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
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
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)
of a Quagga Mussel (size
= about 3/4 inch)
What they are not
Credit: Tom Mosher KDWP
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
Zebra and quagga mussels are the only freshwater mollusk species
that attach via threads to hard surfaces. Zebra and Quagga mussels
shells. Asian clam shells are thicker, with distinct ridges.
Zebra mussels seldom grow larger than 1 inch and typically have
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
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?
they are present in North American waters, they are usually
of them. Zebra/Quagga Mussels are biofoulers that occlude pipes
in municipal and industrial raw-water systems, requiring millions
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
indigenous mussels). In huge numbers, they out-compete other
filter feeders, starving them. They adhere to all hard surfaces,
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
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
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
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?
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.
Aquatic Species (NAS) information resource for the United
States Geological Survey has information on Zebra Mussels as
well as many other problematic species.
Zebra Mussel Druse
Zebra Mussels on Rock
Mussel on Monofilament