Glass eels use an inner compass to navigate the tides

June 19, 2017, 8:22 a.m.
Juvenile fresh water eels are called glass eels because their bodies are transparent.
Photo: InesWiehle /iStockPhoto

Glass eels get their name for an obvious reason, but there's much about these tiny European eels (Anguilla anguilla) that are less apparent. They are actually baby freshwater eels, and they're are only see-through as youngsters. During the larval stage of life, they travel across the Atlantic in a way that had scientists baffled for a long time.

How exactly are they able to find their way from the sea to the coastal estuaries where they live as adults? Thanks to a recent study by the University of Miami's Rosenstiel School of Marine and Atmospheric Science in collaboration with the Norwegian Institute of Marine Research's Austevoll Research Station, that mystery is closer to being solved. They use the planet’s magnetic field like a compass and an internal clock to track the tides that take them to the coast.

The study in Science Advances explains how they reached this conclusion:

To test this hypothesis, we used a unique combination of laboratory tests and in situ behavioral observations conducted in a drifting circular arena. Most (98%) of the glass eels tested in the sea exhibited a preferred orientation that was related to the tidal cycle. Seventy-one percent of the same eels showed the same orientation during ebb tide when tested in the laboratory under a manipulated simulated magnetic field in the absence of any other cue. These results demonstrate that glass eels use a magnetic compass for orientation and suggest that this magnetic orientation system is linked to a circatidal rhythm.

The study authors wanted to look at European eels because they cross the Atlantic twice in their lifetime — once as adults and once as larvae. The fact that larvae can cross an ocean while mostly drifting has been puzzling, and looking at the European eel’s use of geomagnetic sense could reveal clues about how other species navigate using Earth’s magnetic field.

But that’s only one reason the findings are important. Glass eels are severely overfished in this life stage, due to a high demand in Asian markets. The European eel is considered critically endangered.

The study’s findings could assist in improving conservation efforts such as relocation programs.

According to the New York Times:

It’s unknown whether facing south during ebb tide is a universal behavior, or whether glass eels in different regions orient differently, said Caroline Durif, a senior researcher at the Institute of Marine Research in Norway and an author of the study.
If the latter is the case, she added, that might have implications for restoration programs that relocate glass eels from heavily to sparsely populated areas. It’s possible relocating them could "disturb their orientation system," Durif said.

Knowing how glass eels move through the world as they swim to adulthood brings us a step closer to ensuring their survival.