Fact Check: Nobody Knows How Eels Reproduce

Eels have become a hot topic on social media as users discuss a compelling mystery about where they come from.

The Claim

On August 14, TikTok user ColeTheScienceDude published a video titled "We Don't Know Where Eels Come From."

The user relays facts about how little scientists know about the reproductive cycle of the eel species Anguilla anguilla, also known as the European eel or freshwater eels.

He states: "We have been to the moon and yet we still do not know how eels sexually reproduce."

The user goes on to describe other aspects about eels that scientists are puzzled by, including that "if you dissect one and you look inside, you will not find sexual organs".

He adds: "We have never observed eels mating in captivity or in the wild."

The video has proved extremely popular, gaining 1.8 million likes and over 133,000 shares on TikTok. It also received thousands of likes and retweets on Twitter where it was reposted.

The Facts

The lack of data that researchers have on freshwater eel reproduction is well-documented.

In a TED Talk article available online, zoologist Lucy Cooke states that the life of the freshwater eel begins in the Bermuda Triangle and they then make their way to continental waters.

The last stage of their lives, Cooke states, is mysterious, because researchers have been unable to find their breeding grounds and "still haven't observed mating in the wild, or found a single eel egg."

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.

This was corroborated to Newsweek by Sebastian Nikitas Politis, a researcher at the National Institute of Aquatic Resources in Denmark who led a study in breeding eels in captivity.

However, Politis disputed some statements made in the TikTok video and went on to say that eel reproduction has been observed in captivity.

He told Newsweek that the claim that no reproductive organs have been found in Anguilla anguilla eels is "not correct" and that they do exist in specimens in later stages of their life cycles.

"We definitely have seen eels that have reproductive organs—we are reproducing eels in captivity," he said.

"We are treating them with hormones in order to actually sexually mature and then once they are ready, then we can bring females and males together and then we have observed their mating behavior, but it is also not that common as it could or should be."

He described how males and females come into close proximity to one another, and release eggs and sperm.

It should also be noted that other types of eels are better understood than Anguilla anguilla.

However, Politis did confirm that scientists have never found oceanic spawning grounds for freshwater eels in the wild, nor have they found eggs or early hatchlings, nor have they found a mature specimen that is ready to go spawning.

He said: "So there is some truth to the whole story that there are parts of the life cycle that we don't know. And that is the mystery about it."

The Ruling

Fact Check - Mostly True

Mostly True.

It is mostly true that scientists don't know the full reproductive cycle of eels in the wild.

The caveat is that, contrary to claims made on social media, they have been observed reproducing in captivity and their sexual organs have also been observed.

FACT CHECK BY Ed Browne

Eel
A stock photo shows a European eel in a river. Claims that scientists don't know how they reproduce have sparked interest on social media. Michel VIARD/Getty