Walk out onto what used to be the middle of the Charles River at Millennium Park these days and you'll see not just sand and mud and land plants growing everywhere, but the shells of what our local wildlife photographer Mary Ellen says are Chinese mystery snails.
The snails, native to eastern Asia, first showed up in Massachusetts in 1914, either because they hitched a ride with goldfish imported to control mosquitoes or because they were released by Chinese fish markets dumping water from their fish into local waterways (they were, in fact, first introduced in North America as a food in Chinese markets in San Francisco a couple decades earlier)..
They got their name because it was initially a mystery where the adults came from - rather than releasing eggs to hatch in the water, females "give birth to young, fully developed snails that suddenly and 'mysteriously' appear." They're also sometimes called trapdoor snails, because they have the ability to withdraw completely into their shells and then close up the opening with a "trap door."
They love ponds and slow-moving shallow river water covering silt and mud, where they can hoover up algae and diatoms - so the slow-moving Charles River where Boston, Dedham and Needham are ideal for them.
In Asia, they can carry parasites that can infect people, but no such transmission has ever been seen here. They do, however, carry other illnesses that can affect fresh-water mussels, the shells of which are now also easily seen in the mud and silt along the Charles at Millennium Park.
As noted above, they are edible. In fact, in 2012, in Eating Aliens, Jackson Landers described a taste test comparing them to French snails. He reported they were inferior to the French snails in French cuisine, but said that they compared favorably to clam strips, and that "after being tenderized, fried, and served with tartar sauce, they're quite good."