Antarctic krill
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- this stage has been peer reviewed by an academic board - no errors were found - formatting could be better
The Antarctic krill (Euphausia superba [2]) is a species of krill, shrimp-like invertebrates found in the Antarctic waters of the Southern Ocean.
Krill live in large, dense schools, called swarms, with up to 20,000 individual krill per cubic meter. They feed directly on minute phytoplankton, thereby using the primary production energy originally derived from the sun in order to sustain their pelagic life cycle[3][4]. They grow to a length of 6 cm, weigh up to 2 grams, and can live up to six years.
Systematics
All members of the krill order are shrimplike animals of the crustacean superorder Eucarida. Their breastplate units, or thoracomers, are joined with the carapace. The short length of these thoracomers on each side of the carapace makes the gills of the Antartic krill visible to the human eye. The legs do not form a jaw structure, which differentiates this order from the crabs, lobsters and shrimp. Wikispecies
Development
The main spawning time of Antarctic krill is from January through March, both above the continental shelf and also in the upper region of deep sea oceanic areas. In the typical way of all 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[5], which he derived from the results of the expedition of the famous British research vessel "Discovery", the development is as follows: 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 - 3000 m. From the time the egg hatches, the 1st nauplius (i.e., larval stage) starts migrating towards the surface with the aid of its three pairs of legs; the so-called "developmental ascent".
The next two larval stages, termed 2nd nauplius and metanauplius, still do not eat but are nourished by the yolk. After three weeks, the little krill has finished 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 (bristles). At 15 mm, the juvenile krill resembles the habitus of the adults. After two to three years, krill reaches maturity. Like all crustaceans, krill must molt in order to grow. Approximately every 13 to 20 days krill sheds its chitin skin and leaves it behind as exuvia.
Food
The gut of E. superba can often be seen to be shining in green through the animal's 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" [6] (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 is exceptional for animals the size of krill. Likely this is an adaption to the seasonality of its food supply, which is mostly limited to the dark winter months 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; its digestion efficiency is not very high and therefore a lot of carbon is still left in the feces (see below).
Filter feeding
The Antarctic krill manages to utilize directly the minute phytoplankton cells, which no other higher animal of krill size can do. This is accomplished through filter feeding, using the krill's developed front legs, providing for a very efficient filtering apparatus (Kils 1983[7]): the six thoracopods form a very effective "feeding basket" used to collect phytoplankton from the open water. In the movie linked to the right, the krill is hovering at a 55° angle on the spot. In lower food concentrations, the feeding basket is pushed through the water for over half a meter in an opened position, like in the in situ image below, and then the algae are combed to the mouth opening with special setae on the inner side of the thoracopods. See "Details of the feeding basket" below for some electron microscope images showing the fine structure of the feeding basket.
Ice-algae raking
Krill can scrape off the green lawn of ice-algae from the underside of the pack ice [8] (Marschall 1988[9]). The image to the right, taken via a ROV (image from Kils & Marschall 1995[10]), features how most krill swim in an upside-down position directly under the ice. Only a single animal (in the middle) can be seen hovering in the free water. Krill have developed special rows of rake-like setae at the tips of the thoracopods, and graze the ice in a zig-zag 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.
The Biological Pump and Carbon Sequestration
The krill is a highly untidy feeder, and it often spits out aggregates of phytoplankton (spit balls) containing thousands of cells sticking together. It also produces fecal strings that still contain plenty of carbon and the glass shells of the diatoms. Both are heavy and sink very fast into the abyss. This process is called the biological pump. As the waters around Antarctica are very deep (2000 – 4000 m), they act as a Carbon dioxide sink: this process exports large quantities of carbon (fixed Carbon dioxide, CO2) from the biosphere and sequesters it for about 1000 years.
If the phytoplankton is consumed by other components of the pelagic ecosystem, most of the carbon retains in the upper strata. There are speculations that this process is one of the largest bio-feedbacks of the planet, maybe the most sizable of them all, driven by a gigantic biomass. Still more research is needed to quantify the Southern Ocean ecosystem.
Means of survival
The compound eye
Although the uses for 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.
Bioluminescence
Krill are often referred to as light-shrimp because they can emit light, produced by bioluminiscent organs. These organs are located on various parts of the individual krill's body: one pair of organs at the eyestalk (c.f. the image of the head above), another pair on the hips of the 2nd and 7th thoracopods, and singlular organs are located on the four pleonsternites. These light organs will emit a yellow-green light from time to time, for up to 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 lens in the front guide the light produced, and the whole organ can be rotated through muscles. The function of these lights is not yet fully understood, some hypotheses have suggested they serve to compensate the krill's shadow so that they are not visible to predators from below; other speculations maintain that they play a significant role in mating or schooling at night.
