Krill are considered an importanttrophic level connection near the bottom of thefood chain. They feed onphytoplankton and, to a lesser extent,zooplankton, and are also the main source of food for many larger animals. In theSouthern Ocean, one species, theAntarctic krill, makes up an estimatedbiomass of around 379 milliontonnes,[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 largedaily vertical migrations, providing food for predators near the surface at night and in deeper waters during the day.
Krill are fished commercially in theSouthern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, mostly from theScotia Sea. Most krill catch is used foraquaculture andaquarium feeds, asbait insport fishing, or in the pharmaceutical industry. Krill are also used for human consumption in several countries. They are known asokiami (オキアミ) in Japan and ascamarones in Spain and the Philippines. In the Philippines, they are also calledalamang and are used to make a salty paste calledbagoong.
The order Euphausiacea comprises twofamilies. The more abundantEuphausiidae contains 10 differentgenera with a total of 85 species. Of these, the genusEuphausia is the largest, with 31 species.[5] The lesser-known family, theBentheuphausiidae, has only onespecies,Bentheuphausia amblyops, abathypelagic krill living in deep waters below 1,000 m (3,300 ft). It is considered the most primitive extant krill species.[6]
Phylogeny obtained from morphological data, (♠) names coined in,[8] (♣) possibly paraphyletic taxon due toNematobrachion in.[8] (♦) clades differs from Casanova (1984),[9] wherePseudoeuphausia is sister toNyctiphanes,Euphausia is sister toThysanopoda andNematobrachion is sister toStylocheiron.
As of 2013[update], the order Euphausiacea is believed to bemonophyletic 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 ofThysanopode tricuspide byHenri Milne-Edwards in 1830, the similarity of their biramous thoracopods had led zoologists to group euphausiids and Mysidacea in the orderSchizopoda, which was split byJohan Erik Vesti Boas in 1883 into two separate orders.[14] Later,William Thomas Calman (1904) ranked theMysidacea in the superorderPeracarida and euphausiids in the superorderEucarida, although even up to the 1930s the order Schizopoda was advocated.[10] It was later also proposed that order Euphausiacea should be grouped with thePenaeidae (family of prawns) in the Decapoda based on developmental similarities, as noted byRobert Gurney andIsabella 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 asBentheuphausia amblyops in krill andAmphionides 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 withHoplocarida.[18]
No extant fossil can be unequivocally assigned to Euphausiacea. Some extincteumalacostracantaxa have been thought to be euphausiaceans such asAnthracophausia,Crangopsis—now assigned to theAeschronectida (Hoplocarida)[8]—andPalaeomysis.[19] All dating ofspeciation events were estimated bymolecular clock methods, which placed the last common ancestor of the krill family Euphausiidae (order Euphausiacea minusBentheuphausia amblyops) to have lived in theLower Cretaceous about130 million years ago.[12]
In the Antarctic, seven species are known,[28] one in genusThysanoessa (T. macrura) and six inEuphausia. TheAntarctic krill (Euphausia superba) commonly lives at depths reaching 100 m (330 ft),[29] whereas ice krill (Euphausia crystallorophias) reach depth of 4,000 m (13,100 ft), though they commonly inhabit depths of at most 300–600 m (1,000–2,000 ft).[30] Krill perform Diel Vertical Migrations (DVM) in large swarms, and acoustic data has shown these migrations to go up to 400 metres in depth.[31] Both are found atlatitudes south of55° S, withE. crystallorophias dominating south of74° S[32] and in regions ofpack ice. Other species known in theSouthern Ocean areE. frigida,E. longirostris,E. triacantha andE. vallentini.[33]
Krill anatomy explained, usingEuphausia superba as a modelThegills of krill are externally visible
Krill arecrustaceans and, like all crustaceans, they have achitinousexoskeleton. They have anatomy similar to a standarddecapod with their bodies made up of threeparts: the cephalothorax is composed of thehead and thethorax, which are fused, and theabdomen, which bears the ten swimming appendages, and thetail fan. This outer shell of krill is transparent in most species.
