Editing Swarm behaviour (section)

====Copepods====
[[File:Copepodkils.jpg|thumb|right|This copepod has its antenna spread (click to enlarge). The antenna detects the pressure wave of an approaching fish.]]
{{See also|Hunting copepods}}

[[Copepod]]s are a group of tiny [[crustacean]]s found in the sea and lakes. Many species are [[plankton]]ic (drifting in sea waters), and others are [[benthos|benthic]] (living on the ocean floor). Copepods are typically {{convert|1|to|2|mm|in|2}} long, with a teardrop shaped body and large [[antenna (biology)|antennae]]. Although like other crustaceans they have an armoured [[exoskeleton]], they are so small that in most species this thin armour, and the entire body, is almost totally transparent. Copepods have a compound, median single eye, usually bright red, in the centre of the transparent head.

Copepods also swarm. For example, monospecific swarms have been observed regularly around [[coral reef]]s and [[sea grass]], and in lakes. Swarms densities were about one million copepods per cubic metre. Typical swarms were one or two metres in diameter, but some exceeded 30 cubic metres. Copepods need visual contact to keep together, and they disperse at night.<ref>{{cite journal |last1= Hamner |first1= WM |last2= Carleton |first2= JH |year= 1979 |title= Copepod swarms: Attributes and role in coral reef ecosystems |journal= Limnol. Oceanogr. |volume= 24 |issue= 1 |pages= 1–14 |doi= 10.4319/lo.1979.24.1.0001 |bibcode= 1979LimOc..24....1H|doi-access= free }}</ref>

Spring produces [[algal bloom|blooms]] of swarming [[phytoplankton]] which provide food for copepods. Planktonic copepods are usually the dominant members of the [[zooplankton]], and are in turn major food organisms for many other marine animals. In particular, copepods are prey to [[forage fish]] and [[jellyfish]], both of which can assemble in vast, million-strong swarms. Some copepods have extremely fast [[escape response]]s when a predator is sensed and can jump with high speed over a few millimetres (see animated image below).

<gallery>
File:Herringramkils.jpg|Photo: School of herrings [[ram feeding]] on a swarm of copepods.
File:Synchropredation.gif|Animation showing how herrings hunting in a synchronised way can capture the very alert and evasive copepod (click to view).
File:Jelly cc4.jpg|Swarms of [[jellyfish]] also prey on copepods
</gallery>

Planktonic copepods are important to the [[carbon cycle]]. Some scientists say they form the largest animal [[biomass]] on earth.<ref>{{cite web |author1=Johannes Dürbaum |author2=Thorsten Künnemann |name-list-style=amp |date=November 5, 1997 |title=Biology of Copepods: An Introduction |url=http://www.uni-oldenburg.de/zoomorphology/Biologyintro.html |publisher=[[Carl von Ossietzky University of Oldenburg]] |access-date=December 8, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20100526164720/http://www.uni-oldenburg.de/zoomorphology/Biologyintro.html |archive-date=May 26, 2010}}</ref> They compete for this title with [[Antarctic krill]]. Because of their smaller size and relatively faster growth rates, however, and because they are more evenly distributed throughout more of the world's oceans, copepods almost certainly contribute far more to the [[secondary productivity]] of the world's oceans, and to the global ocean [[carbon sink]] than [[krill]], and perhaps more than all other groups of organisms together. The surface layers of the oceans are currently believed to be the world's largest carbon sink, absorbing about 2 billion tonnes of carbon a year, the equivalent to perhaps a third of [[greenhouse gas|human carbon emissions]], thus reducing their impact. Many planktonic copepods feed near the surface at night, then sink into deeper water during the day to avoid visual predators. Their moulted exoskeletons, faecal pellets and respiration at depth all bring carbon to the deep sea.