Why do all eels breed in the Sargasso Sea?
That is not the set up to a joke.
A few weeks ago, researchers finally answered a question that has stumped everyone from Aristotle to Sigmund Freud (yep, before he got into psychoanalysis, he was cutting up eels, fruitlessly looking for their sex organs – figures): where do eels come from?
The reason for this enduring mystery is that eels (specifically freshwater eels) are “catadromous”: they begin life far out in the deep ocean, migrate to live their adult lives in estuaries, freshwater rivers and lakes, and then return to the ocean to breed and die. This is the reverse of salmon, who are “anadromous”, living most of their lives at sea, before returning to the exact freshwater rivers they were born in to breed – and also die.
The problem is that no one knew exactly where in the boundless oceans the eels were coming from.
Back in 1904, Danish biologist Johannes Schmidt came very close. He led a series of expeditions in the Mediterranean and Atlantic Ocean to investigate European eels. He found that the further west he travelled across the Atlantic, the smaller and smaller (ie the younger and younger) the eels got. He sailed as far as the Sargasso Sea, a part of the Atlantic just east of Bermuda that’s characterised by its calm water and the brown Sargassum seaweed that proliferates there. (It’s also the only sea with no land boundaries – one for the pub quiz.) Here, he caught the smallest eel larvae that had ever been found. He deduced that this must be where European eels come from, but he didn’t actually see any spawning, or adult eels ready to spawn.
Schmidt was right. A team of researchers led by the Environment Agency has now managed to track, using satellite tags, European eels making the 10,000km (6,214-mile) journey from their continental freshwater redoubts to the Sargasso Sea. The first direct evidence of eels travelling to their spawning point.
And it’s not just European eels. All freshwater eels in Europe and America are born in the Sargasso Sea. Even eels in landlocked bodies of water can get to the sea. In the right conditions – on a damp night after a little rain – eels can slither along land to another body of water.
But it prompts the obvious question: why do eels – happily living for potentially decades in the depths of rivers and lakes – migrate so far to breed? What is the evolutionary benefit for what seems like a massive cost? Why not just find mates and breed in a habitat that they’re already suited to?
First, let’s look at the eel’s lifecycle (which is fascinating in itself) and then we’ll consider why eels, and other animals, evolved to migrate such a long way to breed.
The eel of life
Eels go through five main life stages:
Leptocephalus (larval stage)
Glass eel
Elver
Yellow eel
Silver eel
First, fully mature adult eels arrive in the Sargasso Sea, spawn some eggs and die. They will reproduce only once in their lives. Having just completed a 10,000km journey from Europe, they’ve got no time for parental care.
The fertilised eggs then drift eastwards on the Gulf Stream towards Europe (the American eels hop off the Gulf Stream earlier to reach the east coast of America), developing into transparent, willow leaf-shaped larvae called leptocephali. It’s thought that during this journey, which can last up to two years, a sort of magnetic map is imprinted on the young eels, which they will later use to navigate back to where it was born.
Upon reaching the European continental shelf, the eels will have transformed into small glass eels, looking a bit like vermicelli.
When the eels reach the brackish water of estuaries, they start to pigment – darken – and become elvers. At around 8cm, these young eels migrate upstream looking for the freshwater habitats they’ll be calling home for the best part of their lives.
The freshwater stage of the eel, known as the yellow eel, is the longest period of its life, ranging from 7 to 85(!) years. The reason for this massive range is that eels wait for the right conditions to begin the journey back to Sargasso Sea. No one is quite sure what “the right conditions” are, but a variety of factors, like temperature, water levels, sunlight and even the phases of the moon seem to play a role.
Once they’ve reached an adequate size and built up enough fat, the yellow eel transforms once again for the final stage of its life: the silver eel. The eel changes colour, its pectoral fins widen, its eyes grow up to 10 times their original size, it puts on muscle, dissolves its own stomach (relying solely on its fat reserves from then on) and, finally, develops its reproductive organs (this is why Freud couldn’t find them!).