Krill feature intricatecompound eyes. Some species adapt to different lighting conditions through the use of screeningpigments.[34]
They have twoantennae and several pairs of thoracic legs calledpereiopods orthoracopods, so named because they are attached to the thorax. Their number varies among genera and species. These thoracic legs include feeding legs and grooming legs.
Krill are probably the sister clade of decapods because all species have five pairs ofswimming legs called "swimmerets" in common with the latter, very similar to those of alobster orfreshwater crayfish.
In spite of having ten swimmerets, otherwise known aspleopods, krill cannot be considered decapods. They lack any true ground-based legs due to all theirpereiopods having been converted into grooming and auxiliary feeding legs. InDecapoda, there are ten functioningpereiopods, giving them their name; whereas here there are no remaining locomotivepereiopods. Nor are there consistently tenpereiopods at all.
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 cornuta, livesdeep in the open ocean.[35] Krill can be easily distinguished from other crustaceans such as trueshrimp by their externally visiblegills.[36]
Except forBentheuphausia amblyops, krill arebioluminescent animals having organs calledphotophores that can emit light. The light is generated by anenzyme-catalysedchemiluminescence reaction, wherein aluciferin (a kind of pigment) is activated by aluciferase enzyme. Studies indicate that the luciferin of many krill species is afluorescenttetrapyrrole similar but not identical todinoflagellate luciferin[37] and that the krill probably do not produce this substance themselves but acquire it as part of their diet, which contains dinoflagellates.[38] Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles.[39] The precise function of these organs is as yet unknown; possibilities include mating, social interaction or orientation and as a form of counter-illumination camouflage to compensate their shadow against overhead ambient light.[40][41]
Phytoplankton convert CO2, which has dissolved from the atmosphere into the surface oceans (90 Gt yr−1) into particulate organic carbon (POC) during primary production (~ 50 Gt C yr−1). Phytoplankton are then consumed by krill and small zooplankton grazers, which in turn are preyed upon by higher trophic levels. Any unconsumed phytoplankton form aggregates, and along with zooplankton faecal pellets, sink rapidly and are exported out of the mixed layer (< 12 Gt C yr−1 14). Krill, zooplankton and microbes intercept phytoplankton in the surface ocean and sinking detrital particles at depth, consuming and respiring this POC to CO2 (dissolved inorganic carbon, DIC), such that only a small proportion of surface-produced carbon sinks to the deep ocean (i.e., depths > 1000 m). As krill and smaller zooplankton feed, they also physically fragment particles into small, slower- or non-sinking pieces (via sloppy feeding, coprorhexy if fragmenting faeces), retarding POC export. This releases dissolved organic carbon (DOC) either directly from cells or indirectly via bacterial solubilisation (yellow circle around DOC). Bacteria can then remineralise the DOC to DIC (CO2, microbial gardening). Diel vertically migrating krill, smaller zooplankton and fish can actively transport carbon to depth by consuming POC in the surface layer at night, and metabolising it at their daytime, mesopelagic residence depths. Depending on species life history, active transport may occur on a seasonal basis as well. Numbers given are carbon fluxes (Gt C yr−1) in white boxes and carbon masses (Gt C) in dark boxes.[42]
Many krill arefilter feeders:[24] their frontmostappendages, the thoracopods, form very fine combs with which they can filter out their food from the water. These filters can be very fine in species (such asEuphausia spp.) that feed primarily onphytoplankton, in particular ondiatoms, which are unicellularalgae. Krill are mostlyomnivorous,[43] although a few species arecarnivorous, preying on smallzooplankton and fishlarvae.[44]
Krill are an important element of the aquaticfood chain. Krill convert theprimary 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 huntcopepods and larger zooplankton.[44]
Many animals feed on krill, ranging from smaller animals like fish or penguins to larger ones likeseals andbaleen whales.[45]
Disturbances of anecosystem resulting in a decline in the krill population can have far-reaching effects. During acoccolithophore bloom in theBering 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 (mainlyE. pacifica) in that region declined sharply. This in turn affected other species: theshearwater population dropped. The incident was thought to have been one reasonsalmon did not spawn that season.[47]
