How do eels reproduce?

On the internet, it’s a popular question. How do eels — those long, squirming creatures you might spot in an aquarium or on a seafood menu — go about creating baby eels? In other words, how exactly do eels reproduce?

Scientists have been researching these fascinating creatures for millennia, yet even after all this time, many aspects of their reproductive behavior remain a mystery.

To uncover the mysteries of eel reproduction, we turned to an expert: Caroline Durif, an ecologist at the Institute of Marine Research in Storebø, Norway, who specializes in studying the fascinating behaviors of these unique fish. Here’s what we discovered.

What Exactly Is an Eel?

Just calling something an "eel" doesn't automatically make it one. For example, you've probably heard of the South American electric eel (Electrophorus electricus). Despite its name, it's not actually an eel. It's more like a peculiar relative of the carp, officially classified as a "knife fish."

The order Anguilliformes is home to the world’s "true" eels. There are over 800 species, including morays (the ones that side with Ursula in "The Little Mermaid"), conger eels, and even the aptly named spaghetti eels.

Typically long and thin, true eels lack pelvic fins, the paired appendages found on the underside of many fish. Eels in the Anguilliformes group are often scaleless, and many feel slippery to the touch.

While many eels thrive in salty water, the majority prefer saltwater habitats. However, there are 19 species known as "freshwater eels," or "Anguillids," that spend most of their lives away from the ocean. As Durif explains in a video interview, "They spawn in the sea and grow in freshwater."

Among the 19 species, three of the most well-known are the European eel (Anguilla anguilla), the American eel (Anguilla rostrata), and the Japanese eel (Anguilla japonica).

Early Life Stages of an Eel

Describing the life cycle of these fish as complex is putting it mildly. If a freshwater eel reaches sexual maturity, it will have undergone five distinct stages, each marked by both physical changes and shifts in behavior.

"The first stage is called the leptocephalus larva. They are named leptocephalus because 'lepto' means leaf and 'cephalus' means head," says Durif.

True to their name, the baby eels have leaf-like bodies that appear broad and flattened when viewed from the side. (In technical terms, they are "laterally compressed.") Their nearly transparent form provides excellent camouflage — think of trying to find a lost contact lens at the bottom of a swimming pool.

Leptocephalus larvae are ocean-dwelling creatures. Eventually, however, something instinctive drives them to seek a new environment, marking the start of the next phase in their life cycle.

"For the anguillids, they travel vast distances, drifting through the Gulf Stream. Once they reach the continental shelf, they undergo a transformation into glass eels," Durif explains. These glass eels remain somewhat transparent but are longer and thinner in comparison.

Eel Maturity

As Durif puts it, the next phase for most anguillid glass eels is their attraction to freshwater, which leads them to migrate inland, swimming up rivers.

This brings us to the third life stage: the "yellow eels." Unlike their transparent larval and glass eel stages, these eels have pigmentation, taking on a yellowish hue. However, this won't be the last color transformation they undergo.

"When they're ready, they become silver eels, which is comparable to puberty," Durif explains. "We call them 'silver' because they sport a silver belly and a dark dorsal side... It's a survival adaptation against predators."

The transformation isn't instant. It can take 20 to 30 years for an eel to transition from a yellow eel to a silver eel. Once the change is complete, they return to the ocean, where they finally reach sexual maturity, marking the fifth and final stage of their life cycle.

An Eel's Final Days

As stated in a 2021 study co-authored by Durif and published in the journal *Fish and Fisheries*, we still know very little about the reproductive phase of anguillid eels, "because sexually mature eels have never been caught alive in the wild."

In the same vein, no one has witnessed these eels spawning in their natural environment. Scientists have yet to observe wild anguillids "in action," so to speak.

Whatever occurs during their mating process, experts believe that freshwater eels die shortly after reproduction. In lab conditions, researchers have been able to induce sexual maturity in silver eels by administering hormones.

Once they undergo the transformation, their health begins to deteriorate. "Their bones become decalcified, much like a woman going through menopause. It's actually quite fascinating. And then their digestive system starts to regress," Durif explains.

Perhaps that’s for the best. Mature freshwater eels gather in areas where their usual food sources (insects and small fish) are likely scarce or completely absent.

Tales of the Sargasso Sea

In the Atlantic Ocean, there's a region known as the Sargasso Sea. Unlike the Mediterranean, the Red Sea, and most other seas, this one isn’t surrounded by land. Instead, it’s bordered by powerful ocean currents.

Both American and European eels come to this region to spawn. Studies suggest they may use magnetic fields to help navigate the vast ocean.

Far across the globe, freshwater eels living around the Pacific Ocean have their own breeding grounds. The Japanese eel is believed to spawn somewhere west of the Mariana Islands, while other species may reproduce between New Caledonia and Fiji.

"The European eel has the longest migration," Durif shares, with some individuals traveling approximately 4,971 miles (8,000 kilometers) from Norway to the Sargasso Sea.

Eels release their eggs underwater, where they are fertilized by sperm clouds. This applies to both freshwater eels and non-freshwater species, such as morays and conger eels.

Speaking of which, Durif mentions that we "know even less about conger eels than anguillid eels" when it comes to their reproduction. "We believe there is at least one spawning area, possibly in the Mediterranean." Hopefully, future research will offer more insights into their secretive lives.