76-485: 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 the bottom of the food chain . They feed on phytoplankton and, to
152-636: 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,
228-421: 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
304-500: 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
380-479: 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
456-458: A different species. If this can happen without the larger mother population also developing a new autapomorphy, then the mother population cannot remain a species under the autapomorphic species concept: it would no longer have any apomorphies not also shared by the daughter species. Phylogenetic similarities: These phylogenetic terms are used to describe different patterns of ancestral and derived character or trait states as stated in
532-610: 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
608-403: A large, diverse group of mainly aquatic arthropods including decapods ( shrimps , prawns , crabs , lobsters and crayfish ), seed shrimp , branchiopods , fish lice , krill , remipedes , isopods , barnacles , copepods , opossum shrimps , amphipods and mantis shrimp . The crustacean group can be treated as a subphylum under the clade Mandibulata . It is now well accepted that
684-468: A larger Pancrustacea clade . The traditional classification of Crustacea based on morphology recognised four to six classes. Bowman and Abele (1982) recognised 652 extant families and 38 orders, organised into six classes: Branchiopoda , Remipedia , Cephalocarida , Maxillopoda, Ostracoda , and Malacostraca . Martin and Davis (2001) updated this classification, retaining the six classes but including 849 extant families in 42 orders. Despite outlining
760-585: 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
836-468: A limbless abdomen, except from a telson and caudal rami which is present in many groups. The abdomen in malacostracans bears pleopods , and ends in a telson, which bears the anus , and is often flanked by uropods to form a tail fan . The number and variety of appendages in different crustaceans may be partly responsible for the group's success. Crustacean appendages are typically biramous , meaning they are divided into two parts; this includes
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#1732872095526912-436: A single taxon, they do not convey information about relationship. Therefore, autapomorphies are not useful to infer phylogenetic relationships. However, autapomorphy, like synapomorphy and plesiomorphy is a relative concept depending on the taxon in question. An autapomorphy at a given level may well be a synapomorphy at a less-inclusive level. An example of an autapomorphy can be described in modern snakes. Snakes have lost
988-428: 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
1064-487: A spiral format. Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, and a collection of major ganglia is found below the gut. In many decapods , the first (and sometimes the second) pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans (such as the Christmas Island red crab ) mate seasonally and return to
1140-1256: 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
1216-415: 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
1292-566: 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
1368-438: Is an open circulatory system , where blood is pumped into the haemocoel by a heart located near the dorsum. Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods have haemoglobins . The alimentary canal consists of a straight tube that often has a gizzard-like "gastric mill" for grinding food and a pair of digestive glands that absorb food; this structure goes in
1444-559: 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
1520-565: Is found only in one taxon , but not found in any others or outgroup taxa , not even those most closely related to the focal taxon (which may be a species , family or in general any clade). It can therefore be considered an apomorphy in relation to a single taxon. The word autapomorphy , introduced in 1950 by German entomologist Willi Hennig , is derived from the Greek words αὐτός, autos "self"; ἀπό, apo "away from"; and μορφή, morphḗ = "shape". Because autapomorphies are only present in
1596-464: Is produced in Asia, with China alone producing nearly half the world's total. Non-decapod crustaceans are not widely consumed, with only 118,000 tons of krill being caught, despite krill having one of the greatest biomasses on the planet. Autapomorphy In phylogenetics , an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it
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#17328720955261672-669: Is some debate as to whether or not Cambrian animals assigned to Ostracoda are truly ostracods , which would otherwise start in the Ordovician . The only classes to appear later are the Cephalocarida , which have no fossil record, and the Remipedia , which were first described from the fossil Tesnusocaris goldichi , but do not appear until the Carboniferous . Most of the early crustaceans are rare, but fossil crustaceans become abundant from
