Zebra mussel with byssal threads
photo credit Phil Moy

What do they look like?

Zebra mussels are small, rarely exceeding about 1 1/2" in length. Their shells have alternating light and dark stripes from which they get their name. They attach to just about any hard surface with byssal threads.

Zebra mussel Frequently Asked Questions

Where did they come from?

Like many of our recent invaders, zebra mussels are native to the Black and Caspian Sea region of Europe.

How did they get here?

They came to the United States in the ballast water of ocean-going ships. Zebra mussels were first found in Lake St. Clair about 1986. In only a few years they had spread throughout the Great Lakes and by 1999 had expanded their range all the way to the Gulf of Mexico.

How do they spread?

Zebra mussels spread easily because they have a microscopic planktonic larva called a veliger. Zebra mussel larvae are invisible to the naked eye and can float in the water for a period of three weeks to three months depending on water temperature. Zebra mussels can reproduce from spring to late fall and their larvae can be present in the water well into winter. Zebra mussels may also spread as juveniles and adults attached to hard substrate such as plants, boat hulls, motors, anchors or any submerged object.

What do they do?

Unlike our native freshwater mussels that burrow in the sediment, zebra mussels attach to hard surfaces like barnacles. This characteristic makes them particularly troublesome for municipal water and power facilities as well as anyone else who uses lake or river water. As the larval zebra mussel grows it gets too heavy to stay in the water column and begins to sink. At this point they settle and stick to just about any submerged hard surface. This may include rocks, sticks, stumps, boat hulls, weeds, other mussels, motors, anchors, buoys, piers and especially the inside of water intake pipes. 

Once inside a water intake pipe the mussels can form layer upon layer of living mussels, enough to clog or significantly impede the flow of water in the pipe. Great Lakes water treatment plants and electricity generating plants spend millions of dollars annually to keep their intake pipes clear of zebra mussels. One of the most effective means is to introduce a low concentration of chlorine at the end of the pipe where the water is sucked in. This kills the zebra mussel larvae preventing them from becoming established and thus keeps the pipe clean.

This same colonizing and clogging ability can also be a problem for boaters if they leave their boat in the water all summer long. Zebra mussels colonize water intake grids and sea lockers on boats too. Fire suppression and cooling systems may become inefficient or inoperable once clogged with the mussels. Fouling of the hull makes the vessel less efficient in the water.

Perhaps of greater interest to anglers is their effects on fish habitat. Zebra mussels are called filter feeders. They pump water over their gills and filter out small bits of animal and plant plankton for food. A single adult zebra mussel can filter a liter (~1.2 qts.) of water per day. As they feed zebra mussels remove plankton from the water. This can make the water clearer. As the water clears light can penetrate deeper into the water. Light sensitive fish such as walleye may move to deeper waters than traditionally found. Greater light penetration can also allow weed beds to increase both in density and depth. At the extreme this can interfere with boating and other recreational activities.

Zebra mussel feeding can also affect the food web of the lake. Zooplankton (animal plankton) forms the first food for many of our favored sport fish. This zooplankton relies on phytoplankton (plant plankton) for food.  As zebra mussels filter plankton from the water they are competing with larval fish or their forage for food. In nutrient rich (eutrophic) lakes this may not have as great an impact as in nutrient poor (oligotrophic) lakes. Zebra mussels are glutinous feeders, filtering more food particles than they can consume. The excess food is egested in a mucous covered packet called pseudofeces. Pseudofeces settles on the bottom and creates a new food supply for bottom dwelling insects, worms and crustaceans. This can increase the food supply for bottom feeding fish or their prey, so the impact of zebra mussels is twofold: they remove food from the water column and deposit it in a different form on the bottom.

Finally, zebra mussels are bioaccumulators; they accumulate contaminants present in the water at three to five or as much as 10 times the concentration in the water. As the zebra mussels themselves become food for fish and birds the concentration of the contaminants can increase in successive steps as they pass up the food chain.

Where are they found?

Zebra mussels are present in lakes of 21 states and in the provinces of Ontario and Quebec. Zebra mussels are found in lakes and rivers of the following states: Minnesota, Wisconsin, Michigan, Illinois, Indiana, Iowa, New York, Pennsylvania, Ohio, Kentucky, West Virginia, Virginia, Maryland, Vermont, Tennessee, Mississippi, Alabama, Louisiana, Arkansas, Missouri and Oklahoma.

Though many states have lists of waters known to be infested, most infestations are not found for one to two years. It is best to assume the water body you are on is infested and to take the necessary precautions to prevent spreading zebra mussels. 

Prevention:

The steps to help prevent the spread of zebra mussels are simple:

• Drain water from bilge, boat, motor, bait wells and live wells
  
• Remove weeds from the trailer, motor, or anywhere they may become tangled
  
• Dispose of live bait in the trash, not in the water
  
• Wash the boat and trailer with hot (104^o F) water or let it dry in the sun for five days before going to a different water body.

Note that the steps to prevent the spread of zebra mussels will prevent the spread of many other unwanted species. Draining the water will help remove larval zebra mussels. Removing weeds will help avoid spreading larval zebra mussels that may be contained in the water associated with the weeds as well as adult mussels that may be attached to the weeds. Removal of the weeds also prevents spreading invasive plant species. Disposing of live bait in the trash prevents the accidental release of non-native fish that might be included with the bait minnows as well as the larval zebra mussels that may be present in the water in the bait bucket. Some night crawlers are now considered to be a threat to some ecosystems; disposing of these in the trash may help prevent their spread as well. Washing the boat with hot water or letting it dry for several days will kill zebra mussel larvae remaining in the interstices of the boat and trailer.

Want to learn more?

Visit Zebra Mussel Watch