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Antarctic krill

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               iAntarctic Krill
           Scientific classification

   Kingdom:   Animalia
   Phylum:    Arthropoda
   Subphylum: Crustacea
   Class:     Malacostraca
   Order:     Euphausiacea
   Family:    Euphausiidae
   Genus:     Euphausia
   Species:   E. superba

                                Binomial name

   Euphausia superba
   Dana, 1850

   The Antarctic krill (Euphausia superba ) is a species of krill found in
   the Antarctic waters of the Southern Ocean. Antarctic krill are
   shrimp-like invertebrates that live in large schools, called swarms,
   sometimes reaching densities of 10,000 - 30,000 individual animals per
   cubic meter. They feed directly on minute phytoplankton, thereby using
   the primary production energy that the phytoplankton originally derived
   from the sun in order to sustain their pelagic (open ocean) life cycle.
   They grow to a length of 6 cm, weigh up to 2 grams, and can live for up
   to six years. They are a key species in the Antarctic ecosystem and
   are, in terms of biomass, likely the most successful animal species on
   the planet (approximately 500 million tonnes).

Systematics

   All members of the krill order are shrimp-like 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
   Antarctic krill visible to the human eye. The legs do not form a jaw
   structure, which differentiates this order from the crabs, lobsters and
   shrimp.

Life cycle

   The eggs are spawned close to the surface and start sinking. In the
   open ocean they sink for about 10 days: the nauplii hatch in ca. 3000
   meter depth
   Enlarge
   The eggs are spawned close to the surface and start sinking. In the
   open ocean they sink for about 10 days: the nauplii hatch in ca. 3000
   meter depth

   The main spawning season 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 (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 fertilized as they pass out of the genital opening
   by sperm liberated from spermatophores which have been attached by the
   males.

   According to the classical hypothesis of Marr, derived from the results
   of the expedition of the famous British research vessel RRS Discovery,
   egg development then proceeds as follows: Gastrulation (development of
   egg into embryo) sets in during the descent of the 0.6 mm eggs on the
   shelf at the bottom, in oceanic areas in depths around 2,000–3,000 m.
   From the time the egg hatches, the 1^st 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 2^nd nauplius and metanauplius,
   still do not eat but are nourished by the remaining yolk. After three
   weeks, the little krill has finished the ascent. Growing larger,
   additional larval stages follow (2^nd and 3^rd calyptopis, 1^st to 6^th
   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. Krill reach
   maturity after two to three years. Like all crustaceans, krill must
   molt in order to grow. Approximately every 13 to 20 days krill shed
   their chitin skin and leave it behind as exuvia.

Food

   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.
   Enlarge
   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.

   The gut of E. superba can often be seen shining green through the
   animal's transparent skin, an indication that this species feeds
   predominantly on phytoplankton—especially very small diatoms (20  μm),
   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 (see "the biological
   pump" below).

   In aquaria, krill 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. 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.

Filter feeding

   Krill feeding under high phytoplankton concentration. A slow motion
   video (from 300 frame/s; 490 kB) is also available.
   Enlarge
   Krill feeding under high phytoplankton concentration. A slow motion
   video (from 300 frame/s; 490 kB) is also available.

   The Antarctic krill manages to directly utilize the 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, providing for an efficient filtering apparatus:
   the six thoracopods (legs attached to the thorax) form a very effective
   "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.
   Scanning electron microscope images of this amazing structure can be
   studied here. In the movie linked to the left, 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, as in the in situ image below, and then the algae are combed
   to the mouth opening with special setae (bristles) on the inner side of
   the thoracopods.

Ice-algae raking

   Antarctic krill feeding off ice algae. The surface of the ice on the
   left side is colored green by the algae.
   Enlarge
   Antarctic krill feeding off ice algae. The surface of the ice on the
   left side is colored green by the algae.

   Antarctic krill can scrape off the green lawn of ice-algae from the
   underside of the pack ice. The image to the right, taken via a ROV,
   shows 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 (1.5 cm²/s). 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 water column below. Krill find an extensive
   energy source here, especially in the spring.

The biological pump and carbon sequestration

   In situ image taken with an ecoSCOPE. A green spit ball is visible in
   the lower right of the image and a green fecal string in the lower
   left.
   Enlarge
   In situ image taken with an ecoSCOPE. A green spit ball is visible in
   the lower right of the image and a green fecal string in the lower
   left.

