-access=free |bibcode=2005ICJMS..62...25D }}</ref> using their swimmerets for propulsion. Their larger migrations are subject to ocean currents. When in danger, they show an [[escape reaction]] called [[Caridoid escape reaction|lobstering]]—flicking their [[Caudal (anatomical term)|caudal]] structures, the [[telson]] and the [[uropod]]s, they move backwards through the water relatively quickly, achieving speeds in the range of 10 to 27 body lengths per second, which for large krill such as ''E. superba'' means around {{cvt|0.8|m/s|ft/s|0}}.<ref>{{cite journal|author=U. Kils|title=Swimming behavior, swimming performance and energy balance of Antarctic krill ''Euphausia superba''|url=http://ecoscope.com/biomass3.htm|journal=BIOMASS Scientific Series 3, BIOMASS Research Series|pages=1–122|year=1982|access-date=11 November 2017|archive-date=2 June 2020|archive-url=https://web.archive.org/web/20200602170105/http://ecoscope.com/biomass3.htm|url-status=dead}}</ref> Their swimming performance has led many researchers to classify adult krill as [[nekton|micro-nektonic]] life-forms, i.e., small animals capable of individual motion against (weak) currents. Larval forms of krill are generally considered zooplankton.<ref>{{cite journal |author1=S. Nicol |author2=Y. Endo |url=http://www.fao.org/docrep/003/w5911e/w5911e00.htm |title=Krill Fisheries of the World |journal=FAO Fisheries Technical Paper |volume=367 |year=1997}}</ref>

==Biogeochemical cycles==
[[File:Role of Antarctic krill in biogeochemical cycles.webp|thumb|upright=2.4|right| {{center|'''Role of Antarctic krill in biogeochemical cycles'''}} Krill (as swarms and individuals) feed on phytoplankton at the surface (1) leaving only a proportion to sink as phytodetrital aggregates (2), which are broken up easily and may not sink below the permanent thermocline. Krill also release faecal pellets (3) whilst they feed, which can sink to the deep sea but can be consumed (coprophagy) and degraded as they descend (4) by krill, bacteria and zooplankton. In the marginal ice zone, faecal pellet flux can reach greater depths (5). Krill also release moults, which sink and contribute to the carbon flux (6). Nutrients are released by krill during sloppy feeding, excretion and egestion, such as iron and ammonium (7, see Fig. 2 for other nutrients released), and if they are released near the surface can stimulate phytoplankton production and further atmospheric CO<sub>2</sub> drawdown. Some adult krill permanently reside deeper in the water column, consuming organic material at depth (8). Any carbon (as organic matter or as CO<sub>2</sub>) that sinks below the permanent thermocline is removed from subjection to seasonal mixing and will remain stored in the deep ocean for at least a year (9). The swimming motions of migrating adult krill that migrate can mix nutrient-rich water from the deep (10), further stimulating primary production. Other adult krill forage on the seafloor, releasing respired CO<sub>2</sub> at depth and may be consumed by demersal predators (11). Larval krill, which in the Southern Ocean reside under the sea ice, undergo extensive diurnal vertical migration (12), potentially transferring CO<sub>2</sub> below the permanent thermocline. Krill are consumed by many predators including baleen whales (13), leading to storage of some of the krill carbon as biomass for decades before the whale dies, sinks to the seafloor and is consumed by deep sea organisms.<ref name=Cavan2019/>]]
{{further|Marine biogeochemical cycles}}

