Anostraca is one of the fourorders ofcrustaceans in the classBranchiopoda; its members are referred to asfairy shrimp. They live invernal pools[3] andhypersaline lakes across the world, and they have even been found indeserts, ice-covered mountain lakes, andAntarctic ice.[4] They are usually 6–25 mm (0.24–0.98 in) long (exceptionally up to 170 mm or 6.7 in). Most species have 20 body segments, bearing 11 pairs of leaf-likephyllopodia (swimming legs), and the body lacks acarapace. They swim "upside-down" andfeed by filtering organic particles from the water or by scrapingalgae from surfaces,[4] with the exception ofBranchinecta 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 8families.[4]
The body of a fairy shrimp is elongated and divided into segments.[5] The whole animal is typically 6–25 millimetres (0.24–0.98 in) long, but one species,Branchinecta gigas does not reachsexual maturity until it reaches 50 mm (2.0 in) long, and can grow to 170 mm (6.7 in) long.[5] Theexoskeleton is thin and flexible,[5] and lacks any sign of acarapace.[6] The body can be divided into three distinct parts (tagmata) – head, thorax and abdomen.[6]
The head is morphologically distinct from the thorax. It bears twocompound eyes on prominent stalks, and two pairs ofantennae.[7] 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 duringmating.[7] In some groups, males have an additional frontal appendage.[7]
The thorax of most anostracans has 13 segments (19 inPolyartemiella and 21 inPolyartemia).[8] All but the last two are very similar, with a pair of biramous phyllopods (flattened, leaf-likeappendages).[6] The last two segments are fused together,[5] and their appendages are specialised for reproduction.[8] Most anostracans have separate sexes (gonochorism), but a few reproduce byparthenogenesis.[9] The abdomen comprises 6 segments without appendages, and atelson,[8] which bears two flattened caudal rami or "cercopods".[5]
The head contains two digestive glands and the small lobatestomach into which they empty. This is connected to a longintestine, which terminates in a shortrectum, with theanus located on thetelson.[7] Thehaemocoel of anostracans is pumped by a long, tubularheart, which runs through most of the animal's length.[7] A series of slits allow haemocoel into the heart, which is then pumped out of the anterior opening byperistalsis.[7] The nervous system consists of two nerve cords which run the length of the body, with twoganglia and two transversecommissures in most of the body segments.[7]
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 forosmotic regulation.[7] Two coiledglands at the bases of the maxillae are used toexcretenitrogenous waste, typically in the form ofurea.[7] Most of the animal's nitrogenous waste is, however, in the form ofammonia, which probably diffuses into the environment through the phyllopodia and gills.[7]
Anostracans inhabit inland waters ranging fromhypersaline lakes to lakes that are almost devoid of dissolved substances;[5] they are "the most archetypal crustaceans" in ephemeral waters.[10] The relatively large size of fairy shrimp, together with their slow means of locomotion, makes them an easy target forpredatoryfish andwaterfowl.[10] This has led to their distribution being restricted to environments with fewer predators, such asvernal pools,salt lakes and lakes at high altitudes or latitudes.[10] The southernmost recorded fairy shrimp isBranchinecta gaini from theAntarctic Peninsula,[11] while the altitude record is held byB. brushi, which lives at 5,930 metres (19,460 ft) in theChileanAndes.[12] Other genera, such asStreptocephalus, occur indeserts throughout the world.[13]
Anostracans swim gracefully by movements of their phyllopodia (thoracic appendages) in ametachronal rhythm.[7] When swimming, the animal's ventral side is normally uppermost (often described as swimming "upside-down").[5] Theyfilter 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.[5]
Another important aspect of the fairy shrimp’s life cycle is their universal ability to enter diapause,[14][15] a state of biological dormancy where growth and metabolism are arrested,[16] as an egg (or cyst). This trait assists in both species' dispersal and in overcoming adverse environmental conditions.[15][16] 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.[17][5][16] It is also the only way for the fairy shrimps to colonize new habitats—facilitated by a variety of conditions including wind, predators, currents[18][19][20]—as the soft-bodied adults are unable to leave the freshwater system.[19] Once in diapause, these cysts can remain viable for centuries,[18] and the mixing of system sediment results in the hatching of different aged cysts in each generation.[21][19][22] 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.[19]
