91-439: Thysanopoda amblyops G. O. Sars, 1883 Bentheuphausia amblyops , the deep sea krill is a species of krill . B. amblyops is the only species within its genus, which in turn is the only genus within the family Bentheuphausiidae . All the 85 other species of krill known are classified in the family Euphausiidae . B. amblyops occurs in the northern Atlantic Ocean in latitudes south of 40° N , and also in
182-639: A bathypelagic krill living in deep waters below 1,000 m (3,300 ft). It is considered the most primitive extant krill species. Well-known species of the Euphausiidae of commercial krill fisheries include Antarctic krill ( Euphausia superba ), Pacific krill ( E. pacifica ) and Northern krill ( Meganyctiphanes norvegica ). Bentheuphausia Thysanopoda (♣) Nematobrachion (♦) Meganyctiphanes Pseudeuphausia Euphausia Nyctiphanes Nematoscelis Thysanoessa Tessarabrachion Stylocheiron As of 2013 ,
273-422: 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, 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
364-503: 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, which contains dinoflagellates. Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles. The precise function of these organs
455-423: A mutation–selection balance . It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the fitness landscape will outcompete a quasispecies located at a higher but narrower fitness peak in which the surrounding mutants are unfit, "the quasispecies effect" or the "survival of the flattest". There is no suggestion that a viral quasispecies resembles
546-405: A ring species . Also, among organisms that reproduce only asexually , the concept of a reproductive species breaks down, and each clone is potentially a microspecies. Although none of these are entirely satisfactory definitions, and while the concept of species may not be a perfect model of life, it is still a useful tool to scientists and conservationists for studying life on Earth, regardless of
637-400: A "classical" method of determining species, such as with Linnaeus, early in evolutionary theory. However, different phenotypes are not necessarily different species (e.g. a four-winged Drosophila born to a two-winged mother is not a different species). Species named in this manner are called morphospecies . In the 1970s, Robert R. Sokal , Theodore J. Crovello and Peter Sneath proposed
728-424: A 'smallest clade' idea" (a phylogenetic species concept). Mishler and Wilkins and others concur with this approach, even though this would raise difficulties in biological nomenclature. Wilkins cited the ichthyologist Charles Tate Regan 's early 20th century remark that "a species is whatever a suitably qualified biologist chooses to call a species". Wilkins noted that the philosopher Philip Kitcher called this
819-428: A connected series of neighbouring populations, each of which can sexually interbreed with adjacent related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing
910-480: A decline in 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. 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
1001-432: A different species from its ancestors. Viruses have enormous populations, are doubtfully living since they consist of little more than a string of DNA or RNA in a protein coat, and mutate rapidly. All of these factors make conventional species concepts largely inapplicable. A viral quasispecies is a group of genotypes related by similar mutations, competing within a highly mutagenic environment, and hence governed by
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#17328525763111092-613: A few species are carnivorous , preying on small zooplankton and fish larvae . 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. Many animals feed on krill, ranging from smaller animals like fish or penguins to larger ones like seals and baleen whales . Disturbances of an ecosystem resulting in
1183-508: A genetic boundary suitable for defining a species concept is present. DNA barcoding has been proposed as a way to distinguish species suitable even for non-specialists to use. One of the barcodes is a region of mitochondrial DNA within the gene for cytochrome c oxidase . A database, Barcode of Life Data System , contains DNA barcode sequences from over 190,000 species. However, scientists such as Rob DeSalle have expressed concern that classical taxonomy and DNA barcoding, which they consider
1274-492: A misnomer, need to be reconciled, as they delimit species differently. Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species. A phylogenetic or cladistic species is "the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)". The empirical basis – observed character states – provides
1365-449: A particular species, including which genus (and higher taxa) it is placed in, is a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be corroborated or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as
1456-400: A short way of saying that something applies to many species within a genus, but not to all. If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. The names of genera and species are usually printed in italics . However, abbreviations such as "sp." should not be italicised. When a species' identity is not clear,
1547-404: A specialist may use "cf." before the epithet to indicate that confirmation is required. The abbreviations "nr." (near) or "aff." (affine) may be used when the identity is unclear but when the species appears to be similar to the species mentioned after. With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including: The naming of
