Vernal Pool Fairy Shrimp

At the time of listing in 1994, the vernal pool fairy shrimp was known from 32 populations stretching from Shasta County, California, south through the Central Valley to Tulare County and along the central coast range from northern Solano County to San Benito County, California, as well as four additional populations in southern California.

Since the vernal pool fairy shrimp’s listing, surveys of vernal pools and other temporary waters throughout the western United States have resulted in an increase in the shrimp’s known range. The vernal pool fairy shrimp is currently found in 32 counties across California’s Central Valley, central coast, as well as southern California and Jackson County, in southern Oregon. The species lives in a variety of vernal pool habitats and occurs in 13 of the 17 vernal pool regions and 45 of the 85 core recovery areas identified in California and Oregon.

Vernal pool 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.

Female vernal pool fairy shrimp carry fertilized eggs in a 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 in the vernal pools.

The lifespan of the vernal pool fairy shrimp is 91 days on average. Vernal pool fairy shrimp can be found in vernal pools starting in November most years, and complete their entire life cycle by early May. On average, vernal pool fairy shrimp take 18 days to mature after hatching and 40 days to reproduce. Multiple cohorts of eggs may hatch in a single vernal pool throughout the wet season, given the right conditions. Vernal pool fairy shrimp disappear before the vernal pools dry.

Vernal pool fairy shrimp are non-migratory and have little ability to disperse on their own. Aquatic birds are the most likely agents of dispersal of vernal pool 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.

The vernal pool fairy shrimp is a small, freshwater crustacean. They have slender bodies; large, stalked compound eyes and 11 pairs of swimming legs that also function as gills. They glide gracefully through the water upside down, swimming by beating their legs in a complex, wavelike movement that passes from front to back. The vernal pool fairy shrimp can be differentiated from other fairy shrimp by the shape of the male’s second antenna and the female’s third thoracic segment, on the middle part of its body. Unlike other types of shrimp, the vernal pool fairy shrimp does not have a hard outer shell.

Measurements
Length: 0.12 to 1.5 in (3 to 38 mm)