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Anostraca is one of the four orders of
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...
s in the class
Branchiopoda Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca (or Cladocera), Notostraca and the Devonian ''Lepidocaris''. They are mostly small, freshwater animals that feed on plankton and detritus. Description ...
; its members are referred to as fairy shrimp. They live in
vernal pool Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe ...
s and
hypersaline lake A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in hi ...
s across the world, and they have even been found in deserts, ice-covered mountain lakes and Antarctic ice. They are usually long (exceptionally up to ). 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. 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 Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...
.


Description

The body of a fairy shrimp is elongated and divided into segments. The whole animal is typically long, but one species, '' Branchinecta gigas'' does not reach
sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definit ...
until it reaches long, and can grow to long. The
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
is thin and flexible, and lacks any sign of a carapace. The body can be divided into three distinct parts ( tagmata) – head, thorax and abdomen.


Head

The head is morphologically distinct from the thorax. It bears two
compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...
s on prominent stalks, and two pairs of antennae. 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 In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...
. In some groups, males have an additional frontal appendage.


Thorax and abdomen

The thorax of most anostracans has 13 segments (19 in '' Polyartemiella'' and 21 in '' Polyartemia''). All but the last two are very similar, with a pair of biramous phyllopods (flattened, leaf-like appendages). The last two segments are fused together, and their appendages are specialised for reproduction. Most anostracans have separate sexes (
gonochorism In biology, gonochorism is a sexual system where there are only two sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric. Gonochorism ...
), but a few reproduce by
parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...
. The abdomen comprises 6 segments without appendages, and a
telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...
, which bears two flattened caudal rami or "cercopods".


Internal anatomy

The head contains two digestive glands and the small lobate
stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
into which they empty. This is connected to a long intestine, which terminates in a short rectum, with the anus located on the
telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...
. The haemocoel of anostracans is pumped by a long, tubular
heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...
, which runs through most of the animal's length. A series of slits allow haemocoel into the heart, which is then pumped out of the anterior opening by
peristalsis Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whic ...
. The nervous system consists of two nerve cords which run the length of the body, with two ganglia and two transverse
commissure A commissure () is the location at which two objects abut or are joined. The term is used especially in the fields of anatomy and biology. * The most common usage of the term refers to the brain's commissures, of which there are five. Such a commi ...
s in most of the body segments.
Gas exchange Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...
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. Two coiled glands at the bases of the maxillae are used to
excrete Excretion is a process in which metabolic waste is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after lea ...
nitrogenous waste Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, ...
, typically in the form of
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
. Most of the animal's nitrogenous waste is, however, in the form of
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
, which probably diffuses into the environment through the phyllopodia and gills.


Ecology and behaviour

Anostracans inhabit inland waters ranging from
hypersaline lake A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in hi ...
s to lakes that are almost devoid of dissolved substances; they are "the most archetypal crustaceans" in ephemeral waters. The relatively large size of fairy shrimp, together with their slow means of locomotion, makes them an easy target for
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
y
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
and
waterfowl Anseriformes is an order of birds also known as waterfowl that comprises about 180 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which i ...
. This has led to their distribution being restricted to environments with fewer predators, such as
vernal pool Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe ...
s, salt lakes and lakes at high altitudes or latitudes. The southernmost recorded fairy shrimp is '' Branchinecta gaini'' from the Antarctic Peninsula, while the altitude record is held by '' B. brushi'', which lives at in the
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
an
Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
. Other genera, such as '' Streptocephalus'', occur in deserts throughout the world. Anostracans swim gracefully by movements of their phyllopodia (thoracic appendages) in a
metachronal rhythm A metachronal rhythm or metachronal wave refers to wavy movements produced by the sequential action (as opposed to synchronized) of structures such as cilia, segments of worms, or legs. These movements produce the appearance of a travelling wave. ...
. When swimming, the animal's ventral side is normally uppermost (often described as swimming "upside-down"). 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. Another important aspect of the fairy shrimp’s life cycle is their universal ability to enter diapause, a state of biological dormancy where growth and metabolism are arrested, as an egg (or cyst). This trait assists in both species' dispersal and in overcoming adverse environmental conditions. 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. It is also the only way for the fairy shrimps to colonize new habitats—facilitated by a variety of conditions including wind, predators, currents—as the soft-bodied adults are unable to leave the freshwater system. Once in diapause, these cysts can remain viable for centuries, and the mixing of system sediment results in the hatching of different aged cysts in each generation. 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. 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 The Prairie Pothole Region (PPR; french: Région des cuvettes/fondrières des prairies) is an expansive area of the northern Great Plains that contains thousands of shallow wetlands known as potholes. These potholes are the result of glacier ac ...
of the Great Plains in North America, especially in years when temporary wetlands are abundant. Similarly, ''
Artemia ''Artemia'' is a genus of aquatic crustaceans also known as brine shrimp. It is the only genus in the family Artemiidae. The first historical record of the existence of ''Artemia'' dates back to the first half of the 10th century AD from Urmia L ...
'' forms an important part of the diet of
flamingo Flamingos or flamingoes are a type of wading bird in the family Phoenicopteridae, which is the only extant family in the order Phoenicopteriformes. There are four flamingo species distributed throughout the Americas (including the Caribbea ...
s wherever it can be found.


