March of the Salamanders: Tennessee Aquarium studies spring mass migration

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Each spring, one of nature's most amazing migrations takes place but goes virtually unnoticed. Now scientists from the Tennessee Aquarium Conservation Institute and Southeast Missouri State University are studying the march of the salamanders to help save these amazing amphibians.

Spotted salamander on a moss-covered rock.

Spotted salamander near Chattanooga, TN.

“Pound for pound, there are more salamanders in these ecosystems than any other vertebrate. They’re an indicator species that reflect the health of their ecosystem as a whole.” - Dr, Josh Ennen, a conservation biologist at the Tennessee Aquarium Conservation Institute

The waning days of fall and the arrival of winter in the Southeast bring with them colder, and decidedly wetter, weather. This dour season may dampen the spirits of the region’s human residents, but days of steady winter drizzle help refill temporary forest pools that serve as a vital habitat for many animals.

These impermanent bodies of water — sometimes called “vernal pools” or “ephemeral ponds” — are important breeding grounds for salamanders and frogs. The annual reappearance of these pools triggers mass migrations of animals such as Spotted and Mole Salamanders. Between February and April, they return, en masse, to the same ponds to lay eggs in fish-free waters.

“I have a long appreciation for vernal pools, which are really cool aquatic systems,” said Dr. Josh Ennen, a conservation biologist at the Tennessee Aquarium Conservation Institute. “You have thousands and thousands of animals coming to one location, and you know they’ll be there within a three-week time period.”

“While often overlooked by the public, ephemeral ponds serve as important havens for aquatic biodiversity,” said Dr. Jon Davenport, an assistant professor of biology at Southeast Missouri State University     (SEMO) and an adjunct scientist at the Tennessee Aquarium Conservation Institute. “It is amazing that generation after generation of these salamanders return to those same fish-free ponds to deposit their eggs.”

In February, Ennen and Davenport embarked on a months-long study analyzing how salamanders use vernal pools in a wildlife management area near downtown Chattanooga.

On a breezy gray late winter morning, Ennen and a pair of SEMO researchers, Kenzi Stemp and Kayli Thomas, worked their way methodically along the edges of a chest-deep ponds in Prentice Cooper State Forest. Along the way, they checked minnow traps they placed in the water to collect salamanders.

The success of their collection effort was evident in a series of plastic tubs along the shore that were overflowing with dozens of wriggling salamanders. Some were still massively swollen with unlaid eggs.

“Today, we found several hundred, if not a thousand Spotted Salamanders and Mole Salamanders,” Ennen said. “Through these mass migrations, they bring a lot of nutrition from the terrestrial environment into aquatic systems that other animals can use in the food chain.”

Most salamanders begin life as purely aquatic larva that later undergo metamorphosis into primarily terrestrial adults. Adult salamanders then complete their life cycle by returning to aquatic breeding grounds, such as vernal pools, where they lay their eggs in large, gelatinous masses. This reliance on water and a moist environment makes salamanders especially populous in the Southeast, where geologic stability and a warm, wet climate have contributed to their abundance, Ennen said.

“Some of the density estimates of salamanders from the terrestrial and aquatic ecosystems show that they make up the greatest percentage of vertebrate biomass,” he said. “Pound for pound, there are more salamanders in these ecosystems than any other vertebrate.”

Because of their sensitivity to environmental changes, scientists consider salamanders and other amphibians to be a kind of living bellwether for the health of the surrounding habitat. Being able to more accurately judge salamander populations will help scientists form a more complete picture of the health of aquatic ecosystems such as vernal pools.

“They’re almost like a canary in a coal mine,” Ennen said. “They’re a kind of indicator species that reflect the health of their ecosystem as a whole.”

The vernal pool study was focused on Mole Salamanders, in particular larvae and paedomorphic adults (those that have retained their juvenile gills). To identify the individuals they collected, researchers implanted tags under the salamanders’ skin.

When exposed to a handheld ultraviolet light, these tags glow. Researchers used different tag colors and implanted the tags in different positions of the salamanders’ bodies at each collection site. By checking salamanders for these tags and noting its color and location, researchers can track the animals’ movement through and around the pond.

In the future, this research will help scientists to understand how larval salamanders use vernal pools, improve the effectiveness of collection methods and help researchers to more accurately estimate salamander abundance in a given location.

Even as scientists seek to better understand salamanders’ migrations, seeing thousands of amphibians making their annual journey, often right through backyards and neighborhoods, is a memorable experience more Southerners — especially children — should experience, Ennen said.

“I think people would be impressed by this behavior,” he said. “I always enjoyed knowing things are in my backyard. My kids do, too. I think it’s just a point of pride when you have anything that’s wild sharing the same space as you.”

But people don’t have to venture out into the wilderness to see this seasonal march of the salamanders. Spotted salamanders are one of the main animal “characters” featured in the new IMAX film, “The Wild Around You 3D” now showing at the Tennessee Aquarium IMAX 3D Theater. Audiences witness the spectacular spring migration of these amphibians. The filmmakers were able to document the mass spawning aggregations and used macro, time-lapse cinematography techniques to capture baby salamanders hatching from their eggs.

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Thom Benson
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