Vernal pools are seasonal bodies of water that typically fill with water when snow melts in the spring and dry up later with the warmer weather in late summer. (“Vernal” means having to do with the spring season.) All that is needed for a vernal pool to form is a depression in the landscape that has no outlet where the water drains out. They range in size from small puddles to much larger wetlands. Most vernal pools don’t have formal names since they are not permanent features of the landscape, but they often are given descriptive nicknames by regular local visitors, such as “Deadfall Pond” or “Eggy-Weggy Pond.” The City of New York Department of Parks and Recreation keeps an eye on several vernal pools in the parks of Queens, Staten Island, and the Bronx. Later in the summer, when there is less rain and no more snowmelt, the pools dry up. Woodland pools also lose a lot of water to the surrounding trees and shrubs when the leaves appear and the water moves up through the plants and and out through the leaves by transpiration.
Vernal pools form temporary aquatic environments that cannot support a population of fish because they dry out for part of the year. They are home to many other fascinating creatures that can take advantage of the watery environment without getting eaten by fish. These creatures spend all or part of their life cycle in the water and the rest on dry land. Many amphibians that live on land at least part of the year rely on vernal pools for their breeding cycle. Organisms that depend on vernal pools must complete the aquatic stage of their life cycle before the water disappears. Some common creatures found in vernal pools are fairy shrimp, spotted salamanders, and wood frogs. All of them have interesting adaptations to their changeable environment.
The Fairy Shrimp
There are numerous species of fairy shrimp that inhabit vernal pools throughout the US. The eastern fairy shrimp occurs east of the Appalachians from North Carolina to Maine, as well as in several other states and part of Canada. They can be up to a centimeter long, and may be orange or translucent white, blue, or green, depending on their diet. They breed in vernal pools in late spring. The eggs of fairy shrimp are very sturdy and can be transported by the wind if the pool dries up. Although they can survive the pool drying up, the pool must be deep enough not to freeze all the way through in winter or heat up to more than about 70 degrees Fahrenheit in summer. The eggs hatch when the pool refills with water in the late fall, and once hatched, the immature and adult shrimp must remain in the same pool and will die when the pool dries out. Fairy shrimp eat algae, diatoms, and tiny aquatic creatures such as the eggs and larva of aquatic worms. They are food for many other creatures such as amphibians, waterfowl, and aquatic insect larvae.
The Spotted Salamander
Spotted salamanders live in the woods and reproduce each spring in swamps, ponds, or vernal pools. They are not often seen, because they spend most of their time hiding in dead leaves, under logs, or in tunnels underground. They have a very strong homing instinct and return to their home pools to breed. The egg masses are covered in a thick jelly that helps keep them from drying out and protects them from being eaten. A special species of one-celled green algae grows on the eggs, giving extra oxygen to the eggs and helping to camouflage them. It was recently discovered that the algae are actually symbiotic with the salamander larvae, the only known example of a vertebrate in symbiosis with a photosynthesizer. Spotted salamander larvae are eaten by fish, frogs, and aquatic insects. They themselves are aggressive predators that eat whatever they can catch. When they first hatch, this is mainly small insects, and tiny crustaceans like daphnia and fairy shrimp. As they get larger they eat larger prey, including invertebrates, larger insects, tadpoles, and other salamander larvae. In times of overcrowding, such as may occur when the vernal pools start to dry up, they may even become cannibalistic and attack each other. The adult spotted salamander uses its sticky tongue to catch food, mainly forest floor invertebrates such as earthworms, snails and slugs, millipedes, centipedes, spiders, and insects, and sometimes other types of smaller salamanders. Adult spotted salamanders secrete a milky poison from glands on the back and tail. Their colorful spots warn predators of this toxic defense.
The Wood Frog
Wood frogs are the most widespread North American amphibian, ranging from northern Georgia through the northeast, across Canada and into Alaska. They are found farther north than any other North American reptile or amphibian and are the only frogs found north of the Arctic Circle! They and their eggs can survive being frozen for up to seven months because their blood contains a “natural antifreeze.” They can be up to about 3 inches long and are light tan to reddish brown brown in color, with a distinctive, dark mask-like marking from the snout to behind the eyes. They have two ridges along the back and their underparts are light-colored. The sound of the wood frog’s call is often compared to a quacking duck or squawking chicken. They call only during the mating season, which occurs in most of their range from February until May. Wood frog eggs, in clusters of 1000-3000, are usually attached to fallen branches in the water. The presence of algae can cause the egg mass to turn green. The eggs develop and hatch more quickly when the water is warm, in as little as a week, and more slowly—up to a month—in cold water. After hatching, they live as algae-eating tadpoles for eight to 16 weeks before becoming froglets. The males mature in one or two years, the females in two to three years, and the females also live longer. They continue growing throughout their lives. Adult wood frogs have many predators including larger frogs, snakes, herons, raccoons, skunks, and mink. Predators of tadpoles include diving beetles, water bugs, and tiger salamander larvae. Leeches, eastern newts, and aquatic insects eat wood frog eggs. Wood frogs eat a variety of insects and other small invertebrates, especially spiders, and slugs and snails.
Next time you are out in the woods in the spring, don’t just step around the puddles, take a closer look at them. You may find something surprising!