| Antarctic krill | |
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Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Malacostraca |
| Order: | Euphausiacea |
| Family: | Euphausiidae |
| Genus: | Euphausia |
| Species: | E. superba |
| Binomial name | |
| Euphausia superba Dana, 1850 | |
| Synonyms[2] | |
Antarctic krill (Euphausia superba) is aspecies ofkrill found in theAntarctic waters of theSouthern Ocean. It is a small, swimmingcrustacean that lives in large schools, calledswarms, sometimes reaching densities of 10,000–30,000 animals per cubic metre.[3] It feeds directly on minutephytoplankton, thereby using theprimary productionenergy that phytoplankton originally derive from the sun in order to sustain itspelagiclife cycle.[4] It grows to a length of 6 centimetres (2.4 in), weighs up to 2 grams (0.071 oz), and can live for up to six years. A key species in the Antarcticecosystem and in terms ofbiomass,E. superba is one of the most abundant animal species on the planet, with a cumulative biomass of approximately 500 million metric tons (550 million short tons; 490 million long tons).[5]
The mainspawning season of Antarctic krill is from January to March, both above thecontinental shelf and also in the upper region of deep sea oceanic areas. In the typical way of all krill, the male attaches aspermatophore to the genital opening of the female. For this purpose, the firstpleopods (legs attached to the abdomen) of the male are constructed as mating tools. Females lay 6,000–10,000eggs at one time. They arefertilised as they pass out of the genital opening.[6]
According to the classical hypothesis of Marriosis De' Abrtona,[7] derived from the results of the expedition of the famous British research vesselRRSDiscovery, egg development then proceeds as follows:gastrulation (development of egg into embryo) sets in during the descent of the 0.6 mm (0.024 in) eggs on the shelf at the bottom, in oceanic areas in depths around 2,000–3,000 metres (6,600–9,800 ft). The egg hatches as anauplius larva; once this has moulted into a metanauplius, the young animal starts migrating towards the surface in a migration known as developmental ascent.[8]
The next two larval stages, termed second nauplius and metanauplius, still do not eat but are nourished by the remainingyolk. After three weeks, the young krill has finished the ascent. They can appear in enormous numbers counting 2 per litre in 60 m (200 ft) water depth. Growing larger, additional larval stages follow (second and third calyptopis, first to sixth furcilia). They are characterised by increasing development of the additional legs, the compound eyes and the setae (bristles). At 15 mm (0.59 in), the juvenile krill resembles the habitus of the adults. Krill reach maturity after two to three years. Like allcrustaceans, krill mustmoult in order to grow. Approximately every 13 to 20 days, krill shed theirchitinousexoskeleton and leave it behind asexuvia.
The gut ofE. superba can often be seen shining green through its transparent skin. This species feeds predominantly onphytoplankton—especially very smalldiatoms (20μm), which it filters from the water with a feeding basket.[9] The glass-like shells of thediatoms are cracked in thegastric mill and then digested in thehepatopancreas. The krill can also catch and eatcopepods,amphipods and other smallzooplankton. The gut forms a straight tube; its digestive efficiency is not very high and therefore a lot ofcarbon is still present in thefeces. Antarctic krill (E. superba) primarily has chitinolytic enzymes in the stomach and mid-gut to break down chitinous spines on diatoms, additional enzymes can vary due to its expansive diet.[10]
Inaquaria, krill have been observed toeat each other. When they are not fed, they shrink in size aftermoulting, which is exceptional for animals this size. It is likely that this is anadaptation to the seasonality of their food supply, which is limited in the dark winter months under the ice. However, the animal's compound eyes do not shrink, and so the ratio between eye size and body length has thus been found to be a reliable indicator of starvation.[11] A krill with ample food supply would have eyes proportional to body length, compared to a starving krill that would have eyes that appeared larger than what is normal.
Antarctic krill directly ingest minutephytoplankton cells, which no other animal of krill size can do. This is accomplished throughfilter feeding, using the krill's highly developed front legs which form an efficient filtering apparatus:[12] the sixthoracopods (legs attached to thethorax) create a "feeding basket" used to collect phytoplankton from the open water. In the finest areas the openings in this basket are only 1 μm in diameter. In lower food concentrations, the feeding basket is pushed through the water for over half a metre in an opened position, and then the algae are combed to the mouth opening with specialsetae (bristles) on the inner side of the thoracopods.
