The changing natural world at our doorsteps | Illustration and text by Patterson Clark
October 2, 2012
Although the fat, exotic mollusk is abundant in the shallow bays of the Potomac River, what's known about it is slim. Among its many mysteries is how the two-inch-long snail affects native aquatic life.
Michelle Ryan, a George Mason University doctoral student, is a gumshoe in a kayak, tracking mystery snails in the river. Funded in part by the Occoquan Watertrail League, Ryan and the volunteers whom she trains nab snails from Occoquan and Belmont bays, measuring shells, pinpointing locations and cataloguing water and weather conditions. "I am focused on why they are here, what niche they are playing in the environment and whether they are adding to or subtracting from our freshwater system," she says.
Her surveillance of the scientific literature reveals a few intriguing facts:
• The snails are gluttons for algae growing on the river's bottom, yet they excrete very little of the pollutants phosphorus and ammonium.
• In their tissue, the snails concentrate spilled oil and other toxins, a trait that makes them potentially valuable as tipsters on pollution.
• They can close their shell and survive out of the water for as long as a month.
• When water temperatures rise above 59 degrees, a female snail starts birthing quarter-inch-long juvenile snails — a hundred at a time. If pressured by predators, she can reproduce at twice the normal rate. Females live for about five years, males for three. They retreat to deep waters for the winter, where they hibernate in the mud.
• Merchants and sailors brought mystery snails to the West Coast in the 1890s for Asian food markets. By 1911, the snails had escaped to California irrigation ditches. Demands by food, aquarium and water-garden markets helped spirit the snail across the country. By 1960, mystery snails had found a haven in the Potomac at Alexandria.
• The snails are native to East Asian rice paddies, where they are snatched up for the dinner table. They're capable of transmitting parasitic flatworms, but no human infections have been recorded.
• Pet stores sometimes sell brightly colored apple snails under the name "mystery snail," but the real deal is olive to dark brown, sometimes using the alias "trapdoor snail." To scientists, the true identity of mystery snails is elusive. Maybe they are Chinese mystery snails, which are a bit squatter in profile, or perhaps they're Japanese mystery snails, which tend to have more angular spires on their shells — or the two species might really be only one species that varies in shape. It's a mystery for now.
And how did that particular gastropod get the nickname "mystery snail"?
"That I don't know," confesses Ryan.