On summer nights, mature eels leave the European rivers in which they've fed for 12 years or more, and physiological changes: notably, their eyes enlarge and their guts shrink, so they can no longer feed.
Then they head out across the continental shelf - and vanish.
The eels' spawning has never been witnessed, nor have adults been caught on their migration.
Indeed, it was only in 1896 that a near-transparent, leaf-shaped fish was identified as the earliest developmental stage of the European eel: the leptocephalus.
Then, in 1922, Danish marine biologist Johannes Schmidt towed nets across the Atlantic, measuring the length of leptocephali captured in various locations. He found the highest concentration of the smallest larvae in the Sargasso Sea, close to the Bahamas, thereby determining the spawning area.
North American eels also migrate to the Sargasso, where their spawning may overlap with that of European eels - hybrids sometimes result.
Here they transform into transparent 'glass eels' and then become elvers, recognisable as miniature eels, congregating in estuaries before ascending to river headwaters, sometimes wriggling across wet grass to reach isolated waters where they feed and grow.
In 2009, researchers from the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) published details of the first 1,300km of the 5,000km outward journey.
The eels swim south towards the Azores; gyre currents then take them west. During the day they dive to 500m - the colder water slows their metabolism and helps them conserve energy.
How eels navigate is unknown. Japanese eels are sensitive to magnetic fields, which might guide them towards spawning grounds with magnetic anomalies. It's been suggested that the enlarged eyes of migratory adults are an adaptation to astral navigation - unlikely, since they don't all migrate at the same time.
Most of the 800 species of eels are oceanic; fresh-water eels are a subgroup comprising species that seem to have evolved to exploit feeding opportunities in river systems, but have retained their affinity for ancestral oceanic breeding sites. Why, we don't know...
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