Between the vastness of the oceans and the meandering rivers, eels complete a life-spanning migration that spans thousands of kilometres. It has taken hundreds of years for scientists to piece together the outlines of the eel's life cycle, but the details of their migration routes, navigation mechanisms and reproduction remain one of the most intriguing mysteries in biology.
The life cycle of the Anguilla anguilla, for example, can be divided into five stages. Eggs and Leptocephalus. in the Sargasso Sea in the North Atlantic, eel eggs hatch in 400 metres of freezing cold water. The larvae are shaped like a transparent willow leaf and their flattened bodies float with the currents, feeding on plankton. This stage lasts for about two years as they travel across the Atlantic with the warm Gulf Stream, eventually reaching the shores of Europe. Glass Eel When the larvae approach land, their bodies gradually shrink into small, slender, transparent eels. At this point, they must swim against the current, crossing estuaries, locks and even waterfalls to enter freshwater rivers. This process has a mortality rate of up to 90 per cent, with only a few ever reaching their habitat. Yellow Eel In rivers and lakes, eels lose their transparency and take on a yellow-brown camouflage. They lurk in the silt and emerge day and night to feed on fish, shrimp and insects, a stage that can last up to 20 years. Silver Ee, at sexual maturity, eels' eyes dilate, the sides of their bodies glow with a metallic sheen, their digestive systems deteriorate, and their body fat reserves surge. They stop eating and begin to migrate towards the ocean. Upon returning to the Sargasso Sea, the eels lay their eggs in the deep sea and then die of exhaustion. Their offspring will repeat the same journey, but no one knows how they will be able to pinpoint the spawning site - there are no landmarks in this part of the ocean, and even humans rely on satellites to navigate.
So how do eels travel thousands of kilometres to find their spawning grounds? Some hypothesised that they could sense geomagnetic fields or chemical gradients in seawater, but experiments have shown that even when transplanted into unfamiliar waters, eels can still adjust their routes, as if they have an inherited 'nautical map' in their brains. It was only in 2010 that scientists photographed the eggs of Anguilla japonica for the first time in the Sargasso Sea, and the reproductive behaviour of European eels has not yet been directly observed. Their mating patterns, spawning depths, and even whether they act collectively, remain open questions. It takes six months to a year for a European eel to return from freshwater to the sea, during which time it does not eat at all. How do they sustain their energy? The study found that the muscle tissue of the silver eel breaks itself down to provide energy, a mechanism of 'autophagy' that is almost tragic.
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