The Antarctic krill is an important species in the context of [[Marine biogeochemical cycles|biogeochemical cycling]]<ref>Ratnarajah, L., Bowie, A.R., Lannuzel, D., Meiners, K.M. and Nicol, S. (2014) "The biogeochemical role of baleen whales and krill in Southern Ocean nutrient cycling". ''PLOS ONE'', '''9'''(12): e114067. {{doi|10.1371/journal.pone.0114067|doi-access=free}}</ref><ref name=Cavan2019/> and in the [[Antarctic food web]].<ref>Hopkins, T.L., Ainley, D.G., Torres, J.J., Lancraft, T.M., 1993. Trophic structure in open waters of the Marginal Ice Zone in the Scotia Weddell Confluence region during spring (1983). Polar Biology 13, 389–397.</ref><ref>Lancraft, T.M., Relsenbichler, K.R., Robinson, B.H., Hopkins, T.L., Torres, J.J., 2004. A krill-dominated micronekton and macrozooplankton community in Croker Passage, Antarctica with an estimate of fish predation. Deep-Sea Research II 51, 2247–2260.</ref> It plays a prominent role in the Southern Ocean because of its ability to [[Nutrient cycle|cycle nutrients]] and to feed penguins and [[Baleen whale|baleen]] and [[blue whale]]s.

[[File:Cycling of nutrients by an individual krill.webp|thumb|upright=1.7|left| {{center|'''Cycling of nutrients by an individual krill'''}} When krill moult they release dissolved calcium, fluoride and phosphorus from the exoskeleton (1). The chitin (organic material) that forms the exoskeleton contributes to organic particle flux sinking to the deep ocean. Krill respire a portion of the energy derived from consuming phytoplankton or other animals as carbon dioxide (2), when swimming from mid/deep waters to the surface in large swarms krill mix water, which potentially brings nutrients to nutrient-poor surface waters (3), ammonium and phosphate is released from the gills and when excreting, along with dissolved organic carbon, nitrogen (e.g., urea) and phosphorus (DOC, DON and DOP, 2 & 4). Krill release fast-sinking faecal pellets containing particulate organic carbon, nitrogen and phosphorus (POC, PON and POP) and iron, the latter of which is bioavailable when leached into surrounding waters along with DOC, DON and DOP (5).<ref name=Cavan2019>Cavan, E.L., Belcher, A., Atkinson, A., Hill, S.L., Kawaguchi, S., McCormack, S., Meyer, B., Nicol, S., Ratnarajah, L., Schmidt, K. and Steinberg, D.K. (2019) "The importance of Antarctic krill in biogeochemical cycles". ''Nature communications'', '''10'''(1): 1–13. {{doi|10.1038/s41467-019-12668-7}}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]]

{{clear}}

==Human uses ==
{{see also|Krill fishery}}
[[File:Krillmeatkils.jpg|thumb|Deep-frozen plates of [[Antarctic krill]] for use as animal feed and raw material for cooking]]

===Harvesting history===
Krill have been harvested as a food source for humans and domesticated animals since at least the 19th century, and possibly earlier in Japan, where it was known as ''okiami''. Large-scale fishing developed in the late 1960s and early 1970s, and now occurs only in Antarctic waters and in the seas around Japan. Historically, the largest krill fishery nations were Japan and the Soviet Union, or, after the latter's dissolution, Russia and [[Ukraine]].<ref name="pri">{{cite web|author1=Grossman, Elizabeth|title=Scientists consider whether krill need to be protected from human over-hunting|url=https://www.pri.org/stories/2015-07-14/scientists-consider-whether-krill-need-be-protected-human-over-hunting|publisher=Public Radio International (PRI)|access-date=1 April 2017|date=14 July 2015}}</ref> The harvest peaked, which in 1983 was about 528,000 tonnes in the Southern Ocean alone (of which the Soviet Union took in 93%), is now managed as a precaution against overfishing.<ref>{{cite web|title=Krill fisheries and sustainability: Antarctic krill (Euphausia superba)|url=https://www.ccamlr.org/en/fisheries/krill-fisheries-and-sustainability|publisher=Commission for the Conservation of Antarctic Marine Living Resources|access-date=1 April 2017|date=23 April 2015}}</ref>

In 1993, two events caused a decline in krill fishing: Russia exited the industry; and the [[Convention for the Conservation of Antarctic Marine Living Resources]] (CCAMLR) defined maximum catch quotas for a [[sustainable fisheries|sustainable exploitation]] of Antarctic krill. After an October 2011 review, the Commission decided not to change the quota.<ref name=nature/>