1638-523: A species as groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Mayr's use of the adjective "potentially" has been a point of debate; some interpretations exclude unusual or artificial matings that occur only in captivity, or that involve animals capable of mating but that do not normally do so in
1729-400: A species as determined by a taxonomist. A typological species is a group of organisms in which individuals conform to certain fixed properties (a type), so that even pre-literate people often recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. This method was used as
1820-488: A species. All species definitions assume that an organism acquires its genes from one or two parents very like the "daughter" organism, but that is not what happens in HGT. There is strong evidence of HGT between very dissimilar groups of prokaryotes , and at least occasionally between dissimilar groups of eukaryotes , including some crustaceans and echinoderms . The evolutionary biologist James Mallet concludes that there
1911-685: A species. Generally the term includes the unknown element of a distinct act of creation. Many authors have argued that a simple textbook definition, following Mayr's concept, works well for most multi-celled organisms , but breaks down in several situations: Species identification is made difficult by discordance between molecular and morphological investigations; these can be categorised as two types: (i) one morphology, multiple lineages (e.g. morphological convergence , cryptic species ) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity , multiple life-cycle stages). In addition, horizontal gene transfer (HGT) makes it difficult to define
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#17328525763112002-428: 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
2093-519: A taxonomic decision at the discretion of cognizant specialists, is not governed by the Codes of Zoological or Botanical Nomenclature, in contrast to the PhyloCode , and contrary to what is done in several other fields, in which the definitions of technical terms, like geochronological units and geopolitical entities, are explicitly delimited. The nomenclatural codes that guide the naming of species, including
2184-506: A traditional biological species. The International Committee on Taxonomy of Viruses has since 1962 developed a universal taxonomic scheme for viruses; this has stabilised viral taxonomy. Most modern textbooks make use of Ernst Mayr 's 1942 definition, known as the Biological Species Concept as a basis for further discussion on the definition of species. It is also called a reproductive or isolation concept. This defines
2275-447: A variation on the morphological species concept, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms. It differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species
2366-515: A variety of reasons. Viruses are a special case, driven by a balance of mutation and selection , and can be treated as quasispecies . Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics . Early taxonomists such as Linnaeus had no option but to describe what they saw: this was later formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts,
2457-1264: Is E. crystallorophias , which is endemic to the Antarctic coastline. Species with endemic distributions include Nyctiphanes capensis , which occurs only in the Benguela current, E. mucronata in the Humboldt current, and the six Euphausia species native to the Southern Ocean. In the Antarctic, seven species are known, one in genus Thysanoessa ( T. macrura ) and six in Euphausia . The Antarctic krill ( Euphausia superba ) commonly lives at depths reaching 100 m (330 ft), 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). Krill perform Diel Vertical Migrations (DVM) in large swarms, and acoustic data has shown these migrations to go up to 400 metres in depth. 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
2548-438: Is "an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies". This differs from the biological species concept in embodying persistence over time. Wiley and Mayden stated that they see the evolutionary species concept as "identical" to Willi Hennig 's species-as-lineages concept, and asserted that
2639-416: Is a defensive mechanism, confusing smaller predators that would like to pick out individuals. In 2012, Gandomi and Alavi presented what appears to be a successful stochastic algorithm 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." Krill typically follow
2730-400: Is a group of sexually reproducing organisms that recognise one another as potential mates. Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms; no matter whether populations can hybridise successfully, they are still distinct cohesion species if
2821-458: Is a set of organisms adapted to a particular set of resources, called a niche, in the environment. According to this concept, populations form the discrete phenetic clusters that we recognise as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters. A genetic species as defined by Robert Baker and Robert Bradley is a set of genetically isolated interbreeding populations. This
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2912-568: Is able to reduce its body size when there is not enough food available, moulting 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. Shrinkage has been postulated for other temperate-zone species of krill as well. Some high-latitude species of krill can live for more than six years (e.g., Euphausia superba ); others, such as
3003-563: 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. Many krill are filter feeders : their frontmost appendages , 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 as Euphausia spp.) that feed primarily on phytoplankton , in particular on diatoms , which are unicellular algae . Krill are mostly omnivorous , although
