The Mud Snail, or Batillaria, is small and spiral-shaped. This two-inch brown and tan creature lives in huge numbers on mud-flats around certain bays in Washington, California, and British Columbia. All are near where Pacific oysters are grown.

Padilla Bay in Skagit County, Washington, is one place you can see millions of Mud Snails. At low tide, they're spread across the mud flats, plowing slowly along scraping up minute plants, called diatoms, off the surface. Watch closely and you may see some Mud Snails moving a bit more quickly. These are actually empty shells hermit crabs have taken as homes.

So why are Mud Snails only found near where Pacific oysters are grown? These are the kind oyster farmers grow and that we buy in the store. They're native to Japan rather than North America. There's a native oyster on the west coast of the U.S., the Olympia oyster, but it's quite small. Although once common, it became rare from over-harvesting. Oyster farmers began importing the much larger Pacific oysters from Japan to grow and sell here. The Mud Snail, which is native to the Asian coast and didn't occur on North American beaches, accidentally came along for the ride. It's what scientists call an "invasive species" and, typical of such creatures, has become very abundant in places. In California, the Mud Snail out-competes a similar, native snail because the Mud Snail is susceptible to fewer parasites and feeds more efficiently too.

Some invasive species cause tremendous problems for native plants and animals. You can help by thoroughly cleaning your boat and gear before moving from one body of water to another. Be sure to follow all regulations related to keeping harmful species out.