All these changes aid the eel on its journey back to the Sargasso Sea to start the cycle again – spawning and dying beneath the brown Sargassum weed, the first and last thing they see.
As an interesting side note, this complex life cycle means that breeding eels in captivity is impossible. It remains the biggest unsolved problem in the global eel farming industry, which currently relies on the constant supply of wild glass eels. So valuable is this industry that the illegal global trade in European eels alone – typically smuggled in plastic bags inside suitcases – is worth up to £2.5bn a year.
Why do eels migrate so far to breed?
So why do they do it? They’re certainly not the only ones.
Arctic Terns, for instance, have the longest migration of any animal. Every year, they travel from their wintering grounds in the Antarctic Circle to their breeding grounds in Arctic Circle and back again – a round trip of up to 81,600km (50,700 miles). The terns are chasing summer: sea and flying conditions are better, and the sunlight makes it much easier to spot their prey on the ground or at sea.
Monarch butterflies in North America migrate up to 4,830km (3,000 miles), from the east and west coasts down towards their wintering grounds in Mexico, before heading back again. They migrate to chase the warmth, because they can’t survive the northern winter. This journey is so long – and the Monarch’s life span so short (less than two months) – that the individuals that complete this journey are four generations removed from the ones that started it. And yet each fulfils the genetic quest they were born to complete.
Eels are trickier because we still know very little about their migratory behaviour. And the little we do know paints a confusing picture.
For one, the migration isn’t a neat annual journey. They can depart any time between August and December, with males starting the migration earlier, and the females later (although the females are faster swimmers). Also, given the large age range of mature adults, it suggests that they don’t migrate as a cohort. Rather, it just depends which individuals happen to find themselves in the right conditions to migrate.
Even once they’ve found their way to the sea, it’s not smooth sailing. Some eels linger around the coast, sometimes for years, before either continuing with the migration, or swimming back upstream(!) to have another go when conditions are better.
And if they do set off in earnest, they don’t travel together. They just turn up one by one at the spawning location, hopefully arriving on time. Some researchers have even supposed that if an eel arrives too late, it might just hang out in the Sargasso Sea and wait for the next spawning season. (Although how it lasts that long after it’s dissolved its own stomach is something of a mystery.)
This journey starts to make sense if we look at the evolution of the freshwater eel.
Back to where it all began
European and American eels derive from a deep-ocean-dwelling ancestor that lived in the warm waters of the Indo-Pacific many millions of years ago. The modern-day tropical eels that still live in that part of the world bear more resemblance to the ancestral eels. They have shorter migrations to their breeding grounds (around 100km, or 62 miles) and, as a result, spawn year-round.
This ancient ancestor is speculated to have entered the North Atlantic around 30 million years ago. Back then, the Atlantic was a much narrower body of water, so these early Atlantic eels could exploit the estuarine and freshwater habitats (where, perhaps, they experienced less competition for prey) on the continents while maintaining a short migration loop around the coasts of the North Atlantic. But as the Atlantic Ocean grew over millions of years, the distance they had to migrate back to their spawning grounds in the ocean gradually increased to the 10,000km journey we see today.
In other words, the original reason for the eels’ migration was the search for a more advantageous environment, and the present distance back to the breeding grounds is an accident of geography.
It still prompts the question, though: why return all that way to the Sargasso Sea? If they’ve adapted to successfully exploit cold freshwater habitats, why haven’t they evolved to mate and spawn in those same habitats?
The need for tropical water is explained by the fact that the ideal water temperature for eel spawning and growth is around 23-25°C. And specifically the Sargasso Sea? Perhaps, just like us, they have a preference for the familiar – a sense of home.
Thank you to reader Jonatan for alerting me to the recent research, and inspiring this piece!
> "the present distance back to the breeding grounds is an accident of geography."
That wouldn't explain Pacific eels though, would it?