Krill
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- For other uses, see Krill (disambiguation).
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| Missing image Krill_swarm.jpg A krill swarm | ||||||||||||
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Krill are shrimp-like marine invertebrate animals and are important organisms of the zooplankton, particularly as food for baleen whales. Their scientific name is Euphausiids, after their taxonomic order Euphausiacea. The word Krill comes from the Norwegian whalefood and young fry of fish.
Krill occur in all oceans of the world. They are considered a keystone species near the bottom of the food chain because they feed on phytoplankton and smaller zooplankton, converting these into a form suitable for many larger animals for whom krill makes up the largest part of their diet.
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.
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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. 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 speciesTemplate:Mn.
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
Krill occurs world-wide in all oceans. 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.
In the Antarctic, seven species are known: five of the genus Thysanoessa and two species of the genus EuphausiaTemplate:Mn. The Antarctic krill (Euphausia superba) commonly lives at depths up to 100 mTemplate:Mn, 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 deepTemplate:Mn. Both are found at latitudes south of 55° S; with E. crystallorophias dominating south of 74° STemplate:Mn and in regions of pack ice.
Morphology
Krillanatomykils.jpg
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 called "cephalothorax". This outer shell of krill is nearly transparent. Krill feature intricate compound eyes; some species can adapt to different lighting conditions through the use of screening pigmentsTemplate:Mn. They have two antennae and several pairs (the numbers vary between species) of swimming legs called "pleopods" or "swimmerets". Most krill are about 1 to 2 cm long as adults, a few species grow to sizes of the order of 6 to 10 cm.
Many krill are filter feeders: their frontmost extremities, the thoracopods (so named because they are attached to the thorax), 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.
Except for the Bentheuphausia amblyops species, krill are bioluminescent animals, i.e. they have fluorescent organs that can emit light. The precise function of these organs is as yet unknown; they might have a purpose in mating, or in orientation.
Behaviour
Most krill are swarming animals; the size and density varies greatly depending on the species and the region. On Euphausia superba, swarms can contain tens of thousands of 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 activityTemplate:Mn, apparently to reduce encounters with predators and to conserve energy. Some species (e.g. Thysanoessa spinifera) also form surface swarms during the day, but it is not entirely clear why they do this, because it makes them extremely vulnerable to predators.
Dense swarms may elicit a feeding frenzy among predators such a 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 carapace behind as a decoyTemplate:Mn.
Krill normally swim at a leisurely pace (a few cm/sec; less than 10 body lengths per second), using their swimmerets for propulsion. Their larger migrations are subject to the currents in the ocean. 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 secondTemplate:Mn, which for large krill such as E. superba means around 1 m/s.
Life cycle
The general life-cycle of krill has been the subject of several studies (e.g. Guerny 1942Template:Mn, or Mauchline & Fisher 1969Template:Mn) 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, metanauplius, calyptopsis, and furcilia stages, each of which is sub-divided into several sub-stages. The larvae grow and molt multiple times during this process, shedding their rigid exoskeleton and growing a new one whenever it becomes too small. During the latter two 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 the furcilia stages may vary even within one species depending on environmental conditionsTemplate:Mn.
After the final furcilia stage, the krill emerges as a fully functional adult. During the mating season, which varies depending on the species and the climate, the male deposits a sperm package at the genital opening 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 massTemplate:Mn. Krill can have multiple broods in one season.
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 eggs into the water, where they float freely, disperse, and are on their own. These species generally hatch in the nauplius 1 stageTemplate:Mn. 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 metanaupliiTemplate:Mn.
Some species of krill can live up to more than six years (e.g. Euphausia superba); others, such as Euphausia pacifica, live only for two yearsTemplate:Mn. Nyctiphanes simplex even lives only for six to eight monthsTemplate:Mn.
Ecology
Coccolithophore_bloom.jpg
Krill are an important element of the food chain. Antarctic krill feed directly on phytoplankton, they convert the primary production energy into a form suitable for consumption by larger animals that cannot feed directly on the minuscule algae but than can feed upon krill. Northern krill has a smaller feeding basket and hunts 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 1998Template:Mn, 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) 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 seasonTemplate:Mn.
Other factors besides predators and food availability also can influence the mortality rate in krill populations. There are several single-celled parasites 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 coastTemplate:Mn.
See also: Carbon sequestration, biological pump.
Economy
Main article: Krill fishery.
Krill has been harvested as a food source for both humans and their domesticated animals since the 19th century, in Japan maybe even earlier. Large-scale fishing has been developed only in the late 1960s and early 1970s and 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, the Russian Federation and Ukraine. A peak in krill harvest had been reached in 1983 with more than 500,000 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, the formerly Soviet krill fishing states abandoned their operations, and second, the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) defined maximum quotas for a sustainable exploitation of krill. Nowadays, the largest krill fishing nations in the Antarctic are Japan, followed by PolandTemplate:Mn. 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 quotaTemplate:Mn. 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 tonnesTemplate:Mn.
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 & FosterTemplate:Mn 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 concentrationsTemplate:Mn.
References
Template:Mnb2 Brinton, E.; Ohman, M. D.; Townsend, A. W.; Knight, M. D.; Bridgeman, A. L.: Euphausiids of the World Ocean (http://www.springeronline.com/sgw/cda/frontpage/0,10735,1-10038-22-1578932-0,00.html), World Biodiversity Database CD-ROM Series; Springer Verlag, 2000. ISBN 3-540-14673-3.
