Antarctic krill
Antarctic krill is a species of krill found in the Antarctic waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 animals per cubic metre. It feeds directly on minute phytoplankton, thereby using the primary production energy that phytoplankton originally derive from the sun in order to sustain its pelagic life cycle. It grows to a length of, weighs up to, and can live for up to six years. A key species in the Antarctic ecosystem and in terms of biomass, E. superba is one of the most abundant animal species on the planet, with a cumulative biomass of approximately.
Life cycle
The main spawning season of Antarctic krill is from January to March, both above the continental shelf and also in the upper region of deep sea oceanic areas. In the typical way of all krill, the male attaches a spermatophore to the genital opening of the female. For this purpose, the first pleopods of the male are constructed as mating tools. Females lay 6,000–10,000 eggs at one time. They are fertilised as they pass out of the genital opening.According to the classical hypothesis of Marriosis De' Abrtona, derived from the results of the expedition of the famous British research vessel RRS Discovery, egg development then proceeds as follows: gastrulation sets in during the descent of the eggs on the shelf at the bottom, in oceanic areas in depths around. The egg hatches as a nauplius larva; once this has moulted into a metanauplius, the young animal starts migrating towards the surface in a migration known as developmental ascent.
The next two larval stages, termed second nauplius and metanauplius, still do not eat but are nourished by the remaining yolk. After three weeks, the young krill has finished the ascent. They can appear in enormous numbers counting 2 per litre in water depth. Growing larger, additional larval stages follow. They are characterised by increasing development of the additional legs, the compound eyes and the setae. At, the juvenile krill resembles the habitus of the adults. Krill reach maturity after two to three years. Like all crustaceans, krill must moult in order to grow. Approximately every 13 to 20 days, krill shed their chitinous exoskeleton and leave it behind as exuvia.
File:Kilsheadkils.jpg|thumb|The head of Antarctic krill. Observe the bioluminescent organ at the eyestalk and the nerves visible in the antennae, the gastric mill, the filtering net at the thoracopods and the rakes at the tips of the thoracopods.
Food
The gut of E. superba can often be seen shining green through its transparent skin. This species feeds predominantly on phytoplankton—especially very small diatoms, which it filters from the water with a feeding basket. The glass-like shells of the diatoms are cracked in the gastric mill and then digested in the hepatopancreas. The krill can also catch and eat copepods, amphipods and other small zooplankton. The gut forms a straight tube; its digestive efficiency is not very high and therefore a lot of carbon is still present in the feces. Antarctic krill 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.In aquaria, krill have been observed to eat each other. When they are not fed, they shrink in size after moulting, which is exceptional for animals this size. It is likely that this is an adaptation 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. 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.
Filter feeding
Antarctic krill directly ingest minute phytoplankton cells, which no other animal of krill size can do. This is accomplished through filter feeding, using the krill's highly developed front legs which form an efficient filtering apparatus: the six thoracopods 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 special setae on the inner side of the thoracopods.Ice-algae raking
Antarctic krill can scrape off the green lawn of ice algae from the underside of pack ice. Krill have developed special rows of rake-like setae at the tips of their thoracopods, and graze the ice in a zig-zag fashion. One krill can clear an area of a square foot in about 10 minutes. 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.Biological pump and carbon sequestration
Krill are thought to undergo between one and three vertical migrations from mixed surface waters to depths of 100 m daily. The krill is a very untidy feeder, and it often spits out aggregates of phytoplankton containing thousands of cells sticking together. It also produces fecal strings that still contain significant amounts of carbon and 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, they act as a carbon dioxide sink: this process exports large quantities of carbon from the biosphere and sequesters 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.
Biology
Bioluminescence
Krill are often referred to as light-shrimp because they emit light through bioluminescent organs. These organs are located on various parts of the individual krill's body: one pair of organs at the eyestalk, another pair are on the hips of the second and seventh thoracopods, and singular organs on the four pleonsternites. 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 in mating or schooling at night.The krill's bioluminescent organs contain several fluorescent substances. The major component has a maximum fluorescence at an excitation of 355 nm and emission of 510 nm.
Escape reaction
Krill use an escape reaction to evade predators, swimming backwards very quickly by flipping their rear ends. This swimming pattern is also known as lobstering. Krill can reach speeds of over. The trigger time to optical stimulus is, despite the low temperatures, only 55 ms.Genome
The genome of E. superba spans about 48 GB and is thus one of the largest in the animal kingdom and the largest that has been assembled to date. Its content of repetitive 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 of polyploidy. Shao et al. annotated 28,834 protein-coding genes in the Antarctic krill genome, which is similar to other animal genomes. The gene and intron lengths of Antarctic krill are notably shorter than those of lungfishes and Mexican axolotl, two other animals with giant genomes.Geographic distribution
Antarctic krill has a circumpolar distribution, being found throughout the Southern Ocean, and as far north as the Antarctic Convergence. At the Antarctic Convergence, the cold Antarctic surface water submerges below the warmer subantarctic waters. This front runs roughly at 55° south; from there to the continent, the Southern Ocean covers 32 million square kilometres. 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 become ice free in summer. The water temperature fluctuates at.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 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. One swarm covered an area of of ocean, to a depth of and was estimated to contain over 2 million tons of krill. Recent research suggests that krill do not simply drift passively in these currents but actually modify them. 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.