SN C 1D/1DR B I OL O GY: T HE Z E B R A M USS E L ( AN E X O T IC S P E C I E S) The identification of the zebra m ussel in Lake Erie in the early 1990s set off a series of alarms that captured the attention of news media for nearly eight years. Biologists believe that this tiny bivalve (m ollusk), a native of the Caspian Sea in western Asia, entered the Great Lakes from bilge water discharged from ships. In the Great Lakes, this exotic species found lots of food and so spread quickly.

In 1991 there were extensive colonies of zebra m ussels in Lake Ontario and sm all groups could be found in Georgian Bay on Lake Huron. By 1994, the zebra mussel was com m on in the Rideau Canal and throughout the Trent-Severn W aterway. By 1995, the invading m ussels had m oved through the Ohio River to the Mississippi, and could be found all the way to the Gulf of Mexico.

The m ussel attaches to alm ost any hard object standing in water. The m ussels blocked water intake pipes from the Great Lakes, choking hydroelectric plants and freshwater supplies for a num ber of industries. Ontario Hydro, municipalities and the Ontario Ministry of the Environment all undertook massive cam paigns to prevent the m ussels from m oving up intake pipes into generating stations, water treatm ent plants, and industrial plants. These efforts may also have diverted attention and resources aw ay from pollution issues in the Great Lakes.

ECOLOGY & THE ZEBRA M USSELSpeculations about what this rapidly reproducing organism m ight do to the food webs of the lake ecosystem s were even m ore alarm ing. For exam ple, ecologists noted that the pearly m ussel, a natural inhabitant of m ost of O ntario’s freshwater lakes, had difficulty com peting with the zebra m ussel. In every place that zebra m ussels invaded, a decline in the num ber of pearly m ussels followed. Zebra m ussels and pearly m ussels should not compete for the sam e niche: pearly m ussels burrow into mud along the shores of freshwater lakes, while zebra mussels attach them selves to hard surfaces. However, the shells of the pearly mussels are a hard surface – layers of zebra mussels form on top of pearly m ussels.

Mussels are filter feeders. They put out sm all threads covered w ith a sticky mucous, and com b the water to rem ove sm all organism s for food. Bacteria, algae, and very tiny anim als are taken into the mollusk for food. If m any zebra mussels attach to the shells of the pearly mussels, little food filters down to the pearly mussels. Also, the zebra m ussels attached to their shells prevent them from moving to a m ore favourable location.

The introduction of zebra m ussels has not been detrim ental to other species. Ducks, especially the less scaup, and other aquatic birds feed on the mussel. The discarded shells of the zebra m ussel also provide underwater shelter for snails, aquatic insects, sm all crustaceans, and w ater mites. Hydra (a sm all freshw ater relative of the jellyfish) also benefits: the larvae of the zebra m ussel provide a ready source of food.

The invading mussels have indeed caused problem s, but the Great Lakes ecosystem has not been devastated. Zebra m ussels do not cover the shoreline, nor have they elim inated com peting species. One study, of yellow perch in western Lake Erie, indicates that they do not interfere with the spawning of fish. In fact, they may even help yellow perch by creating a m ore favourable habitat for sm all crustaceans, such as crayfish, that feed on the m ussel. Perch eat crustaceans.

Som e studies even credit the zebra mussel with long-term benefits. The num ber of algae had been increasing greatly owing to fertilizers and other hum an pollutants that were carried into the Great Lakes with the spring runoff. The growth in algae was affecting aquatic plants, as the algae were blocking light from the Sun. Mussels eat algae, and now aquatic plants are thriving once again.

The m ussels not only reduced the am ount of algae in the Great Lakes, but they also rem oved pollutants from the water. Each adult m ussel draws in as m uch as 1.5 L of water daily, retaining the pollutants and expelling the water. So much water is filtered that Lake Erie is now 60% clearer than it was before the arrival of the mussel. However, all of this filtering of pollutants does not com e without a cost. The pollutants stored in the zebra mussels are passed on to predators, for whom they can be toxic.

Ecologists are also speculating about the negative effects of reducing the algae population, because algae are the m ost im portant producers in the lakes, and so are im portant in the food web. Even clearer water may pose a threat. More sunlight would penetrate the w ater, causing greater w arm ing. Because warm water holds less oxygen, fish such as trout, which require higher levels of oxygen, could suffer.

As you can see, assessing the im pact of the zebra m ussel is a complicated business, and it has recently become m ore complicated. Another exotic species, the goby, w as found in the W elland Canal in 1996. The stow aw ay likely entered the Great Lakes from ballast water held in a freighter that had visited the Black Sea. Gobies eat zebra m ussels, but they also have other effects – w e cannot rely on them to restore the original ecosystem . The goby chases other fish aw ay from their spawning grounds and feeds on the eggs of native fish such as walleye, perch, and sm all-m outh bass. Other invasive species, including the Eurasian ruffe, spiny water flea, and purple loosestrife, have also been introduced to the Great Lakes through ballast water.

