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Antarctic krill (Euphausia superba) is a species of krill found in Antarctic waters in the Southern Ocean.

Krill live in large and dense schools called swarms or clouds with up to 20,000 individuals per cubic meter. They are an important factor in primary production, feeding on phytoplankton and converting the energy so obtained to sustain their pelagic life style . They grow to a length of 6 cm, weigh up to 2 grams, and can live up to six years. Euphausia superba is the key species in the Antarctic Ecosystem, it is, in terms of biomass, the most successful animal of the earth, roughly twice the total biomass of humans, and it is the central figure in the just starting experiments of Ocean engineering.

Geographical Distribution

Krill can be found thronging the surface waters of the Southern Ocean; they have a circumpolar distribution, with the highest concentrations in the Atlantic sector. The Antarctic convergence defines more or less the northern boundary of the Southern Ocean. That is the circumpolar front where the cold Antarctic surface water submerges below the warmer subantarctic waters.

The Southern Ocean with its Atlantic, Pacific and Indian sectors stretches from the polarfront at ca. 55 degree South to the edge of the continent, covering 32 million square kilometers. This is 65 times the size of the North Sea. In the winter season more than three quarters of this area is covered by ice compared to the 24 million square kilometers which become icefree in the summer. The water temperatures are between - 1.3 and 3 degree Centigrade.

The waters of the Southern Ocean form a system of currents. When there is a West Wind Drift the surface strata travels round Antarctica in an easterly direction. Near to the continent the East Wind Drift runs counterclockwise. At the front between both, large eddies develop, for example in the Weddell Sea. The krill schools drift with these watermasses, and it is one stock all around Antarctica with gene exchange over the whole area. There is not much knowledge about migration patterns because it is not possible to put tags on krill yet.

Position in the Antarctic ecosystem

Antarctic 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. The size-step between krill and its prey is unusually large, normally it takes 3 or 4 steps from the 20 micrometer small phytoplankton to organisms of krill size (via copepods and small fish). The next size-step in the food chain to the whales is also enormous, a phenomenon of the Antarctic ecosystem which is found nowhere else in the world. E. superba lives only in the Southern Ocean, in the North Atlantic, Meganyctiphanes norvegica and in the Pacific, Euphausia pacifica are dominant species.

Systematic

The order euphausiacea are shrimplike eucarida. All thoracomers are joined to the carapace. The shortness of the thoracomers on each side of the carapace makes the gills of the Antartic krill visible. The thoracopods do not form a gnathopod, this differentiates this order against the decapoda. Wikispecies

Biomass

Their biomass is estimated to be between 100 and 800 million tonnes, making E. superba 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. Why can krill build up such a high biomass? The waters around the icy continent harbor one of the the largest plankton assemblages of our world, maybe the biggest of all. It is full of phytoplankton, as the water rises from the depths to the light flooded surface, bringing nutrients from all oceans back to the photic zone.

Decline with shrinking pack ice

There are fears that the Antartic Krill's overall biomass has been declining rapidly over the last few decades, some scientists have speculated this value is as high as 80%. This could be caused by the reduction of the pack ice zone due to the consequence of global warming (review in Gross 2005). The graph shows the rising temperatures in the Southern Ocean and the loss of pack ice (inverted scale) over the last years 40 years. Antartic Krill, especially in the early stages of development seem to need the pack ice structures in order to have a good chance of survival. The pack ice provides natural cave-like features, the Krill use these cave-like features to evade their predators. In years where there has been a decline in the average amount of pack ice the Krill's natural predators will substitute the Krill for Salps in order to feed. (Atkinson et. al., 2004).

Fisheries

The fishery of Antarctic krill is on the order of 90,000 tonnes per year. The products are used in Japan and for feeds. Krill fisheries are difficult in two aspects: because a krill net needs to have very fine meshes as it has a very high drag, producing a bow wave, deflecting the krill to the sides. Also fine meshes clog very fast. A fine net is also a very delicate net, and the first krill nets exploded while fishing through the schools. Another problem is how to bring the catch on board.

