Zebra Mussel
(Dreissena polymorpha Pallas)
Quagga Mussel
(Dreissena rostriformis bugensis)
Ecosystem impacts
Physical Impacts
Quagga Mussels
Zebra Mussel (Dreissena polymorpha)
This clam-like bivalve mollusk, commonly called the zebra mussel, is a native of
the Ponto-Caspian region of Eastern Europe and western Asia – the Black, Caspian
and Aral Seas and the Ural River drainage. It was introduced into several
European freshwater ports during the late 1700s; within 150 years Dreissena was
found throughout European inland waterways. Dreissena was first found in the
Great Lakes in Lake St. Clair in 1988, where it is believed to have been
introduced as the result of the discharge of freshwater international shipping
ballast water.
AI map 5d 2008-02-26.pdf
"While there is still some disagreement as to whether the zebra
mussel was introduced in a single location or whether the North American
population is the result of multiple introductions in various locations over a
number of years, what is certain is that once established in the Great Lakes,
the zebra mussel spread rapidly across the eastern half of the continent,
reaching 23 states and two provinces in only 15 years. Once established in the
lakes, the mussel spread via intra- and interbasin ballast water movement;
attachment to commercial ship and recreational boat hulls and boat trailers;
movement of construction equipment; larvae carried by currents, in bait buckets
and in recreational boat live wells; commercial bait and aquaculture shipments;
and, natural movement in lake and river currents and flows. Zebra mussels have
now crossed the 100th Meridian and are now found in 25 states nationwide.
Ecosystem Impacts
Zebra mussels are small (generally under 5 cm), with elongated shells typically
marked by alternating light and dark bands. Eggs (as many as one million per
season per female) are fertilized outside the body in the spring or summer.
Larvae (veligers) are free-swimming for up to 30 days, being dispersed by
currents. Juvenile mussels settle to the bottom and attach to suitable hard
substratum (rock, wood, shells of native mussels, and human-made surfaces such
as concrete, steel, fiberglass, etc.) by secreting durable elastic strands
called byssal threads; if no hard substrate is available, zebra mussels will
also attach to vegetation. The mussels are generally found within 2 to 7 meters
of the water surface but have been found as deep as 50 meters. Zebra mussels
will colonize lakeshores and riverbanks where they attach to rock or gravel
substrates, forming broad mats up to 10 to 15 cm in thickness. Colony densities
may reach 20,000 per square meter. Extensive colonization of shoal areas could
impair reproduction of species of fish (such as walleye and lake trout) that
spawn only on rocky-bottomed areas.
Zebra mussels are filter feeders, capable of filtering up to two liters of water
per day per adult, feeding on phytoplankton, small zooplankton, detritus, and
even bacteria down to approximately the one-micron size range. Such filtration
can dramatically increase water clarity and significantly reduce lake
productivity, changing aquatic plant and animal habitat value. It has been
estimated that the zebra mussel population of the western basin of Lake Erie has
the capability of filtering the entire volume of the basin daily. Since
phytoplankton and detritus are major food sources for pelagic lake and riverine
food webs, fisheries-related impacts can result from zebra mussel filtration
activity. Excessive removal of phytoplankton, detritus and small zooplankton
from the water column can result in a decline in zooplankton species that feed
upon those food particles. Larger zooplankton species and larval fish of all
species preying on smaller zooplankton face reduced survival as mussel
populations expand. Changes in water transparency can favor a shift towards
increased production of benthic algae.
Physical Impacts
Because of an affinity for water currents, zebra mussels extensively colonize
water pipelines and canals, such as those in drinking water treatment plants,
industrial facilities and electric power generation plants. Once inside an
intake, the mussels are protected from predation and the ravages of the weather,
resulting in very large densities of mussels (one Great Lakes power plant canal
had up to 750,000 mussels per square meter). Such mussel growth can severely
reduce water flow, result in a loss of intake head, obstruct valves, clog
condensers and heat enchangers, result in noxious tastes and smells in treated
water, produce nuisance methane gas, and increase electro-corrosion of steel and
cast iron pipelines. Zebra mussels attached to a commercial or recreational
vessel’s hull increase drag and fuel consumption. Recreational use of beaches is
impacted by colonization of cobble in nearshore areas and by littering of
beaches by shells washed up by storm waves. Bathers on Great Lakes beaches have
adopted the use of beach/bathing footgear to prevent cuts from zebra mussel
shells. It has been estimated that since their introduction into North America,
zebra mussels have caused $1 billion to $1.5 billion worth of economic harm.
Quagga Mussel (Dreissena rostriformis bugensis)
Originally thought to be a variant of the zebra mussel (D. polymorpha, see
species profile, above), the quagga mussel (nicknamed after a now-extinct
zebra-like quadruped), is now known to be genetically a separate, distinct
species. For the general description of introduction, range expansion, and
ecological and physical impacts, see the discussion in the profile of D.
polymorpha. Zebra mussels possess a distinctly flattened ventral (hinge) surface
as compared to the same side on quagga mussels, which presents a smoothly
rounded surface. A simple field test to distinguish the two as large adult
specimens is that a zebra mussel will usually sit upright on its ventral side,
whereas a quagga mussel will tip over. On smaller mussels, with shells that have
not finished forming, this is not a definitive test. A second test is that the
valves (shell halves) on zebra mussels are symmetrical with the two valves
meetings along a straight line, whereas on quagga mussels they are asymmetrical,
with the two valves meeting along a curved line.
Shell shape comparison of zebra mussel (
D. polymorpha) on left and quagga
mussel (
D. bugensis) on right
Quagga mussels have been found to be able to survive and colonize in areas of
soft substrate, such as sandy or silty lake bottoms, whereas zebra mussels that
settle onto such bottoms usually do not survive. D. polymorpha is more tolerant
of warm temperatures than is D. bugensis, capable of sustaining water
temperatures of 30°C for extended periods of time and up to 39°C for several
hours, while D. bugensis has an upper thermal tolerance limit of approximately
25°C. D. bugensis appears more efficient at growing under lower water
temperatures than D. polymorpha. Quagga mussels can filter and incorporate food
down to the sub-micron level, thereby including bacteria too small for zebra
mussels or native clams and mussels to consume, giving the quaggas a competitive
advantage over zebra mussels and native species when levels of larger plankton
are too low to sustain large populations. There are no apparent salinity
tolerance differences between the two species, each having chronic salinity
tolerance levels of approximately 2 parts per thousand, allowing some, but not
much, range expansion into tidal estuaries.
A marked distribution replacement of D. polymorpha by D. bugensis is taking
place in the Great Lakes; since the mid-1990s, D. bugensis has almost totally
replaced D. polymorpha in Lakes Erie and Ontario, to the extent that the
original zebra mussels are now very rare in most of eastern Lake Erie and
throughout Lake Ontario. It is theorized that this might be due do the quagga
mussel’s ability to colonize softer substrates than zebra mussels, to the
quagga’s ability to grow more efficiently in colder water temperatures than
zebra mussels, particularly in deeper waters, and to the quagga’s more efficient
feeding capability.
IMAGES:
Mussel grouping: Scott Camazine, New York Sea Grant
Clogged pipe: Don Schloesser, USGS, Biological Resources Division
Shell shape comparison:US Army Corps of Engineers