High Quality Content by WIKIPEDIA articles! The midvalley fairy shrimp, Branchinecta mesovallensis, is a small (7–20 millimeters or 0.28–0.79 inch) freshwater crustacean in the Branchinectidae family endemic to shallow ephemeral pools (pools that seasonally fill and dry up) near the middle of California`s Central Valley. These vernal pool ecosystems are home to other unique organisms adapted to the ephemeral nature of the water cycle in the pools in California`s mediterranean climate.