Antarctic krill can scrape off the green lawn ofice algae from the underside ofpack ice.[13][14] Krill have developed special rows of rake-like setae at the tips of theirthoracopods, and graze the ice in a zig-zag fashion. One krill can clear an area of a square foot in about 10 minutes (1.5 cm2/s). Recent discoveries have found that the film of ice algae is well developed over vast areas, often containing much more carbon than the whole water column below. Krill find an extensive energy source here, especially in the spring after food sources have been limited during the winter months.
Krill are thought to undergo between one and three vertical migrations from mixed surface waters to depths of 100 m daily.[15] The krill is a very untidy feeder, and it often spits out aggregates ofphytoplankton (spitballs) containing thousands of cells sticking together. It also produces fecal strings that still contain significant amounts ofcarbon andglass shells of thediatoms. Both are heavy and sink very fast into the abyss. This process is called thebiological pump. As the waters aroundAntarctica are very deep (2,000–4,000 metres or 6,600–13,100 feet), they act as acarbon dioxide sink: this process exports large quantities of carbon (fixedcarbon dioxide, CO2) from the biosphere andsequesters it for about 1,000 years.
If the phytoplankton is consumed by other components of the pelagic ecosystem, most of the carbon remains in the upper layers of the ocean. There is speculation that this process is one of the largest biofeedback mechanisms of the planet, maybe the most sizable of all, driven by a gigantic biomass. Still more research is needed to quantify the Southern Ocean ecosystem.
Krill are often referred to aslight-shrimp because they emit light throughbioluminescent organs. These organs are located on various parts of the individual krill's body: one pair of organs at theeyestalk (cf. the image of the head above), another pair are on the hips of the second and sevenththoracopods, and singular organs on the fourpleonsternites. These light organs emit a yellow-green light periodically, for up to 2–3 s. They are considered so highly developed that they can be compared with a flashlight. There is a concave reflector in the back of the organ and a lens in the front that guide the light produced. The whole organ can be rotated by muscles, which can direct the light to a specific area. 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 inmating orschooling at night.
The krill's bioluminescent organs contain several fluorescent substances. The major component has a maximumfluorescence at an excitation of 355 nm and emission of 510 nm.[16]
Krill use anescape reaction to evadepredators, swimming backwards very quickly by flipping their rear ends. This swimming pattern is also known aslobstering. Krill can reach speeds of over 0.6 metres per second (2.0 ft/s).[17] Thetrigger time to opticalstimulus is, despite the low temperatures, only 55 ms.
Thegenome ofE. superba spans about 48 GB and is thus one of the largest in theanimal kingdom and the largest that has been assembled to date. Its content ofrepetitive DNA is about 70% and may reach up to 92.45% after additional repeat annotation, which is also the largest fraction known of any genome. There is no evidence ofpolyploidy. Shao et al. annotated 28,834protein-codinggenes in the Antarctic krill genome, which is similar to other animal genomes. The gene andintron lengths of Antarctic krill are notably shorter than those oflungfishes andMexican axolotl, two other animals with giant genomes.[18]
Antarctic krill has a circumpolar distribution, being found throughout theSouthern Ocean, and as far north as theAntarctic Convergence.[19] At the Antarctic Convergence, the cold Antarctic surface water submerges below the warmersubantarctic waters. This front runs roughly at55° south; from there to the continent, the Southern Ocean covers 32 million square kilometres. This is 65 times the size of theNorth Sea. In the winter season, more than three-quarters of this area become covered by ice, whereas 24,000,000 square kilometres (9,300,000 sq mi) become ice free in summer. The water temperature fluctuates at −1.3–3 °C (29.7–37.4 °F).
