European eel

The European eel is a long, snake-like fish that spends most of its life in rivers before migrating to the ocean to breed. Scientists still haven’t seen them mate in the wild. Their numbers have dropped sharply due to dams, fishing, and parasites.

• Their life cycle includes five distinct stages, once mistaken for separate species.
• They travel from freshwater rivers to the ocean to reproduce.
• Hydroelectric dams block their migration routes, threatening their survival.