Anostracans are an important food source for many birds and fish. For example, they provide much of the food for femalepintails andmallards in thePrairie Pothole Region of theGreat Plains inNorth America, especially in years when temporary wetlands are abundant.[23] Similarly,Artemia forms an important part of the diet offlamingos wherever it can be found.[24]
Brine shrimp are used as food forfish and other organisms inaquaria andaquaculture.[25] 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 theGreat Salt Lake inUtah andSan Francisco Bay inCalifornia;[26] adults are collected fromMono Lake and transported frozen.[25]
Fairy shrimp are believed to have diverged from the main line of Branchiopoda during the Ordovician period,[27][28] around the same time it is thought they colonized freshwater and estuarine ecosystems.[16] This transition is believed to have resulted from selection pressure to escape predation in the Early Paleozoic seas.[29][5][15]Lepidocaris from the earlyDevonianRhynie chert is likely a close relative of Anostraca. The oldest known modern-looking ansotracan isHaltinnaias from the late Devonian (Famennian) Strud locality of Belgium, around 365 million years old.[30][31]
Some studies point to fossils resembling fairy shrimp in the Upper Cambrian,[32][33] specifically the oldest known branchiopod fossil,Rehbachiella kinnekullensis, from Orsten marine deposits.[34] 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.[27]
The monophyly of this order is well supported,[29][35][36][27][32][37][38] and the scientific community has reached consensus that Anostraca was the first group to branch off from the Branchiopoda.[14][32][15][38][28]
The radiation hypothesis championing rapid spread and colonization during the Gondwana fragmentation closely echoes the current distribution of the order.[28][19] Presently, Anostraca are found on all seven continents.[29] Most extant genera have restricted geographical distributions. Only three genera are widespread across the remnants of the former supercontinentPangaea:Artemia,Branchinella andBranchinecta, while the remaining genera are found only throughout former Laurasia.[39] 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.[19] Studies have found Anostraca capable of rapid colonization[40] and speciation.[21]
Fairy Shrimp has been a student favorite as the mascot ofUC Merced. There have been several efforts to make this animal the official mascot of the campus. Still, in 2001 the bobcat was chosen instead.[41][42] 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.[43][44]
^abcWilliam David Williams (1980)."Arachnids and Crustaceans".Australian Freshwater Life: the Invertebrates of Australian Inland Waters (2nd ed.). Palgrave Macmillan Australia. pp. 118–184.ISBN978-0-333-29894-7.
^abHairston, Nelson G.; Cáceres, Carla E. (1996-03-01). "Distribution of crustacean diapause: micro- and macroevolutionary pattern and process".Hydrobiologia.320 (1–3):27–44.doi:10.1007/bf00016802.ISSN0018-8158.S2CID44914111.
^abcdAlekseev, Victor R.; Starobogatov, Yaroslav I. (1996-03-01). "Types of diapause in Crustacea: definitions, distribution, evolution".Hydrobiologia.320 (1–3):15–26.doi:10.1007/bf00016801.ISSN0018-8158.S2CID253609.
^abParnov, V.; Krylov, P.; Riccardi, N. (2004). "Role of diapause in dispersal and invasion success by aquatic invertebrates".Journal of Limnology.63:59–69.
^Brendonck, L. (1996). "Diapause, quiescence, hatching requirements: what we can learn from large freshwater branchiopods (Crustacea Branciopoda: Anostraca, Notostraca, Conchostraca)".Hydrobiologia.320 (1–3):85–97.doi:10.1007/bf00016809.S2CID26327230.
^Weekers, P (2002). "Phylogenetic analysis of anostracans (Branchiopoda: Anostraca) inferred from nuclear 18S ribosomal DNA (18S rDNA) sequences".Molecular Phylogenetics and Evolution.25 (3):535–544.doi:10.1016/s1055-7903(02)00289-0.PMID12450757.
^abRegier, Jerome C.; Shultz, Jeffrey W.; Zwick, Andreas; Hussey, April; Ball, Bernard; Wetzer, Regina; Martin, Joel W.; Cunningham, Clifford W. (2010-02-10). "Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences".Nature.463 (7284):1079–1083.Bibcode:2010Natur.463.1079R.doi:10.1038/nature08742.ISSN1476-4687.PMID20147900.S2CID4427443.
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