Krill is the Norwegian word for whale food. It is also used as synonym for euphausiids, which are shrimp-like marine invertebrates, important organisms of the plankton (zooplankton).

In the literal sense krill is used as common name for the most spectacular species: the Antarctic Krill (Euphausia superba) of the Antarctic waters in the Southern Ocean. It is an euphausiid (Arthropoda / Crustacea / Malacostraca / Euphausiacea) [1] (http://www.biosis.org.uk/zrdocs/zoolinfo/grp_crus.htm#Euphausiacea). Krill live in large schools (swarms) and convert the primary production directly into a relatively large animal [2] (http://www.ecoscope.com/naturwie.htm)[3] (http://www.ecoscope.com/polarbe2.htm): they grow to a length of 6 cm, weigh 2 grammes, and live probably for 6 years.

Krill is the keystone species of the ecosystem of Antarctica, and is an important food organism for whales, seals, Leopard Seals, fur seals, Crabeater Seals, squid, icefish, penguins, albatrosses and many other birds. Their biomass is estimated to be between 100 and 800 million tonnes, making E. superba probably the most successful animal on the planet; for comparison, the total non-krill yield from all world fisheries is about 100 million tonnes per year. The fishery of krill is on the order of 90,000 tonnes per year.

The gut of E. superba can often be seen shining in green through its transparent skin, an indication that this species feeds predominantly on phytoplankton, e.g. diatoms, which it filters from the water with a "feeding basket" [4] (http://www.ecoscope.com/krill/filter/index.htm). Antarctic krill can also scrape algae from the undersurface of the pack ice [5] (http://www.ecoscope.com/icecave2.htm) and prey on copepods. Krill is also called light-shrimp because it can produce a yellow green light with light-organs at the eyes and base of the swimming legs (bioluminescence).

E. superba lives only in the Southern Ocean, in the North Atlantic, Meganyctiphanes norvegica and in the Pacific, Euphausia pacifica are other Euphausiacea species.

Filter feeding: slow motion movie (300 frames per second) of pump filtering of the feeding basket formed by the six thoracopods shown by krill collecting phytoplankton from the open water. The krill is hovering at a 55 degree angle at the spot. This behavior is shown under very high phytoplankton concentrations. In lower food concentrations the feeding basket is pushed through the water over half a meter in an opened position, like indicated in the watercolor sketch at night.

Detail of the filter formed by the thoracopods. Like a comb long setae stretch forwards to cover over the gap between the thoracopods.

The first degree filter setae carry in v-form two rows of second degree setae, pointing towards the inside of the feeding basket (electron microscope image). To display the total area of this fascinating structure one would have to tile (http://www.ecoscope.com/krill/filter/filter7/index.htm) 7500 times this image.

Into these gaps are then third degree setae reach half the distance. In some parts of the net the openings are only 1 micrometer wide (electron microscope image).

In the spring krill can scrape off the green lawn of ice algae from the underside of the pack ice in Antarctica. In this image, taken by an ROV, most krill swim in an upside down position directly under the ice. Only one animal (in the middle) is hovering in the open water.

Escape reaction: Krill can evade predators by very fast backward swimming (lobstering), flipping its telson. They reach speeds of over 60 cm per second. The trigger time to optical stimulus is in spite of the low temperatures only 55 milliseconds.

External link

• "virtual microscope" (http://www.ecoscope.com/krill) of Antarctic krill for interactive dives into their morphology and behavior, along with other peer-reviewed information

This species is often misspelled Euphasia superba [6] (http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&q=%22euphasia+superba%22&btnG=Google+Search) or Eupausia superba [7] (http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&q=%22eupausia+superba%22&btnG=Google+Search).

"Krill" is also the name of an Elfquest character, the WaveDancer maiden who's Brill's twin sister.