   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 significant
   amounts 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
   (2,000–4,000 m), 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 strata. 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.

Biological peculiarities

Bioluminescence

   Watercolor of bioluminescent krill
   Enlarge
   Watercolor of bioluminescent krill

   Krill are often referred to as light-shrimp because they can emit
   light, produced by bioluminescent 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 2^nd and 7^th thoracopods, and singular organs on the four
   pleonsternites. These light organs emit a yellow-green light
   periodically, 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 a lens in the front guide the
   light produced, and the whole organ can be rotated by 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.

   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

   Lobstering krill
   Enlarge
   Lobstering krill

   Krill use an escape reaction to evade predators, swimming backwards
   very quickly by flipping their telson. This swimming pattern is also
   known as lobstering. Krill can reach speeds of over 60 cm per second.
   The trigger time to optical stimulus is, despite the low temperatures,
   only 55 milliseconds.

The compound eye

   Electron microscope image of the compound eye - the eyes are deep black
   in the living animal
   Enlarge
   Electron microscope image of the compound eye - the eyes are deep black
   in the living animal

   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.

   As mentioned above, krill can shrink in size from one molt to the next,
   which is generally thought to be a survival strategy to adapt to scarce
   food supplies (a smaller body needs less energy, i.e., food). However,
   the animal's eyes do not shrink when this happens. The ratio between
   eye size and body length has thus been found to be a reliable indicator
   of starvation.

Geographical distribution

   Krill distribution on a NASA SeaWIFS image - the main concentrations
   are in the Scotia Sea at the Antarctic Peninsula
   Enlarge
   Krill distribution on a NASA SeaWIFS image - the main concentrations
   are in the Scotia Sea at the Antarctic Peninsula

   Antarctic 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
   at 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 yet be tagged 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. Crabeater seals have even
   developed special teeth as an adaptation to catch this abundant food
   source: its most 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; their diet consists to 98% of E. superba.
   These seals consume over 63 million tonnes of krill each year. 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.

   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 copepods, large copepods, mysids
   to 5 cm fish). 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 and production

   The Antarctic krill's biomass is estimated to be between 125 to 725
   million tonnes, making E. superba the most successful animal species on
   the planet. It should be noted that of all animals visible to the naked
   eye some biologists speculate that ants provide the largest biomass
   (but this speculation adds up hundreds of different species) whilst
   others speculate that it could be the copepods, but this too would be
   the sum of many hundreds of species that exist over the planet. To get
   an impression of the biomass of E. superba against that of other
   species: The total non-krill yield from all world fisheries, finfish,
   shellfish, cephalopods and plankton is about 100 million tonnes per
   year whilst estimates of the Antarctic krill production are between 13
   million to several billion tonnes per year.

   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 nutrients 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 between 1 and 2 g/m² in the open ocean. Close to the ice it
   can reach 30 to 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 just enormous, even
   compared to other large primary producers such as rainforests. 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

   after data compiled by Loeb et al. 1997 - temperature and pack ice area
   - the scale for the ice is inverted to demonstrate the correlation -
   the horizontal line is the freezing point - the oblique line the
   average of the temperature - in 1995 the temperature reached the
   freezing point
   Enlarge
   after data compiled by Loeb et al. 1997 - temperature and pack ice area
   - the scale for the ice is inverted to demonstrate the correlation -
   the horizontal line is the freezing point - the oblique line the
   average of the temperature - in 1995 the temperature reached the
   freezing point

   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 global warming. 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.
   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 Salps, a barrel-shaped
   free-floating filter feeder that also grazes on plankton.

Fisheries

   Annual world catch of E. superba, compiled from FAO data.
   Enlarge
   Annual world catch of E. superba, compiled from FAO data.

   The fishery of the Antarctic krill is on the order of 100,000 tonnes
   per year. The major catching nations are Japan and Poland. The products
   are used largely in Japan as a delicacy and worldwide 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.
   Additionally, fine nets also tend to be very delicate, and the first
   krill nets tore apart while fishing through 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. Processing 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 lack of knowledge available about the whole Antarctic
   ecosystem, large scale experiments involving krill are 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 termed HNLC (high
   nutrient, low carbon). The phenomenon is called the Antarctic Paradox,
   and occurs because iron is missing. 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. Krill is the
   key player in this process, collecting the minute plankton cells which
   fix carbon dioxide and converting the substance to rapidly-sinking
   carbon 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 air conditioners running.
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