For a large portion of history, the life cycle of the migratory European Eel (Anguilla anguilla) remained a mystery. We owe what we today know about the critically endangered eels to Johannes Schmidt. Fishermen of North American and Europe had wondered, before Schmidt’s discovery, why they never caught baby eels.

A leptocephalus

In reality, the fishermen were catching eels, but they did not recognize them as eels because of the drastic change the animals go through during their juvenile and mature stages. Eels are reversely catadromous; they breed in the ocean and migrate in to fresh water to live, while salmon, for example breed in fresh water and spend most of their lives in the ocean.

Schmidt conducted expeditions in to the North Atlantic and Mediterranean sea, and found leptocephali; transparent, leaf shaped organisms. He would late discover that these small creatures were actually the eel’s initial phase. The deeper in to the Atlantic Schmidt travelled, the smaller the larvae got, until he finally caught the smallest leptocephali ever seen in a 1922 expedition off the south of Bermuda in the Sargasso Sea. After his research, Johannes Schmidt named the Sargasso Sea the most likely breeding grounds for the eels, though he never observed spawning directly.

Eel life cycle

The eel’s life cycle consists of a few characteristic stages. After spawning, a baby eel leptocephalus hatches. On a 300-day migration, wherein the transparent eels must resist predation and feed, likely by aid of sargassum weed, they drift toward Europe. At the European coast, the eels metamorphosize in to a transparent larval stage called a “glass eel” since this is the first stage where the eels are recognizable as eels. After entering fresh water estuaries and migrating upstream, the eel grows in to an elver, a miniature version of a yellow adult eel. After 5-10 years in fresh water the eels become sexually mature, become silver in colour, and begin migration back in to the ocean to spawn and die.

Glass eels

There is still much to learn about the European Eel, and today it is listed as a critically endangered species. The decline in the number of eels reaching Europe is thought to be due to overfishing, parasites, and barriers to migration like dams.

It is thought that there is a small population of European Eels in the Spittle Pond Nature reserve. Because of the main pond’s closeness to the ocean and its brackish water, eels are able to make a comfortable home there.