Petromyzon, a jawless fish, lays eggs in fresh water and its ammocoete larvae return to the ocean after metamorphosis. Sea lampreys are anadromous and migrate up rivers to spawn. Spawning is followed by the death of the adults. After several years in freshwater habitats, the ammocoete larvae undergo a metamorphosis that allows young, post-metamorphic lampreys to migrate to the seas, and start the adult hematophagous method of feeding.
Eptatretus is an inshore hagfish found in the northwest pacific. For spawning, it migrates into the deep waters of the sea.
Myxine is a typical hagfish occasionally called as slime eel.