A vernal pool is a temporary freshwater pool that forms seasonally and provides critical habitat for fairy shrimp.
These wetlands commonly fill during winter rains or spring snowmelt and dry out during summer. They are especially characteristic of Mediterranean climates, including California’s Central Valley, though similar pools occur in forests, grasslands and rocky plains elsewhere.
Many vernal pools have clay, hardpan or volcanic layers that slow drainage. Their temporary nature often prevents fish from becoming established, giving amphibians and invertebrates a refuge from aquatic predators and competition.
Fairy shrimp survive the dry season as drought-resistant eggs. When rain refills a pool, the eggs hatch rapidly, and the shrimp complete their short life cycle before the water disappears. The term “vernal” refers to spring, although the timing and duration of inundation vary by region.