Snail Armies Mow Down Drought-Stricken Marshes

Adrianne Appel
for National Geographic News

December 16, 2005

When a good salt marsh goes bad, voracious snails finish off the job,
new research reveals.

Periwinkle snails ordinarily live in ecological harmony with salt marshes, eating only dead cordgrass and a fungus that grows on the plants.

But when salt marshes are stressed by drought, snails graze by the millions in a feeding frenzy that mows down the living plants.

During the past six years, salt marshes in the southeastern United States—in Georgia and Louisiana especially—have experienced unprecedented die-offs.

Wetlands help protect coastal areas from storm surges and flooding brought by tropical storms and hurricanes.

More than 250,000 acres (100,000 hectares) have been lost, mostly due to a severe drought that peaked between 1999 and 2001.

But periwinkle snails also played a role, says Brian Silliman, study author and University of Florida, Gainesville ecologist.

"The snails finish [the salt marsh] off," he said. "They graze it down and completely kill it, change the marsh from a green habitat to a brown habitat," he said.

Silliman and colleagues describe their study in today's issue of the journal Science.

Cheese and Crackers

Periwinkle snails feed on a diet of dead marsh grass and fungus that Silliman likens to "cheese and crackers."

"The plant provides the carbs, and the fungus is the protein and fat," he said. "If [the snails] don't get that 'cheese,' they don't grow."

While snails use dead grass for food, they use living grass to hide from predators and escape high tide.

Each day, large numbers of snails feasting on dead marsh grass in coastal salt marshes flee the incoming tide and their main predator, the blue crab, by climbing up live marsh grass.

Once there, the gastropods use their belt of serrated teeth, called radula, to slice the living cordgrass. Sporting about 40 rows with five teeth each, the snails are well-equipped for the job. "Gruesome under the microscope," Silliman said.

Fungus, ever present and normally harmless, then takes hold in the grasses' wounds. About a week later, with the fungi flourishing and the grass plants weakening, the next wave of snails has itself a ready-made meal.

Snail Onslaught

In their experiment, Silliman and colleagues cleared an area of snails and protected it from further snail invasions by enclosing it with a three-foot-by-three-foot (one-meter-by-one-meter) snail-proof, wire mesh cage.

The researchers were then able to compare the fate of the grass in the protected areas with those open to snails. The scientists observed grass growth for 14 months at 12 sites in Georgia, South Carolina, and Louisiana.

The results "revealed these gastropod consumers significantly damage live [cordgrass] when grazing their fungal food," the authors wrote.

Silliman says millions of snails concentrated in areas he calls "snail fronts," waves of gastropods about 3 to 10 feet (1 to 3 meters) wide and 33 to 660 feet (10 to 200 meters) long.

The researchers counted the snails by hand when they cleared areas for the cages. "There were layers and layers of snails on grass. All over stems, on the base [of plants] and everything," Silliman said.

Run Away Grazing

Speaking of periwinkle snails, the ecologist said: "The climate can turn a seemingly stable system to an unstable one."

"The physical stress [of drought-stricken marshes] is like a light switch—it turns on the run-away grazing of the snail" and more fungus growth, Silliman said.

In a vicious cycle, the bountiful fungus helps fuel a growing snail population, which results in more marsh loss, Silliman said.

Any future droughts from climate change may make cordgrass vulnerable to snails again, the authors say.

Meanwhile, the snail's primary predator, the blue crab, has declined 40 to 85 percent in recent years.

The crab loss may be a factor influencing the snail population, though research has yet to fully explore this possibility, Silliman said.

The study authors said their findings "stress that interactions between food web dynamics and climate must be considered when investigating community collapse in coastal systems."

Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).