Online, it has become a frequently asked question. How do eels – those long, writhing fish you may have seen in an aquarium or seafood restaurant – do baby eels? In other words, how do eels reproduce?
People have been studying the creatures for literally thousands of yearsbut even after all this time, certain aspects of their reproductive life are still shrouded in mystery.
To get the lean on eel reproductionwe spoke to an expert: Caroline Durif, an ecologist at the Marine Research Institute in Storebø, Norway, who studies the habits of these incredible fish. Here’s what we learned.
What exactly is an eel?
Calling something an “eel” does not make it one, per se. You’ve probably heard of the South American electric eel (Electric electrophore). Despite its common name, this dramatic fish is not really an eel. Instead, it is a relative of the bizarro carp that is classified as a “knifefish”.
order Anguilliformes contains everyone “real” eels. Over 800 species exist, including moray eels (you know, Ursula’s Sidekicks of “The Little Mermaid”), conger eels and the aptly named spaghetti eels.
Usually slender and elongated, true eels do not have pelvic finswhich are twin appendages often found on the bellies of other fish. Anguilliformes may be without scale; many species are slippery to the touch.
Regardless of their texture, salt water is the preferred habitat for the majority of eels. However, there are 19 species of “freshwater eels”, also known as “Anguillidae“, which spend much of their lives away from saline environments. “They spawn in the sea and grow in fresh water,” Durif explains in a video interview.
Of these 19 species, perhaps the three best known are the European eel (Anguilla anguilla), American eel (Anguilla rostrata) and Japanese eel (Anguilla japonica).
Early life stages of an eel
To say that these fish have a complex life cycle is an understatement. If it survives to reach sexual maturity, a freshwater eel will have passed through five distinct stages. With each new phase, the animal undergoes both a physical transformation and a change in its way of life.
“The first one [stage] is called a leptocephalus larva. They are called leptocephali because ‘lepto‘ means leaf and ‘headache‘ means head,” says Durif.
True to their name, hatchling larvae have leaf-like bodies that appear broad and flattened in profile. (In technical jargon, their bodies are “laterally compressed.”) Being almost transparent, they are also very well camouflaged – imagine trying to retrieve a lost contact lens from the bottom of a swimming pool.
Leptocephalus larvae are ocean animals. Eventually, however, instinct drives them to find a change of scenery, where the transition to the second phase of the life cycle occurs.
“For the anguillids, they migrate over great distances, they drift through the Gulf Stream. Then, when they reach the continental shelf, they metamorphose into glass eels,” says Durif. The elvers are still (more or less) transparent, but they are longer and thinner in comparison.
Being “drawn to fresh water” in Durif’s words is the next step for most anguillide elvers is to head inland up the rivers.
This brings us to life stage n°3: “yellow eels”. Unlike transparent larvae and transparent glass eels, these types have body pigment, with an overall yellowish complexion. This is not the last color change the fish will experience.
“When they’re ready, they turn into silver eels, which is like puberty,” says Durif. “We call them ‘silver’ because they have a silver belly and a black dorsal area… It’s an adaptation to predation.”
Change doesn’t happen overnight. Going from a yellow eel to a silver eel can take 20 or 30 years. Once the process is complete, they return to their roots and head out to sea. Only then can eels reach sexual maturity, the fifth and final stage of their life cycle.
The last days of an eel
To quote a 2021 article (co-authored by Durif) that was published in the journal Fish and Fisheries, we don’t know much about the reproductive stage of anguillide eels”because sexually mature eels have never been caught alive in the wild.”
Likewise, no one has observed these eels spawning in their natural habitat. Scientists have yet to catch wild anguillids “in the act”, if you will.
Whatever happens there, experts believe that freshwater eels die soon after mating. Researchers in the laboratory have managed to induce sexual maturity in silver eels by injecting them with hormones.
After the transition, their health declines. “The bones decalcify like a woman in menopause. It’s super interesting, actually. And then their digestive tract, their intestine, regresses,” says Durif.
Maybe that’s just as well. Breeding-age freshwater eels congregate in places where their usual food options (insects and small fish) are likely to be scarce or non-existent.
Tales from the Sargasso Sea
In the Atlantic Ocean, there is a region called the Sargasso Sea. Unlike the Mediterranean, the Red Sea and most other seas, this one is not bordered by any land mass. Instead, its peculiar boundaries are formed by strong ocean currents.
On the other side of the world, the freshwater eels that live in and around the Pacific Ocean have their own spawning grounds. The Japanese eel is believed to breed at a site west of the Mariana Islands. Other species could breed somewhere between New Caledonia and Fiji.
“The European eel has the longest migration,” Durif tells us, with some individuals crossing about 4,971 miles (8,000 kilometers) to reach the Sargasso Sea from Norway.
Eels release their eggs underwater, to be fertilized by clouds of expelled sperm. This applies to both freshwater and non-freshwater eels, like moray eels and congresses.
Speaking of which, Durif says we “know even less about congresses than anguillide eels” when it comes to reproduction. “We believe there is at least one spawning ground, perhaps in the Mediterranean.” Hopefully future research will shed some light on their private lives.