How do eels reproduce? It’s a question that’s been shrouded in mystery, but we are getting closer to understanding how eels mate in the wild? Eels make an epic migration out to sea in order to reproduce, but where science has fallen behind is in understanding how they get there, and how on Earth they know where they're going. We spoke to Dr Matthew Gollock, an expert in eel behavior and scientist with the Zoological Society of London (ZSL), to find out more about what we really know about how eels mate in the wild.

Research has allowed us to track the love lives of European eels (Anguilla anguilla) and the American eel (Anguilla rostrata) as far as their spawning ground in the Sargasso Sea (a nursery ground that’s popular with juvenile sea turtles, too). However, the mystery endures because nobody really knows how they make the journey, or what they do when they arrive.

Do we know how eels reproduce?

While the ins and outs of eel reproduction are still something of a mystery in the wild, these animals have been used for research as well as farming. Studies from these contexts have enabled us to peek behind the curtain of eel reproduction, but they have only shone a light on small aspects of the mysterious interaction in captivity.

As for what happens on those thousands-of-miles-long migrations out in the big wide world? We’re still pretty stumped.

“There have been some studies that have given us insight into some elements of eel reproduction, but these are often limited to studying a single aspect of what is a very complex situation,” said ZSL Marine and Freshwater Senior Programme Manager and Chair of The IUCN SSC Anguillid Eel Specialist Group Dr Matthew Gollock to IFLScience.

“For example, swim tunnels have been used to determine energy use in the spawning migration, but these are unable to take into account the pressure associated with these migrations and also the diurnal movement within the water column.”

“There are also efforts to stimulate reproduction in the lab using hormone treated eels. This has led to successful spawning, but again, to what extent this mimics the wild is uncertain.”

Why is finding out how eels mate in the wild so difficult?

If you’re slapping on a pair of flippers with the intention of settling the Great Eel Reproduction Mystery once and for all, we regret to inform you that solving it isn’t as simple as popping out to take a look. This is because of the weird and wonderful lives of eels.

“Anguillid eels have a complex life-cycle that involves migrations between continental waters where they feed and grow, and the open ocean where they breed,” explained Gollock, however, we are making progress.

“Tracking these migrations is challenging and expensive and as such there is only a limited amount of research that has been carried out on this. In the past 10 to 20 years our knowledge has increased greatly due to improvements in technology and, in the species that are most studied, such as the American, European and Japanese eel, we believe that breeding occurs below 500 meters [1,640 feet] and is influenced by a range of factors that we are only just beginning to understand.”

What theories do we have about the great eel reproduction mystery?

As for existing theories regarding the intricacies of eel reproduction, here too we are making waves but only a single hunch has been proven so far.

“Tagging studies have been carried out on several eel species and our understanding of where they breed is improving all the time,” Gollock told IFLScience.

“However, only the Japanese and Giant Mottled eel has been found at a suspected spawning site, and for other species we are still working to understand specific locations and associated conditions.”

For now, research continues into the mystery of eel reproduction, a goal which Gollock is driven by in his work on a joint project with the UK Environment Agency. It aims to tag European eels in an effort to better understand their spawning migration, but for now, at least, the eels’ sexy secrets remain safe.

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