What Are Glass Eels? The Eel Life Cycle Explained

The five stages of the eel life cycle

Freshwater eels such as the European eel change shape, color and lifestyle so completely that early naturalists mistook some stages for different animals. The table gives typical figures for the European eel.

Leptocephalus: the leaf-shaped larva

European eels hatch in the Sargasso Sea in the western North Atlantic as leptocephali, a name meaning “thin head”. These larvae look nothing like eels. They are flat, transparent and shaped like a willow leaf, with a tiny head and forward-pointing teeth, and their bodies are filled with a watery jelly that helps them float.

Leptocephali drift northeast on the Gulf Stream and the North Atlantic Drift, feeding on tiny particles of marine snow and small jellyfish-like animals. Estimates of how long this drift takes range from under a year to about two years. Until the 1890s, scientists thought they were a separate species of fish; how the truth was discovered is told in Where do eels come from?

Glass eels: arriving at the coast

When a leptocephalus nears the edge of the European continental shelf, it goes through a dramatic metamorphosis. Its flat leaf shape becomes a rounded, eel-shaped body, its larval teeth are lost and it actually shrinks in length and weight. The result is a glass eel: a perfect miniature eel with no pigment, about 6–8 cm (2.5–3 in) long and weighing only about 0.3 g, so that roughly 3,000 fit in a kilogram.

Glass eels reach Europe’s coasts mainly in winter and spring. They arrive first on the Atlantic shores of Spain, Portugal and western France, from autumn onward, and later farther north, reaching Britain around spring and Scandinavia and the Baltic later still. Many feed little at this stage and live on their energy reserves.

To move upriver against the current, glass eels use the tides. On a rising tide they swim up into the water and are carried inland; when the tide turns, they sink to the bottom and hold on until the next flood. Huge numbers can pour into an estuary on a single night, and they can climb damp walls and weirs, sometimes piling over each other in masses to get past obstacles.

Elvers: turning dark and moving upstream

Within a few weeks of reaching brackish or fresh water, glass eels begin to develop dark pigment, starting with spots that spread over the body. As they darken and start feeding actively on tiny insect larvae, worms and crustaceans, they become elvers. Terms vary, though: in Britain, the word elver is often used for glass eels as well.

Elvers spread out into rivers, streams, lakes and coastal lagoons. Some stay in estuaries or the sea, while others travel hundreds of kilometers inland, climbing past small waterfalls and weirs. Dams that are too high to climb can stop them completely, which is one reason eel passes are being added to barriers across Europe.

Yellow eels: the long growing years

Once settled, the young eel enters the longest stage of its life. Called yellow eels because of their yellowish-brown flanks and belly, these eels feed mainly at night on insect larvae, snails, crustaceans, small fish and frogs, and hide by day under stones or in mud.

The yellow eel stage usually lasts about 5–20 years, depending on temperature and food, and can be much longer in cold northern waters. During this time the eel’s sex is settled. Eels growing in crowded waters, such as estuaries, more often become males, while eels at low density far upstream more often become females, which grow much larger.

Silver eels: the return journey

When a yellow eel has stored enough fat, it transforms once more into a silver eel. Its back darkens, its sides and belly turn silvery white, and its eyes grow much larger and become more sensitive to the dim blue light of the deep ocean. Its pectoral fins lengthen and darken, its skin thickens, and its digestive system shrinks until it can no longer feed.

Silver eels leave their rivers mainly in autumn and early winter, often on dark, rainy nights when the water is high. They then swim roughly 5,000–10,000 km (3,000–6,000 mi) to the Sargasso Sea, living on stored fat, where they are thought to spawn once and die. Their eggs hatch into new leptocephali, and the cycle begins again.

Why are glass eels so valuable?

Eels have never been bred commercially through their whole life cycle in captivity, so every farmed eel starts life as a wild-caught glass eel. Demand from eel farms, especially in East Asia, has made glass eels one of the most valuable fish in Europe, worth hundreds to thousands of euros per kilogram.

That value has fueled both overfishing and large-scale smuggling, at a time when the number of glass eels reaching Europe has fallen to a few percent of its levels in the 1970s. The full story of the eel’s decline is told in Why are European eels endangered? Glass eels are true eels; the very different animal that only borrowed the name is explained in Is an electric eel an eel?