Zebra Mussels

Zebra mussels, A. Benson, USGS

What Are Zebra Mussels?

Zebra mussels (Dreissena polymorpha), native to fresh waters in Eurasia, are an invasive species in North America. They were first introduced to the Great Lakes in the late 1980s in cargo ship ballast water and have since spread to many other waterways throughout the US.  They are small freshwater mussels, with adults ranging from 1⁄4 to 1 1⁄2 inches long.  They have yellow and brown striped shells.  Unlike native mussels, they can attach themselves to hard surfaces in the water.

What Is Their Life Cycle?

• Eggs: Adult zebra mussels release their eggs into the water column. The eggs are fertilized by sperm from male mussels. A single female can produce up to 1 million eggs per year, giving zebra mussels a very high reproductive rate.
• Larvae: The fertilized eggs hatch into larvae called veligers. Veliger larvae are free-swimming and can be transported long distances by currents.
• Juvenile: After about two weeks, the veligers settle to the bottom of a body of water and attach themselves to a hard surface. They then metamorphose into juvenile mussels.
• Adult: Juvenile mussels grow and develop into adult mussels within a few years. Adult zebra mussels can live for up to 10 years.

Why Are They A Problem?

The high reproductive rate of zebra mussels is one of the reasons why they have been so successful in colonizing new waterways. They can quickly establish large populations and outcompete native species for food and habitat.

Zebra mussels can:

• clog water intakes and other pipes, (management costs in electrical power and other water-consuming facilities amount to several hundred $M per year)
• attach to boat motors and boat hulls, reducing performance and efficiency,
• attach to rocks, swim rafts and ladders where swimmers can cut their feet on the shells,
• attach to and smother native mussels and clams, and
• eat tiny food particles that they filter out of the water, which can reduce available food for larval fish and other animals. (They are responsible for the near extinction of many species in the Great Lake system by outcompeting native species for food and by growing on top of and suffocating the native clams and mussels.)

How Are They Transported From Lake To Lake?

• Under the right conditions, adult zebra mussels can survive for up to a week out of water.
• Veligers can survive for several days in a dry well or ballast tank with water.
• As boats that are not properly cleaned, drained and dried are transported from lake to lake, they can carry zebra mussels as ‘stow-aways’ to infect another lake.
• Zebra mussels have infested waterways in more than 25 states in the US. They have been found in hundreds of lakes in the Midwest. They are found in many NY lakes, but as of 2023, have not been found in lakes in New Hampshire and Maine.
• In Vermont, zebra mussels are infesting Lake Champlain. Veligers and adults have been found in Lake Bomoseen and adults only, no veligers, in Lake Dunmore.  Zebra mussels have been reported in the Canadian waters of Lake Memphramagog.

Are All Lakes Vulnerable To Infestation?

In 2021 the Vermont DEC issued a report entitled Zebra Mussel Inland Survey Field Report.  This report described a survey of 45 inland Vermont Lakes. In each lake, plankton net tow samples were collected along with water samples. Only Lake Bomoseen net tow samples contained veligers. The water samples were analyzed for total calcium.

Other studies have shown that zebra mussels cannot reproduce or grow when calcium levels are too low, below about 15-20 mg/l. Among the lakes surveyed, Vermont inland lakes with calcium levels greater than 20 mg/l include Beebe Pond, Burr Pond, Derby Lake, Harvey’s Lake, Lake Hortonia, and Shelburne Pond.   The calcium levels in Lake Bomoseen and in Lake Dunmore were 19.6 and 8.0 mg/l respectively. This may explain why only adults were found on Lake Dunmore while tow net samples found no veligers.

However, it should be noted that zebra mussel survivability depends on many parameters, and vigilance is needed, even when calcium concentrations may indicate low risk.

Here Are Some Ways To Prevent The Spread Of Zebra Mussels:

• Clean, drain, and dry your boat, trailer, and other equipment before moving them from one body of water to another.
• Check your boat for zebra mussels before launching it.
• Boats leaving a lake with ballasts and inboard engines that cannot be completely emptied of water, should not be relaunched into a new waterbody for at least a week or until these volumes can be decontaminated. This will assure that any trapped veligers will not survive.
• Report any zebra mussels you find to your local authorities. You can report the discovery of an invasive species on the Vermont Invasives website.

By taking these steps, you can help to prevent the spread of zebra mussels and other aquatic invasive species, and protect Vermont lakes and ponds.