Complex sentence: The zebra mussel, which arrived in North American waters in 1985 originated in the Black Sea and Caspian Sea area and is thought to have been brought to this continent in the ballast of foreign freighters.
Split into simpler sentences: The zebra mussel, which arrived in North American waters in 1985. It originated in the Black Sea and Caspian Sea area and is thought to have been brought to this continent in the ballast of foreign freighters.