Krill
Krill | |
---|---|
Northern krill (Meganyctiphanes norvegica) | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | Malacostraca |
Superorder: | Eucarida |
Order: | Euphausiacea Dana, 1852 |
Families and genera | |
|
Krill (Euphausiids),
Krill are considered an important trophic level connection—near the bottom of the food chain. They feed on phytoplankton and, to a lesser extent, zooplankton, and are also the main source of food for many larger animals. In the Southern Ocean, one species, the Antarctic krill, makes up an estimated biomass of around 3.79 billion tonnes,[4] making it among the species with the largest total biomass. Over half of this biomass is eaten by whales, seals, penguins, seabirds, squid, and fish each year. Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.
Krill are fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, most of this from the Scotia Sea. Most of the krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry. In Japan, the Philippines, and Russia, krill are also used for human consumption and are known as okiami (オキアミ) in Japan. They are eaten as camarones in Spain and Philippines. In the Philippines, krill are also known as alamang and are used to make a salty paste called bagoong.
Krill are also the main prey of baleen whales, including the blue whale.
Taxonomy
Krill belong to the large
The order Euphausiacea comprises two
Well-known species of the Euphausiidae of commercial
Phylogeny
Proposed phylogeny of Euphausiacea[8] | |||||||||||||||||||||||||||||||||
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Phylogeny obtained from morphological data, (♠) names coined in,[8] (♣) possibly paraphyletic taxon due to Nematobrachion in.[8] (♦) clades differs from Casanova (1984),[9] where Pseudoeuphausia is sister to Nyctiphanes, Euphausia is sister to Thysanopoda and Nematobrachion is sister to Stylocheiron. |
As of 2013[update], the order Euphausiacea is believed to be monophyletic due to several unique conserved morphological characteristics (autapomorphy) such as its naked filamentous gills and thin thoracopods[10] and by molecular studies.[11][12][13]
There have been many theories of the location of the order Euphausiacea. Since the first description of Thysanopode tricuspide by Henri Milne-Edwards in 1830, the similarity of their biramous thoracopods had led zoologists to group euphausiids and Mysidacea in the order Schizopoda, which was split by Johan Erik Vesti Boas in 1883 into two separate orders.[14] Later, William Thomas Calman (1904) ranked the Mysidacea in the superorder Peracarida and euphausiids in the superorder Eucarida, although even up to the 1930s the order Schizopoda was advocated.[10] It was later also proposed that order Euphausiacea should be grouped with the Penaeidae (family of prawns) in the Decapoda based on developmental similarities, as noted by Robert Gurney and Isabella Gordon.[15][16] The reason for this debate is that krill share some morphological features of decapods and others of mysids.[10]
Molecular studies have not unambiguously grouped them, possibly due to the paucity of key rare species such as Bentheuphausia amblyops in krill and Amphionides reynaudii in Eucarida. One study supports the monophyly of Eucarida (with basal Mysida),[17] another groups Euphausiacea with Mysida (the Schizopoda),[12] while yet another groups Euphausiacea with Hoplocarida.[18]
Timeline
No extant fossil can be unequivocally assigned to Euphausiacea. Some extinct
Distribution
Krill occur worldwide in all oceans, although many individual species have
Species of the genus Thysanoessa occur in both Atlantic and Pacific oceans.[21] The Pacific is home to Euphausia pacifica. Northern krill occur across the Atlantic from the Mediterranean Sea northward.
Species with neritic distributions include the four species of the genus Nyctiphanes.[22] They are highly abundant along the upwelling regions of the California, Humboldt, Benguela, and Canarias current systems.[23][24][25] Another species having only neritic distribution is E. crystallorophias, which is endemic to the Antarctic coastline.[26]
Species with endemic distributions include Nyctiphanes capensis, which occurs only in the Benguela current,[22] E. mucronata in the Humboldt current,[27] and the six Euphausia species native to the Southern Ocean.
