   #copyright

Krill

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

                        iEuphausiacea
   A northern krill (Meganyctiphanes norvegica)
   A northern krill (Meganyctiphanes norvegica)
                  Scientific classification

   Kingdom:    Animalia
   Phylum:     Arthropoda
   Subphylum:  Crustacea
   Class:      Malacostraca
   Superorder: Eucarida
   Order:      Euphausiacea
               Dana, 1852

                                  Families

     * Euphausiidae
          + Euphausia Dana, 1852
          + Meganyctiphanes Holt and W. M. Tattersall, 1905
          + Nematobrachion Calman, 1905
          + Nematoscelis G. O. Sars, 1883
          + Nyctiphanes G. O. Sars, 1883
          + Pseudeuphausia Hansen, 1910
          + Stylocheiron G. O. Sars, 1883
          + Tessarabrachion Hansen, 1911
          + Thysanoessa Brandt, 1851
          + Thysanopoda Latreille, 1831
     * Bentheuphausiidae
          + Bentheuphausia amblyops G. O. Sars, 1883

   Krill are shrimp-like marine invertebrate animals. These small
   crustaceans are important organisms of the zooplankton, particularly as
   food for baleen whales, mantas, whale sharks, crabeater seals and other
   seals, and a few seabird species that feed almost exclusively on them.
   Another name is euphausiids, after their taxonomic order Euphausiacea.
   The name krill comes from the Norwegian word krill meaning "young fry
   of fish".

   Krill occur in all oceans of the world. They are considered keystone
   species near the bottom of the food chain because they feed on
   phytoplankton and to a lesser extent zooplankton, converting these into
   a form suitable for many larger animals for whom krill makes up the
   largest part of their diet. In the Southern Ocean, one species, the
   Antarctic Krill, Euphausia superba, makes up a biomass of hundreds of
   millions of tonnes, similar to the entire human consumption of animal
   protein. Over half of this biomass is eaten by whales, seals, penguins,
   squid and fish each year, and replaced by growth and reproduction. Most
   of the species display large daily vertical migrations making a
   significant amount of biomass available as food for predators near the
   surface at night and in deeper waters during the day.

   Commercial fishing of krill is done in the Southern Ocean and in the
   waters around Japan. The total global production amounts to 150 –
   200,000 tonnes annually, most of this from the Scotia Sea. Most krill
   is used for aquaculture and aquarium feeds, as bait in sport fishing,
   or in the pharmaceutical industry. In Japan and Russia, krill is also
   used for human consumption and known as okiami (オキアミ) in Japan.

Taxonomy

   The order Euphausiacea is split into two families. The family
   Bentheuphausiidae has only one species, Bentheuphausia amblyops, a
   bathypelagic krill living in deep waters below 1,000 m. It is
   considered the most primitive living species of all krill. The other
   family — the Euphausiidae — contains ten different genera with a total
   of 85 species. Of these, the genus Euphausia is the largest, with 31
   species.

   Well-known species—mainly because they are subject to commercial krill
   fishery—include Antarctic krill (Euphausia superba), Pacific krill
   (Euphausia pacifica) and Northern krill (Meganyctiphanes norvegica).

Distribution

   A krill swarm
   Enlarge
   A krill swarm

   Krill occur worldwide in all oceans; most species have transoceanic
   distribution and several species have endemic or neritic restricted
   distribution. Species of the genus Thysanoessa occur in both the
   Atlantic and the Pacific Ocean, which is also home to Euphausia
   pacifica. Northern krill occurs across the Atlantic, from the north to
   the Mediterranean Sea. The four species of the genus Nyctiphanes are
   highly abundant along the upwelling regions of the California, Humbolt,
   Benguela, and Canarias Current Systems, where occur most of the largest
   fisheries' activities of fish, molluscs and crustaceans.

   In the Antarctic, seven species are known: one of the genus Thysanoessa
   (T. macrura) and six species of the genus Euphausia. The Antarctic
   krill (Euphausia superba) commonly lives at depths up to 100 m, whereas
   Ice krill (Euphausia crystallorophias) has been recorded at a depth of
   4,000 m but commonly lives in depths at most 300 to 600 m deep. Both
   are found at latitudes south of 55° S; with E. crystallorophias
   dominating south of 74° S and in regions of pack ice. Other species
   known in the Southern Ocean are E. frigida, E. longirostris, E.
   triacantha, and E. vallentini.

