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Populations of the zebra mussel, Dreissena polymorpha (Pallas), were first found in the Laurentian Great Lakes in 1988 (Hebert et al., 1989). This species is native to the Caspian, Aral, and Black Seas and the rivers that drain into them but has spread throughout Europe, principally during the 18th century. Since it is restricted to estuarine and freshwater habitats, it is presumed that it was introduced into North America by ballast waters of transoceanic vessels. Based on the substantial amount of genetic variation found in these initial populations, as estimated from electrophoretic variation of allozymes, the colonization of the Great Lakes was by a large number of immigrants and not just a few founders (Hebert et al., 1989). Despite this recency of establishment of this species, the zebra mussel has rapidly spread throughout a large portion of the United States. It has been reported from all the Great Lakes and Lake Champlain, the Mississippi River from St. Paul down to New Orleans, the Illinois River, the Ohio River, and many others (National Aquatic Nuisance Species Clearinghouse; Web site at: http://cce.cornell.edu/aquaticinvaders).
This spread has produced a number of negative economical and ecological consequences. Because of its high fecundity and its ability to tightly adhere to surfaces, it is a very serious fouling organism. The weight of attached mussels can become so great that marker buoys can sink. They can also interfere with the workings of lock gates. The main problem for industry is that zebra mussels can line the interiors of intake pipes to such an extent that water flow is blocked or greatly reduced. This blockage can result in heat damage to power plants and necessitates costly removal or replacement of intake pipes (Minchin and Moriarty, 1998).
Colonization by zebra mussels has devastating ecological impacts on native bivalves (Mackie, 1991; Haag et al., 1993), frequently driving them to local extinction. Zebra mussels readily, perhaps preferentially, settle on native bivalves and eventually cover them over. They filter the water so efficiently that they can lower the amount of suspended food organisms below levels needed to sustain native unionids. Their success in spreading rapidly and colonizing new areas is due to two features of zebra mussel biology. Firstly, zebra mussels produce large numbers (> 30,000 per female) of planktotrophic veliger larvae which can survive for 10-15 days (Hebert et al., 1989). Such larvae are well suited for efficient dispersal by water currents, especially in riverine systems. Additionally, they don't require the presence of suitable host species of fish, as do native unionid larvae. Secondly, the spread of zebra mussels is aided by their ability to attach to objects by byssal threads (the same ability that leads to their fouling problems).
Because of this ability, adult zebra mussels can attach to and be spread by ships, barges, and fishing boats. They are also able to survive outside of water for several hours, due to their ability to close their shells tightly, and so can even be spread by attachment to boats which are transported by trailer from one body of water to another. This mode of dispersal has been termed boater-mediated jump dispersal. Thus, the distribution of zebra mussels reflects the combined effects of two distinct mechanisms of dispersal: by larvae via water currents, and by adults via human intervention. Larval dispersal would be expected to produce mussel settlement more or less continuously along suitable substrate which is interconnected by water. Human-mediated dispersal has the potential to generate sporadic, long-distance dispersal which may involve many or just a few initial colonists. The extent to which these two dispersal mechanisms is operating, and which type of dispersal is responsible for founding particular populations, are important questions to be answered in trying to understand this highly invasive nuisance species.
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