1748-521: Is thought to be just 1 ⁄ 10 to 1 ⁄ 100 of the total number as most species remain as yet undiscovered . Although most crustaceans are small, their morphology varies greatly and includes both the largest arthropod in the world – the Japanese spider crab with a leg span of 3.7 metres (12 ft) – and the smallest, the 100- micrometre -long (0.004 in) Stygotantulus stocki . Despite their diversity of form, crustaceans are united by
1824-490: Is useful in that it avoids non-monophyletic groupings, it has its criticisms as well. N.I. Platnick, for example, believes the autapomorphic species concept to be inadequate because it allows for the possibility of reproductive isolation and speciation while revoking the "species" status of the mother population. In other words, if a peripheral population breaks away and becomes reproductively isolated, it would conceivably need to develop at least one autapomorphy to be recognized as
1900-611: The Marmorkrebs crayfish. In many crustaceans, the fertilised eggs are released into the water column , while others have developed a number of mechanisms for holding on to the eggs until they are ready to hatch. Most decapods carry the eggs attached to the pleopods , while peracarids , notostracans , anostracans , and many isopods form a brood pouch from the carapace and thoracic limbs. Female Branchiura do not carry eggs in external ovisacs but attach them in rows to rocks and other objects. Most leptostracans and krill carry
1976-698: The Carboniferous , as are the first true mantis shrimp. In the Decapoda , prawns and polychelids appear in the Triassic, and shrimp and crabs appear in the Jurassic . The fossil burrow Ophiomorpha is attributed to ghost shrimps, whereas the fossil burrow Camborygma is attributed to crayfishes. The Permian–Triassic deposits of Nurra preserve the oldest (Permian: Roadian) fluvial burrows ascribed to ghost shrimps (Decapoda: Axiidea, Gebiidea) and crayfishes (Decapoda: Astacidea, Parastacidea), respectively. However,
2052-465: The Carboniferous period onwards. Within the Malacostraca, no fossils are known for krill , while both Hoplocarida and Phyllopoda contain important groups that are now extinct as well as extant members (Hoplocarida: mantis shrimp are extant, while Aeschronectida are extinct; Phyllopoda: Canadaspidida are extinct, while Leptostraca are extant ). Cumacea and Isopoda are both known from
2128-401: The Japanese spider crab with a leg span of up to 3.8 m (12.5 ft) and a mass of 20 kg (44 lb). Like other arthropods , crustaceans have an exoskeleton , which they moult to grow. They are distinguished from other groups of arthropods, such as insects , myriapods and chelicerates , by the possession of biramous (two-parted) limbs, and by their larval forms , such as
2204-430: 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
2280-503: The hexapods ( insects and entognathans ) emerged deep in the Crustacean group, with the completed pan-group referred to as Pancrustacea . The three classes Cephalocarida , Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans ( oligostracans and multicrustaceans ). The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to
2356-629: The nauplius stage of branchiopods and copepods . Most crustaceans are free-living aquatic animals , but some are terrestrial (e.g. woodlice , sandhoppers ), some are parasitic (e.g. Rhizocephala , fish lice , tongue worms ) and some are sessile (e.g. barnacles ). The group has an extensive fossil record , reaching back to the Cambrian . More than 7.9 million tons of crustaceans per year are harvested by fishery or farming for human consumption, consisting mostly of shrimp and prawns . Krill and copepods are not as widely fished, but may be
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2432-413: The pereon or thorax , and the pleon or abdomen . The head and thorax may be fused together to form a cephalothorax , which may be covered by a single large carapace . The crustacean body is protected by the hard exoskeleton , which must be moulted for the animal to grow. The shell around each somite can be divided into a dorsal tergum , ventral sternum and a lateral pleuron. Various parts of
2508-486: 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
2584-515: The zoea (pl. zoeæ or zoeas ). This name was given to it when naturalists believed it to be a separate species. It follows the nauplius stage and precedes the post-larva . Zoea larvae swim with their thoracic appendages , as opposed to nauplii, which use cephalic appendages, and megalopa, which use abdominal appendages for swimming. It often has spikes on its carapace , which may assist these small organisms in maintaining directional swimming. In many decapods , due to their accelerated development,
2660-702: The Asian shore crab, Hemigrapsus sanguineus . Since the opening of the Suez Canal , close to 100 species of crustaceans from the Red Sea and the Indo-Pacific realm have established themselves in the eastern Mediterranean sub-basin, with often significant impact on local ecosystems. Most crustaceans have separate sexes , and reproduce sexually . In fact, a recent study explains how the male T. californicus decide which females to mate with by dietary differences, preferring when