Morphology

   Krill anatomy explained, using Euphausia superba as a model
   Enlarge
   Krill anatomy explained, using Euphausia superba as a model

   Krill are crustaceans and have a chitinous exoskeleton made up of three
   segments: the cephalon (head), thorax, and the abdomen. The first two
   segments are fused into one segment, the cephalothorax. This outer
   shell of krill is transparent in most species. Krill feature intricate
   compound eyes; some species can adapt to different lighting conditions
   through the use of screening pigments. They have two antennae and
   several pairs of thoracic legs called pereiopods or thoracopods (so
   named because they are attached to the thorax; their number varies
   among genera and species). These thoracic legs include the feeding legs
   and the grooming legs. Additionally all species have five swimming legs
   called pleopods or "swimmerets," very similar to those of the common
   freshwater lobster. Most krill are about 1 to 2 cm long as adults, a
   few species grow to sizes of the order of 6 to 15 cm. The largest krill
   species is the mesopelagic Thysanopoda spinicauda. Krill can be easily
   distinguished from other crustaceans such as true shrimps by their
   externally visible gills.
   The gills of krill are externally visible.
   Enlarge
   The gills of krill are externally visible.

   Many krill are filter feeders: their front-most extremities, the
   thoracopods, form very fine combs with which they can filter out their
   food from the water. These filters can be very fine indeed in those
   species (such as Euphausia spp.) that feed primarily on phytoplankton,
   in particular on diatoms, which are unicellular algae. However, it is
   believed that all krill species are mostly omnivorous and some few
   species are carnivorous preying on small zooplankton and fish larvae.

   Except for the Bentheuphausia amblyops species, krill are
   bioluminescent animals having organs called photophores that are able
   to emit light. The light is generated by an enzyme-catalyzed
   chemiluminescence reaction, wherein a luciferin (a kind of pigment) is
   activated by a luciferase enzyme. Studies indicate that the luciferin
   of many krill species is a fluorescent tetrapyrrole similar but not
   identical to dinoflagellate luciferin and that the krill probably do
   not produce this substance themselves but acquire it as part of their
   diet that contains dinoflagellates. Krill photophores are complex
   organs with lenses and focusing abilities, and they can be rotated by
   muscles. The precise function of these organs is as yet unknown; they
   might have a purpose in mating, social interaction or in orientation.
   Some researchers (e.g. Lindsay & Latz or Johnsen) have proposed that
   krill use the light as a form of counter-illumination camouflage to
   compensate their shadow against the ambient light from above to make
   themselves more difficult to be seen by predators from below.

Behaviour

   Most krill are swarming animals; the size and density of such swarms
   vary greatly depending on the species and the region. Of Euphausia
   superba, there have been reports of swarms of up to 10,000 to 30,000
   individuals per cubic meter. Swarming is a defensive mechanism,
   confusing smaller predators that would like to pick out single
   individuals.

   Krill typically follow a diurnal vertical migration. They spend the day
   at greater depths and rise during the night towards the surface. The
   deeper they go, the more they reduce their activity, apparently to
   reduce encounters with predators and to conserve energy. Some species
   (e.g. Euphausia superba, E. pacifica, E. hanseni, Pseudeuphausia
   latifrons, or Thysanoessa spinifera) also form surface swarms during
   the day for feeding and reproductive purposes even though such
   behaviour is dangerous because it makes them extremely vulnerable to
   predators.
   Beating pleopods of a swimming Antarctic krill.
   Enlarge
   Beating pleopods of a swimming Antarctic krill.

   Dense swarms may elicit a feeding frenzy among predators such as fish
   or birds, especially near the surface, where escape possibilities for
   the krill are limited. When disturbed, a swarm scatters, and some
   individuals have even been observed to molt instantaneously, leaving
   the exuvia behind as a decoy.

   Krill normally swim at pace of a few centimetres per second (0.2 – 10
   body lengths per second), using their swimmerets for propulsion. Their
   larger migrations are subject to the currents in the ocean. When in
   danger, they show an escape reaction called lobstering: flipping their
   caudal appendages, i.e. the telson and uropods, they move backwards
   through the water relatively quickly, achieving speeds in the range of
   10 to 27 body lengths per second, which for large krill such as E.
   superba means around 0.8 m/s. Their swimming performance has led many
   researchers to classify adult krill as micro-nektonic lifeforms, i.e.
   small animals capable of individual motion against (weak) currents.
   Larval forms of krill are generally considered zooplankton.