Template:Mnb2 Brodeur, R.D.; Kruse, G.H.; et al.: Draft Report of the FOCI International Workshop on Recent Conditions in the Bering Sea, pp. 22 – 26; NOAA 1998.
Template:Mnb2 Brueggeman, P.: Euphausia crystallorophias (http://scilib.ucsd.edu/sio/nsf/fguide/arthropoda10.html), from the Underwater Field Guide to Ross Island & McMurdo Sound, Antarctica (http://scilib.ucsd.edu/sio/nsf/fguide/).
Template:Mnb2 CCAMLR: Harvested species: Krill (Eupausia superba) (http://www.ccamlr.org/pu/E/sc/fish-monit/hs-krill.htm). Accessed June 20, 2005.
Template:Mnb2 Gaten, E.: Meganyctiphanes norvegica (http://www.le.ac.uk/biology/gat/krill.html); accessed Jun 15, 2005.
Template:Mnb2 Gómez-Gutiérrez, J.: Personal communication (http://www.rain.org/pipermail/sanctuary-naturalist-corps/2002-August/000839.html); 2002.
Template:Mnb2 Gómez-Gutiérrez, J.: Euphausiids (http://www.geocities.com/jgomez64/euphausiids.html); accessed Jun 16, 2005.
Template:Mnb2 Gurney, R.: Larvae of decapod crustacea. Royal Society Publ. 129; London 1942.
Template:Mnb2 Haberman, K: Answers to miscellaneous questions about krill (http://quest.arc.nasa.gov/antarctica2/ask/new/Miscellaneous_questions_about_krill.txt), February 26, 1997. Accessed June 17, 2005.
Template:Mnb2 Howard, D.: Krill in Cordell Bank National Marine Sanctuary (http://oceanexplorer.noaa.gov/explorations/02quest/background/krill/krill.html), NOAA. Last accessed June 15, 2005.
Template:Mnb2 Ignatyev, S. M.: Functional-Morphological Adaptations of the Krill to Active Swimming (http://www.ibss.iuf.net/people/ignat/ikrill99.html), Poster on the 2nd International Symposium on Krill, Santa Cruz, California, USA; August 23-27, 1999.
Template:Mnb2 Taxonomy of Euphausiacea (http://www.itis.usda.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=95496) from ITIS.
Template:Mnb2 Jaffe, J.S.; Ohmann, M. D.; De Robertis, A.: Sonar estimates of daytime activity levels of Euphausia pacifica in Saanich Inlet (http://jaffeweb.ucsd.edu/pubs/Sonar%20estimates%20of%20daytime%20activity%20levels%20of%20Euphausia%20pacifica%20in%20Saanich%20Inlet.pdf), Can. J. Fish. Aquat. Sci. 56, pp. 2000 – 2010; 1999.
Template:Mnb2 Kirkwood, J.A.: A Guide to the Euphausiacea of the Southern Ocean. Australian National Antarctic Research Expedition; Australia Dept of Science and Technology, Antarctic Division; 1984.
Template:Mnb2 Knight, M. D.: Variation in Larval Morphogenesis within the Southern California Bight Population of Euphausia pacifica from Winter through Summer, 1977-1978 (http://www.calcofi.org/newhome/publications/CalCOFI_Reports/v25/pdfs/Vol_25_Knight.pdf), CalCOFI Report Vol. XXV, 1984.
Template:Mnb2 Krill (http://marinebio.org/species.asp?id=518) at MarineBio.
Template:Mnb2 Mauchline, J.; Fisher, L.R.: The biology of euphausiids. Adv. Mar. Biol. 7; 1969.
Template:Mnb2 Nicol, S.; Endo, Y.: Krill Fisheries of the World (http://www.fao.org/documents/show_cdr.asp?url_file=//DOCREP/003/W5911E/w5911e00.htm), FAO Fisheries Technical Paper 367; 1997.
Template:Mnb2 Nicol, S.; Foster, J.: Recent trends in the fishery for Antarctic krill (http://www.edpsciences.org/articles/alr/pdf/2003/01/alr3065.pdf?access=ok), Aquat. Living Resour. 16, pp. 42 – 45; 2003.
Template:Mnb2 Roach, J.: Scientists Discover Mystery Krill Killer (http://news.nationalgeographic.com/news/2003/07/0717_030717_krillkiller.html), National Geographic News, July 17, 2003. See also the base article: Gómez-Gutiérrez, J.; Peterson, W. T.; De Robertis, A.; Brodeur, R. D.: Mass Mortality of Krill Caused by Parasitoid Ciliates, Science Vol 301; issue 5631, pp. 339f; July 18, 2003.
Template:Mnb2 Ross, R. M.; Quetin, L. B.: How Productive are Antarctic Krill? Bioscience 36, pp. 264 – 269; 1986.
Template:Mnb2 Sala, A.; Azzali, M.; Russo, A.: Krill of the Ross Sea: distribution, abundance and demography of Euphausia superba and Euphausia crystallorophias during the Italian Antarctic Expedition (January-February 2000) (http://www.icm.csic.es/scimar/662sal.html), Scientia Marina 66(2), pp. 123 – 133. 2002.
Template:Mnb2 Weier, J.: Changing Currents color the Bering Sea a new shade of Blue (http://earthobservatory.nasa.gov/Study/Coccoliths/bering_sea.html), NOAA Earth Observatory, 1999. Last accessed June 15, 2005.da:krill de:Krill pt:Krill sv:Lysräkor