Zebra/Quagga Mussels

Informational Brief on Western Quagga Mussels (407 KB PDF)

What are they?

Zebra and Quagga Mussels are freshwater, bivalve mollusks that typically have a dark and white (zebra-like) pattern on their shells. They are alien to North America but have invaded many of our waters. There are two species of Dreissena in North America: Dreissena polymorpha, commonly called "Zebra Mussels" and Dreissena rostiformis bugensis, commonly called "Quagga Mussels" but may also be referred to as "Zebra Mussels," which is sometimes used as a general term for all Dreissenid mussels. Despite some minor morphological and ecological differences, both species are very similar and pose a significant threat to our waters. Both species, Zebra Mussels and Quagga Mussels, in general, are usually about an inch or less long, but may be larger. When healthy, they attach to hard substrates. much like marine (saltwater) mussels but unlike any native freshwater bivalve. They are often found in clusters.

Zebra Mussels (size = about 3/4 inch each)

Enlarged Photo of a Quagga Mussel (size = about 3/4 inch)

What they are not

The clams on the right are not zebra mussels. Instead, these are Asian clams (Corbicula fluminea), commonly found in many areas around the United States. Also an invasive species, Asian clams reached the United States in the 1930's and have subsequently established populations in 38 states. Although they are a nuisance species, particularly for industrial and municipal operations using raw-water systems, their range in North America is already fairly well established (coast to coast). More information on Asian Clams is available from the USGS

How do Zebra/Quagga Mussels and Asian Clams Differ?

Zebra and Quagga mussels attach to hard surfaces via hair-like threads. Asian clams do not, rather, they burrow in soft sediments and/or gravel. Zebra and quagga mussels are the only freshwater mollusk species that attach via threads to hard surfaces. Zebra and Quagga mussels have thin, fragile shells. Asian clam shells are thicker, with distinct ridges. Zebra mussels seldom grow larger than 1 inch and typically have alternating light and dark strips (but some individuals do not have stripes). Quagga mussels may grow slightly larger than zebra mussels, but generally are about the same size. Asian clams can grow to 1.5 inches and are light yellow-brown to dark brown. Asian clams generally do not have stripes. However, damage to the proteinaceous outer layer of Asian clam shells sometimes exposes a lighter calcareous layer beneath that may appear to be stripes. Zebra mussel shells are D-shaped. Quagga mussel shels are more rounded, but both species of Dreissena have generally asymmetrical shells compared to Asian clams. Asian clam shells are more round and symmetrical.

Why are Zebra and Quagga Mussels a Problem?

When they are present in North American waters, they are usually millions of them. Zebra/Quagga Mussels are biofoulers that occlude pipes in municipal and industrial raw-water systems, requiring millions of dollars annually to treat. Zebra Mussel densities have been reported to be over 700,000 individuals per square meter in some facilities in the Great Lakes area. They produce microscopic larvae that float freely in the water column, and thus can pass by screens installed to exclude them. Monitoring and control of Zebra and Quagga Mussels costs millions of dollars annually.

Zebra/Quagga Mussels also negatively impact aquatic ecosystems, harming native organisms (including already imperiled indigenous mussels). In huge numbers, they out-compete other filter feeders, starving them. They adhere to all hard surfaces, including the shells of native mussels, turtles, and crustaceans. Zebra/Quagga mussels actively feed on green-algae and may increase the proportion of foul-smelling blue-green algae in water systems.

Where did they come From?

Zebra and Quagga mussels are native to Eurasia. Until the mid 1980s there were no zebra mussels in North America. Quickly that changed when they were inadvertently introduced into waters near the Great Lakes region. It is suspected that zebra mussels hitched a ride in ballast water tanks of commercial ships. Zebra Mussels (Dreissena polymorpha) were first discovered in the United States in Lake St. Clair near Detroit, Michigan in 1988. Since the '80s, zebra mussels have spread, unchecked by natural predators, throughout much of the eastern United States (see the range map). They currently infest much of the Great Lakes basin, the St. Lawrence Seaway, and much of the Mississippi River drainage system. The have spread up the Arkansas River into eastern Oklahoma. Quagga mussels invaded North America later than zebra mussels and have been confirmed in fewer waters, including the Great Lakes, the St. Louis area and now in the Lower Colorado River.

Where can I learn more?

US Army Corps of Engineers
The US Army Corps of Engineers hosts a website called the Zebra Mussel Information System. This excellent resource covers topics including identification, life history and biology, distribution, slowing the spread, impacts and risk assessment, monitoring, management and control, and bioaccumulation.

US Geological Survey
The Nonindigenous Aquatic Species (NAS) information resource for the United States Geological Survey has information on Zebra Mussels as well as many other problematic species.