Zebra mussels (Dreissena polymorpha) are native to large parts of Europe, but are exotic species in North America. They were first identified in North America in 1988 in Lake St. Clair (between lakes Erie and Huron). In 1991, a second species was identified in Lake Ontario: the quagga mussel (Dreissena bugensis).

The zebra mussel was likely introduced to North America via the ballast water of large transport ships. It is thought (partly as a result of genetic evidence) that the zebra mussels now present in North America were from populations in the southern part of their native range; most likely, from the Black Sea. In North America, they have successfully invaded all of the Great Lakes, the St. Lawrence River, the Mississippi river and countless other bodies of freshwater in the northern U. S. and southern Canada.

They are small creatures (maximum size of about 35 mm) that are mostly sessile. They do move (very slowly), but spend most of their time attached to some hard substrate. They remain attached by secreting an adhesive protein mixture from their byssus (foot). Their life cycle differs from native bivalves in that their larval stage (called the veliger stage) is not parasitic, but rather herbivorous. Thus, they are capable of incredible rates of reproduction, given that their life cycle is not dependent on the success of another species. They also have nearly no predators in North America, and as such are able to expand their populations at a nearly geometric rate*.

The impact of zebra mussels on North American ecosystems is extreme. They severely damage native species in two manners. The first is by direct interference; one of the preferred attachment sites for adult zebra mussels is native unionid mussles, which are almost uniformly endangered species. The high rate of flow created by these powerful filter feeders makes their shells perfect locations for zebra mussels, since the food literally comes right to them. However, they tend to attach in such numbers as to kill the native species in a very short period of time. The second impact the zebra mussels have on native species is due to their huge numbers (populations can reach sizes of ~5000 individuals per m2). They can clear the water column in shallow lakes several times per day, and thus consume huge amounts of algae. This takes the biomass from the pelagic zone and transfers it to the benthic zone. As such, zooplankton are heavily disadvantaged, and fish species such as lake trout, walleye, rainbow trout and others that feed in the offshore areas tend to go into decline. However, the mussels produce a copious amount of nutrient rich pseudofaeces, which is a valuable food resource to benthic invertebrates. As such, near shore fish species are favoured, and populations of bass, pike and yellow perch can actually increase in numbers.

*North America was recently invaded by three species of gobies from the same region as the zebra mussels. These fish have a hard, bony upper palate and are molluscivores. They eat zebra mussels at a remarkable rate, and thus may help to control the exploding populations. However, research has shown that they only eat the juvenile stages, and thus will not be able to control the mussels completely. Also, they are driving another suite of native species into extinction: the sculpins.