European eel

At night, in a Mediterranean lagoon, a brown silhouette undulates over a sandy-muddy bottom before returning to the shadows. The European eel seems made for this discreet setting, yet some individuals can complete their entire growth phase there without ever truly living in a river.

Then comes silvering: the belly pales, the back darkens, the eyes enlarge, and the digestive tract regresses. The fish is preparing to leave its coastal world.

A study published in 2016 provided the first direct proof that silver eels departing from southern France could cross Gibraltar: two tagged females reached the Atlantic and traveled more than 2,000 km from their starting point. In the open sea, their behavior shifts once again. The tags show them ascending several hundred meters at night and descending deep during the day, a cycle repeated throughout the migration.

The oceanic journey then relies on the energy reserves accumulated before departure. A study published in 2022 finally provided the first direct proof that an adult eel could reach the presumed spawning area in the Sargasso Sea.

Yet, after more than a century of research, neither an egg nor a spawning adult has ever been directly observed there. Somewhere in this coastless sea, the rendezvous toward which it crosses the Atlantic remains still invisible.