The zebra mussel is a small freshwater shellfish native to the Black & Caspian seas of western Russia. They were introduced into European waters in the 18th Century. By 1986 the mollusks were transported to North America from freshwater European ports, through the discharge of ballast tanks from international shippers.
They are prolific breeders. Each female can produce up to 40,000 eggs each year. Using elastic-like fibers they can attach to any hard surface and quickly colonize large areas, reaching densities of more than 100,000 per square meter. They feed by filtering water containing microorganisms through their gill system.
Once the zebra mussels invaded Lake Erie they spread like wildfire. Their impact on Lake Erie has been profound. Nearly all particulate matter is strained from the lake’s water. Uneaten suspended matter is bound with mucous and amassed among the shells in its immense colonies. Because of this filtering activity, the clarity of Lake Erie has greatly improved, allowing light to penetrate much deeper, and with much greater intensity than ever before.
Unfortunately this phenomenon has serious consequences to the lake’s ecosystem and water quality. Besides severely affecting the aquatic food chain, this increase in light intensity causes the foul summertime taste and odor problem. The additional light entering the lake causes a steep acceleration in the blue-green algae growing cycle, the main source of taste and odor problems.
Quagga mussels (Dressena bugensis), a close relative of the zebra mussel, were first discovered in the Great Lakes region in September 1989, when one was spotted near Port Colborne, Lake Erie; however, the recognition of the quagga type as a distinct species did not occur until 1991. Their arrival to the Great Lakes region, like the zebra mussels, appears to be the result of ballast water discharge from transoceanic ships into the Great Lakes.
The quagga mussel shell is striped, as is that of the zebra mussel, but the quagga shell is paler toward the hinge. There is a large range of shell features, including a distinct species in Lake Erie that is pale or completely white. The quagga is slightly larger than the zebra mussel, about 20mm (0.8 in) wide, roughly about the size of an adult human's thumbnail. A fully mature female mussel is capable of producing up to one million eggs per year.
Quagga mussels feed all year, even in winter when its cousin the zebra mussel lies dormant. Some researchers believe that Lake Erie's dead zone is likely the partial work of the tiny quagga mussel’s non-stop feeding, its ability to live in deep water (it has been found at depths up to 130m in the Great Lakes) and the excretion of phosphorous with its waste.