Glass Eels: See-Through, Slippery And Guided By Magnetism And Tides

2017-06-14T12:45:43+00:00 June 14, 2017|
At the early stage of their lives, European eels are known as "glass eels" because they are transparent. (Credit: Alessandro Cresci)

(Click to enlarge) At the early stage of their lives, European eels are known as “glass eels” because they are transparent. (Credit: Alessandro Cresci)

European eels are born and die in the North Atlantic Ocean, but spend most of their lives in rivers or estuaries across Europe and North Africa.

(From The New York Times / by Steph Yin)– In between, they traverse thousands of miles of ocean, where it’s often unclear which way is up or down. Scientists have therefore long suspected that these critically endangered fish use magnetism to help guide them.

A study published Friday in Science Advances shows, for the first time, that European eels might link magnetic cues with the tides to navigate. Studying juveniles during the crucial stage when they move toward land from open ocean, the authors found that eels faced different directions based on whether the tide was flowing in (flood tide) or out (ebb tide).

Changing orientation might help eels take advantage of tides to travel from the ocean to the coast, and into fresh water, more efficiently, said Alessandro Cresci, a graduate student at the University of Miami and lead author of the study.

Previous studies have shown that eels can detect magnetic fields, but how they use this sixth sense “has remained a matter of speculation” until now, said Michael J. Miller, an eel biologist at Nihon University in Japan who was not involved in the study.

When transitioning from sea to coast, European eels are in a stage of their lives where they are about the size of a finger and transparent along their bodies, thus the name “glass eels.”

Mr. Cresci’s group studied glass eels from the coast of Norway, observing the animals in the field by putting 54 slippery, see-through eels, one by one, in a drifting chamber equipped with cameras and compasses. When the tide ebbed, these animals generally faced south, but when it flowed in, they showed no consistent orientation.

The researchers then studied 49 of the same eels in laboratory tanks. They subjected some of the eels to reoriented magnetic fields, rotating magnetic north to the east, south or west. 

During the time of day corresponding to ebb tide, eels still tended to face whichever direction meant south to them under their assigned magnetic field, even though there was no change in the water around them — suggesting they paired a biological compass with an internal tide clock to maintain a consistent behavior. During flood tide, they tended to face magnetic north.

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