3094-414: Is between 8 and 8.7 million. About 14% of these had been described by 2011. All species (except viruses ) are given a two-part name , called a "binomial". The first part of a binomial is the genus to which the species belongs. The second part is called the specific name or the specific epithet (in botanical nomenclature , also sometimes in zoological nomenclature ). For example, Boa constrictor
3185-414: Is called speciation . Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species . Speciation depends on a measure of reproductive isolation , a reduced gene flow. This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Reproductive isolation
3276-403: Is described formally, in a publication that assigns it a unique scientific name. The description typically provides means for identifying the new species, which may not be based solely on morphology (see cryptic species ), differentiating it from other previously described and related or confusable species and provides a validly published name (in botany) or an available name (in zoology) when
3367-671: Is further weakened by the existence of microspecies , groups of organisms, including many plants, with very little genetic variability, usually forming species aggregates . For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies—perhaps 400 in the case of the blackberry and over 200 in the dandelion, complicated by hybridisation , apomixis and polyploidy , making gene flow between populations difficult to determine, and their taxonomy debatable. Species complexes occur in insects such as Heliconius butterflies, vertebrates such as Hypsiboas treefrogs, and fungi such as
3458-726: Is hard or even impossible to test. Later biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, and the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: (1) agamospecies for asexual organisms (2) biospecies for reproductively isolated sexual organisms (3) ecospecies based on ecological niches (4) evolutionary species based on lineage (5) genetic species based on gene pool (6) morphospecies based on form or phenotype and (7) taxonomic species,
3549-403: Is no easy way to tell whether related geographic or temporal forms belong to the same or different species. Species gaps can be verified only locally and at a point of time. One is forced to admit that Darwin's insight is correct: any local reality or integrity of species is greatly reduced over large geographic ranges and time periods. The botanist Brent Mishler argued that the species concept
3640-468: Is not valid, notably because gene flux decreases gradually rather than in discrete steps, which hampers objective delimitation of species. Indeed, complex and unstable patterns of gene flux have been observed in cichlid teleosts of the East African Great Lakes . Wilkins argued that "if we were being true to evolution and the consequent phylogenetic approach to taxa, we should replace it with
3731-400: Is one of the species of the genus Boa , with constrictor being the species' epithet. While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example, the boundaries between closely related species become unclear with hybridisation , in a species complex of hundreds of similar microspecies , and in
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3822-586: Is similar to Mayr's Biological Species Concept, but stresses genetic rather than reproductive isolation. In the 21st century, a genetic species could be established by comparing DNA sequences. Earlier, other methods were available, such as comparing karyotypes (sets of chromosomes ) and allozymes ( enzyme variants). An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation. In palaeontology , with only comparative anatomy (morphology) and histology from fossils as evidence,
3913-434: Is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity . Other ways of defining species include their karyotype , DNA sequence, morphology , behaviour, or ecological niche . In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. The most recent rigorous estimate for the total number of species of eukaryotes
4004-594: Is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model . A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form (a chronospecies ), without increasing the number of resultant species. Horizontal gene transfer between organisms of different species, either through hybridisation , antigenic shift , or reassortment ,
4095-523: The ICZN for animals and the ICN for plants, do not make rules for defining the boundaries of the species. Research can change the boundaries, also known as circumscription, based on new evidence. Species may then need to be distinguished by the boundary definitions used, and in such cases the names may be qualified with sensu stricto ("in the narrow sense") to denote usage in the exact meaning given by an author such as
4186-487: The Southern Ocean are E. frigida , E. longirostris , E. triacantha and E. vallentini . Krill are crustaceans and, like all crustaceans, they have a chitinous exoskeleton . They have anatomy similar to a standard decapod with their bodies made up of three parts : the cephalothorax is composed of the head and the thorax , which are fused, and the abdomen , which bears the ten swimming appendages, and
4277-399: The fly agaric . Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they can hybridise where their geographical ranges overlap. A ring species is
