The Secret World of Vernal Pools
March 1 is when I anticipate the earliest arrival of amphibians at shallow, ephemeral forest pools in north-central Pennsylvania. In some years, when I make the first daytime visit to these vernal pools, I find the ground frozen and snow-covered. Then, on an early March night, Jefferson salamanders begin their migration across that snow-covered landscape toward frozen pools. When a warm day melts the ice at the pool margins, the sallies slither beneath the ice to begin an annual ritual that brings new life and ensures the continued existence of these large, beautiful salamanders, their battleship-gray skin adorned with blue speckles. The males always appear first to lay packets of sperm atop soggy leaves on the bottom of the pools. Then the females arrive, picking up the sperm packets with their cloacal (a multi-functional posterior orifice) “lips,” fertilizing the eggs inside their swollen abdomens, then releasing and wrapping the eggs in a mass around a twig or stick protruding from the bottom of the pool. One year, I could see all of this happening through clear ice. Usually, though, what goes on beneath the ice is largely unnoticed, except, perhaps, by the salamanders themselves.
Forest—or vernal—pools are actually small, seasonal wetlands that serve an important role in the forest ecosystem. It is in the vernal or spring season that they serve as the sole breeding sites for some amphibians and invertebrates, and as an optional breeding site for others. Energy and nutrients are exchanged between the pools and the forest, each contributing to the success of the other. The forest shades the pools, keeping the water cool and slowing evaporation. Leaves serve as sites for attaching sperm packets and providing hiding places for the emerging tadpoles and larvae. Fallen twigs hold the eggs of most amphibian species until they hatch. Emerging tadpoles and salamander larvae enter the forest, contributing energy and nutrients to the forest community.
Back to the spring activities at the pools, another migration begins as the ice vanishes—a hoard of hopping masked frogs that seem to appear out of nowhere. The male wood frogs arrive first and initiate a chorus of quacks that from a distance sounds like ducks. These noisy males dive for cover during the day, but sit quietly by the pool and they reappear, entertaining with their quacking cacophony. When the females arrive, the males beckon them to mate and yield their large clusters of floating egg masses. The eggs, being near the warm surface of the pool, will, out of necessity, develop and hatch quickly. As spring advances into summer, the water evaporates, leaving the tadpoles high and dry if they do not develop hopping legs with great haste.
Another momentous but easily missed happening takes place on the first warm spring night with heavy rainfall. Spotted salamanders—dark blue skin and bright yellow spots—charge to the pools. Grab a flashlight, don a raincoat, and be patient. About two hours after dark the male sallies arrive, depositing their whitish sperm packets atop leaves. They’re followed by the females, who pick up the packets and wrap the fertile eggs in a mass around a convenient stick or twig, much like the Jefferson salamanders. The salamander larvae remain in the pool until they, too, develop legs and venture into the forest.
One unique ephemeral pool species is the fairy shrimp, a freshwater crustacean found nowhere else. Swimming upside down, fairy shrimps deposit their eggs among the leaves, where they will survive the summer dry season. When the rains return in autumn, the eggs hatch.
Vernal pools are miniature ecosystems within the forested landscape, and each should be treasured. They are great places to take children for a wildlife adventure, but take care not to disturb the animals, their eggs, or that delicate aquatic system that seemingly disappears with the warmth of summer, only to magically reappear again the following spring.