Zebra Mussels

Zebra Mussel Ecology

Dreissena polymorpha, better known by their common name, zebra mussels, are native to Caspian and Black seas, first described in 1769 by Peter Simon Pallas. They are a small bivalve mollusk that grows to be about the size of a finger nail, in the shape of a D with alternating black and brown strips all along the shells. They are famous for growing in large colonies that can block native aquatic species and human water pipes.

Reproduction

Zebra mussels are normally gonochorist, though a small number of hermaphrodites have been described, in the amounts of 4-8%. Zebra mussel’s reproduction happens via external fertilization in the water column. This is a viable option for zebra mussel due to their growth in large colonies in close proximity to one another. During reproduction, it is important for egg and sperm release to occur at the roughly the same time for external fertilization to be successful. This spawning of gametes seems to stem from a chemical response of serotonin (Ram, Fong and Garton, 1996). Female zebra mussel usually begin reproduction in their second year. Oogenesis for zebra mussels typically occurs in the autumn for release in the spring. A female zebra mussel can release up to one million eggs a year that can be fertilized in the water by one of the two million sperm a male can release. These them form free-swimming larvae called veligers (Benson et al, 2014). The free swimming veliger is an unusual life stage for a freshwater benthic invertebrate. This larvae is fast and capable of riding river currents to new lakes and sources of water. This is the primary cause of the rapid spread of zebra mussels in North America, especially in the Great Lakes region (Nalepa and Schloesser, p. 144).

Growth and Feeding

Soon, the veliger falls to the bottom of the waterbody and crawls along the ground by means of a foot until it finds a suitable surface to attach to. Zebra mussels attach to the substrate by

means of a fibrous threaded organ called a byssus. The veligers do prefer a hard and firm surface over soft ones such as plants or silt of a lake or river bottom (Benson et al, 2014). According to studies done in Lake St. Clair on zebra mussel settlement, the organisms tend to avoid toxic metals such as brass, copper and aluminum. The toxicity of the metals zinc, copper and aluminum have been known to affect the growth of mollusks. However, there is still growth on these materials, it is just reduced compared to other materials such as PVC pipe or other polymers (Nalepa and Schloesser, p. 172).

Zebra mussels are filter feeders and they get their food by using siphons on their bodies to pump water into their shells to gather food from the water column and expel the remainder in a mucus covered mass that they drop to the ground as pseudofeces. They primarily feed on microscopic algae and phytoplankton. Each zebra mussel can filter an estimated 1 liter of water a day (Johnson and Padilla, 1996). Zebra mussels are capable of growing in many different types of lakes, despite their trophic state. One study found that zebra mussels grew in both mesooligotropic lakes as well as eutrophic lakes. The two populations did grow differently, however. The mesooligotropic populations were smaller but much more numerous. The eutrophic population were larger, due to their increased access to food and calcium to grow larger, but had smaller numbers. This is due to a lack of reproduction of this population as well as an inability for postveliger survival (Nalepa and Schloesser, p. 90).

Invasive Species

Origin

Zebra mussels are best known throughout the world as an invasive species. They first spread to North America in the late 1980’s, with the first large colony being discovered in Lake St. Clair of the Great Lakes in 1988. They are believed to have arrived via European cargo ships carrying larval stages of the organism in the ballast water that was discharged when the ship reached America. This likely happened two years prior to the discovery of zebra mussels in 1986 (Johnson and Padilla, 1996).

Problems

Zebra mussels as an invasive species multiply very fast and are capable of causing great harm to natural ecosystems. They can completely cover native mussels, clams and other shellfish with a hard outer surface to attach to. They can even attach to vegetation on the waterbody floor. The larval stages also swim into water intake pipes at water treatment facilities, settle there and build up until they clog the pipes people use for their water supply. Their sharp edges cut swimmer’s feet and cut

fishing line. They also settle on docks and buoys and need to be removed every year. This can be especially detrimental to research buoys as it can interfere with gathering data. Zebra mussels have been estimated to have caused humans up to $1 billion per year of costs associated with damages to property and for cleaning up waterbodies that contain them (Pimentel, Zuniga and Morrison, 2005).

Effects

The effects of zebra mussels on waterbodies, especially lakes, is very pronounced. Their effectiveness as filter feeder has a significant effect on water clarity and particles in the water column, especially phytoplankton. A study done in Saginaw Bay in Lake Huron tested for water clarity using a Secchi disc as well as a kPAR test and also tested total phosphorus content and total chlorophyll in the water column, which can show the amount of phytoplankton. The researchers tested many different stations, in both the inner and outer bays. The inner bay has more zebra mussel activity as it is considered eutrophic. The outer bay has less activity and is considered oligotrophic. The study showed that the water clarity in the inner bay was much higher and the chlorophyll and phosphorus content were much lower. This is drastically changed from earlier readings before zebra mussels were introduced. The outer bay reading also showed these results, but to a much lesser extent and were not as drastically different from previous years readings before the introduction of zebra mussels (Fahnenstiel et al, 1995). This decline in food can be a problem for native filter feeders as they will not be able to get enough food to survive.

Citations

Benson, A. J., D. Raikow, J. Larson, A. Fusaro, and A. K. Bogdanoff. "Dreissena Polymorpha." Nonindigenous Aquatic Species Database. USGS, 6 June 2014. Web. 22 Nov. 2016.

Fahnenstiel, Gary L., Gregory A. Lang, Thomas F. Nalepa, and Thomas H. Johengen. "Effects of Zebra Mussel (Dreissena Polymorpha) Colonization on Water Quality Parameters in Saginaw Bay, Lake Huron." Journal of Great Lakes Research 21.4 (1995): 435-48. ScienceDirect. Elsevier. Web. 24 Nov. 2016.

Johnson, Ladd E., and Dianna K. Padilla. "Geographic Spread of Exotic Species: Ecological Lessons and Opportunities from the Invasion of the Zebra Mussel Dreissena Polymorpha." Biological Conservation 78.1-2 (1996): 23-33. ScienceDirect. Elsevier. Web. 24 Nov. 2016.

Nalepa, T. F., and Donald W. Schloesser. Zebra Mussels: Biology, Impacts, and Control. Boca Raton, FL: Lewis, 1993. Print.

Pimentel, David, Rodolfo Zuniga, and Doug Morrison. "Update on the Environmental and Economic Costs Associated with Alien-invasive Species in the United States." Ecological Economics 52.3 (2005): 273-88. ScienceDirect. Elsevier. Web. 23 Nov. 2016.

Ram, Jeffrey L., Peter P. Fong, and David W. Garton. "Physiological Aspects of Zebra Mussel Reproduction: Maturation, Spawning, and Fertilization." American Zoologist 36.3 (1996): 326-38. Oxford Journals. Oxford University, 1 June 1996. Web. 23 Nov. 2016.

Image Citations

https://www.wildlife.ca.gov/portals/0/Images/Conservation/Laboratory/QuaggaNZebraMussel/ZM09-7-12-zebra-veliger.jpg

http://www.lakegeorgeassociation.org/what-we-do/Invasive-Species/images/byssalcordscopy_000.jpg

https://nas.er.usgs.gov/UserImages/current_zm_quag_map.jpg

http://www.okbassfednation.com/ZebraMussel.jpg

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