During hauling of the full net out of the water the organisms compress each other, and much juice is lost. Experiments have been carried out to pump krill, still in water, through a large tube on board. A special krill net is under development too. The processing of the krill has to be very quick because it deteriorates within a few hours. One goal is to split the muscular hind part from the front part and to separate the chitin armor, in order to produce frosted products and concentrate powders. Its high protein and vitamin content makes krill quite suitable for direct human consumption and for the animal-feed industry.

Food

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, especially very small diatoms (20 micrometer), which it filters from the water with a "feeding basket" (see below), but they can also catch copepods, amphipods and other small zooplankton. In aquaria they have been observed eating each other. When they are not fed in aquaria, they shrink in size after molting, which in nature is exceptional for a higher animal the size of krill. Likely this is an adaption for the seasonality of its food supply, which is extremely limited in the dark winter months and under the ice. The glass shells of the diatoms are cracked in the "gastric mill" and then digested in the hepatopancreas. The gut forms a strait tube, the digestion efficiency is not very high, therefore a lot of carbon is still left in the feces (see below).

Bioluminescence

Krill are often referred to as light-shrimp because they can emit light - which is produced by light emitting organs - see (bioluminescence). These organs are located on various parts of its body: one pair of organs at the eyestalk (a high magnification image of the head can be accessed here) another pair on the hips of the 2nd and 7th thoracopods and singlular organs are located on the four pleonsternites. These lightorgans will emit from time to time a yellow-green light for upto 2 to 3 seconds. They are considered so highly developed that they can be compared with a torchlight: A concave reflector in the back of the organ and a lens in the front guide the produced light, and the whole organ can be rotated by muscles. The function of this light is not quite clear, some arguments have suggested that they compensate their shadow so that they are not visible for predators from below, other speculations are that it is important for mating or schooling at night.

Development

The main spawning time of krill is from January to March, over the shelf, but also in oceanic areas over deep waters. As it is typical for euphausiaceans, the male attaches a sperm package to the genital opening of the female. For this purpose, the first pleopods of the male are constructed as tools. According to the classical hypothesis of MARR 1962, which he derived from the results of the great Discovery-Expedition, the development is this: Gastrulation sets in during the descent of the 0.6 mm eggs, on the shelf at the bottom, in oceanic areas in depths around 2000 m. From the egg hatches the 1st nauplius and starts the migration towards the surface with the aid of its three pairs of legs ("developmental ascent"). The next two larval stages, 2nd nauplius and metanauplius, do not eat but are nourished by the yolk. After three weeks the little krill has finished as 1st calyptopis the ascent. Growing larger, additional larval stages follow (2nd and 3rd calyptopis, 1st to 6th furcilia). They are characterized by increasing development of the additional legs, the compound eyes and the setae. At 15 mm the juvenile krill resembles the habitus of the adults. After two, maybe three years, krill reaches maturity. As characteristic for all crustaceans krill must molt in order to grow. Approximately every 13 to 20 days krill ejects from its chitin skin and leaves it as exuvia behind.

Filter feeding

How do they manage to utilize directly the minute phytoplankton cells (when no other higher animal of krill size can do such)? They developed in their front legs a very efficient filtering apparatus (Kils 1983): 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 in the in situ image below, and then they comb the algae to the mouth opening with special setae on the inner side of the thoracopods. The fine structure of the feeding basket on electron microscope images can be seen here.

Ice-algae raking

Krill can scrape off the green lawn of ice-algae from the underside of the pack ice (Marschall 1988). In this image, taken via a ROV (image from Kils & Marschall 1995), most krill swim in an upside-down position directly under the ice. Only one animal (in the middle) is hovering in the free water. They have developed special rows of rake like setae at the tips of the thoracopods and graze the ice in a zig-zak fashion, akin to a lawnmower. One krill can clear an area of a square foot in about 10 minutes. It is relatively new knowledge that the film of ice algae is very well developed over vast areas, often containing much more carbon than the whole watercolumn below. Especially in the spring krill finds here an extensive energy source.