In the Antarctic, seven species are known,
Anatomy and morphology
Krill are
Krill feature intricate compound eyes. Some species adapt to different lighting conditions through the use of screening pigments.[34]
They have two
Krill are probably the sister clade of decapods because all species have five pairs of
Most krill are about 1–2 centimetres (0.4–0.8 in) long as adults. A few species grow to sizes on the order of 6–15 centimetres (2.4–5.9 in). The largest krill species, Thysanopoda spinicaudata, lives
Except for
Ecology
Feeding
Many krill are
Krill are an important element of the aquatic food chain. Krill convert the primary production of their prey into a form suitable for consumption by larger animals that cannot feed directly on the minuscule algae. Northern krill and some other species have a relatively small filtering basket and actively hunt copepods and larger zooplankton.[44]
Predation
Many animals feed on krill, ranging from smaller animals like fish or penguins to larger ones like seals and baleen whales.[45]
Disturbances of an ecosystem resulting in a decline in the krill population can have far-reaching effects. During a coccolithophore bloom in the Bering Sea in 1998,[46] for instance, the diatom concentration dropped in the affected area. 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. The incident was thought to have been one reason salmon did not spawn that season.[47]
Several single-celled
Climate change poses another threat to krill populations.[51]
Plastics
Preliminary research indicates krill can digest
Life history and behavior
The life cycle of krill is relatively well understood, despite minor variations in detail from species to species.
By the calyptopsis stages differentiation has progressed far enough for them to develop a mouth and a digestive tract, and they begin to eat phytoplankton. By that time their yolk reserves are exhausted and the larvae must have reached the photic zone, the upper layers of the ocean where algae flourish. 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 moult. The number of segments added during any one of the furcilia stages may vary even within one species depending on environmental conditions.[53] After the final furcilia stage, an immature juvenile emerges in a shape similar to an adult, and subsequently develops gonads and matures sexually.[54]
Reproduction
During the mating season, which varies by species and climate, the male deposits a sperm sack at the female's genital opening (named thelycum). 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.[55] Krill can have multiple broods in one season, with interbrood intervals lasting on the order of days.[25][56]
Krill employ two types of spawning mechanism.[25] The 57 species of the genera Bentheuphausia, Euphausia, Meganyctiphanes, Thysanoessa, and Thysanopoda are "broadcast spawners": the female releases the fertilised eggs into the water, where they usually sink, disperse, and are on their own. These species generally hatch in the nauplius 1 stage, but have recently been discovered to hatch sometimes as metanauplius or even as calyptopis stages.[57] The remaining 29 species of the other genera are "sac spawners", where the female carries the eggs with her, attached to the rearmost pairs of thoracopods until they hatch as metanauplii, although some species like Nematoscelis difficilis may hatch as nauplius or pseudometanauplius.[58]
Moulting
Moulting occurs whenever a specimen outgrows its rigid exoskeleton. Young animals, growing faster, moult more often than older and larger ones. The frequency of moulting varies widely by species and is, even within one species, subject to many external factors such as latitude, water temperature, and food availability. The subtropical species Nyctiphanes simplex, for instance, has an overall inter-moult period of two to seven days: larvae moult on the average every four days, while juveniles and adults do so, on average, every six days. For E. superba in the Antarctic sea, inter-moult periods ranging between 9 and 28 days depending on the temperature between −1 and 4 °C (30 and 39 °F) have been observed, and for Meganyctiphanes norvegica in the North Sea the inter-moult periods range also from 9 and 28 days but at temperatures between 2.5 and 15 °C (36.5 and 59.0 °F).[59] E. superba is able to reduce its body size when there is not enough food available, moulting also when its exoskeleton becomes too large.[60] 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. Shrinkage has been postulated for other temperate-zone species of krill as well.[61]
Lifespan
Some high-latitude species of krill can live for more than six years (e.g., Euphausia superba); others, such as the mid-latitude species Euphausia pacifica, live for only two years.[7] Subtropical or tropical species' longevity is still shorter, e.g., Nyctiphanes simplex, which usually lives for only six to eight months.[62]
Swarming
Most krill are
Vertical migration
Krill typically follow a diurnal vertical migration. It has been assumed that they spend the day at greater depths and rise during the night toward the surface. The deeper they go, the more they reduce their activity,[66] apparently to reduce encounters with predators and to conserve energy. Swimming activity in krill varies with stomach fullness. Sated animals that had been feeding at the surface swim less actively and therefore sink below the mixed layer.[67] As they sink they produce feces which employs a role in the Antarctic carbon cycle. Krill with empty stomachs swim more actively and thus head towards the surface.