Several single-celledendoparasitoidicciliates of the genusCollinia can infect species of krill and devastate affected populations. Such diseases were reported forThysanoessa inermis in the Bering Sea and also forE. pacifica,Thysanoessa spinifera, andT. gregaria off the North American Pacific coast.[48][49] Someectoparasites of the familyDajidae (epicarideanisopods) afflict krill (and also shrimp andmysids); one such parasite isOculophryxus bicaulis, which was found on the krillStylocheiron affine andS. longicorne. It attaches itself to the animal's eyestalk and sucks blood from its head; it apparently inhibits the host's reproduction, as none of the afflicted animals reached maturity.[50]
Preliminary research indicates krill can digestmicroplastics under 5 mm (0.20 in) in diameter, breaking them down and excreting them back into the environment in smaller form.[52]
The life cycle of krill is relatively well understood, despite minor variations in detail from species to species.[15][24] After krill hatch, they experience several larval stages—nauplius,pseudometanauplius,metanauplius,calyptopsis, andfurcilia, each of which divides into sub-stages. The pseudometanauplius stage is exclusive to species that lay their eggs within an ovigerous sac: so-called "sac-spawners". The larvae grow andmoult repeatedly as they develop, replacing their rigid exoskeleton when it becomes too small. Smaller animals moult more frequently than larger ones.Yolk reserves within their body nourish the larvae through metanauplius stage.
By the calyptopsis stagesdifferentiation 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 thephotic 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 developsgonads and matures sexually.[54]
The head of a female krill of the sac-spawning speciesNematoscelis difficilis with her brood sac. The eggs have a diameter of 0.3–0.4 millimetres (0.012–0.016 in)
During the mating season, which varies by species and climate, the male deposits asperm sack at the female's genital opening (namedthelycum). The females can carry several thousand eggs in theirovary, 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 generaBentheuphausia,Euphausia,Meganyctiphanes,Thysanoessa, andThysanopoda 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 likeNematoscelis difficilis may hatch as nauplius or pseudometanauplius.[58]
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 speciesNyctiphanes 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. ForE. 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 forMeganyctiphanes norvegica in theNorth 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 forE. 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]
Some high-latitude species of krill can live for more than six years (e.g.,Euphausia superba); others, such as the mid-latitude speciesEuphausia 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]
Most krill areswarming animals; the sizes and densities of such swarms vary by species and region. ForEuphausia superba, swarms reach 10,000 to 60,000 individuals per cubic metre.[63][64] Swarming is a defensive mechanism, confusing smaller predators that would like to pick out individuals. In 2012, Gandomi and Alavi presented what appears to be asuccessful stochastic algorithm[broken anchor] for modelling the behaviour of krill swarms. The algorithm is based on three main factors: " (i) movement induced by the presence of other individuals (ii) foraging activity, and (iii) random diffusion."[65]
Krill typically follow adiurnalvertical 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 producefeces which employs a role in the Antarcticcarbon 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, andThysanoessa 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 usingArtemia 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 afeeding frenzy among fish, birds and mammal predators, especially near the surface. When disturbed, a swarm scatters, and some individuals have even been observed to moult instantly, leaving theexuvia behind as a decoy.[70]
Krill normally swim at a pace of 5–10 cm/s (2–3 body lengths per second),[71] using their swimmerets for propulsion. Their larger migrations are subject to ocean currents. When in danger, they show anescape reaction calledlobstering—flicking theircaudal structures, thetelson and theuropods, 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 asE. superba means around 0.8 m/s (3 ft/s).[72] Their swimming performance has led many researchers to classify adult krill asmicro-nektonic life-forms, i.e., small animals capable of individual motion against (weak) currents. Larval forms of krill are generally considered zooplankton.[73]
