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Page 4.5: Zebra Mussel Facts

It is appropriate to establish some facts about the zebra mussel, so as to enhance ones understanding of this mollusk.

1.0 History and Distribution

Zebra mussels (Dreissena polymorpha) are a temperate, freshwater species. They are native to an area in Russia near the Caspian Sea. Canals built during the 1700s and 1800s across Europe made bulk shipping much easier but also helped to spread the zebra mussel. By 1830, zebra mussels were present in most of Europe.

The probable introduction of zebra mussels into the Great Lakes of North America occurred in 1985 or 1986 via the ballast water of ocean-going ships from Europe. Zebra mussels were discovered in Lake St. Clair in 1988 and since then have spread throughout the Great Lakes and are still spreading as shown in Figure 1.0.

Figure 1.0 - Zebra Mussel (Dreissena polymorpha) Distribution in the United States and the Great Lakes area of Canada.

Source: National Biological Service, Southeastern Biological Science Center, Gainesville, Florida, 1996

2.0 - The Problem With Zebra Mussels

The following will highlight the main areas of concern that will support the fact that zebra mussels are having a significant impact on the Canadian and US economies.

2.1 - Industrial, Commercial, and Recreational Concerns

The main problem with zebra mussels since the introduction of this non-indigenous species to North America is that they have infested the water systems of many industrial, utility, and municipal plants (such as manufacturing, power, and water treatment plants). These infestations have resulted in reduced pipe bores and blockage, electro-corrosion, screen blockages, heat exchanger blockages, interference with valve action, etc. This has resulted in significant reductions in pumping capabilities and occasional shutdowns. Some examples of the
zebra mussels impact are:

the Detroit Edison power plant in Monroe, Michigan, had to shutdown in 1989 when its intake pipe become clogged with millions of mussels,
the Monroe drinking water plant was shutdown for 2-1/2 days in 1989 because its intake pipe , a mile out into Lake Erie, got blocked by mussels, and
the Niagara Mohawk Power Corporation - Nine Mile nuclear power plant on Lake Ontario experienced valve problems in 1995.

Even recreation based industries have been impacted by zebra mussels. Unprotected docks, breakwalls, boat bottoms, and boat engine cooling systems have been infested. Even beaches have been impacted by the zebra mussel. The sharp edged mussel shells along beaches are sharp and are therefore a hazard to unprotected feet.

2.2 - Biological and Ecological Concerns

Zebra mussels have also interfered with the aquatic food chain. The filtering capacity of each mussel (depending on its size) is on average approximately 1 Liter per day. They remove nearly all particulate matter in the 15 to 40 micrometer range, including phytoplankton, zooplankton, and even their own veligers. Instead of passing any undesired particulate matter back into the water column, mussels bind it with mucous into loose pellets called pseudofeces that are ejected and settle.

By removing large amounts of phytoplankton from the water column, zebra mussels remove the food source for microscopic zooplankton, which in turn are food for other organisms and therefore affects all other aquatic organisms further up the food chain as represented in Figure 2.0. This competition for phytoplankton at the base of the food chain could have a long-term negative impact on the Great Lakes fisheries industry.

In heavy mussel infestation areas, the accumulation of pseudofeces is also a concern. The pseudofeces slowly decompose and in so doing large amounts of oxygen is removed in the decomposition process (ie. very high Biochemical Oxygen Demand). During the decomposition process the pH in the local area becomes very acidic. However, flushing currents have been found to prevent any serious impacts on bottom and/or reef spawning fish (ie. walleye and bass).

Zebra mussels have also significantly impacted on the indigenous (native) mussels because they compete for food and space. Mussels have even been found to attach to the larger native mussels which sometimes results in extreme stresses on the native mussels and usually results
in death. There is concern that some rarer native mussel species may be eliminated.

Zebra mussels have improved water clarity in the Great Lakes. In Lake Erie, the water clarity has increased from 10 to 17 feet (a 70% improvement) since the zebra mussels introduction in the early 1990s. It can be argued that a portion of this improvement is due to phosphorus abatement programs that have been ongoing since the 1970s. The result of this improvement in clarity is that light can now penetrate to deeper depths and there has been a significant increase in aquatic plants and fauna.

Another concern is that since zebra mussels are such efficient high volume biofilters there is a concern about bioaccumulation of organic and inorganic pollutants in their tissues. In addition, they deposit a large portion of these pollutants in their pseudofeces and can thus contaminate sediments. These contaminants can be passed up the food chain so that any fish or birds that feed on mussels will bioaccumulate these contaminants. This can also affect man who is at the top of the food chain.

Figure 2.0 A schematic presentation of the biomanipulation cascade and the role of the zebra mussel in lake restoration.

