Eels are known as freshwater fish, but their life cycle takes on that of a 'migratory fish,' which means they lay eggs and the eggs hatch in the sea, and the young eels return to the freshwater.
ウナギは淡水魚として知られているが、海で産卵・孵化を行い、淡水にさかのぼってくる「回遊（こうかかいゆう）」という生活形態をとる。