Vernal Pools

Spring has sprung!  Now that we’re enjoying warmer and longer days, it’s the perfect time to explore vernal pools.  What are they?  The name says it all; vernal (meaning spring) pools are a unique type of temporary wetland.  They can be found anywhere in the U.S., most commonly near river flood plains.

First formed in fall by overflowing groundwater, they freeze during winter and are then added to by melting snow and rainwater in early spring.  Once thawed, these pools typically last a few months before drying up in summer.  The best conditions for a healthy vernal pool are a cold and snowy winter followed by a rainy spring.

All of the leaves, sticks and other organic material accumulated through the fall and winter create an excellent food source within the pools. These nutrients are essential for many specialized animals, who begin or end their lives here.  Fish do not live in vernal pools, as they would have no way of accessing them and would be in trouble once the pool dries up!  The lack of fish is advantageous to amphibians and invertebrates, which reproduce in the pools and benefit from no fish predation.

Some animals, like the wood frog (Rana sylvatica), spotted salamander (Ambystoma maculatum), and fairy shrimp (Eubranchipus vernalis) are adapted to completely depend on vernal pools to survive.  The temporary quality of these pools dictate they must live or reproduce quickly!  Other animals such as the spotted turtle make use of the pools when they are available, feeding on eggs and larvae.  Many insects will visit a vernal pool, including dragonflies, damselflies, and beetles.  Mosquitoes especially thrive in the wet conditions of this habitat. Birds, including ducks, herons and owls and mammals, like coyotes and rabbits also sometimes visit.

Fairy shrimp Eubranchipus vernalis
These tiny ( ½ to 1.5”) reddish organisms, related to brine shrimp, are amazingly resilient!  Eggs can be thin-shelled for summertime or thick-shelled for winter.  The winter variety eggs survive in dried-up mud through the cold of winter, hatching out when the pool refills in spring.  Larvae quickly develop into adults, which in turn mate and die within the few months before the pool dries up.  Lab experiments show that eggs can survive in dry conditions for up to 15 years, and can endure temperatures ranging from 99 C to – 190 C!

Wood frog Rana sylvatica
Throughout the winter, these small frogs (about 2”) remain alive in an impressive hibernation, which allows them to freeze.  While frozen, wood frogs’ breathing, blood flow and heart beat cease.  As soon as temperatures rise for thawing in spring, the frog revives and seeks out a vernal pool to mate.  Once eggs are laid in the water, adults return to the surrounding woods, leaving them to grow on their own.  Eggs cluster in large numbers within a jelly-like substance, helping them to remain warm.  Tadpoles can be found as early as April, and fully developed frogs leave the pools by late June.

Spotted salamander Ambystoma maculatum
The spotted salamander is in the family of mole salamanders.  These animals spend most of their lives in upland woods, living underground in burrows that were made by small rodents.  These salamanders are actually quite common, but are rarely seen as they remain very well-hidden and feed at night.  In the springtime, they do as the wood frog does; they find their way to the vernal pool where they began life to mate and lay eggs.  As with the wood frog, once eggs are laid, the adults leave and head back to their burrow homes.  They, too spend winters in a frozen hibernation.

You can see an adult wood frog and spotted salamander in the tanks at the Lloyd Center, as well as a sample from our local vernal pool which contains eggs, tadpoles and larvae.