Source: H. H. Reeders & A. Bij de Vaate, Zebra mussels (Driesenna polymorpha): a new perspective for water quality management, Hydrobiologia 200/201: 437-450, 1990

2.3 - The Anatomy and Life Cycle of the Zebra Mussel

The common zebra mussel (Dreissena polymorpha) is easily identified by its characteristic striped shell, which may be white, brown, and yellow, or shades of these colours. These animals are members of the mollusca family. Since they have two shells or valves they are known as bivalves. They exist as either males or females with a relative population ratio of from 1:1 to 7:1, respectively (depending on your literature source). A close cousin of the D. polymorpha is the recently discovered Quagga mussel. The differences between these two mussels are that the D. polymorpha has a concave ventral edge and the Quagga has a convex ventral edge as shown in Figure 3.0. Figure 4.0 shows a flow chart to help identify zebra mussels and other native biofoulers.

The major internal organs of the zebra mussel are shown in Figure 5.0. The mantle that surrounds the internal tissues secrete the shell. Water is drawn into the incurrent siphon (which also has a ring of tentacles that assist in food particulate selection and conveyance) by the action of cilia on the gills that generate a current and pull water over the gills. The gill cilia removes solid particles and food from the passing water and digestible food particles are directed by the cilia towards the mouth for ingestion. All other undigestible particles are wrapped in mucous that is secreted by the cells in the gills and released as pseudofeces, via the excurrent siphon. The digestible food passes into the mouth, through the esophagus, and into the stomach and digestive diverticula (or liver). Digestive enzymes are stimulated to release into the stomach by the action of the rotating crystalline style (a secreted rod of gelatinous material) rubbing against the walls of the crystalline sac, located against the stomach wall. Any undigested food is passed down the intestine and out the anal papilla, located just upstream of the excurrent siphon. The end byproducts of mussel ingestion is a mixture of pseudofeces and feces from the excurrent siphon. This material is denser than water and will settle in the water column.

The mussel also has a muscular foot which permits crawling. The foot also contains the byssal gland and byssal canal that runs to the base of the foot. This gland secretes one byssal thread at a time and each thread terminates with an adhesive pad or plaque as shown in Figure 6.0. A mussel of 2.5 cm in length can have as many as 600 threads holding it to a hard substrate surface. The byssal growth rate is approximately 1 to 9 per day depending on mussel age and environmental conditions (ie. Temperature, pH, contaminant stressors, etc.). The primary component of the byssal threads is 3,4-dihydroxylphenylalanine (DOPA), which along with other protein compounds, such as cystine (that helps the byssal threads bind readily with sulphides in the hard substrates), helps the byssal threads have strength, durability, and good adhesive strength to composite surfaces.

Zebra mussels are epifaunal in that they attach to the surface of solid substrates (ie. rock, concrete, most metals, most plastics, as well as other mussels, just to name a few). Reports indicate that mussels (of varying sizes) can reach densities as high as 3 x 106/m2. In general,
zebra mussels will attach to most hard, non-toxic surfaces and they even attach to each other and other types of mussels, forming thick layers and/or clumps. These clumps (or druses) sometimes break off and form the substrate for the settlement of new mussels.

The range of habitat tolerances for zebra mussels are presented in Table 1.0.

Table 1.0 - Zebra Mussel Environmental Tolerances

Source: Adapted from Chuck O'Neill, Colonization, Dreissena Polymorpha Journal, Zebra Mussel Information Exchange, 6:5, Winter, 1995/96, p. 9.

Zebra mussels cannot settle in water velocities greater than 1.5 m/s and will die if exposed to water temperatures greater than 30 to 35 oC for prolonged periods. They also tend to occupy the solid surfaces down to a depth a approximately 15 m (because below that the temperature
gets to cold for reproduction and growth.

The life cycle of the zebra mussel is divided up into five development stages. These stages are the egg, the veliger, the post-veliger, the settling, and the adult stages, as shown in Figure 7.0. When climatic conditions are ideal, a single female can produce up to 40,000 eggs at one time and as many as one million eggs over a two year period. The eggs are externally fertilized by male sperm. A typical egg is 40 to 50 m in diameter. The fertilized eggs (or zygotes) go through embryonic development and the embryo develops and the eggs hatch after about 6 to 20 hours to release free swimming larvae called veligers. Veligers propel themselves by the controlled movement of cilia (hair-like appendages on the body surface). The veligers are extremely small and are capable of passing through 80 to 100 m mesh screens. After
approximately two to three weeks the veliger enters the post-veliger stage of development. After an additional two to three weeks the post-veligers enter the settling stage where they start to settle as a result of growth causing an increase in mussel density. In approximately
three weeks, all the settling stage veligers have settled and begin to develop into adults. In the adult stage the zebra mussel produces byssa (fine protein threads) which are used for surface attachment (as previously described). The lifespan of the zebra mussel is approximately 2 to 3 years in the Great Lakes but can live as long as 5 years. Table 2.0 summarizes the development duration and size of each of the development stages.

