Fairy shrimp (Order Anostraca) are fascinating freshwater crustaceans found in vernal pools, those temporary ponds that fill with water in the fall or spring and dry up by summertime. Since fairy shrimp require these pools for their existence, they are known as an “obligate species” of vernal pools. They are found worldwide, with seven known families occurring in North America.
Fairy shrimp in our area can be observed in late winter and early spring in the cold waters of their woodland pools. They grow to be about an inch long and are often orange or reddish when young, with adult females turning bluish and males green. At first glance you may think you are observing small fish as the fairy shrimp zip through the water. They appear comical as they swim upside down, using their ten pairs of swimming legs to propel them. When they stop moving they sink to the bottom of the pond. Their diet includes all sorts of microscopic life, from algae and bacteria, to tiny crustaceans and dead plant and animal matter. A fairy shrimp feeds on its back, filtering water towards its mouth with the rhythmic movement of its legs.
The adult fairy shrimp mate and die before the vernal pool begins to warm – sometimes months, sometimes only weeks after they first appear. The female either releases eggs that fall to the bottom of the pool, or the eggs remain in her brood sac. When she dies, the eggs fall with her to the bottom. These eggs can survive in the leaf litter of the dried pool, tolerating extreme temperatures of heat and cold, because they are encased in a cyst. They can colonize another pool by traveling with windblown particulates from the bottom of their dried pool. Or they can be carried along on the fur or skin of a passing animal, and can even survive passing through the animal’s gut before being deposited in a new location. Cyst-covered eggs have been known to persist for twenty years, hatching when conditions are right and a vernal pool has refilled with water.
One of the secrets of the fairy shrimp’s life cycle still to be uncovered is why a vernal pool will hatch these tiny creatures for several consecutive years only to be absent of shrimp in another year. So much is yet to be learned of the denizens of our temporary woodland pools.