Life cycle

   A nauplius of Euphausia pacifica hatching, emerging backwards from the
   egg.
   Enlarge
   A nauplius of Euphausia pacifica hatching, emerging backwards from the
   egg.

   The general life-cycle of krill has been the subject of several studies
   (e.g. Guerny 1942, or Mauchline & Fisher 1969) performed on a variety
   of species and is thus relatively well understood, although there are
   minor variations in details from species to species. When krill hatch
   from the eggs, they go through several larval stages called the
   nauplius, pseudometanauplius, metanauplius, calyptopsis, and furcilia
   stages, each of which is sub-divided into several sub-stages. The
   pseudometanauplius stage is exclusive of species that lay their eggs
   within an ovigerous sac (so-called sac-spawners). The larvae grow and
   molt multiple times during this process, shedding their rigid
   exoskeleton and growing a new one whenever it becomes too small.
   Smaller animals molt more frequently than larger ones. Until and
   including the metanauplius stage, the larvae nourish on yolk reserves
   within their body. Only by the calyptopsis stages, differentiation has
   progressed far enough for them to develop a mouth and a digestive
   tract, and they begin to feed upon phytoplankton. By that time, the
   larvae must have reached the photic zone, the upper layers of the ocean
   where algae flourish, for their yolk reserves are exhausted by then and
   they would starve otherwise. During the furcilia stages, segments with
   pairs of swimmerets are added, beginning at the frontmost segments.
   Each new pair becomes functional only at the next molt. The number of
   segments added during any one of the furcilia stages may vary even
   within one species depending on environmental conditions.

   After the final furcilia stage, the krill emerges in a shape similar to
   an adult, but is still immature. During the mating season, which varies
   depending on the species and the climate, the male deposits a sperm
   package at the genital opening (named thelycum) of the female. The
   females can carry several thousand eggs in their ovary, which may then
   account for as much as one third of the animal's body mass. Krill can
   have multiple broods in one season, with interbrood periods of the
   order of days.
   The head of a female krill of the sac-spawning species Nematoscelis
   difficilis with her brood sac. The eggs have a diameter of 0.3 –
   0.4 mm.
   Enlarge
   The head of a female krill of the sac-spawning species Nematoscelis
   difficilis with her brood sac. The eggs have a diameter of 0.3 – 0.4
   mm.

   There are two types of spawning mechanisms. The 57 species of the
   genera Bentheuphausia, Euphausia, Meganyctiphanes, Thysanoessa, and
   Thysanopoda are "broadcast spawners": the female eventually just
   releases the fertilized eggs into the water, where they usually sink
   into deeper waters, disperse, and are on their own. These species
   generally hatch in the nauplius 1 stage, but recently have been
   discovered to hatch sometimes as metanauplius or even as calyptopis
   stages. The remaining 29 species of the other genera are "sac
   spawners", where the female carries the eggs with her attached to its
   rearmost pairs of thoracopods until they hatch as metanauplii, although
   some species like Nematoscelis difficilis may hatch as nauplius or
   pseudometanauplius.

   Some high latitude species of krill can live up to more than six years
   (e.g. Euphausia superba); others, such as the as mid-latitude species
   Euphausia pacifica, live only for two years. Subtropical or tropical
   species' longevity is still smaller, like e.g. Nyctiphanes simplex that
   usually lives only for six to eight months.

   Molting occurs whenever the animal outgrows its rigid exoskeleton.
   Young animals, growing faster, therefore molt more often than older and
   larger ones. The frequency of molting varies wildly from species to
   species and is, even within one species, subject to many external
   factors such as the latitude, the water temperature, or the
   availability of food. The subtropical species Nyctiphanes simplex, for
   instance, has an overall intermolt period in the range of two to seven
   days: larvae molt on the average every three days, while juveniles and
   adults do so on the average every five days. For E. superba in the
   Antarctic sea, intermolt periods ranging between 9 and 28 days
   depending on the temperature between -1° C to 4°C have been observed,
   and for Meganyctiphanes norvegica in the North Sea the intermolt
   periods range also from 9 and 28 days but at temperatures between 2.5°C
   to 15°C. E. superba is known to be able to reduce its body size when
   there is not enough food available, molting also when its exoskeleton
   becomes too large. Similar shrinkage has also been observed for E.
   pacifica (a species occurring in the Pacific Ocean from polar to
   temperate zones) as an adaptation to abnormally high water
   temperatures, and has been postulated for other temperate species of
   krill, too.

