They swim "upside-down" and feed by filtering organic particles from the water or by scraping algae from surfaces,[4] with the exception of Branchinecta gigas, or "giant fairy shrimp", which is itself a predator of other species of anostracans.

[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 for osmotic regulation.

[10] The relatively large size of fairy shrimp, together with their slow means of locomotion, makes them an easy target for predatory fish and waterfowl.

[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).

[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.

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.

[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.

The oldest known modern-looking ansotracan is Haltinnaias from the late Devonian (Famennian) Strud locality of Belgium, around 365 million years old.

[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.

It comprises around 313 species, grouped into 26 genera in eight families:[29] Fairy Shrimp has been a student favorite as the mascot of UC Merced.

[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.