Brine shrimp are crustaceans that inhabit salty waters around the world, both inland and on the coast. It is currently accepted that one species, Artemia franciscana, is the only brine shrimp species that inhabits the Great Salt Lake, but there is discussion of genetic and life-history variability that could result in more species being discovered (see Parthenogenesis and shrimp genetics). The average adult male brine shrimp is 0.3–0.4 inches long, and the average female is 0.4–0.5 inches long. They feed by directing food towards their mouth via a series of undulating appendages and digest food through a simple digestive tract. In the process, they ingest a lot of salt water, which must be excreted through gills called "branchia". They can survive in water with salinities ranging from 30–330 g/l (3% to 33% salinity).
Around 10 million birds, representing around 330 species, use the Great Salt Lake annually. A variety of these birds feed on brine shrimp, either exclusively or opportunistically, to fuel long migrations. Eared grebes, Wilson's phalaropes, and Red-necked phalaropes forage almost exclusively on brine shrimp during staging, or preparation for migration. Both of these birds undergo a large weight gain during this period; DWR has found that Wilson's phalaropes gain as much as 2 grams per day, doubling their weight before migration, while Eared grebes increase their body mass from 375 ± 50g to 575 ± 25g during their staging period. Without this food source, their long migrations would not be possible. Avocets, stilts, and waterfowl opportunistically feed on brine shrimp.
Waterboatman, a type of aquatic insect, also feed on brine shrimp. However, these bugs only survive in fresh or slightly brackish water, and are not a significant source of mortality. No natural parasites or pathogens have been identified either, meaning that human-created impacts are the greatest threat to brine shrimp in the Great Salt Lake.
Geologic core samples show that brine shrimp have been present in the Great Salt Lake area for at least 600,000 years. Scientists believe that they arrived as cysts, an embryo covered in a protective shell, on the feet of migrating birds (see the life cycle). Based on physiological traits, scientists believe that brine shrimp were originally a freshwater species that adapted to saline water. Predation by fish restricted the periods of time when brine shrimp were present in Lake Bonneville. The present day Great Salt Lake is too salty for fish and provides an optimal habitat for brine shrimp.