The annual Antarctic catch stabilised at around 100,000 tonnes, which is roughly one fiftieth of the CCAMLR catch quota.<ref name=ccamlr/> The main limiting factor was probably high costs along with political and legal issues.<ref>{{cite journal|author=Minturn J. Wright |title=The Ownership of Antarctica, its Living and Mineral Resources |journal=Journal of Law and the Environment |year=1987 |volume=4 |issue=2 |pages=49–78 |url=http://heinonline.org/HOL/Page?handle=hein.journals/jlen4&div=14&collection=journals&set_as_cursor=0&men_tab=srchresults}}</ref> The Japanese fishery saturated at some 70,000 tonnes.<ref name="nicol2">{{cite journal |author1=S. Nicol |author2=J. Foster |title=Recent trends in the fishery for Antarctic krill |journal=Aquatic Living Resources |volume=16 |pages=42–45 |year=2003 |issue=1 |doi=10.1016/S0990-7440(03)00004-4|bibcode=2003AqLR...16...42N |url=http://www.alr-journal.org/10.1016/S0990-7440(03)00004-4/pdf }}</ref>

Although krill are found worldwide, fishing in Southern Oceans are preferred because the krill are more "catchable" and abundant in these regions. Particularly in Antarctic seas which are considered as [[wikt:pristine|pristine]], they are considered a "clean product".<ref name=pri/>

In 2018 it was announced that almost every krill fishing company operating in Antarctica will abandon operations in huge areas around the Antarctic Peninsula from 2020, including "buffer zones" around breeding colonies of penguins.<ref>{{Cite news|url=https://www.independent.co.uk/environment/antarctica-krill-fishing-industry-marine-protected-zone-greenpeace-whales-seals-penguins-a8439311.html|title=Krill fishing industry backs massive Antarctic ocean sanctuary to protect penguins, seals and whales|last=Josh|first=Gabbatiss|date=10 July 2018|work=The Independent|access-date=10 July 2018}}</ref>

===Human consumption===
{{See also|Shrimp paste}}
[[File:08634jfPamarawan, Malolos City, Bulacan River Districtfvf 27.jpg|thumb|Dried [[fermentation (food)|fermented]] krill, used to make ''Bagoong alamang'', a type of [[shrimp paste]] from the [[Philippines]]]]  
Although the total [[Biomass (ecology)|biomass]] of Antarctic krill may be as abundant as 400 million tonnes, the human impact on this [[keystone species]] is growing, with a 39% increase in total fishing yield to 294,000 tonnes over 2010–2014.<ref name="ccamlr">{{cite web|title=Krill – biology, ecology and fishing|url=https://www.ccamlr.org/en/fisheries/krill-%E2%80%93-biology-ecology-and-fishing|publisher=Commission for the Conservation of Antarctic Marine Living Resources|access-date=1 April 2017|date=28 April 2015}}</ref> Major countries involved in krill harvesting are [[Norway]] (56% of total catch in 2014), the [[Republic of Korea]] (19%), and [[China]] (18%).<ref name=ccamlr/>

Krill is a rich source of [[protein]] and [[omega-3 fatty acids]] which are under development in the early 21st century as human food, [[dietary supplement]]s as oil capsules, livestock food, and [[pet food]].<ref name=pri/><ref name="nature">{{cite journal|pmid=20811427|year=2010|last=Schiermeier|first=Q|title=Ecologists fear Antarctic krill crisis|journal=Nature|volume=467|issue=7311|pages=15|doi=10.1038/467015a|doi-access=free}}</ref><ref name="noaa">{{cite web|url=https://swfsc.noaa.gov/textblock.aspx?Division=AERD&id=11462|title=Why krill?|publisher=Southwest Fisheries Science Center, US National Oceanic and Atmospheric Administration|date=22 November 2016|access-date=1 April 2017}}</ref> Krill tastes salty with a somewhat stronger fish flavor than shrimp. For mass consumption and commercially prepared products, they must be peeled to remove the inedible [[exoskeleton]].<ref name=noaa/>

