Green tree frogs have shown an ability to thrive in saltier water.

BODIE ISLAND, N.C. 11/8/18

 

Molly Albecker conducted a tree frog survey in the salty marshes near Bodie Island Lighthouse, but she didn’t think she’d find any frogs.

The doctoral candidate and her East Carolina University adviser, Michael McCoy, knew the water salinity was 23 parts per thousand, too salty for frogs.

“Then we heard one call,” Albecker said.

The tree frog was perched on a boardwalk over the marsh. Albecker spotted a small, fishlike creature swimming in the marsh water below. It was a green tree frog tadpole.

In all the textbooks and research papers they knew of, no one had ever found a green tree frog tadpole in water that salty.

“Amphibians are the only class of vertebrate animals in which there are no saltwater species,” said McCoy, an assistant professor at ECU. “So finding frogs breeding in saline water was a huge surprise to me.”

Typically, the tiny, lime-green amphibians like freshwater with a salinity around one part per thousand. The ocean is 35 parts per thousand.

Albecker has devoted her studies since that day on July 3, 2014, to how the green tree frog is adapting to higher salinities.

Green tree frogs might be the great amphibian hope, she said. Scientists assume if the oceans rise and inland waters become more salty, freshwater species will become extinct if they can’t get away. The little tadpole in the salty marsh may change that perspective.

“Maybe they can adapt,” Albecker said.

Her studies have shown that green tree frogs on the coast flourish in more saline water while the same species further inland does not. Frogs around Greenville, about 70 miles from the sea, breed and thrive in freshwater but avoid salty water.

Green tree frogs are vulnerable to sun and wind and may have developed a better capacity to retain water even in salty conditions, McCoy said.

Too much salt for the unaccustomed frog could dry out its insides. But coastal green tree frogs could have more of the protein that expels salt through its urine, she said.

Saltier water may also help frogs carry fewer gut parasites, and prompt young to develop faster, her studies indicated. Tests showed that coastal tadpoles developed into adults in about a month, while freshwater tadpoles took up to three months to mature.

But the coastal frogs were smaller than their inland counterparts, possibly making competition to get a female tougher.

Male green tree frogs bellow a distinctive call to attract females. Bigger frogs have a more attractive sound, she said. They leave the water and climb up a few feet where the repetitive hornlike sound can carry long distances through the night air. People often see the creatures clinging to front doors in their effort to get higher and louder.

Smaller frogs beat the competition by hiding near a larger fellow and stealing the female who comes calling, Albecker said.

The green tree frog is common and ranges along the southeastern United States as far north as Maryland and as far west as Texas. Its diet includes about anything it can snap with its tongue, including flies, ants, crickets, spiders and moths, according to the National Wildlife Federation.

While it’s becoming clear that green tree frogs have adapted to their coastal environment, researchers still don’t know how.

Albecker will continue her studies into the next year on the little green frog that is a proven survivor.

“There are many more questions to answer,” she said.