Editing Swarm behaviour (section)

====Krill====
[[File:Krill swarm.jpg|right|thumb|Swarming [[krill]]]]
Most [[krill]], small shrimp-like [[crustacean]]s, form large swarms, sometimes reaching densities of 10,000–60,000 individual animals per cubic metre.<ref name="H+83">{{cite journal |last1=Hamner |first1=WM |last2=Hamner |first2=PP |last3=Strand |first3=SW |last4=Gilmer |first4=RW |title=Behavior of Antarctic Krill, ''Euphausia superba'': Chemoreception, Feeding, Schooling and Molting' |journal=[[Science (journal)|Science]] |volume=220 |pages=433–5 |year=1983 |doi=10.1126/science.220.4595.433 |pmid=17831417 |issue=4595|bibcode= 1983Sci...220..433H|s2cid=22161686 }}</ref><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 |year=2007 |isbn=978-0-313-33922-6}}</ref> Swarming is a defensive mechanism, confusing smaller predators that would like to pick out single individuals. The largest swarms are visible from space and can be tracked by satellite.<ref>Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. {{ISBN|978-0-565-09243-6}}.</ref> One swarm was observed to cover an area of 450 square kilometres (175 square miles) of ocean, to a depth of 200 meters (650 feet) and was estimated to contain over 2 million tons of krill.<ref name="Hoare, Ben 2009 p. 107">Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. {{ISBN|978-0-565-09243-6}}</ref> Recent research suggests that krill do not simply drift passively in these currents but actually modify them.<ref name="Hoare, Ben 2009 p. 107"/> Krill typically follow a [[diurnality|diurnal]] [[diel vertical migration|vertical migration]]. By moving vertically through the ocean on a 12-hour cycle, the swarms play a major part in mixing deeper, nutrient-rich water with nutrient-poor water at the surface.<ref name="Hoare, Ben 2009 p. 107"/> Until recently it has been assumed that they spend the day at greater depths and rise during the night toward the surface. It has been found that 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 |issue=11 |pages=2000–10 |year=1999 |doi=10.1139/cjfas-56-11-2000 |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=2011-07-20}}</ref> apparently to reduce encounters with predators and to conserve energy.

Later work suggested that swimming activity in krill varied with stomach fullness. Satiated animals that had been feeding at the surface swim less actively and therefore sink below the mixed layer.<ref>{{cite journal |journal=Current Biology |year=2006 |volume=16 |pmid=16461267 |issue=3 |pages=83–4 |doi=10.1016/j.cub.2006.01.044 |title=Satiation gives krill that sinking feeling |author1=Geraint A. Tarling |author2=Magnus L. Johnson |name-list-style=amp|doi-access=free |bibcode=2006CBio...16..R83T }}</ref> As they sink they produce faeces which may mean that they have an important role to play in the Antarctic carbon cycle. Krill with empty stomachs were found to swim more actively and thus head towards the surface. This implies that vertical migration may be a bi- or tri-daily occurrence. Some species 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">Howard, D.: "[http://pubs.usgs.gov/circ/c1198/chapters/133-140_Krill.pdf Krill]", pp.&nbsp;133–140 in Karl, H.A. et al. (eds): ''[http://pubs.usgs.gov/circ/c1198/ Beyond the Golden Gate – Oceanography, Geology, Biology, and Environmental Issues in the Gulf of the Farallones]'', [[U.S. Geological Survey|USGS]] Circular 1198, 2001. URLs last accessed 2010-06-04.</ref> Dense swarms may 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 [[ecdysis|moult]] instantaneously, 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|NOAA]] |access-date=15 June 2005}}</ref> 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 Algorithm: A New Bio-Inspired Optimization Algorithm |journal= Communications in Nonlinear Science and Numerical Simulation |doi=10.1016/j.cnsns.2012.05.010|year=2012 |bibcode= 2012CNSNS..17.4831G |volume=17 |issue=12 |pages=4831–4845}}</ref>