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 musselzebra 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.



Luckily, these mussels aren’t known to be present in the Northwest — yet.

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.

New Zealand mudsnail

Mussels aren’t the only scary thing hitching a ride with unwitting people.
The New Zealand mudsnail (photo credit: Portland State University) is another, though it most famously rides on wading boots rather than boat trailers. The snails live on the bottom of streams or lakes, and crawl onto boots as people fish or collect scientific data. The impacts of the mudsnail are not well-studied yet, but they can reproduce very quickly – and, in fact, clone themselves so that only one is needed to start a new population. They can build to densities of more than 500,000 per square meter, and can pass through fish undigested. Mudsnails impact food webs by consuming the algae that would normally be eaten by aquatic insects that feed the fish.


DidymoAnother 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 watermilfoilEurasian 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.

more information

On the Lookout for Aquatic Invaders

Identification guide developed for community-based groups to increase their understanding of aquatic invasive species, and to initiate monitoring efforts. 92 pages. $8.95 + s/h 

You Can Stop the Spread of Aquatic Invasive Species

A booklet/brochure (4 pages, 5 1/2 x 8 1/2) to download are receive by mail. 

New Zealand Mudsnails (2nd edition)

A  field detection and gear treatment guide for researchers, monitoring crews, watershed survey groups, and anyone else who travels frequently between aquatic or riparian locations. (10 pages, 5 1/2 x 8 1/2)