A walk in any deciduous forest in early March is certain to reveal a variety of vernal pools.
Every depression, large and small, fills with snow melt and rain. At night these pools become hubs of activity for a variety of amphibians and aquatic invertebrates. Vernal pools can be as small as a kiddy swimming pool to acres in size. But they must be shallow, and they must, at some point in late spring or summer, dry up completely. Because water ultimately disappears from vernal pools, they cannot support fish. And that makes them safe for aquatic organisms that fish would otherwise eat.
Vernal pools are about to explode with life. As days get longer and daytime temperatures flirt with the 50s, aquatic invertebrates that have been dormant for months and amphibians waking from hibernation return to their birth ponds to reproduce.
Eggs of aquatic insects and crustaceans hatch to form the base of the food chain.
In February, wood frogs, whose bodies had been frozen solid all winter long, found their way to snow-ringed pools. By mid-March, spring peepers will arrive and in April, American toads appear.
Perhaps the most impressive visitor to vernal pools are never heard and seldom seen.
Spotted salamanders spend most of their lives in underground burrows, but on rainy late winter nights they surface and return to their natal vernal pools to breed. At some locations, these mass migrations can number in the hundreds or even thousands of individuals.
Sometimes these salamanders can be observed en masse crossing highways that separate their terrestrial habitat from the vernal pools. Watch for them crossing country roads on rainy March evenings.
Spotted salamanders are easy to recognize. They are dark with two rows of prominent yellow spots running the length of the body. Adults can grow to 6-8 inches long.
To find spotted salamanders, curious naturalists must get outside on rainy, late winter nights. It means wearing raingear and getting wet feet. The reward can be finding masses of salamanders writhing in vernal pools.
Visit a breeding pool after dark, during the rain, and when the temperature is above 40 degrees. Use a flashlight to scan the pool. Rain allows salamanders to stay moist during their brief stay above ground.
Males arrive at pools hours or a few days before females. When the females arrive, the salamanders “dance.” Males and females swim in circles and rub against each other. Eventually, males deposit a spermatophore (a packet of sperm) on a piece of vegetation. A female then picks up the spermatophore with her cloacal lips, and eggs are fertilized as they pass through the cloaca.
As many as 200 eggs form a gelatinous mass that can be as large as a baseball. The eggs hatch in 30-60 days, depending on temperature. The carnivorous larval salamanders live for two to four months before transforming into adults and then migrating back to their subterranean habitat. As larva, these salamanders eat a variety of tiny zooplankton and aquatic invertebrates.
Because vernal pools dry up completely at some point later in the year, they cannot support populations of fish. And since fish are major predators of aquatic invertebrates, amphibian eggs, tadpoles, and larval salamanders, vernal pools are a relatively predator-free habitat. This means breeding success is usually high.
On the other hand, using this predator-free habitat means betting that eggs will hatch and larva will transform into terrestrial adult forms before the pools dry up. Dry springs mean that salamanders must wait until the following year to breed. Fortunately spotted salamanders can live 20-30 years, so it’s OK if they fail to breed occasionally.
Vernal pools are much more than mud holes in the woods. They are thriving communities of aquatic organisms perfectly adapted to these specialized ephemeral habitats.
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