Ecology

   NASA SeaWiFS satellite image of the large phytoplankton bloom in the
   Bering Sea in 1998.
   Enlarge
   NASA SeaWiFS satellite image of the large phytoplankton bloom in the
   Bering Sea in 1998.

   Krill are an important element of the food chain. Antarctic krill feed
   directly on phytoplankton, converting the primary production energy
   into a form suitable for consumption by larger animals that cannot feed
   directly on the minuscule algae, but that can feed upon krill. Some
   species like the Northern krill have a smaller feeding basket and hunt
   for copepods and larger zooplankton. Many other animals feed on krill,
   ranging from smaller animals like fish or penguins to larger ones like
   seal and even baleen whales.

   Disturbances of an ecosystem resulting in a decline of the krill
   population can have far-reaching effects. During a coccolithophore
   bloom in the Bering Sea in 1998, for instance, the diatom concentration
   dropped in the affected area. However, krill cannot feed on the smaller
   coccolithophores, and consequently, the krill population (mainly E.
   pacifica) in that region declined sharply. This in turn affected other
   species: the shearwater population dropped, and the incident was even
   thought to have been a reason for salmon not returning to the rivers of
   western Alaska in that season.

   Other factors besides predators and food availability also can
   influence the mortality rate in krill populations. There are several
   single-celled endoparasitoidic ciliates of the genus Collinia that can
   infect different species of krill and cause mass dying in affected
   populations. Such diseases have been reported for Thysanoessa inermis
   in the Bering Sea, but also for E. pacifica, Thysanoessa spinifera, and
   T. gregaria off the North-American Pacific coast. There are also some
   ectoparasites of the family Dajidae (epicaridean isopods) that afflict
   krill (and also shrimps and mysids); one such parasite is Oculophryxus
   bicaulis which has been found on the krill Stylocheiron affine and S.
   longicorne. It attaches itself to the eyestalk of the animal and sucks
   blood from its head; it is believed that it inhibits the reproduction
   of its host as none of the afflicted animals found reached maturity.

   See also: Carbon sequestration, biological pump.

Economy

   Deep frozen plates of Antarctic krill for use as animal feed and raw
   material for cooking
   Enlarge
   Deep frozen plates of Antarctic krill for use as animal feed and raw
   material for cooking

   Krill has been harvested as a food source for both humans (okiami) and
   their domesticated animals since the 19th century, in Japan maybe even
   earlier. Large-scale fishing developed only in the late 1960s and early
   1970s, and now occurs only in Antarctic waters and in the seas around
   Japan. Historically, the largest krill fishery nations were Japan and
   the Soviet Union, or, after the latter's dissolution, Russia and
   Ukraine. A peak in krill harvest had been reached in 1983 with more
   than 528,000 tonnes in the Southern Ocean alone (of which the Soviet
   Union produced 93%). In 1993, two events led to a drastic decline in
   krill production: first, Russia abandoned its operations, and second,
   the Commission for the Conservation of Antarctic Marine Living
   Resources ( CCAMLR) defined maximum catch quotas for a sustainable
   exploitation of Antarctic krill. Nowadays, the largest krill fishing
   nations in the Antarctic are Japan, followed by South Korea, Ukraine,
   and Poland. The annual catch in Antarctic waters seems to have
   stabilized around 100,000 tonnes of krill, which is roughly one
   fiftieth of the CCAMLR catch quota. The main limiting factor is
   probably the high cost associated with Antarctic operations. The
   fishery around Japan appears to have saturated at some 70'000 tonnes.

   Experimental small-scale harvesting is being carried out in other
   areas, too, e.g. fishing for Euphausia pacifica off British Columbia or
   harvesting Meganyctiphanes norvegica, Thysanoessa raschii and
   Thysanoessa inermis in the Gulf of St. Lawrence. These experimental
   operations produce only a few hundred tonnes of krill per year. Nicol &
   Foster consider it unlikely that any new large-scale harvesting
   operations in these areas will be started due to the opposition from
   local fishing industries and conservation groups.

   Krill taste salty and somewhat stronger than shrimp. For
   mass-consumption and commercially prepared products, they must be
   peeled because their exoskeleton contains fluorides, which are toxic in
   high concentrations. Excessive intake of okiami may cause diarrhea.
   Retrieved from " http://en.wikipedia.org/wiki/Krill"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
