Vernal Pool Fairy Shrimp

Status

Vernal pool fairy shrimp (Branchinecta lynchi) is a small, freshwater crustacean that is Federally-listed as Threatened under the Federal Endangered Species Act (September 19, 1994). Currently, this species is known to occur throughout much of the length of the Central Valley of California and in isolated locations on the Santa Rosa plateau and in Riverside County. Vernal pool fairy shrimp has also recently been found within the Medford area in southern Oregon. Current threats to this species include habitat loss and fragmentation due to ongoing development, agricultural conversion, and changes in hydrology.

Description

Adult vernal pool fairy shrimp range in size from 0.5 inch to 1.0 inch in length. This species has a delicate elongate body, large stalked compound eyes, no carapace, and eleven pairs of swimming legs. Vernal pool fairy shrimp glides gracefully upside down, swimming by beating its legs in a complex, wavelike movement that passes from front to back.

Habitat Requirements

Vernal pool fairy shrimp occur in a variety of vernal pool habitats, from small, clear, sandstone rock pools to large, turbid, alkaline grassland valley floor pools. This species has never been found in permanent bodies of water such as riverine or marine habitats. While vernal pool fairy shrimp have been found in large vernal pools, individuals tend to occur in smaller pools often measuring less than 0.05 acres in size. This species has been collected from early December through early May.

Vernal pool fairy shrimp are adapted to survive the seasonal flooding and drying of the pools in which they live. After maturing, this species breeds and produces eggs that are either dropped to the bottom of the pool or retained in the female until she dies and sinks to the bottom. Adult fairy shrimp die at the end of the wet season after the pool dries out. Remaining eggs are then protected throughout the dry season by a thick outer covering. Resting fairy shrimp eggs are known as “cysts”. The cysts are capable of withstanding heat, cold and prolonged dry conditions. When the pools refill during the following rainy season some, but not all, of the cysts hatch. The average time of maturity of vernal pool fairy shrimp is 41 days, but can be as little as 18 days in warmer pools.

Vernal pool fairy shrimp feed on algae, bacteria, protozoa, rotifers, and pieces of detritus. They are sometime preyed upon by vernal pool tadpole shrimp, another small, Federally-listed crustacean.

Local Occurrence

Over 10 WHF preserves were designed to provide for habitat for the vernal pool fairy shrimp in the Sacramento and San Joaquin Valleys. A few include BCAG Verna Pool Preserve, Montelena, Werre Ranch, 3-D South, Deadman Creek, Sand Creek, Great Valley, and Twin Cities.