ECONOM ICS & THE ZEBRA M USSELThe predicted hum an disaster of clogged pipes, resulting in m ulti-billion-dollar clean-up bills, has proven to be slightly exaggerated. Evidence seem s to indicate that chlorine has prevented the zebra m ussel from choking off water intake system s. However, controlling the m ussels has not come without financial cost. One estim ate for Ontario Hydro has pegged the cost of the initial control efforts at $20 m illion, w ith an annual cost of as much as $1 million for maintenance. Com m ercial fishing has survived the influx of invading mussel, but only barely. The industry, which generated $600 m illion before the zebra m ussel, now generates only $200 million.

PRACTICEAnswer the following questions on a separate piece of paper. Be sure to use full sentences when required!1. Make a chart listing the positive and negative effects of the introduction of the zebra m ussel to the

Great Lakes (5 each).2. Im agine you were given the opportunity to speak to the Ministry of Natural Resources about the

issue of invasive or exotic species. W hich hum an activities would you recom m end should be discontinued in the Great Lakes to reduce the introduction of non-native species. Alternatively, you m ay propose potential solutions to reduce the introduction of invasive organism s to the Great Lakes.

SN C 1D B I OL O GY: T HE Z E B R A M USS E L ( AN E X O T IC S P E C I E S) WS#4 INSTRUCTIONS:

1. The ( 2 1 a) was identified in Lake Erie in the early 90's. Biologists believe this ( 2 8 a) , a native of

@ Read the handout “The Zebra Mussel (An Exotic Species)” Com plete each statem ent by finding the word that fits inthe space. The number in the bracket indicates the

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® space. The letter represents (a)cross or (d)own.Answer the practice question (see handout) on the back.

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the Caspian Sea, entered the Great Lakes from ( 10d ) water discharged (or dum ped) from ships.In the Great Lakes this ( 22d ) found lots of food and spread quickly. By 1995, zebra mussels could be found all the way to the Gulf of ( 2 4 a) .

2. The m ussel attaches itself to alm ost any hard object standing in ( 17d ) . The m ussels ( 8d ) water intake pipes, choking ( 1 4 a) plants and freshw ater supplies for industries.

3. ( 15d ) (theories) about w hat this rapidly reproducing organism might do to the food webs of the lake ecosystem s were even more alarm ing. In every place that zebra m ussels invaded, a decline in the num ber of ( 3 1 a) mussels followed - but they should not com pete for the sam e ( 11d ) because pearly mussels burrow into m ud while zebra m ussels attach to hard surfaces. The answer- zebra m ussels attached them selves to the pearly mussels. Mussels are ( 3d ) feeders - bacteria, algae, and tiny anim als are food for the mollusk. If many zebra mussels attach to the pearly m ussels, little if any food filters down. As well, the zebra m ussels prevent the pearly mussel from ( 9a) .

4. However, not all is lost. ( 2d ) and other aquatic birds feed on the zebra m ussel. The discarded ( 3 0 a) of the zebra m ussel also provide underwater shelter for many organism s. The ( 1 8 a) of the zebra m ussel is also a source of food for the ( 1d ) (a freshwater relative of the jellyfish).

5. W hile the invading m ussels have caused problem s the Great Lakes ( 25d ) has not been devastated (as forecast). Some studies even credit the zebra mussel with long-term ( 20d ) .

6. The num ber of ( 1 9 a) had been increasing greatly owing to ( 23d ) and other hum an pollutants that were carried into the Great Lakes with spring ( 6d ) . The growth in algae was affecting aquatic plants, as the algae were blocking ( 7d ) from the Sun. Mussels eat algae, and now aquatic plants are thriving once again.

7. The m ussels not only reduced the am ount of algae but they also rem oved ( 13d ) from the water.Each adult mussel draws in as much as 1.5 L of water daily, retaining the pollutants and expelling the w ater. So m uch water is filtered that Lake Erie is now 60% clearer than is was before the arrival of the mussel. However, all this filtering of pollutant does not com e w ithout a cost. The

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pollutants stored in the zebra m ussels are passed onto ( 5a) , for whom they can be ( 2 9 a) .

8. Ecologists are also speculating about the negative effects of reducing the algae population, because algae are the most im portant ( 3 2 a) in the lakes, and so are im portant in the food web. Even clearer water may pose a threat. More sunlight would penetrate the water, causing greater ( 12d ) . Because warm water holds less ( 27d ) , fish such as trout, which require higher levels of oxygen, could suffer.

9. Another exotic species, the ( 2 6 a) , likely entered the Great Lakes from ( 16d ) water held in a freighter that had visited the Black Sea. W hile gobies eat zebra m ussels, they chase other fish away form their spawning grounds and feed on the eggs of native fish.

10. Evidence seem s to indicate that ( 4a) has prevented the zebra m ussel from choking w ater intake system s. However, controlling the mussels has not com e w ithout financial cost - Ontario Hydro has pegged the initial cost at $20 m illion with an annual cost of $1 million. In addition, comm ercial fishing which generated $600 m illion before the zebra m ussel now only generates $200 m illion.