Escape reaction

Krill evades predators by very fast backward swimming (lobstering), flipping its telson. They reach speeds of over 60 cm per second (Kils 1982). The trigger time to optical stimulus is, in spite of the low temperatures, only 55 milliseconds.

The compound eye

Although the uses and reasons behind the development of their massive black compound eyes remain a mystery, there is no doubt that antarctic krill have one of the most fantastic structures for vision seen in nature.

The Biological Pump and Carbon Sequestration

Krill is a highly untidy feeder, and it often spits out aggregates of phytoplankton (spit balls) containing thousands of cells sticking together. Also it produces fecal strings, which still contain a lot of carbon and the glass shells of the diatoms. Both are heavy and sink very fast into the abyss. This process is called Biological pump. As the waters around Antarctica are very deep (2000 - 4000 m), this process exports large quantities of carbon (fixed CO2) from the biosphere and sequesters it for about 1000 years (Carbon Sequestration, Carbon_dioxide_sink). If the phytoplankton is consumed by other components of the pelagic ecosystem, most carbon stays in the upper strata. There are speculations that this process is one of the big bio-feedbacks of the planet, maybe the strongest of them all, driven by a gigantic biomass. But much more research needs to be done for a conclusion to be reached.

Future visions and Ocean Engineering

Regardless of the diminished available knowledge, there are large scale experiments already being performed to increase Carbon Sequestration: in vast areas of the Southern Ocean there are plenty of nutrients but still the phytoplankton does not grow much. These areas are coined HNLC (high nutrient, low carbon). The phenomenon is called Antarctic Paradoxon. The reason is that iron is missing . Relatively small injections of iron from research vessels trigger very large blooms covering many miles. The hope is that such Ocean engineering in large scale will draw down carbon dioxide as compensation for the burning of fossil fuels . Krill is the key player in collecting the minute plankton cells so they sink fast in the form of spit balls and fecal strings. The general idea is that in the future a fleet of tankers is going to circle the Southern Seas injecting iron, so an animal, which nearly nobody knows, might help keep cars and air-conditioners running.

References

Atkinson A, Siegel V, Pakhomov E, Rothery P 2004 Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature 432:100-103

Gross L 2005 As the Antarctic Ice Pack Recedes, a Fragile Ecosystem Hangs in the Balance. PLoS Biol 3(4):127

Kils U & Klages N 1979 Der Krill. Naturwissenschaftliche Rundschau 10:397-402 (English translation: The Krill)

Kils U 1982 Swimming behavior, Swimming Performance and Energy Balance of Antarctic Krill Euphausia superba. BIOMASS Scientific Series 3, BIOMASS Research Series, 1-122

Kils U 1983 Swimming and feeding of Antarctic Krill, Euphausia superba - some outstanding energetics and dynamics - some unique morphological details. In: Berichte zur Polarforschung, Alfred Wegener Institut fuer Polarforschung, Sonderheft 4 (1983) On the biology of Krill Euphausia superba, Proceedings of the Seminar and Report of Krill Ecology Group, Editor S. B. Schnack, 130-155 and title page image

Kils U & Marschall P 1995 Der Krill, wie er schwimmt und frisst - neue Einsichten mit neuen Methoden (The antarctic krill - feeding and swimming performances - new insights with new methods) In Hempel I, Hempel G, Biologie der Polarmeere - Erlebnisse und Ergebnisse (Biology of the polar oceans) Fischer Jena - Stuttgart - New York, 201-207 (and images p 209-210)

Loeb V, Siegel V, Holm-Hansen O, Hewitt R, Fraser W, et al. 1997 Effects of sea-ice extent and krill or salp dominance on the Antarctic food web. Nature 387:897-900

Marr J W S 1962 The natural history and geography of the Antarctic Krill Euphausia superba - Discovery report 32:33-464

Marschall P 1988 The overwintering strategy of Antarctic krill under the pack ice of the Weddell Sea - Polar Biol 9:129-135

• long list

External links

• "Virtual microscope" of Antarctic krill for interactive dives into their morphology and behavior, along with other peer-reviewed information
• high resolution images on Wikisource

Notes

This species is often misspelled Euphasia superba or Eupausia superba .

• Crustaceans