New Zealand mud snails are only a few millimeters long but can reproduce to densities of hundreds of thousands per square meter.
New Zealand mud snails are only a few millimeters long but can reproduce to densities of hundreds of thousands per square meter. (U.S. Geological Survey/Maryland Department of Natural Resources)

A tiny but nonetheless concerning invader has been discovered in the Gunpowder River: the New Zealand mud snail.

The Maryland Department of Natural Resources confirmed the species' first known appearance in the state this month.


Gunpowder Riverkeeper Theaux Le Gardeur said he discovered them while on a walk with his 2-year-old son in early September.

"He picked up a rock and said, 'Rock!' and I looked at it and it was covered with snails," Le Gardeur said. "I said, 'Oh, my.' "

The cone-shaped mollusk is only 4 to 6 millimeters long but has proliferated to densities of hundreds of thousands of snails per square meter in some lakes and rivers around the world. It first appeared in the United States in Idaho's Snake River in 1987 and has since spread across the western U.S. and the Great Lakes. The closest place to Maryland that the snails have been found is in Spring Creek, a popular fly-fishing spot in central Pennsylvania.

The species' ecological impacts are still not fully known, but biologists fear the mud snails could outcompete and displace native species of snails and insects that live on slimy stream and river bottoms.

The snails have already reproduced in large numbers in Gunpowder Falls, in the first five miles to the south of Prettyboy Dam, Le Gardeur said.

When biologists investigated, they were able to collect several hundred of the snails "in a matter of minutes," state natural resources officials said.

The discovery raises concern for fly fishermen, who often travel around the country or the world to different "hot spots," said Le Gardeur, who also operates Backwater Angler, a fishing shop in Parkton. The snails could easily spread to other popular fishing spots on the Savage and Potomac rivers if anglers aren't careful, he said.

State officials are urging boaters and anglers on the Gunpowder to remove all water, dirt, mud and sand and any visible animals, fish or plants from their vessels, equipment and gear before using them in another body of water. It's important that any water on or inside boats or equipment be drained before leaving the river, to avoid introducing the snails to other waterways, officials said.

There are already 11 wash stations along the Gunpowder where anglers are encouraged to rinse their waders and other gear in a salt-water solution, Le Gardeur said. He said he hopes they're used diligently.

"The Gunpowder is now a place where folks are going to have to be really careful," he said.