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Anostraca is one of the four orders of crustaceans in the class Branchiopoda; its members are referred to as fairy shrimp. They live in vernal pools<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and hypersaline lakes across the world, and they have even been found in deserts, ice-covered mountain lakes, and Antarctic ice.<ref name=":10">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> They are usually Template:Convert long (exceptionally up to Template:Convert). Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia (swimming legs), and the body lacks a carapace. They swim "upside-down" and feed by filtering organic particles from the water or by scraping algae from surfaces,<ref name=":10" /> with the exception of Branchinecta gigas, or "giant fairy shrimp", which is itself a predator of other species of anostracans. They are an important food for many birds and fish, and some are cultured and harvested for use as fish food. There are 300 species spread across 8 families.<ref name=":10" />
DescriptionEdit
The body of a fairy shrimp is elongated and divided into segments.<ref name="Belk"/> The whole animal is typically Template:Convert long, but one species, Branchinecta gigas does not reach sexual maturity until it reaches Template:Convert long, and can grow to Template:Convert long.<ref name="Belk"/> The exoskeleton is thin and flexible,<ref name="Belk"/> and lacks any sign of a carapace.<ref name="Australia">Template:Cite book</ref> The body can be divided into three distinct parts (tagmata) – head, thorax and abdomen.<ref name="Australia"/>
HeadEdit
The head is morphologically distinct from the thorax. It bears two compound eyes on prominent stalks, and two pairs of antennae.<ref name="Pennak">Template:Cite book</ref> The first pair of antennae are small, usually unsegmented, and uniramous. The second pair are long and cylindrical in females, but in males they are enlarged and specialised for holding the female during mating.<ref name="Pennak"/> In some groups, males have an additional frontal appendage.<ref name="Pennak"/>
Thorax and abdomenEdit
The thorax of most anostracans has 13 segments (19 in Polyartemiella and 21 in Polyartemia).<ref name="Khanna">Template:Cite book</ref> All but the last two are very similar, with a pair of biramous phyllopods (flattened, leaf-like appendages).<ref name="Australia"/> The last two segments are fused together,<ref name="Belk"/> and their appendages are specialised for reproduction.<ref name="Khanna"/> Most anostracans have separate sexes (gonochorism), but a few reproduce by parthenogenesis.<ref name="Bell">Template:Cite book</ref> The abdomen comprises 6 segments without appendages, and a telson,<ref name="Khanna"/> which bears two flattened caudal rami or "cercopods".<ref name="Belk"/>
Internal anatomyEdit
The head contains two digestive glands and the small lobate stomach into which they empty. This is connected to a long intestine, which terminates in a short rectum, with the anus located on the telson.<ref name="Pennak"/> The haemocoel of anostracans is pumped by a long, tubular heart, which runs through most of the animal's length.<ref name="Pennak"/> A series of slits allow haemocoel into the heart, which is then pumped out of the anterior opening by peristalsis.<ref name="Pennak"/> The nervous system consists of two nerve cords which run the length of the body, with two ganglia and two transverse commissures in most of the body segments.<ref name="Pennak"/>
Gas exchange is thought to take place through the entire body surface, but especially that of the phyllopodia and their associated gills, which may also be responsible for osmotic regulation.<ref name="Pennak"/> Two coiled glands at the bases of the maxillae are used to excrete nitrogenous waste, typically in the form of urea.<ref name="Pennak"/> Most of the animal's nitrogenous waste is, however, in the form of ammonia, which probably diffuses into the environment through the phyllopodia and gills.<ref name="Pennak"/>
Ecology and behaviourEdit
Anostracans inhabit inland waters ranging from hypersaline lakes to lakes that are almost devoid of dissolved substances;<ref name="Belk">Template:Cite book</ref> they are "the most archetypal crustaceans" in ephemeral waters.<ref name="Dumont">Template:Cite book</ref> The relatively large size of fairy shrimp, together with their slow means of locomotion, makes them an easy target for predatory fish and waterfowl.<ref name="Dumont"/> This has led to their distribution being restricted to environments with fewer predators, such as vernal pools, salt lakes and lakes at high altitudes or latitudes.<ref name="Dumont"/> The southernmost recorded fairy shrimp is Branchinecta gaini from the Antarctic Peninsula,<ref>Template:Cite journal</ref> while the altitude record is held by B. brushi, which lives at Template:Convert in the Chilean Andes.<ref name="Hegna">Template:Cite journal</ref> Other genera, such as Streptocephalus, occur in deserts throughout the world.<ref>Template:Cite book</ref>
Anostracans swim gracefully by movements of their phyllopodia (thoracic appendages) in a metachronal rhythm.<ref name="Pennak"/> When swimming, the animal's ventral side is normally uppermost (often described as swimming "upside-down").<ref name="Belk"/> They filter food indiscriminately from the water as they swim, but also scrape algae and other organic materials from solid surfaces, for which they turn to have their ventral side against the food surface.<ref name="Belk"/>