4368-498: The jaguar ( Panthera onca ) of Latin America or the leopard ( Panthera pardus ) of Africa and Asia. In contrast, the scientific names of species are chosen to be unique and universal (except for some inter-code homonyms ); they are in two parts used together : the genus as in Puma , and the specific epithet as in concolor . A species is given a taxonomic name when a type specimen
4459-487: The tail fan . This outer shell of krill is transparent in most species. Krill feature intricate compound eyes . Some species 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 feeding legs and grooming legs. Krill are probably
4550-406: The "cynical species concept", and arguing that far from being cynical, it usefully leads to an empirical taxonomy for any given group, based on taxonomists' experience. Other biologists have gone further and argued that we should abandon species entirely, and refer to the "Least Inclusive Taxonomic Units" (LITUs), a view that would be coherent with current evolutionary theory. The species concept
4641-522: The Decapoda based on developmental similarities, as noted by Robert Gurney and Isabella Gordon . The reason for this debate is that krill share some morphological features of decapods and others of mysids. 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
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#17328525763114732-522: The Euphausiidae. Adults reach a length of 4 to 5 centimetres (1.6 to 2.0 in). Krill Krill (Euphausiids) ( sg. : krill) are small and exclusively marine crustaceans of the order Euphausiacea , found in all the world's oceans. The name "krill" comes from the Norwegian word krill , meaning "small fry of fish", which is also often attributed to species of fish. Krill are considered an important trophic level connection near
4823-504: The Philippines. In the Philippines, they are also called alamang and are used to make a salty paste called bagoong . Krill are also the main prey of baleen whales , including the blue whale . Krill belong to the large arthropod subphylum , the Crustacea . The most familiar and largest group of crustaceans, the class Malacostraca , includes the superorder Eucarida comprising
4914-405: The abbreviation "sp." in the singular or "spp." (standing for species pluralis , Latin for "multiple species") in the plural in place of the specific name or epithet (e.g. Canis sp.). This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology . Authors may also use "spp." as
5005-570: The amount of hybridisation is insufficient to completely mix their respective gene pools . A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology , genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists have assumed that members of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA–DNA hybridisation to decide if they belong to
5096-518: 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). E. superba
5187-474: The biological species concept, "the several versions" of the phylogenetic species concept, and the idea that species are of the same kind as higher taxa are not suitable for biodiversity studies (with the intention of estimating the number of species accurately). They further suggested that the concept works for both asexual and sexually-reproducing species. A version of the concept is Kevin de Queiroz 's "General Lineage Concept of Species". An ecological species
5278-505: The biological species concept, a cladistic species does not rely on reproductive isolation – its criteria are independent of processes that are integral in other concepts. Therefore, it applies to asexual lineages. However, it does not always provide clear cut and intuitively satisfying boundaries between taxa, and may require multiple sources of evidence, such as more than one polymorphic locus, to give plausible results. An evolutionary species, suggested by George Gaylord Simpson in 1951,
5369-599: 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 379 million tonnes , 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 , providing food for predators near
5460-433: The concept of a chronospecies can be applied. During anagenesis (evolution, not necessarily involving branching), some palaeontologists seek to identify a sequence of species, each one derived from the phyletically extinct one before through continuous, slow and more or less uniform change. In such a time sequence, some palaeontologists assess how much change is required for a morphologically distinct form to be considered
5551-678: The environment in smaller form. The life cycle of krill is relatively well understood, despite minor variations in detail from species to species. After krill hatch, they experience several larval stages— nauplius , pseudometanauplius , metanauplius , calyptopsis , and furcilia , 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 and moult 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
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#17328525763115642-435: The evidence to support hypotheses about evolutionarily divergent lineages that have maintained their hereditary integrity through time and space. Molecular markers may be used to determine diagnostic genetic differences in the nuclear or mitochondrial DNA of various species. For example, in a study done on fungi , studying the nucleotide characters using cladistic species produced the most accurate results in recognising
5733-606: The family Dajidae (epicaridean isopods ) afflict krill (and also shrimp and mysids ); one such parasite is Oculophryxus bicaulis , which was found on the krill Stylocheiron affine and S. 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. Climate change poses another threat to krill populations. Preliminary research indicates krill can digest microplastics under 5 mm (0.20 in) in diameter, breaking them down and excreting them back into
