A pretty pest

A winter walk along the Lake Ontario shoreline at Presqu’ile Provincial Park – I’m taking pictures and enjoying they day, especially the crystal clear waters and the abundance of pretty shells. I know that zebra mussels are an invasive species and causing many problems, but I had never understood the scope of the problem until I sat down, went through the images and did a bit of research.

The zebra mussel (Dreissena polymorpha) is a small aquatic animal that resembles freshwater clams. Zebra mussels grow up to five centimetres (two inches) in length and may live up to five years. Their name originated from the brown and white striped colour of its shell. One mature female zebra mussels can produce up to one million eggs per year depending on water quality conditions.

Lake Ontario Zebra mussel

Zebra mussels are native to the Caspian, Black and Azov seas of Eastern Europe; they were initially discovered in the Caspian Sea in 1769. Scientists first discovered a population of zebra mussels in Lake St. Clair in 1988. The zebra mussels were likely transported to North America in the ballast water of a Eurasian transoceanic ship. This ballast water was then discharged in Lake St. Clair, Ontario, likely in 1985 or 1986. Normally a freshwater organism, the zebra mussel has even shown signs of being able to survive the slightly salty conditions of the lower St. Lawrence River.

The prolific mussel, with a voracious appetite for plankton, uses a special byssal gland to secrete highly adhesive threads that attach it to rocks, debris, water- pipes and screens, boat hulls, and – most destructively – native mussels. The zebra mussel’s remarkable attachment ability causes bio-fouling, or build-up of masses of mussels (and mucous-like mussel excrement), that can clog pipes and choke off oxygen and food supplies to other organisms. The mussel also reproduces quickly, and filters vast quantities of water, monopolizing the phytoplankton that formerly sustained other aquatic life.

Lake Ontario Zebra mussel

Zebra mussels are particularly insidious in that they undermine the very foundation of the Great Lakes’ food web.  Being filer feeders, zebra mussels rob fish and other organisms of the food they need.  They offer nothing in return; they provide no value as a prey organism and throw the natural ecosystem out of balance.  Zebra mussels are implicated in the alarming disappearance of Diporeia, a key native zooplankter that is vital to the diet of many native fish species. Toxic contaminants, such as PCBs and PAHs, are heavily concentrated in the tissues of the musse;, and excreted in dangerously high amounts in their feces. Through the process of bio-magnification, ever-increasing concentrations of pollutants are passed up through the food chain, to sport fish, waterfowl and humans. Zebra mussels are also being implicated in Great Lakes outbreaks of bottom-dwelling botulism.

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