2736-519: 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
2812-683: The Hexapoda is distinctly closer to e.g. a Multicrustacean than an Oligostracan is. Crustaceans have a rich and extensive fossil record , which begins with animals such as Canadaspis and Perspicaris from the Middle Cambrian age Burgess Shale . Most of the major groups of crustaceans appear in the fossil record before the end of the Cambrian, namely the Branchiopoda , Maxillopoda (including barnacles and tongue worms ) and Malacostraca ; there
2888-504: 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
2964-406: The animals with the greatest biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology (alternatively, malacostracology , crustaceology or crustalogy ), and a scientist who works in carcinology is a carcinologist . The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head ,
3040-517: 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
3116-452: The basis of amount of divergence associated with reproductive incompatibility, which is measured essentially by number of autapomorphies. This grouping method is often referred to as the " monophyletic species concept" or the "phylospecies" concept and was popularized by D.E. Rosen in 1979. Within this definition, a species is seen as "the least inclusive monophyletic group definable by at least one autapomorphy". While this model of speciation
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3192-448: The eggs between their thoracic limbs; some copepods carry their eggs in special thin-walled sacs, while others have them attached together in long, tangled strings. Crustaceans exhibit a number of larval forms, of which the earliest and most characteristic is the nauplius . This has three pairs of appendages , all emerging from the young animal's head, and a single naupliar eye. In most groups, there are further larval stages, including
3268-675: 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
3344-679: The evidence that Maxillopoda was non-monophyletic, they retained it as one of the six classes, although did suggest that Maxillipoda could be replaced by elevating its subclasses to classes. Since then phylogenetic studies have confirmed the polyphyly of Maxillipoda and the paraphyletic nature of Crustacea with respect to Hexapoda. Recent classifications recognise ten to twelve classes in Crustacea or Pancrustacea, with several former maxillopod subclasses now recognised as classes (e.g. Thecostraca , Tantulocarida , Mystacocarida , Copepoda , Branchiura and Pentastomida ). The following cladogram shows
3420-444: The exoskeleton may be fused together. Each somite , or body segment can bear a pair of appendages : on the segments of the head, these include two pairs of antennae , the mandibles and maxillae ; the thoracic segments bear legs , which may be specialised as pereiopods (walking legs) and maxillipeds (feeding legs). Malacostraca and Remipedia (and the hexapods) have abdominal appendages. All other classes of crustaceans have
3496-604: 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
3572-580: 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
3648-456: The females are algae-fed instead of yeast-fed. A small number are hermaphrodites , including barnacles , remipedes , and Cephalocarida . Some may even change sex during the course of their life. Parthenogenesis is also widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male. This occurs in many branchiopods , some ostracods , some isopods , and certain "higher" crustaceans, such as
3724-434: 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,
3800-637: 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
3876-662: The great radiation of crustaceans occurred in the Cretaceous , particularly in crabs, and may have been driven by the adaptive radiation of their main predators, bony fish . The first true lobsters also appear in the Cretaceous. Many crustaceans are consumed by humans, and nearly 10,700,000 tons were harvested in 2007; the vast majority of this output is of decapod crustaceans : crabs , lobsters , shrimp , crawfish , and prawns . Over 60% by weight of all crustaceans caught for consumption are shrimp and prawns, and nearly 80%
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#17328720955263952-435: 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
4028-510: 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
4104-422: The layer allow light to reach the retina. As the larvae mature into adults, the layer migrates to a new position behind the retina where it works as a backscattering mirror that increases the intensity of light passing through the eyes, as seen in many nocturnal animals. In an effort to understand whether DNA repair processes can protect crustaceans against DNA damage , basic research was conducted to elucidate
4180-509: 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":
4256-408: 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