Table 2.0 - Zebra Mussel Development Duration & Size for each Development Stage

Development Stage	Length (mm)	Development Duration
Egg	40 to 50	6 to 20 hrs
Veliger	<100	2 to 3 weeks
Post-veliger	<200	2 to 3 weeks
Setting Veliger	<300	3 weeks
Adult	<500	2 to 3 years

Source: Adapted from Renata Claudi and Gerald Mackie, Practical Manual for Zebra Mussel Monitoring and Control (Baco Raton, Florida: CRC Press Inc., 1994), p. 28 - 35.

As previously mentioned, mussels are either male or female. They do not propagate or grow at temperatures less than 12 oC. Significant numbers of mussel zygotes appear when the water temperature is in the range of 15 to 17 oC which coincides with the spring and fall periods in the Great Lakes. The ideal temperature for larval development is 18 to 25 oC, which occurs from June to October. In extremely cold water temperatures (over the winter months) the mussels go into a dormant stage, with extremely slow respiration rates and wait for the optimal
seasonal conditions for growth and reproduction.

Mussels can attain a shell length of 15 to 20 mm by the end of the first year and can grow at a rate of 0.5 mm/day under ideal conditions but typically grow at 0.1 to 0.15 mm/day. Reproductive size is in the range of 8 to 10 mm in length. As previously mentioned, most mussels live for two to three years and attain a shell length of approximately 20 mm on average. The mussels high reproductive capacity, rapid growth rate, and ability to survive for a number of days out of the water (ie. on the hulls of drydocked boats) are the three key features that have resulted in the mussels successful invasion of North America.

In the Great Lakes region studies have found that individual mussel biomass will cycle during the year with maximum biomass just before spawning in the spring (in May and June) to minimum biomass at the end of the breeding season, just prior to the winter months (in approximately September and October). This biomass cycling is a result of mussels building up lipid energy stores and then utilizing the lipid energy for spawning and growth. Since most of the contaminants build up in the lipids there is therefore a cycling effect in the mussel tissue
contaminant concentrations that parallel the biomass cycle.

2.4 - General Mitigation Options

The following lists some of the main mitigation options for the control of zebra mussels:

Mitigation Option	Process Summary
Chlorination	continuous exposure to 0.3 to 0.5 mg/L as Total Residual Chlorine for at least 10 days results in 100% mortality.
Hot Water Recirculation	exposure to 37 oC water for at least 2 hours results in 100% mortality.
Oxygen Deprivation	exposure to anoxic environment continuously for at least 4 days results in 100% mortality.
Cathodic Protection	keeps mussels off protected surfaces.
Mechanical Filtration	40 micron mesh self cleaning filters to remove veligers and zygotes.
Anti-fouling Coatings	substrate coatings that are toxic and/or not conducive to mussel attachment.

Other mitigation options that utilize ultraviolet light, potassium salts, advanced oxidation methods (ie. Peroxones), molluscicides, and biocides to name a few are still being researched and are not commonly used.

The above mitigation options are great for controlling the mussels but there is one step that is common to all and that is the manual removal (or mechanical cleaning) of accumulated zebra mussel materials (both alive and dead) from the water supply systems, such as from filters,
intake pipes, forebays, and pump wells. The mechanical cleaning process is important to prevent the accumulation of mussels in a system that may result in system blockages. The following case study will highlight a site specific mechanical cleaning situation and describe how the waste was handled, as well as future uses for this waste.

Figure 3.0 - End views of (a) the zebra mussel and (b) quagga mussel.

Source: Renata Claudi and Gerald L. Mackie, Practical Manual for Zebra Mussel Monitoring and Control(Baco Raton, Florida: CRC Press Inc., 1994), p. 16.

Figure 4.0 - Flow chart for rapid identification of exotic and native bivalves. Biofoulers are shown with an asterisk.

Source: Renata Claudi and Gerald L. Mackie, Practical Manual for Zebra Mussel Monitoring and Control(Baco Raton, Florida: CRC Press Inc., 1994), p. 16.

Figure 5.0 - Diagram of the location of major internal organs of the adult zebra mussel.

Source: Renata Claudi and Gerald L. Mackie, Practical Manual for Zebra Mussel Monitoring and Control(Baco Raton, Florida: CRC Press Inc., 1994), p. 23.

Figure 6.0 - Diagram of the major components of the byssal apparatus.

Source: Renata Claudi and Gerald L. Mackie, Practical Manual for Zebra Mussel Monitoring and Control(Baco Raton, Florida: CRC Press Inc., 1994), p. 25.

Figure 7.0 - Life cycle of the zebra mussel (Dreissena polymorpha).

Source: Renata Claudi and Gerald L. Mackie, Practical Manual for Zebra Mussel Monitoring and Control(Baco Raton, Florida: CRC Press Inc., 1994), p. 32.

[Figures and Tables will be posted by next Wednesday]

If you have any questions please send e-mail to John at: jahibberd@hei-group.com

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Revised on July 23, 2006