Despite knowledge of their round trip migration, scientists still haven't observed mating in the wild, or found a single eel egg. Leading theories suggest that eels reproduce in a flurry of external fertilization, in which clouds of sperm fertilize free-floating eggs.

Where do all eels breed?

Eels have a complex lifecycle that begins far offshore in the Sargasso Sea where adults spawn. After eggs hatch, young eels drift inland with ocean currents into streams, rivers and lakes for over 3,700 miles. This journey may take many years.

How do eels mate?

Cooke adds that the leading theory of eel reproduction is that they reproduce by external fertilization, in which clouds of sperm fertilize free-floating eggs.