Probably the best eel theory in the world

A FEW weeks ago a reader emailed me with a query about the life-cycle of eels.

The specific question was: “Is it true that all eels are spawned in the Sargasso Sea?” The long answer to the question involves Aristotle, Freud and Carlsberg beer. The short answer is that all European freshwater eels (Anguilla anguilla) are spawned in the Sargasso Sea — an area of the Caribbean — as are all North American freshwater eels (Anguilla rostrata). But there are many other eel species in the world, some living in freshwater, some in the sea and some spending part of their lives in each, that do not spawn in the Sargasso.

So it’s not true that all eels in the world originate there but all freshwater eels in Ireland do. The same species is found all over Europe, in the near East and parts of Africa.

But these bald facts fail to do justice to the extraordinary story of the life cycle of the eel and of the scientists who devoted themselves to unravelling its mysteries.

Aristotle had a go at the problem and concluded that they were “born of earthworms” which grew from “the guts of wet soil”. This was pretty much accepted until the 19th century when scientists started to look for a more modern explanation. Sigmund Freud had a go but became so frustrated by his lack of success that he turned his attention to other things.

Then there was a breakthrough when a number of European scientists discovered that an odd little sea creature called Leptocephalus brevirostris was in fact the larval form of the eel. Italian scientist Giovanni Grassi clinched the matter in 1896 when he managed to grow a Leptocephalus into an eel in an aquarium.

Grassi was a brilliant scientist who also played a large part in the research that proved the link between malaria and a species of mosquito and he was also hugely patriotic about the newly-formed state of Italy. But, partly because of his patriotism, be became convinced that eels spawned in the Mediterranean.

Danish scientist Johannes Schmidt thought Grassi was wrong and, starting in 1904, set out to prove it. He had the advantage of massive funding from a foundation set up by the Carlsberg beer company. This paid for marine research vessels in which he cruised first the Mediterranean and then the north Atlantic looking for the missing pieces of the puzzle.

It took him until 1922 when he read a triumphant paper to the Royal Society of London that clinched the matter. The tiny larvae, shaped like willow leaves, take three years to swim, at great depth, across the Atlantic. In Irish coastal waters they turn into semi-transparent ‘glass eels’.

The next transformation is into miniature eels called elvers which ascend rivers and streams. Their bellies turn gold and, travelling at night, they can crawl across wet grass and burrow through sand to reach isolated lakes and ponds. They live with us for 10 to 15 years before their bellies go silver again and, on stormy winter nights, they descend rivers and streams tail first to complete the epic cycle.

They are of great economic importance from the moment the larvae turn into glass eels. A kilo of glass eels can fetch $150,000 on the Japanese market. But in the past 25 years the number of glass eels reaching Europe has dropped by over 90%. The species may be on the verge of extinction and the reason is not fully understood.

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