In 2011, the US [[Food and Drug Administration]] published a letter of no objection for a manufactured [[krill oil]] product to be [[generally recognized as safe]] (GRAS) for human consumption.<ref>{{cite web|url=https://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/NoticeInventory/ucm267323.htm|archive-url=https://web.archive.org/web/20140811041131/http://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/NoticeInventory/ucm267323.htm|url-status=dead|archive-date=11 August 2014|publisher=US FDA|author=Cheeseman MA|date=22 July 2011|title=Krill oil: Agency Response Letter GRAS Notice No. GRN 000371|access-date=3 June 2015}}</ref>

Krill (and other [[planktonic]] [[shrimp]], notably ''[[Acetes]]'' spp.) are most widely consumed in Southeast Asia, where it is [[fermentation (food)|fermented]] (with the shells intact) and usually ground finely to make [[shrimp paste]]. It can be stir-fried and eaten paired with white rice or used to add [[umami]] flavors to a wide variety of traditional dishes.<ref>{{cite journal |last=Omori |first=M. |title=Zooplankton fisheries of the world: A review |journal=Marine Biology |date=1978 |volume=48 |issue=3 |pages=199–205 |doi=10.1007/BF00397145|bibcode=1978MarBi..48..199O |s2cid=86540101 }}</ref><ref>{{cite journal |last1=Pongsetkul |first1=Jaksuma |last2=Benjakul |first2=Soottawat |last3=Sampavapol |first3=Punnanee |last4=Osako |first4=Kazufumi |last5=Faithong |first5=Nandhsha |title=Chemical composition and physical properties of salted shrimp paste (Kapi) produced in Thailand |journal=International Aquatic Research |date=17 September 2014 |volume=6 |issue=3 |pages=155–166 |doi=10.1007/s40071-014-0076-4|doi-access=free |bibcode=2014InAqR...6..155P }}</ref> The liquid from the fermentation process is also harvested as [[fish sauce]].<ref>{{cite journal |last1=Abe |first1=Kenji |last2=Suzuki |first2=Kenji |last3=Hashimoto |first3=Kanehisa |title=Utilization of Krill as a Fish Sauce Material |journal=Nippon Suisan Gakkaishi |date=1979 |volume=45 |issue=8 |pages=1013–1017 |doi=10.2331/suisan.45.1013|doi-access=free }}</ref>

== Bio-inspired robotics ==
Krill are agile swimmers in the intermediate [[Reynolds number]] regime, in which there are not many solutions for uncrewed underwater robotics, and have inspired robotic platforms to both study their locomotion as well as find design solutions for underwater robots.<ref>{{cite journal |last1=Oliveira Santos |first1=Sara |last2=Tack |first2=Nils |last3=Su |first3=Yunxing |last4=Cuenca-Jimenez |first4=Francisco |last5=Morales-Lopez |first5=Oscar |last6=Gomez-Valdez |first6=P. Antonio |last7=M Wilhelmus |first7=Monica |title=Pleobot: a modular robotic solution for metachronal swimming |journal=Scientific Reports |date=June 13, 2023 |volume=13 |issue=1 |page=9574 |doi=10.1038/s41598-023-36185-2 |pmid=37311777 |pmc=10264458 |arxiv=2303.00805 |bibcode=2023NatSR..13.9574O }}</ref>

== See also ==
{{Portal|Crustaceans}}
* [[Antarctic krill]]
* [[Cold-water shrimp]]
* [[Crustacean]]
* [[Krill fishery]]
* [[Krill oil]]
* [[Northern krill]]
* [[Krill paradox]]