5824-582: 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. 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. Moulting occurs whenever
5915-436: 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. After the final furcilia stage, an immature juvenile emerges in a shape similar to an adult, and subsequently develops gonads and matures sexually. During the mating season, which varies by species and climate,
6006-642: The genus Thysanoessa occur in both Atlantic and Pacific oceans. 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 . They are highly abundant along the upwelling regions of the California , Humboldt , Benguela , and Canarias current systems . Another species having only neritic distribution
6097-435: The larvae through metanauplius stage. 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
6188-512: The last common ancestor of the krill family Euphausiidae (order Euphausiacea minus Bentheuphausia amblyops ) to have lived in the Lower Cretaceous about 130 million years ago . Krill occur worldwide in all oceans, although many individual species have endemic or neritic ( i.e., coastal) distributions. Bentheuphausia amblyops , a bathypelagic species, has a cosmopolitan distribution within its deep-sea habitat. Species of
6279-513: 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. Krill can have multiple broods in one season, with interbrood intervals lasting on the order of days. Krill employ two types of spawning mechanism. The 57 species of the genera Bentheuphausia , Euphausia , Meganyctiphanes , Thysanoessa , and Thysanopoda are "broadcast spawners":
6370-412: The mid-latitude species Euphausia pacifica , live for only two years. Subtropical or tropical species' longevity is still shorter, e.g., Nyctiphanes simplex , which usually lives for only six to eight months. Most krill are swarming animals; the sizes and densities of such swarms vary by species and region. For Euphausia superba , swarms reach 10,000 to 60,000 individuals per cubic metre. Swarming
6461-417: The mixed layer. 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. Species A species ( pl. : species) is a population of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring , typically by sexual reproduction . It
6552-625: The monophyly of Eucarida (with basal Mysida), another groups Euphausiacea with Mysida (the Schizopoda), while yet another groups Euphausiacea with Hoplocarida . No extant fossil can be unequivocally assigned to Euphausiacea. Some extinct eumalacostracan taxa have been thought to be euphausiaceans such as Anthracophausia , Crangopsis —now assigned to the Aeschronectida (Hoplocarida) —and Palaeomysis . All dating of speciation events were estimated by molecular clock methods, which placed
6643-478: The numerous fungi species of all the concepts studied. Versions of the phylogenetic species concept that emphasise monophyly or diagnosability may lead to splitting of existing species, for example in Bovidae , by recognising old subspecies as species, despite the fact that there are no reproductive barriers, and populations may intergrade morphologically. Others have called this approach taxonomic inflation , diluting
6734-520: The order Schizopoda , which was split by Johan Erik Vesti Boas in 1883 into two separate orders. 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. It was later also proposed that order Euphausiacea should be grouped with the Penaeidae (family of prawns) in
6825-488: 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 and by molecular studies. 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
6916-472: The order of 6–15 centimetres (2.4–5.9 in). The largest krill species, Thysanopoda spinicaudata , lives deep in the open ocean . Krill can be easily distinguished from other crustaceans such as true shrimp by their externally visible gills . Except for Bentheuphausia amblyops , krill are bioluminescent animals having organs called photophores that can emit light. The light is generated by an enzyme -catalysed chemiluminescence reaction, wherein
7007-585: The paper is accepted for publication. The type material is usually held in a permanent repository, often the research collection of a major museum or university, that allows independent verification and the means to compare specimens. Describers of new species are asked to choose names that, in the words of the International Code of Zoological Nomenclature , are "appropriate, compact, euphonious, memorable, and do not cause offence". Books and articles sometimes intentionally do not identify species fully, using
7098-674: The person who named the species, while the antonym sensu lato ("in the broad sense") denotes a wider usage, for instance including other subspecies. Other abbreviations such as "auct." ("author"), and qualifiers such as "non" ("not") may be used to further clarify the sense in which the specified authors delineated or described the species. Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct. The evolutionary process by which biological populations of sexually-reproducing organisms evolve to become distinct or reproductively isolated as species
7189-487: The result of misclassification leading to questions on whether there really are any ring species. The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat, Felis catus , or the cat family, Felidae . Another problem with common names is that they often vary from place to place, so that puma, cougar, catamount, panther, painter and mountain lion all mean Puma concolor in various parts of America, while "panther" may also mean
