The jawless fish of the superclass Agnatha live in fresh and saltwater environments. Hagfish and lamprey are in this class. While hagfish are found only in the ocean, lampreys are also found in freshwater rivers and lakes.
Whether it's shallow waters or those as deep as 5,500 feet, hagfish live on the ocean floor, feeding on sea worms and dead or dying fish. Lampreys start out as freshwater larvae in waters near the coast, and they eventually move out to sea and return to spawn. Lampreys often make their way farther inland, where the invasive species attack and decrease the population of native fish, disrupting the natural balance of the ecosystem.