Uses

Brine shrimp ''Artemia'' is a genus of aquatic crustaceans also known as brine shrimp. It is the only genus in the family Artemiidae. The first historical record of the existence of ''Artemia'' dates back to the first half of the 10th century AD from Urmia L ...
are used as food for
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
and other organisms in aquaria and aquaculture. 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 Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
and San Francisco Bay in
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
; adults are collected from
Mono Lake Mono Lake ( ) is a saline soda lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in an endorheic basin. The lack of an outlet causes high levels of salts to accumulate in the lake which make its water a ...
and transported frozen.


Fossil record and evolution

Fairy shrimp are believed to have diverged from the main line of Branchiopoda during the Ordovician period, around the same time it is thought they colonized freshwater and estuarine ecosystems. This transition is believed to have been the result of selection pressure to escape predation in the Early Paleozoic seas. ''
Lepidocaris ''Lepidocaris rhyniensis'' is an extinct species of crustacean. It is the only species known from the order Lipostraca, and is the only abundant animal in the Pragian-aged Rhynie chert deposits. It resembles modern Anostraca, to which it is proba ...
'' from the early Devonian
Rhynie chert The Rhynie chert is a Lower Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is locate ...
is likely a close relative of Anostraca. The oldest known modern-looking ansotracan is '' Haltinnaias'' from the late Devonian (
Famennian The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used b ...
) Strud locality of Belgium, around 365 million years old. Some studies point to fossils resembling fairy shrimp in the Upper Cambrian, specifically the oldest known branchiopod fossil, ''Rebachiella kinnekullensis'', from Orsten marine deposits. 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 a true fairy shrimp. The monophyly of this order is well supported, and the scientific community has reached consensus that Anostraca was the first group to branch off from the Branchiopoda. The radiation hypothesis championing rapid spread and colonization during the Gondwana fragmentation closely echoes the current distribution of the order. Presently, Anostraca are found on all seven continents. Most of the extant genera have restricted geographical distributions. Only three genera are widespread across the remnants of the former supercontinent
Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
: ''
Artemia ''Artemia'' is a genus of aquatic crustaceans also known as brine shrimp. It is the only genus in the family Artemiidae. The first historical record of the existence of ''Artemia'' dates back to the first half of the 10th century AD from Urmia L ...
'', '' Branchinella'' and ''
Branchinecta ''Branchinecta'' is a genus of crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipod ...
'', while the remaining genera are found only throughout former Laurasia. This suggests that much of the potential habitat in this supercontinent, now occupied by Anostraca, to have been unoccupied by ecologically similar species, or to have been inhabited by species with less adaptive ability. Studies have found Anostraca capable of rapid colonization and speciation.


Diversity

Anostraca is the most diverse of the four orders of Branchiopoda. It comprises around 313 species, grouped into 26 genera in eight families: * 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


References


External links

* * {{Taxonbar, from=Q853383 Crustacean orders Freshwater crustaceans Extant Ordovician first appearances