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Spring 2009

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Spring 2009 Edition

Pools of the season

by Greg Podniesinski, Betsy Leppo, Sally Ray and Ephram Zimmerman - Pennsylvania Natural Heritage Program

It was a dark and stormy night, and hundreds of slimy creatures began crawling and hopping toward the icy-cold pool ...

Sounds like the beginning of a scary tale, but it’s actually the beginning of the annual breeding migration of salamanders and frogs to seasonal pools all across Pennsylvania.

vernal pond
Photos by Bob Steiner/
Spotted salamander artwork by Linda Steiner

Seasonal, a.k.a. “vernal,” pools  typically are shallow pools or ponds that hold water for only a portion of the year and often are completely dry by mid to late summer. Some deeper pools may hold water all year round. Seasonal pools come in all sorts of shapes and sizes and may be full of grasses, shrubs and wildflowers. They may also have no vegetation at all, just a layer of blackened leaves (referred to as a black-leaf pool).

An important characteristic of most seasonal pools is the absence of fish. Since most seasonal pools go dry every year, fish cannot survive. In the absence of fish, the pools are used by a variety of amphibians and invertebrates that would otherwise be snack food for fish.

Who’s swimming in my seasonal pool?

Several types of salamanders and frogs are dependent on seasonal pools as breeding grounds and nurseries for their young.  Characteristic vernal pool amphibians include marbled salamanders, spotted salamanders, Jefferson salamanders, wood frogs and spadefoot toads. Other amphibians that breed in both permanent and temporary aquatic habitats include green frogs, spring peepers, chorus frogs and American toads.

Breeding for most Pennsylvania amphibians begins in late winter to early spring, during the first few mild, rainy nights, when the air temperature is above 40 degrees F.

wood frog

Photo by Bob Steiner
A wood frog is swimming in the vernal pool on the above photo. Wood frogs are among the most vocal visitors to seasonal pools each spring, going there to mate and lay eggs.

One of the earliest migrators is the Jefferson salamander, which may move across snow to pools still covered with ice. You may ask why any animal would purposely go out on such a cold, miserable night, but seasonal pool amphibians have several good reasons. Amphibians must keep their skin moist, and the rain wets the ground and animals as they move to the pools, preventing stress and injury that might result from drying out on a rainless night. 

By moving so early in the season, some of the amphibians’ predators, especially snakes, are still hibernating, allowing the adults to migrate in safety. Other predators, such as small mammals and birds, are unlikely to be on the hunt on a cold, rainy night. As a result, the salamanders, frogs and toads can move to the ponds unmolested, except for the occasional hardy and curious human with a flashlight and raincoat. The greatest danger they may face during their migration is a close encounter with the wheels of a car.

Once they  have laid and fertilized their eggs, the adults leave the ponds and return to the nearby forests, fields and meadows. The eggs and resulting larvae then begin a race against time. Since most seasonal pools dry up by mid to late summer, amphibian larvae may have only weeks to months to grow and develop into juveniles that are capable of leaving the water.

Right, the gray treefrog sings loudly at mating time. Below, a spotted salamander. Below right, a seasonal pond on a wooded hilltop. Photo by Bob Steiner
gray treefrog
spotted salamander
Photo courtesy Charlie Eichelberger,PNHP

One species, the spadefoot toad, has evolved to take advantage of even the most temporary of pools. The eggs may hatch in as little as two days and the larvae may metamorphose into juvenile toads in several weeks, albeit with tails still attached. If they don’t leave the pool at this point, the weight of the spadefoot toad’s tail may actually drown them. 

The marbled salamander has quite the opposite strategy from the spadefoot toad. Rather than waiting for springtime, the marbled salamander will move into pools in early fall, often when the pools are still quite dry. The marbled salamanders will mate and nest on dry ground, with the females staying to guard the eggs until autumn rains come and fill the pool, after which the eggs will hatch. The larvae slowly grow over the fall and winter and come springtime are ready to prey upon the newly hatched larvae of other amphibians

Shrimp, fleas, flies and tigers

Seasonal pools are also home to a host of interesting insects and other invertebrates, some of which are food for amphibian larvae and some of which prey upon amphibian larvae themselves.

One of the most interesting invertebrates is the fairy shrimp, a small crustacean that is 1/2 to 1-1/2 inches long, depending on the species. These graceful shrimp live only in seasonal pools, swimming “upside-down,” with their many legs beating in a rhythmic pattern.

Fairy shrimp and other crustaceans typical of seasonal pools, such as seed shrimp and water fleas, produce eggs protected in tough cysts. The cysts can withstand drought and severe cold. Seasonal pool crustaceans are carried to new pool habitats on the wind or by hitching a ride with an animal. They may cling to the animal’s exterior or pass unharmed through the digestive track. When water returns to the pools in autumn or spring, the cysts hatch and the cycle begins again.

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Pennsylvania Wild Resource Program