Spawning Migration of the European Eel

Freshwater eels are almost infinitely improbable creatures. They spawn and die in
the middle of the ocean, often associated with undersea mountains. Their transparent,
leaf-like larvae move with ocean currents for months or years until they
approach the mouths of freshwater rivers. Then they undergo a dramatic transformation
in morphology, physiology and behavior. They move from their planktonic
oceanic environment, migrate upstream and live for several years as apex freshwater
predators. Then, almost impossibly, as they become sexually mature, they
reverse their migration downstream to the ocean and back to spawning grounds to
complete their life cycle.