How Do Eels Reproduce? Their Methods Explained

Quick Take

• Freshwater eels travel thousands of miles to one specific ocean location to breed, but once they arrive, they completely vanish from scientific sight. Discover the migration mystery →
• No one has ever witnessed freshwater eels mating in the wild, a mystery that has stumped scientists for millennia. Explore the unseen spawning →
• Saltwater and freshwater eels share almost everything, with one crucial reproductive difference setting them apart and sealing the fate of one species after a single spawn. See how saltwater eels differ →
• Freshwater eel larvae have no brain yet somehow navigate thousands of miles back to the exact rivers their parents left, a feat that scientists still cannot explain. Read about larval navigation →

There are approximately 800 distinct species of true eels distributed across the globe. Most of these eels are marine creatures that inhabit tropical and temperate oceans. There are only about 16 to 19 recognized species and subspecies of true freshwater eels worldwide. True eels have a worldwide distribution, spanning nearly every global ocean and accessible river system, avoiding only the extreme polar ice caps. Although eels vary significantly in size, appearance, and habitat, all true eels share one trait: they spawn in the ocean. Continue reading to learn how eels reproduce.

Overview: Eels

Across the globe, eels vary from miniature bottom-dwellers to massive ocean giants. At the smallest end of the spectrum is the one-jawed eel (Monognathus ahlstromi), a deep-sea marine species that matures at a mere 2 inches in length, while the slender giant moray (Strophidon sathete) can reach lengths of up to 13 feet. Even within individual families, such as freshwater eels, size variations are highly pronounced.

While all true eels share an elongated body, they do not all look alike. Eels display diversity in color and shape, which is influenced by their specific environment.

Coral reef inhabitants like the ribbon eel (Rhinomuraena quaesita) have striking, electric blues and yellows, while moray eels have leopard spots or zebra stripes. However, the majority of eel species are drab, muted, or darkly colored to help them blend into their environment. Freshwater eels are generally shades of olive green, brown, and gray, which helps them remain hidden from predators in muddy sediment.

How do Saltwater Eels Reproduce?

Marine eels complete their reproductive life cycle in the ocean, typically spawning in warm, tropical, or deep subtropical waters. Many marine eel species, such as moray eels, survive the breeding process and can spawn multiple times throughout their adult lives, unlike freshwater eels which typically die after spawning.

They use external fertilization, simultaneously releasing millions of buoyant eggs and sperm directly into the open water. Once fertilized, these eggs hatch into transparent, leaf-like larvae known as glass eels, which drift on oceanic currents for months before metamorphosing into juvenile eels and settling into their permanent saltwater habitats.

How Do Freshwater Eels Reproduce?

For millennia, scientists had no idea how or where freshwater eels mated. Research is ongoing, and thanks to advancements in technology, more is being learned about freshwater eel reproduction.

Researchers were baffled by how freshwater eels returned to the sea to spawn. With the advent of 21st-century technology, they have been able to follow eel migration patterns using tracking devices.

Scientists have tracked adult European and American eels traveling thousands of miles across the Atlantic Ocean straight into the Sargasso Sea. The remaining mystery of freshwater eel reproduction is not how they travel to the ocean, but what happens once they arrive at their spawning grounds in the Sargasso Sea.

While modern satellite tags have successfully tracked adult eels on their long migration, no one has ever witnessed them mating in the wild, found a single wild eel egg, or discovered an adult carcass after spawning.

Once they enter these deep ocean waters, the adults completely vanish. This leaves scientists questioning how eels fertilize their eggs, at what depth they spawn, and how their brainless, drifting larvae manage to navigate thousands of miles back to the same freshwater rivers their parents left.