Fat but Travelling Light

Wildlife biologist Guy Verreault describes a remarkable metamorphosis.

Photograph of a silver eel lying in a semi-circular position on sandy ground covered by a thin layer of water.

[Guy Verreault] Migrating eels don’t need to eat.

All the organs that were useful to them during the 20 or 30 years when they lived in fresh water are now totally useless. Why maintain a system that no longer serves a purpose, that’s no longer of any use? We know that the eels will die in the end. Their stomach has atrophied. A stomach that was used for years to eat and digest food is now of no use.

A close-up head and shoulder view of Guy Verreault appears on the screen. He is facing the camera and gestures with his hands from time to time as he speaks.

When we see eels passing through here in the fall, their stomach is simply resembles a small piece of string. There’s practically nothing in it. In other words, their stomach has atrophied. Their gonads, on the other hand, take up a lot of room. Well, the eels are going to reproduce. And even their anus – because eels have an anus for evacuating waste – is completely closed. One way to find out if an eel is migrating is to look at its anus. If its anus is completely closed, you know the eel is migrating.

Eels have a remarkable physiological adaptation that is not found in any other type of fish. It’s truly specific to eel. It’s precisely all of these adaptations that have once again enabled eel to be so successful.