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Anostraca is one of the four orders of in the class Branchiopoda; its members are referred to as fairy shrimp. They live in and across the world, and they have even been found in , ice-covered mountain lakes, and Antarctica 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 filter feeder organic particles from the water or by scraping algae from surfaces, 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.
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 osmoregulation. Two coiled at the bases of the maxillae are used to excretion nitrogenous waste, typically in the form of urea. 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.
Anostracans swim gracefully by movements of their phyllopodia (thoracic appendages) in a metachronal rhythm. When swimming, the animal's ventral side is normally uppermost (often described as swimming "upside-down"). They filter feeder 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 Northern pintail and in the Prairie Pothole Region of the Great Plains in North America, especially in years when temporary wetlands are abundant. (1992). 9780816620012, University of Minnesota Press. ISBN 9780816620012 Similarly, Artemia forms an important part of the diet of wherever it can be found. (2025). 9781402005046, Springer. ISBN 9781402005046
Some studies point to fossils resembling fairy shrimp in the Upper Cambrian, specifically the oldest known branchiopod fossil, Rehbachiella, 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 an actual 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 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. 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. Studies have found Anostraca capable of rapid colonization and speciation.
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