Zebra Mussels: The Invaders of the Great Lakes

By: Shaina Grover


When you hear the word mussel, you probably think of seafood. However, do you really know what mussels are? Well, essentially, mussels are a type of mollusk that you can usually find in freshwater. They filter the water, keep it clean, and are an essential part of the freshwater ecosystem. However, like many other species, there’s a catch. After all, no species is perfect. One of the main reasons for this is that sometimes, some species can have tremendously negative impacts when they are introduced. They are typically non-native, unwanted, and unnecessary creatures, called invasive species. One example of an invasive species that has spread at an exponential rate is zebra mussels, which have invaded the ecosystems of the Great Lakes.


While it may not seem like a big deal if a new organism is introduced to an ecosystem, the effects can be enormous if it is an invasive species. According to the National Wildlife Federation, a species is considered invasive if it can “grow and reproduce quickly, and spread aggressively, with potential to cause harm.” The way most invasive species prosper is by outcompeting the native organisms of the ecosystem. Generally, they do this by reproducing at a rate much faster than the native species, allowing them to consume a majority of the food and take up a majority of the space. This, of course, can destroy an ecosystem. In fact, if the invasive species has a large enough impact, it can result in the diverse ecosystem becoming dominated by the one species. The effects of invasive species don’t just apply to animals, either. Invasive species can also harm the economy. If they lead to the extinction of a certain organism in an area, or if the growth of a certain plant leads to the closure of a port/dock, people’s businesses could be impacted.

As mentioned before, one key example of invasive species are zebra mussels (Dreissena polymorpha). Zebra mussels got their name from the zigzag, zebra-like stripes on their backs. They only grow to about two inches long and are almost microscopic when they’re in their larvae stage. Don’t let their small size deceive you, however; they can cause a lot of damage. Just look at the statistics. Although they only live for four to five years on average, every year a female zebra mussel will lay up to one million eggs. And so, they will lay approximately five million in their lifetime. Of these five million, only approximately 100,000 of them will live to adulthood. And while that may not sound like a lot compared to five million, each female that survives will repeat the cycle, causing the zebra mussel population to grow exponentially.


Zebra mussels initially arrived in the U.S. in 1986 as stowaways in an Eastern European commercial vessel’s ballast water. However, they were not formally discovered until June 1988, when the first zebra mussel population was found in the Great Lakes in St. Clair. Unfortunately, not enough action was taken to prevent their spread, and as a result, within two years zebra mussels had invaded all five Great Lakes. Nowadays, they can be found in 29 states.


One of the main reasons as to why zebra mussels are able to survive in foreign environments is that they are able to attach to a variety of surfaces, including vegetation, rocks, and boats. They can even attach to other organisms, through their movement and feeding capabilities, which in turn can reduce their chances of survival. In fact, sometimes native mussels can be found with up to 10,000 zebra mussels attached to them!


Let’s now talk about how zebra mussels can harm an ecosystem. Since there are so many of them, they can filter the water they’re in at an extremely rapid rate. For reference, each one can filter approximately one liter of water a day. And while this isn’t entirely a bad thing, there are several downsides of it. First, it results in less plankton in the ecosystem, which means less food for the other organisms. Additionally, the clearer water allows more sunlight to pass through the water, facilitating an increase in the growth of harmful plants and algae. This can result in the decline of native species, as well as an overall decline in the health of the environment. Studies have shown that after only 2 years of the zebra mussels being in the Great Lakes, the population of the native mussels has been on the decline. Moreover, zebra mussels also increase biomagnification because the harmful things they filter out from the water are concentrated in their body and then get passed on to their predators and the predators of those predators (a type of fish), and so on, eventually reaching the humans who consume the fish.


Overall, zebra mussels are a major threat to dozens of ecosystems throughout all of North America. They not only put the native species in the area at risk, but can also have a major economic impact on the people living in the area. The amount of money this invasive species has costed communities throughout North America's Great Lakes’ economy is in the billions, per year. Their tendency to attach to boats, water pipes, and other living organisms results in the blocked natural flow of water. This, in turn, can cause damage to factories, power plants, and water treatment plants. And, because there are so few natural predators of zebra mussels, they are able to reproduce much faster than any animal should, resulting in their damage growing exponentially. Even though they are barely bigger than a coin, they have the ability to destroy ecosystems and economies. It’s like the old saying goes: size isn’t everything.


Citations:

https://molluskconservation.org/MUSSELS/What_Mussel.html

https://www.fws.gov/midwest/endangered/clams/mussels.html

https://www.nationalgeographic.org/encyclopedia/invasive-species/

https://www.nps.gov/articles/zebra-mussels.htm

https://www.csu.edu/cerc/documents/TheIntroductionandSpreadoftheZebraMusselinNorthAmerica.pdf

https://www.kingcounty.gov/services/environment/water-and-land/lakes/facts/zebramussels.aspx#:~:text=In%20spite%20of%20their%20small,water%20structures%20and%20native%20ecosystems.&text=They%20also%20negatively%20impact%20aquatic,other%20filter%20feeders%2C%20starving%20them.

https://people.uwec.edu/piercech/zebra/index_files/Page581.htm

https://www.earthrangers.com/top-10/ten-of-the-worlds-most-invasive-species/



Image Credit:

Zebra Mussels image: "File:Mexilhão-zebra.jpg." Wikimedia Commons, the free media repository. 28 Oct 2020, 20:23 UTC. 1 Dec 2020, 21:50 <https://commons.wikimedia.org/w/index.php?title=File:Mexilh%C3%A3o-zebra.jpg&oldid=505519182>.



What Did You Learn?

1. What are some other examples of invasive species?

Another common example of an invasive species is the Asian Carp. It’s a type of fish native to Eastern Russia and China. It eats the eggs of other fish species and can reproduce quickly. When they eat, they move up the sediments in the water, making it murky instead of clear. This changes which types of organisms can survive in the water. Furthermore, a different example could be the water hyacinth. Although it is a beautiful plant, it grows extremely quickly and can double in size in under a week. Due to their thickness, they can clog rivers, restricting the movement of other animals and blocking sunlight and oxygen.


2. Are all non-native species invasive?

No, not all non-native species are invasive. Sometimes, a non-native species will blend well into the environment and will not cause a significant change in the ecosystem, either because it has many predators or because it is unable to reproduce at a fast rate. However, if the non-native species can reproduce faster than the predators can eat it, it may have a large impact on the environment.


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