Longhorn fairy shrimp are extremely rare and only found in California’s Central Valley. They live in clear to turbid freshwater vernal pools, as well as water-filled depressions in sandstone, near Tracy, California, grass bottomed pools in Merced County or claypan pools around Soda Lake in San Luis Obispo County.

There are only five known locations today:

• Claypan pools in and adjacent to the Carrizo Plain National Monument, San Luis Obispo County
• Vernal pools and grass bottomed pools in the San Luis National Wildlife Refuge Complex, Merced County
• Vernal pools in the Brushy Peak Preserve, Alameda County
• Vernal pools in the Vasco Caves Preserve, near the town of Byron, Contra Costa County
• Vernal pools in the Alkali Sink Conservation Bank east of Mendota, Fresno County

The extent of the historical range and variation in vernal pool habitats where the species occurred is not known. Extensive surveys have never revealed populations in southern California. It is possible that their range included the San Joaquin and Sierra Foothill Vernal Pool Regions in the past, but they are no longer found there. 

Longhorn fairy shrimp are opportunistic filter feeders. They eat algae, bacteria, protozoa, rotifers, and bits of waste from other plants and animals present in their environments. They face competition from other fairy shrimp species and western spadefoot toad tadpoles.

Female longhorn fairy shrimp carry fertilized eggs in sac on the underside of their body. The eggs are either dropped to the pool bottom or remain in the brood sac until the mother dies and sinks to the bottom of the pool.

When the pool dries out, so do the eggs. Resting fairy shrimp eggs are known as cysts. Cysts may remain viable for multiple years due to their protective coverings that help them withstand extreme environmental conditions and even digestion by predators. The cysts remain in the dry pool bed until hatching begins in response to rains and the return of water to the vernal pools.

The life span of the longhorn fairy shrimp is relatively short. Longhorn fairy shrimp can be found in vernal pools starting in November most years, and complete their entire life cycle by early summer. On average, longhorn fairy shrimp take 22 days to mature after hatching and 43 days to reproduce. Multiple cohorts of eggs may hatch in a single vernal pool throughout the wet season, given the right conditions. Longhorn fairy shrimp disappear before the vernal pools dry. Males die first and appear to be less tolerant of stressful conditions than females.

Longhorn fairy shrimp are nonmigratory and have relatively little ability to disperse on their own. Aquatic birds are the most likely agents of dispersal of longhorn fairy shrimp. Large mammals are also known to act as distributors by wallowing in dirt, getting cysts caught in their fur and transporting the cysts to another wallow. Additionally, cysts can be ingested, passed through the digestive tract and then deposited in new habitats when the animal urinates. Cysts can also be dispersed by reptiles, small mammals and insects. Cysts can spread with strong wind, such as in Altamont Pass in Brushy Peak Preserve.

The longhorn fairy shrimp is a small, freshwater crustacean. They have slender bodies; large, stalked compound eyes and 11 pairs of swimming legs. They glide gracefully upside down, swimming by beating their legs in a complex, wavelike movement that passes from front to back. Their legs also function as gills. This species is easily identified by the male's very long second antennae, which is about twice as long as its body. They do not have a hard outer shell.