== References ==
{{Reflist|colwidth=30em}}

== Further reading ==
{{Refbegin}}
* Boden, Brian P.; [[Martin W. Johnson|Johnson, Martin W.]]; Brinton, Edward: [http://escholarship.org/uc/item/62h3k734 "Euphausiacea (Crustacea) of the North Pacific"]. ''Bulletin of the Scripps Institution of Oceanography''. Volume 6 Number 8, 1955.
* [[Edward Brinton|Brinton, Edward]]: [http://escholarship.org/uc/item/90g09364 "Euphausiids of Southeast Asian waters"]. ''Naga Report'' volume 4, part 5. La Jolla: University of California, Scripps Institution of Oceanography, 1975.
* Conway, D. V. P.; White, R. G.; Hugues-Dit-Ciles, J.; Galienne, C. P.; Robins, D. B.: ''[http://www.mba.ac.uk/nmbl/publications/occpub/occasionalpub15.htm Guide to the coastal and surface zooplankton of the South-Western Indian Ocean] {{Webarchive|url=https://web.archive.org/web/20121023115546/http://www.mba.ac.uk/nmbl/publications/occpub/occasionalpub15.htm |date=23 October 2012 }}'', [https://web.archive.org/web/20070927161104/http://www.mba.ac.uk/nmbl/publications/occpub/guide/section13.pdf ''Order'' Euphausiacea], Occasional Publication of the [[Marine Biological Association of the United Kingdom]] No. 15, Plymouth, UK, 2003.
* Everson, I. (ed.): ''Krill: biology, ecology and fisheries''. Oxford, Blackwell Science; 2000. {{ISBN|0-632-05565-0}}.
* {{cite magazine|title=Krill — Untapped Bounty From the Sea?|magazine=[[National Geographic (magazine)|National Geographic]]|first=William M.|last=Hamner|pages=626–642|volume=165|issue=5|date=May 1984|issn=0027-9358|oclc=643483454}}
* Mauchline, J.: [http://www.ices.dk/products/fiche/Plankton/SHEET134.PDF Euphausiacea: ''Adults''] {{Webarchive|url=https://web.archive.org/web/20110515085757/http://www.ices.dk/products/fiche/Plankton/SHEET134.PDF |date=15 May 2011 }}, Conseil International pour l'Exploration de la Mer, 1971. Identification sheets for adult krill with many line drawings. [[PDF]] file, 2&nbsp;[[Megabyte|Mb]].
* Mauchline, J.: [http://www.ices.dk/products/fiche/Plankton/SHEET135-137.PDF Euphausiacea: ''Larvae''] {{Webarchive|url=https://web.archive.org/web/20120419210608/http://www.ices.dk/products/fiche/Plankton/SHEET135-137.PDF |date=19 April 2012 }}, Conseil International pour l'Exploration de la Mer, 1971. Identification sheets for larval stages of krill with many line drawings. PDF file, 3&nbsp;Mb.
* Tett, P.: ''[https://web.archive.org/web/20030823090126/http://www.lifesciences.napier.ac.uk/teaching/MB/Euphausiid03.html The biology of Euphausiids]'', lecture notes from a [https://web.archive.org/web/20050902210008/http://www.lifesciences.napier.ac.uk/teaching/MB/Index.html 2003 course in Marine Biology] from [[Napier University]].
* Tett, P.: ''[https://web.archive.org/web/20051001074904/http://www.lifesciences.napier.ac.uk/teaching/MB/MB9.html Bioluminescence]'', lecture notes from the 1999/2000 edition of that same course.
{{Refend}}

==External links==
{{Commons}}
{{Wikispecies|Euphausia}}
{{wiktionary}}
* [http://www.aad.gov.au/webcams/krill/ Webcam of Krill Aquarium at Australian Antarctic Division]
* [https://web.archive.org/web/20090531064701/http://www.antarcticanimation.com/content/animation/energies/energies.php 'Antarctic Energies'] animation by Lisa Roberts

{{Malacostraca}}
{{forage fish|state=expanded}}
{{commercial fish topics}}
{{Edible crustaceans}}
{{Taxonbar|from=Q29498}}
{{Authority control}}

[[Category:Krill| ]]
[[Category:Commercial crustaceans]]
[[Category:Edible crustaceans]]
[[Category:Extant Early Cretaceous first appearances]]
[[Category:Taxa named by James Dwight Dana]]