As vernal pool season approaches here in Vermont, the mystical fairy shrimp will also be making their yearly appearance. Yes, you read that correctly, Vermont shrimp exist, but we don't suggest you eat them! This might be hard to believe as we have no ocean exposure. What we do have, are lots and lots of vernal pools!
As Vermont Center for Ecostudies notes, fairy shrimp are “permanent residents of temporary pools”. These fascinating creatures rely strictly on vernal pools and other wetlands with ephemeral characteristics. In the early spring, when rain and snow melt collect in depressions and pools, fairy shrimp cysts that have been resting in the sediment and leaf litter, are awakened from their many month-long rest. These cysts are not eggs exactly but fully developed embryos. Cysts wait for conditions that are just right. If it's a particularly dry season, they hold off for another year. This behavior evolved to prevent mass die offs and false alarms that may occur during unusual years and explains why one year, a vernal pool may be teeming with fairy shrimp and then next they are nowhere to be found. Fairy shrimp tend to be consistent in some locations and never appear in others. Mary Holland shares in her book Naturally Curious, the name fairy shrimp is thought to have come from their magical and unexplained appearance and then disappearance in vernal pools. Fossil records date fairy shrimp back to the Cambrian Period approximately 400 million years ago. Fairy shrimp are invertebrates, meaning they do not have a backbone and are further classified as crustaceans-think crabs, lobsters, prawns, and barnacles.
The fairy shrimp species found in Vermont are often a transparent orangey red but occasionally, they will have a blue tint. They range from ¾ to an inch long and have large dark eyes. Adults have 11 pairs of legs, with each leg containing a set of gills. Fairy shrimp swim upside down with their legs facing the sky. Females tend to have visible eggs sacs and males have large clasping antennae that are used during mating. Food sources include bacteria, plankton, and other small bits of organic matter.
Vermont Center for Ecostudies (VCE) has done extensive work on vernal pool monitoring and mapping. Scientists from VCE explain that there are at least two known species of fairy shrimp in Vermont, with the recognition that there could be many more. They are the Vernal Fairy Shrimp (Eubranchipus vernalis) and the Knob-lipped Fairy Shrimp (Eubranchipus bundyi). Evidence suggests that E. bundyi prefer large vernal pools in relatively undisturbed forests.
In the spring when vernal pools fill with water and the conditions are just right, the previously mentioned cysts that have been in a state of suspended development, also known as diapause, begin to stir. There are still some unknowns about how fairy shrimp determine when the conditions are just right and how those ideal conditions vary between species, and even populations. Once the cysts know when the time is right and there is plenty of water in the vernal pool, they go through a series of molts and transformations until they reach adulthood. Mating occurs as adults and cysts, the fully developed embryos, settle to the bottom of vernal pools. They rest in the sediment and leaf litter for many months or even years until the conditions are perfect again. In mid to late May, water temperatures in vernal pools rise, resulting in less oxygen in the water for the fairy shrimp to breathe. The number of predators swimming around vernal pools increases as spring progresses and as late spring arrives, vernal pools begin to dry up for the season. Because fairy shrimp are enclosed in a safe capsule that is resistant to desiccation, they are not affected when the pool begins to dry up!
Most of a fairy shrimp’s life is spent in the diapause cyst phase, as they are only active for several weeks at a time during the spring. Cysts are extremely small and light and are thought to be transported on the feet, fur, and feathers of animals frequenting the vernal pool. They also are thought to travel by wind.
The presence of fairy shrimp indicates unpolluted and undisturbed forests and wetlands. Along with amphibians such as the yellow spotted salamander and wood frog, fairy shrimp depend on vernal pools and other ephemeral wetlands for reproduction and continued survival, illustrating the importance of protecting these vulnerable habitats.