4332-439: 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. Crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are a part of the subphylum Crustacea ( / k r ə ˈ s t eɪ ʃ ə / ),
4408-620: 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
4484-401: The oceans as insects are on land. Most crustaceans are also motile , moving about independently, although a few taxonomic units are parasitic and live attached to their hosts (including sea lice , fish lice , whale lice , tongue worms , and Cymothoa exigua , all of which may be referred to as "crustacean lice"), and adult barnacles live a sessile life – they are attached headfirst to
4560-518: 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
4636-484: 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
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#17328720955264712-470: 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
4788-429: The repair mechanisms used by Penaeus monodon (black tiger shrimp). Repair of DNA double-strand breaks was found to be predominantly carried out by accurate homologous recombinational repair. Another, less accurate process, microhomology-mediated end joining , is also used to repair such breaks. The expression pattern of DNA repair related and DNA damage response genes in the intertidal copepod Tigriopus japonicus
4864-446: The sea to release the eggs. Others, such as woodlice , lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs until they hatch into free-swimming larvae. Most crustaceans are aquatic, living in either marine or freshwater environments, but a few groups have adapted to life on land, such as terrestrial crabs , terrestrial hermit crabs , and woodlice . Marine crustaceans are as ubiquitous in
4940-510: The second pair of antennae, but not the first, which is usually uniramous , the exception being in the Class Malacostraca where the antennules may be generally biramous or even triramous. It is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Trilobites , for instance, also possessed biramous appendages. The main body cavity
5016-725: 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
5092-473: The special larval form known as the nauplius . The exact relationships of the Crustacea to other taxa are not completely settled as of April 2012 . Studies based on morphology led to the Pancrustacea hypothesis, in which Crustacea and Hexapoda ( insects and allies) are sister groups . More recent studies using DNA sequences suggest that Crustacea is paraphyletic , with the hexapods nested within
5168-449: The substrate and cannot move independently. Some branchiurans are able to withstand rapid changes of salinity and will also switch hosts from marine to non-marine species. Krill are the bottom layer and most important part of the food chain in Antarctic animal communities. Some crustaceans are significant invasive species , such as the Chinese mitten crab, Eriocheir sinensis , and
5244-668: 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
5320-493: 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 ,
5396-477: The two pairs of legs that characterize all of Tetrapoda , and the closest taxa to Ophidia – as well as their common ancestors – all have two pairs of legs. Therefore, the Ophidia taxon presents an autapomorphy with respect to its absence of legs. The autapomorphic species concept is one of many methods that scientists might use to define and distinguish species from one another. This definition assigns species on
5472-412: The updated relationships between the different extant groups of the paraphyletic Crustacea in relation to the class Hexapoda . Ostracoda Mystacocarida Branchiura Pentastomida Malacostraca Copepoda Tantulocarida Thecostraca Cephalocarida Branchiopoda Remipedia Hexapoda According to this diagram, the Hexapoda are deep in the Crustacea tree, and any of
5548-426: The zoea is the first larval stage. In some cases, the zoea stage is followed by the mysis stage, and in others, by the megalopa stage, depending on the crustacean group involved. Providing camouflage against predators, the otherwise black eyes in several forms of swimming larvae are covered by a thin layer of crystalline isoxanthopterin that gives their eyes the same color as the surrounding water, while tiny holes in
5624-447: Was analyzed after ultraviolet irradiation. This study revealed increased expression of proteins associated with the DNA repair processes of non-homologous end joining , homologous recombination , base excision repair and DNA mismatch repair . The name "crustacean" dates from the earliest works to describe the animals, including those of Pierre Belon and Guillaume Rondelet , but the name
5700-412: Was not used by some later authors, including Carl Linnaeus , who included crustaceans among the " Aptera " in his Systema Naturae . The earliest nomenclatural valid work to use the name "Crustacea" was Morten Thrane Brünnich 's Zoologiæ Fundamenta in 1772, although he also included chelicerates in the group. The subphylum Crustacea comprises almost 67,000 described species , which
5776-540: 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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