Complex sentence: The zebra mussel, native to the Caspian and Black Seas arrived in Lake St. Clair in the ballast water of a transatlantic freighter in 1988 and within 10 years spread to all of the five neighbouring Great Lakes.
Split into simpler sentences: The zebra mussel, which is native to the Caspian and Black Seas, arrived in Lake St. Clair in the ballast water of a transatlantic freighter in 1988. Within 10 years it had spread to all of the five neighbouring Great Lakes.