Escape reaction
Krill evade predators with the aid of a very fast backward swimming escape reaction, flipping its telson (this swimming pattern is also known as lobstering). They can reach speeds of over 60 cm per second (Kils 1982[11]). The trigger time to optical stimulus is, despite the low temperatures, only 55 milliseconds.
Geographical distribution
Krill are found thronging the surface waters of the Southern Ocean; they have a circumpolar distribution, with the highest concentrations located in the Atlantic sector.
The northern boundary of the Southern Ocean with its Atlantic, Pacific Ocean and Indian Ocean sectors is defined more or less by the Antarctic convergence, a circumpolar front where the cold Antarctic surface water submerges below the warmer subantarctic waters. This front runs roughly a 55° South; from there to the continent, the Southern Ocean covers 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 become covered by ice, whereas 24 million square kilometers become ice free in summer. The water temperatures range between - 1.3 and 3° C.
The waters of the Southern Ocean form a system of currents. Whenever there is a West Wind Drift, the surface strata travels around Antarctica in an easterly direction. Near 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 water masses, to establish one single stock all around Antarctica, with gene exchange over the whole area. Currently, there is little knowledge of the precise migration patterns since individual krill cannot be tagged yet to track their movements.
Position in the Antarctic ecosystem
The Antarctic krill is the keystone species of the Antarctica ecosystem, and provides an important food source for whales, seals, Leopard Seals, fur seals, Crabeater Seals, squid, icefish, penguins, albatrosses and many other species of birds. The size-step between krill and its prey is unusually large, normally taking three or four steps from the 20 micrometer-small phytoplankton for krill-sized organisms (via copepods and small fish)[12]. The next size-step in the food chain to the whales is also enormous, a phenomenon only found in the Antarctic ecosystem. E. superba lives only in the Southern Ocean. In the North Atlantic, Meganyctiphanes norvegica and in the Pacific, Euphausia pacifica are the dominant species.
Biomass
The Antarctic krill's 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. The reason krill are able to build up such a high biomass is that the waters around the icy continent harbor one of the the largest plankton assemblages in the world, possibly the largest. It is filled with phytoplankton, as the water rises from the depths to the light flooded surface, bringing nutrients from all the oceans back into the photic zone.
Decline with shrinking pack ice
There are concerns that the Antarctic krill's overall biomass has been declining rapidly over the last few decades. Some scientists have speculated this value being 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[13]). The graph on the right depicts the rising temperatures of the Southern Ocean and the loss of pack ice (on an 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 fair chance of survival. The pack ice provides natural cave-like features which the krill uses to evade their predators. In the years of low pack ice conditions the krill is substituted by Salps (Atkinson et. al., 2004[14]).
Fisheries
The fishery of the Antarctic krill is on the order of 90,000 tonnes per year. The products are used largely in Japan as delicatess and worldwide as animal food. Krill fisheries are difficult in two important respects: first, because a krill net needs to have very fine meshes as it has a very high drag, producing a bow wave that deflects the krill to the sides. Second, fine meshes tend to clog very fast. Additionally, a fine net is also, by definition, a very delicate net, and the first krill nets designed literally exploded while fishing through the krill schools.
Yet another problem is bringing the krill catch on board. When the full net is hauled out of the water, the organisms compress each other, resulting in great loss of the krill's liquids. Experiments have been carried out to pump krill, while still in water, through a large tube on board. Special krill nets also are currently under development. The processing of the krill must be very rapid since the catch deteriorates within several hours. Aims are splitting the muscular hind part from the front part and separating the chitin armor, in order to produce frosted products and concentrate powders. Its high protein and vitamin content makes krill quite suitable for both direct human consumption and the animal-feed industry.
Future visions and Ocean Engineering
Despite the scarce knowledge available about the whole Antarctic ecosystem, 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 the Antarctic Paradox. The reason for this is that iron is missing [15]. Relatively small injections of iron from research vessels trigger very large blooms, covering many miles. The hope is that such large scale exercises will draw down carbon dioxide as compensation for the burning of fossil fuels [16]. Krill is the key player in collecting the minute plankton cells so as to sink faster, in the form of spit balls and fecal strings. The vision is that in the future a fleet of tankers would circle the Southern Seas, injecting iron, so this relatively unknown animal might help keep cars and airconditioners running.
Additional pictures
Details of the feeding basket
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Click on the images for higher resolutions.
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Notes
^ This species is often misspelled Euphasia superba [17] or Eupausia superba [18].
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
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