Because people know that I work on freshwater shellfish, they send me shells. I get blurry jpegs attached to emails with subject lines like “What are these?” and little cardboard boxes full of dirty shells and cotton the way that the Antiques Road Show guys get Queen Anne chairs and silver creamers. Lately, most of these of these emails and boxes have contained Chinese mystery snails.
Chinese mystery snails have thin, olive shells as big as golf balls (if you see such a big freshwater snail around here, it’s a mystery snail — you don’t need to send it to me), and have become abundant in our ponds, lakes and marshes. They are native to East Asia, but were brought into the U.S. in the late 19th century as a possible food source (ick), and appeared in New York a few decades later. Their rapid spread through the Hudson Valley and the U.S. suggests that many people are releasing them from their aquariums and water gardens.
When you hear a name like “Mystery Snail”, you have to ask “so what’s the mystery?” Unlike most freshwater snails, these snails are live-bearers — instead of laying eggs, the mother carries the developing eggs inside her body, then gives birth to cute baby snails. The mystery is simply where the new baby snails in your aquarium came from — you didn’t even know that your snail Bertram was expecting, and suddenly your aquarium is full of little snails.
But there are two other interesting mysteries about these snails. The first is: what role do they play in our ecosystems? In addition to being big, Chinese mystery snails often are abundant. We recorded a population density of 20 adults per square foot in one local wetland. This means that you literally couldn’t put your hand on the bottom without touching a mystery snail.
Although ecologists usually are pretty good at sniffing out unanswered questions to study, there has been very little scientific work on mystery snails, and some feel that this is an invading species without much of an impact. But when you see 20 of these big snails per square foot, it’s hard to believe that they don’t have an impact.
For example, they've got to be eating something, and a lot of it. Mystery snails can either browse algae off the bottom or filter edible particles from the water. A large population of mystery snails must greatly change the amount and type of such food that is available to native animals.Other impacts seem likely as well. Freshwater snails carry a lot of parasites, so invading snails can increase diseases in an ecosystem.
For instance, when the European faucet snail appeared in the upper Mississippi River in 2002, it caused an explosion of a parasitic disease that kills thousands of ducks each year. No one knows what parasites the Chinese mystery snail might carry, but I know that I come down with a bad case of swimmer's itch (a snail-borne disease) every time I work in the wetland that is so full of mystery snails.
The second interesting mystery is why people like to buy mystery snails, keep them in their aquaria and move them around from lake to lake. Do they think that mystery snails are pretty, or beneficial, or just cool? I suspect that few of the people who like mystery snails harbor any similar fondness for their close cousins, the orange slugs that slither through our gardens. Clearly not all mollusks are created equal.
An even broader mystery is why we so often move species around from place to place. The last few centuries should have taught us that moving species around is a risky business that can lead to ecological and economic disaster. We should have learned from zebra mussels, carp, kudzu, rabbits in Australia, lampreys in the Great Lakes, West Nile virus, hemlock woolly adelgids and hundreds more.
However, as the mystery snail shows, these bitter experiences have failed to kill our optimism that moving a species like the mystery snail will make the world a better place. This is the central mystery of the mystery snail.