On a dark, rainy, and moonlit night, sexually maturing (15- 20 year old) eels leave their juvenile habitat for good. Before them lies ahead a 5 to 6 month-long journey destined for the Sargasso Sea — a large region south of Bermuda and the middle of the North Atlantic Ocean — where they will spawn for the first and last time in their life.
Both European and American eels spawn in the deep, salty waters of the Sargasso Sea. Why they embark on such an arduous journey is not fully understood. Perhaps the Sargasso Sea was their preferred spawning area at the time when continents lay closer to their breeding grounds. But to this day, no one has ever observed an adult eel in the Sargasso Sea.
The Sargasso Sea is wide and deep, making it costly and difficult to conduct investigations on eels there. Tagging studies on eels, of which there have been many, have thus far been unsuccessful in determining their final destination. Eels are likely to swim at 600 to 700 meter depths, says Caroline Durif (scientist at Norway’s Institute of Marine Research).
A mythical creature
There is still much that scientists don’t understand about eels. They have been viewed as legendary species as long as 2000 years ago, when neither eggs nor larvae from eels had been observed; Aristotle believed that eels emerged from mud and were neither male nor female. During the late 1800s, the first eel larvae were observed in the Mediterranean Sea. Since that time, eel larvae have been observed in increasing numbers further and further out into the Atlantic Ocean. A scientific breakthrough occurred when newly hatched eel larvae were collected and measured throughout the Atlantic Ocean. The smallest individuals were mainly found in the Sargasso Sea; thus, it was deduced that eels spawned in this area. The distance between the Norwegian coast and the Sargasso Sea is approximately 6,000 kilometers, and eels swim in deep water masses mostly at night.
Consequently, the next question arises: How is it possible for eels to find their way to the remote waters of the Sargasso Sea?
A magnetic sense used to navigate
Over the years, many theories on how eels navigate this long and difficult journey have been postulated, including that they use: the stars, the coastline, their sense of smell, and the ocean currents. At present, it is believed that eels can sense the Earth’s magnetic field, and use this sense as a compass to chart their course.
Eels require a finely-tuned system of navigation to identify the appropriate spawning area. When one draws a map using the Earth’s magnetic field as a starting point, it becomes apparent that it can lead to the Sargasso Sea. The Earth’s magnetic field can provide the necessary cues - compass orientation and navigation - needed to travel long distances in an environment with few, or no, alternate guideposts. This suggests that eels have developed a kind of mapping sense that allows them to orient themselves using the Earth’s magnetic field, says Caroline Durif.
In collaboration with colleagues, Anne Berit Skiftesvik and Howard Browman, Durif has studied magneto-reception theory in eels over a two-year period. The study was conducted at the IMR research station in Austevoll.
Durif C. M. F., Browman, H. I., Phillips, J. B., Skiftesvik, A. B., Vøllestad, L. A., et al. 2013. Magnetic Compass Orientation in the European Eel. PLoS ONE 8(3): e59212. doi:10.1371/journal.pone.0059212