Antarctic krill ( Euphausia superba) is a species of krill found in the Antarctica waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called , sometimes reaching densities of 10,000–30,000 animals per cubic metre.
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
(legs attached to the abdomen) of the male are constructed as mating tools. Females lay 6,000–10,000 eggs at one time. They are
fertilisation as they pass out of the genital opening.
According to the classical hypothesis of Marriosis De' Abrtona,
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 (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 , the juvenile krill resembles the habitus of the adults. Krill reach maturity after two to three years. Like all , krill must ecdysis in order to grow. Approximately every 13 to 20 days, krill shed their exoskeleton and leave it behind as exuvia.
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
(20
Micrometre), which it filters from the water with a feeding basket.
The glass-like shells of the
are cracked in the
gastric mill and then digested in the
hepatopancreas. The krill can also catch and eat
,
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 (
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.
In Aquarium, krill have been observed to Cannibalism. When they are not fed, they shrink in size after Ecdysis, 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.
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
(legs attached to the
thorax) 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 (bristles) on the inner side of the thoracopods.. The surface of the ice on the left side is coloured green by the algae.]]
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
, 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 cm
2/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.
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 (spitballs) containing thousands of cells sticking together. It also produces fecal strings that still contain significant amounts of
carbon and
glass shells of the
. 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 (fixed
carbon dioxide, CO
2) 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
Bioluminescence organs. These organs are located on various parts of the individual krill's body: one pair of organs at the
eyestalk (cf. the image of the head above), another pair are on the hips of the second and seventh
, and singular organs on the four
. 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 Nanometre and emission of 510 nm.
Escape reaction
Krill use an
escape reaction to evade
, swimming backwards very quickly by flipping their rear ends. This swimming pattern is also known as lobstering. Krill can reach speeds of over .
The
Induction period time to optical stimulus is, despite the low temperatures, only 55
millisecond.
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
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
and
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.[Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. .] One swarm covered an area of of ocean, to a depth of and was estimated to contain over 2 million tons of krill.[Hoare, Ben (2009). Animal Migration. London: Natural History Museum. p. 107. ] Recent research suggests that krill do not simply drift passively in these currents but actually modify them.
Ecology
Antarctic krill is the
keystone species of the
Antarctic ecosystem beyond the coastal shelf,
and provides an important food source for
, seals (such as
,
, and
),
squid,
Notothenioidei,
,
and many other species of
. 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 million
of krill each year.
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.
The size step between krill and its prey is unusually large: generally it takes three or four steps from the 20 μm small phytoplankton cells to a krill-sized organism (via small , large copepods, to 5 cm fish).
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 and production
The biomass 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).
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 largest
plankton assemblages in the world, possibly
the largest. The ocean is filled with
phytoplankton; as the water rises from the depths to the light-flooded surface, it brings
from all of the world's oceans back into the
photic zone where they are once again available to living organisms.
Thus primary 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 the North Sea or upwelling regions, but the area over which it takes place is enormous, even compared to other large primary producers such as . 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.
Decline with shrinking pack ice
A possible decline in Antarctic krill biomass may have been caused by the reduction of the
pack ice zone due to
global warming.
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 to
,
a barrel-shaped free-floating
filter feeder that also grazes on plankton.
Ocean acidification
Another challenge for Antarctic krill, as well as many calcifying organisms (corals, bivalve mussels, snails etc.), is the acidification of the oceans caused by increasing levels of carbon dioxide.
Krill exoskeleton contains carbonate, which is susceptible to dissolution under low pH 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.
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.
Fisheries
The fishery of Antarctic krill is on the order of 100,000 tonnes per year. The major catching nations are
South Korea,
Norway,
Japan and
Poland.
[ CCAMLR Statistical Bulletin vol. 20 (1998-2007) , CCAMLR, Hobart, Australia, 2008. URL last accessed July 3, 2008.] 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 high drag, which generates a
bow 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.
Fishing and potentially overfishing krill is an issue of increasing concern.
Further reading
External links
