Zebra and Quagga Mussels
These small freshwater mussels have a free-swimming young stage (called a veliger) that can be found in open water, as well as the more well-known adults that attach to any more-or-less solid surface. They rapidly reproduce -- (up to a million eggs per year per female) and grow on top of each other. In one Michigan power plant, 700,000 zebra mussels were found per square meter. At this density, they cause many problems as they fill pipes and clog screens at dams and water delivery structures. Each mussel can filter around a quart of water a day, so when their numbers skyrocket, they filter out a huge proportion of the algae that forms the base of much of the aquatic food web. The waterbodies they infest may clear up, but only at great cost to native plants and animals.
If and when they do arrive, they would likely inflict tens to hundreds of millions of dollars worth of damage each year on dams, water delivery systems, and fish and wildlife habitat in the Columbia River system and beyond. Because they attach to everything solid, they are easily carried to new waterbodies by boats.
Mussels aren’t the only scary thing hitching a ride with unwitting people.
Another hitchhiker, one that commonly rides on felt-soled wading boots, is the algae didymo (photo credit: Mark Hoddle). This is a type of algae called a diatom, with a long stalk. It is native to Washington but its populations can occasionally explode in low-nutrient streams, covering the whole streambed. Many states have banned felt-soled wading boots in an attempt to stop its spread.
Eurasian watermilfoil (photo credit: National Park Service) is also commonly spread by boating activities. It has long, flexible stems, and easily grows from fragments that remain damp — and thus viable — while riding between waterbodies. Other invasive aquatic plants also spread easily when seeds or fragments are accidentally carried from waterbody to waterbody. Diseases that harm fish and amphibian species can also be carried in mud or other debris that may cling to boats, trailers, boots and other gear.