Vertical migration may be a 2–3 times daily occurrence. Some species (e.g., Euphausia superba, E. pacifica, E. hanseni, Pseudeuphausia latifrons, and Thysanoessa spinifera) 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.[68]
Experimental studies using Artemia salina as a model suggest that the vertical migrations of krill several hundreds of metres, in groups tens of metres deep, could collectively create enough downward jets of water to have a significant effect on ocean mixing.[69]
Dense swarms can elicit a
Krill normally swim at a pace of 5–10 cm/s (2–3 body lengths per second),
Biogeochemical cycles
The Antarctic krill is an important species in the context of
Human uses
Harvesting history
Krill have been harvested as a food source for humans and domesticated animals since at least the 19th century, and possibly earlier in Japan, where it was known as okiami. Large-scale fishing developed 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.[77] The harvest peaked, which in 1983 was about 528,000 tonnes in the Southern Ocean alone (of which the Soviet Union took in 93%), is now managed as a precaution against overfishing.[78]
In 1993, two events caused a decline in krill fishing: Russia exited the industry; and the
The annual Antarctic catch stabilised at around 100,000 tonnes, which is roughly one fiftieth of the CCAMLR catch quota.[80] The main limiting factor was probably high costs along with political and legal issues.[81] The Japanese fishery saturated at some 70,000 tonnes.[82]
Although krill are found worldwide, fishing in Southern Oceans are preferred because the krill are more "catchable" and abundant in these regions. Particularly in Antarctic seas which are considered as pristine, they are considered a "clean product".[77]
In 2018 it was announced that almost every krill fishing company operating in Antarctica will abandon operations in huge areas around the Antarctic Peninsula from 2020, including "buffer zones" around breeding colonies of penguins.[83]
Human consumption
Although the total
Krill is a rich source of
In 2011, the US Food and Drug Administration published a letter of no objection for a manufactured krill oil product to be generally recognized as safe (GRAS) for human consumption.[85]
Krill (and other
Bio-inspired robotics
Krill are agile swimmers in the intermediate Reynolds number regime, in which there are not many solutions for uncrewed underwater robotics, and have inspired robotic platforms to both study their locomotion as well as find design solutions for underwater robots.[89]
See also
- Antarctic krill
- Cold-water shrimp
- Crustacean
- Krill fishery
- Krill oil
- Northern krill
References
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- ^ E. Brinton (1962). "The distribution of Pacific euphausiids". Bull. Scripps Inst. Oceanogr. 8 (2): 51–270.
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- ^ E. Brinton (1953). "Thysanopoda spinicauda, a new bathypelagic giant euphausiid crustacean, with comparative notes on T. cornuta and T. egregia". Journal of the Washington Academy of Sciences. 43: 408–412.
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Further reading
- Boden, Brian P.; Johnson, Martin W.; Brinton, Edward: "Euphausiacea (Crustacea) of the North Pacific". Bulletin of the Scripps Institution of Oceanography. Volume 6 Number 8, 1955.
- Brinton, Edward: "Euphausiids of Southeast Asian waters". Naga Report volume 4, part 5. La Jolla: University of California, Scripps Institution of Oceanography, 1975.
- Conway, D. V. P.; White, R. G.; Hugues-Dit-Ciles, J.; Galienne, C. P.; Robins, D. B.: Guide to the coastal and surface zooplankton of the South-Western Indian Ocean Archived 23 October 2012 at the Wayback Machine, Order Euphausiacea, Occasional Publication of the Marine Biological Association of the United Kingdom No. 15, Plymouth, UK, 2003.
- Everson, I. (ed.): Krill: biology, ecology and fisheries. Oxford, Blackwell Science; 2000. ISBN 0-632-05565-0.
- Hamner, William M. (May 1984). "Krill — Untapped Bounty From the Sea?". OCLC 643483454.
- Mauchline, J.: Euphausiacea: Adults Archived 15 May 2011 at the Wayback Machine, Conseil International pour l'Exploration de la Mer, 1971. Identification sheets for adult krill with many line drawings. PDF file, 2 Mb.
- Mauchline, J.: Euphausiacea: Larvae Archived 19 April 2012 at the Wayback Machine, Conseil International pour l'Exploration de la Mer, 1971. Identification sheets for larval stages of krill with many line drawings. PDF file, 3 Mb.
- Tett, P.: The biology of Euphausiids, lecture notes from a 2003 course in Marine Biology from Napier University.
- Tett, P.: Bioluminescence, lecture notes from the 1999/2000 edition of that same course.
External links
- Webcam of Krill Aquarium at Australian Antarctic Division
- 'Antarctic Energies' animation by Lisa Roberts