Answer: transoceanic ships.
Explanation:
Zebra mussels are originally native of lakes in Russia and Ukraine.
They are ordinary found in the bottom of ships. For example in the ballast water of tansoceanic ships. So, it is likely that the zebra mussels adhered to those ships were accidentally introduced in the Great Lakes.
Zebra mussels are an invasice speicies so they a great concern for the ecology.
Some of the effects of Zebra mussels in the Great Lakes ecosystem, among others, are:
- they spread into streams and rivers throughout the US
- they cover the substrate of some areas
- they block pipelines
- they are believed to be the source of deadly avian botulism (responbile of killing birds)
- they are responsible for the near extinction of many species