7280-573: The ring. Ring species thus present a difficulty for any species concept that relies on reproductive isolation. However, ring species are at best rare. Proposed examples include the herring gull – lesser black-backed gull complex around the North pole, the Ensatina eschscholtzii group of 19 populations of salamanders in America, and the greenish warbler in Asia, but many so-called ring species have turned out to be
7371-508: The same species. This concept was narrowed in 2006 to a similarity of 98.7%. The average nucleotide identity (ANI) method quantifies genetic distance between entire genomes , using regions of about 10,000 base pairs . With enough data from genomes of one genus, algorithms can be used to categorize species, as for Pseudomonas avellanae in 2013, and for all sequenced bacteria and archaea since 2020. Observed ANI values among sequences appear to have an "ANI gap" at 85–95%, suggesting that
7462-529: The same species. When two species names are discovered to apply to the same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymy . Dividing a taxon into multiple, often new, taxa is called splitting . Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognising differences or commonalities between organisms. The circumscription of taxa, considered
7553-725: The sister clade of decapods because all species have five pairs of swimming legs called "swimmerets" in common with the latter, very similar to those of a lobster or freshwater crayfish . In spite of having ten swimmerets, otherwise known as pleopods , krill cannot be considered decapods. They lack any true ground-based legs due to all their pereiopods having been converted into grooming and auxiliary feeding legs. In Decapoda , there are ten functioning pereiopods , giving them their name; whereas here there are no remaining locomotive pereiopods . Nor are there consistently ten pereiopods at all. Most krill are about 1–2 centimetres (0.4–0.8 in) long as adults. A few species grow to sizes on
7644-536: The southern seas of the Atlantic, in the Indian Ocean and in the Pacific . It is a bathypelagic krill that lives in deep waters below 1,000 metres (3,300 ft). It is distinguished from the Euphausiidae by several morphological features, the most apparent being that they are not bioluminescent and that their first pair of pleopods is not modified as copulatory tool organs. Also, their eyes are smaller than those of
7735-502: The species concept and making taxonomy unstable. Yet others defend this approach, considering "taxonomic inflation" pejorative and labelling the opposing view as "taxonomic conservatism"; claiming it is politically expedient to split species and recognise smaller populations at the species level, because this means they can more easily be included as endangered in the IUCN red list and can attract conservation legislation and funding. Unlike
7826-668: 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, mostly from the Scotia Sea . Most krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing , or in the pharmaceutical industry. Krill are also used for human consumption in several countries. They are known as okiami ( オキアミ ) in Japan and as camarones in Spain and
7917-485: The theoretical difficulties. If species were fixed and clearly distinct from one another, there would be no problem, but evolutionary processes cause species to change. This obliges taxonomists to decide, for example, when enough change has occurred to declare that a lineage should be divided into multiple chronospecies , or when populations have diverged to have enough distinct character states to be described as cladistic species. Species and higher taxa were seen from
8008-494: The three orders, Euphausiacea (krill), Decapoda (shrimp, prawns, lobsters, crabs), and the planktonic Amphionidacea . The order Euphausiacea comprises two families . The more abundant Euphausiidae contains 10 different genera with a total of 85 species. Of these, the genus Euphausia is the largest, with 31 species. The lesser-known family, the Bentheuphausiidae , has only one species , Bentheuphausia amblyops ,
8099-873: The time of Aristotle until the 18th century as categories that could be arranged in a hierarchy, the great chain of being . In the 19th century, biologists grasped that species could evolve given sufficient time. Charles Darwin 's 1859 book On the Origin of Species explained how species could arise by natural selection . That understanding was greatly extended in the 20th century through genetics and population ecology . Genetic variability arises from mutations and recombination , while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures . Genes can sometimes be exchanged between species by horizontal gene transfer ; new species can arise rapidly through hybridisation and polyploidy ; and species may become extinct for
8190-537: The wild. It is difficult to define a species in a way that applies to all organisms. The debate about species concepts is called the species problem. The problem was recognised even in 1859, when Darwin wrote in On the Origin of Species : I was much struck how entirely vague and arbitrary is the distinction between species and varieties. He went on to write: No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of
8281-543: Was thought to have been one reason salmon did not spawn that season. Several single-celled endoparasitoidic ciliates of the genus Collinia can infect species of krill and devastate affected populations. Such diseases were reported for Thysanoessa inermis in the Bering Sea and also for E. pacifica , Thysanoessa spinifera , and T. gregaria off the North American Pacific coast. Some ectoparasites of
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