Krill (as swarms and individuals) feed on phytoplankton at the surface (1) leaving only a proportion to sink as phytodetrital aggregates (2), which are broken up easily and may not sink below the permanent thermocline. Krill also release faecal pellets (3) whilst they feed, which can sink to the deep sea but can be consumed (coprophagy) and degraded as they descend (4) by krill, bacteria and zooplankton. In the marginal ice zone, faecal pellet flux can reach greater depths (5). Krill also release moults, which sink and contribute to the carbon flux (6). Nutrients are released by krill during sloppy feeding, excretion and egestion, such as iron and ammonium (7, see Fig. 2 for other nutrients released), and if they are released near the surface can stimulate phytoplankton production and further atmospheric CO2 drawdown. Some adult krill permanently reside deeper in the water column, consuming organic material at depth (8). Any carbon (as organic matter or as CO2) that sinks below the permanent thermocline is removed from subjection to seasonal mixing and will remain stored in the deep ocean for at least a year (9). The swimming motions of migrating adult krill that migrate can mix nutrient-rich water from the deep (10), further stimulating primary production. Other adult krill forage on the seafloor, releasing respired CO2 at depth and may be consumed by demersal predators (11). Larval krill, which in the Southern Ocean reside under the sea ice, undergo extensive diurnal vertical migration (12), potentially transferring CO2 below the permanent thermocline. Krill are consumed by many predators including baleen whales (13), leading to storage of some of the krill carbon as biomass for decades before the whale dies, sinks to the seafloor and is consumed by deep sea organisms.[42]
When krill moult they release dissolved calcium, fluoride and phosphorus from the exoskeleton (1). The chitin (organic material) that forms the exoskeleton contributes to organic particle flux sinking to the deep ocean. Krill respire a portion of the energy derived from consuming phytoplankton or other animals as carbon dioxide (2), when swimming from mid/deep waters to the surface in large swarms krill mix water, which potentially brings nutrients to nutrient-poor surface waters (3), ammonium and phosphate is released from the gills and when excreting, along with dissolved organic carbon, nitrogen (e.g., urea) and phosphorus (DOC, DON and DOP, 2 & 4). Krill release fast-sinking faecal pellets containing particulate organic carbon, nitrogen and phosphorus (POC, PON and POP) and iron, the latter of which is bioavailable when leached into surrounding waters along with DOC, DON and DOP (5).[42]
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 asokiami. 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 andUkraine.[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]
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 aspristine, 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]
Although the totalbiomass of Antarctic krill may be as abundant as 400 million tonnes, the human impact on thiskeystone species is growing, with a 39% increase in total fishing yield to 294,000 tonnes over 2010–2014.[80] Major countries involved in krill harvesting areNorway (56% of total catch in 2014), theRepublic of Korea (19%), andChina (18%).[80]
Krill is a rich source ofprotein andomega-3 fatty acids which are under development in the early 21st century as human food,dietary supplements as oil capsules, livestock food, andpet food.[77][79][84] Krill tastes salty with a somewhat stronger fish flavor than shrimp. For mass consumption and commercially prepared products, they must be peeled to remove the inedibleexoskeleton.[84]
Antarctic krill show seasonal metabolic flexibility, storing high levels of lipids during the summer and then relying on these reserves during winter when phytoplankton is limited,[85] and recent aquaculture research has also identified krill meal as a promising sustainable marine lipid source.[86][87]
Krill (and otherplanktonicshrimp, notablyAcetes spp.) are most widely consumed in Southeast Asia, where it isfermented (with the shells intact) and usually ground finely to makeshrimp paste. It can be stir-fried and eaten paired with white rice or used to addumami flavors to a wide variety of traditional dishes.[89][90] The liquid from the fermentation process is also harvested asfish sauce.[91]
Krill are agile swimmers in the intermediateReynolds 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.[92]
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Mauchline, J.:Euphausiacea:AdultsArchived 15 May 2011 at theWayback 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:LarvaeArchived 19 April 2012 at theWayback 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.
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