The waters of the Southern Ocean form a system of currents. Whenever there is aWest Wind Drift, the surface strata travels around Antarctica in an easterly direction. Near the continent, theEast Wind Drift runs counterclockwise. At the front between both, largeeddies develop, for example, in theWeddell Sea. The krill swarms swim 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 yet be tagged to track their movements. The largest shoals are visible from space and can be tracked by satellite.[20] One swarm covered an area of 450 square kilometers (170 square miles) of ocean, to a depth of 200 meters (660 feet) and was estimated to contain over 2 million tons of krill.[21] Recent research suggests that krill do not simply drift passively in these currents but actually modify them.[21] 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.[21]
Antarctic krill is thekeystone species of theAntarctic ecosystem beyond the coastal shelf,[22] and provides an important food source forwhales,seals (such asleopard seals,fur seals, andcrabeater seals),squid,icefish,penguins,albatrosses and many other species ofbirds. Crabeater seals have even developed special teeth as an adaptation to catch this abundant food source: its unusual multilobed teeth enable this species to sieve krill from the water. Its dentition looks like a perfect strainer, but how it operates in detail is still unknown. Crabeaters are the most abundant seal in the world; 98% of their diet is made up of E. superba. These seals consume over 63 milliontonnes of krill each year.[23]Leopard seals have developed similar teeth (45% krill in diet). All seals consume 63–130 million tonnes, all whales 34–43 million tonnes, birds 15–20 million tonnes, squid 30–100 million tonnes, and fish 10–20 million tonnes, adding up to 152–313 million tonnes of krill consumption each year.[24]
The size step between krill and its prey is unusually large: generally it takes three or four steps from the 20 μm smallphytoplankton cells to a krill-sized organism (via smallcopepods, large copepods,mysids to 5 cmfish).[4]
E. superba lives only in the Southern Ocean. In the North Atlantic,Meganyctiphanes norvegica and in the Pacific,Euphausia pacifica are the dominant species.
Thebiomass of Antarctic krill was estimated in 2009 to be 0.05 gigatons of carbon (Gt C), similar to the total biomass of humans (0.06 Gt C).[25] The reason Antarctic krill are able to build up such a high biomass and production is that the waters around the icy Antarctic continent harbour one of the largestplankton assemblages in the world, possiblythe largest. The ocean is filled withphytoplankton; as the water rises from the depths to the light-flooded surface, it bringsnutrients from all of the world's oceans back into thephotic zone where they are once again available to living organisms.
Thusprimary production—the conversion of sunlight into organic biomass, the foundation of the food chain—has an annual carbon fixation of 1–2 g/m2 in the open ocean. Close to the ice it can reach 30–50 g/m2. These values are not outstandingly high, compared to very productive areas like theNorth Sea orupwelling regions, but the area over which it takes place is enormous, even compared to other large primary producers such asrainforests. In addition, during the Austral summer there are many hours of daylight to fuel the process. All of these factors make the plankton and the krill a critical part of the planet's ecocycle.
A possible decline in Antarctic krill biomass may have been caused by the reduction of thepack ice zone due toglobal warming.[27] Antarctic 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 tend to give way tosalps,[28] a barrel-shaped free-floatingfilter feeder that also grazes on plankton.
Another challenge for Antarctic krill, as well as many calcifying organisms (corals, bivalve mussels, snails etc.), is theacidification of the oceans caused by increasing levels of carbon dioxide.[29] Krill exoskeleton contains carbonate, which is susceptible to dissolution under lowpH conditions. It has already been shown that increased carbon dioxide can disrupt the development of krill eggs and even prevent the juvenile krill from hatching, leading to future geographically widespread decreases in krill hatching success.[30][31] The further effects of ocean acidification on the krill life cycle however remains unclear but scientists fear that it could significantly impact on its distribution, abundance and survival.[32][33]
The fishery of Antarctic krill is on the order of 100,000 tonnes per year. The major catching nations areSouth Korea,Norway,Japan andPoland.[35] The products are used as animal food and fish bait. Krill fisheries are difficult to operate in two important respects. First, a krill net needs to have very fine meshes, producing a very highdrag, which generates abow wave that deflects the krill to the sides. Second, fine meshes tend to clog very fast.
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. Its high protein and vitamin content makes krill quite suitable for both direct human consumption and the animal-feed industry.[36]
Fishing and potentially overfishing krill is an issue of increasing concern.[37][38]
Media related toEuphausia superba at Wikimedia Commons
Data related toEuphausia superba at Wikispecies| International | |
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