Another important aspect of the fairy shrimp’s life cycle is their universal ability to enter diapause,<ref name=":0">Template:Cite journal</ref><ref name=":1">Template:Cite journal</ref> a state of biological dormancy where growth and metabolism are arrested,<ref name=":2">Template:Cite journal</ref> as an egg (or cyst). This trait assists in both species' dispersal and in overcoming adverse environmental conditions.<ref name=":1" /><ref name=":2" /> Once dormant, these cysts can withstand conditions as harsh and diverse as droughts, frosts, hypersalinity, complete desiccation, exposure to UV radiation and the vacuum of space.<ref>Template:Cite journal</ref><ref name="Belk" /><ref name=":2" /> It is also the only way for the fairy shrimps to colonize new habitats—facilitated by a variety of conditions including wind, predators, currents<ref name=":3">Template:Cite journal</ref><ref name=":4">Template:Cite journal</ref><ref>Template:Cite journal</ref>—as the soft-bodied adults are unable to leave the freshwater system.<ref name=":4" /> Once in diapause, these cysts can remain viable for centuries,<ref name=":3" /> and the mixing of system sediment results in the hatching of different aged cysts in each generation.<ref name=":5">Template:Cite journal</ref><ref name=":4" /><ref>Template:Cite journal</ref> This inbreeding slows the rate of selection by resisting gene flow and minimizing phenotypic variation, in turn promoting the stability of the existing, successful phenotype.<ref name=":4" />
Anostracans are an important food source for many birds and fish. For example, they provide much of the food for female pintails and mallards in the Prairie Pothole Region of the Great Plains in North America, especially in years when temporary wetlands are abundant.<ref name="Batt">Template:Cite book</ref> Similarly, Artemia forms an important part of the diet of flamingos wherever it can be found.<ref>Template:Cite book</ref>
UsesEdit
Brine shrimp are used as food for fish and other organisms in aquaria and aquaculture.<ref name="Jørgensen">Template:Cite book</ref> Their drought-resistant eggs are collected from lakeshores and are stored and transported dry. They hatch readily when submerged in salt water. This is a multimillion-dollar industry, centred on the Great Salt Lake in Utah and San Francisco Bay in California;<ref>Template:Cite book</ref> adults are collected from Mono Lake and transported frozen.<ref name="Jørgensen"/>
Fossil record and evolutionEdit
Fairy shrimp are believed to have diverged from the main line of Branchiopoda during the Ordovician period,<ref name=":6">Template:Cite journal</ref><ref name=":7">Template:Cite journal</ref> around the same time it is thought they colonized freshwater and estuarine ecosystems.<ref name=":2" /> This transition is believed to have resulted from selection pressure to escape predation in the Early Paleozoic seas.<ref name="Diversity" /><ref name="Belk" /><ref name=":1" /> Lepidocaris from the early Devonian Rhynie chert is likely a close relative of Anostraca. The oldest known modern-looking ansotracan is Haltinnaias from the late Devonian (Famennian) Strud locality of Belgium, around 365 million years old.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Some studies point to fossils resembling fairy shrimp in the Upper Cambrian,<ref name=":8">Template:Cite journal</ref><ref>Template:Cite journal</ref> specifically the oldest known branchiopod fossil, Rehbachiella kinnekullensis, from Orsten marine deposits.<ref>Template:Cite journal</ref> Despite its seeming resemblance to modern fairy shrimp, this fossil is still considered by most to be an outlying member of the ancestral marine Branchiopoda rather than an actual fairy shrimp.<ref name=":6" />
The monophyly of this order is well supported,<ref name="Diversity" /><ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref name=":6" /><ref name=":8" /><ref>Template:Cite journal</ref><ref name=":9">Template:Cite journal</ref> and the scientific community has reached consensus that Anostraca was the first group to branch off from the Branchiopoda.<ref name=":0" /><ref name=":8" /><ref name=":1" /><ref name=":9" /><ref name=":7" />
The radiation hypothesis championing rapid spread and colonization during the Gondwana fragmentation closely echoes the current distribution of the order.<ref name=":7" /><ref name=":4" /> Presently, Anostraca are found on all seven continents.<ref name="Diversity" /> Most extant genera have restricted geographical distributions. Only three genera are widespread across the remnants of the former supercontinent Pangaea: Artemia, Branchinella and Branchinecta, while the remaining genera are found only throughout former Laurasia.<ref name="Williams">Template:Cite book</ref> This suggests that much of the potential habitat in this supercontinent, now occupied by Anostraca, was previously unoccupied by ecologically similar species, or inhabited by species with less adaptive ability.<ref name=":4" /> Studies have found Anostraca capable of rapid colonization<ref>Template:Cite journal</ref> and speciation.<ref name=":5" />
DiversityEdit
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Anostraca is the most diverse of the four orders of Branchiopoda. It comprises around 313 species, grouped into 26 genera in eight families:<ref name="Diversity">Template:Cite book</ref>
- Artemiidae – 1 genus, 8 species
- Branchinectidae – 1 genus, 45 species
- Branchipodidae – 5 genera, 35 species
- Chirocephalidae – 9 genera, 81 species
- Parartemiidae – 1 genus, 18 species
- Streptocephalidae – 1 genus, 56 species
- Tanymastigidae – 2 genera, 8 species
- Thamnocephalidae – 6 genera, 62 species
Fairy Shrimp in Popular CultureEdit
Fairy Shrimp has been a student favorite as the mascot of UC Merced. There have been several efforts to make this animal the official mascot of the campus. Still, in 2001 the bobcat was chosen instead.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Fairy shrimp had also been the focus of a challenge to the location of where the campus would be built because of their nearby vernal pool habitat.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
ReferencesEdit
External linksEdit
Template:Branchiopoda Template:Taxonbar Template:Authority control