Eel Migration and Energetics: A 2300-Year Detective Story

European eels (Anguilla anguilla) are currently believed to breed in the western Atlantic Ocean near Bermuda, although the distance between Europe and this purported breeding area is about 5500 km (3400 mi). Skeptics have questioned whether adult eels—which do not eat in the ocean—could swim that far and still have enough stored energy for reproduction. Recently, investigators placed eels in a water tunnel for a simulated 5500-km migration. The eels had to swim for almost 6 months to cover the equivalent of 5500 km. Not only did they succeed, but they did so at a remarkably low cost. As seen in the figure, the metabolic rates of the swimming eels tended to be only about twice as great as those of resting eels. The migrating eels achieved a cost of transport that was only about one-fifth as great as expected for their body size, pointing to exceptional swimming efficiency. As a result, they lost only about 20% of their body weight and would have had reasonable energy stores remaining on arrival in Bermuda.

European eels have been a focal mystery in marine biology ever since the time of the ancient Greeks, when Aristotle, in his writing on natural history, highlighted the literal statement that no one in Europe had ever seen sperm or eggs in a European eel or seen the eels breed! Box Extension 9.3 outlines the long story of how scientists ultimately concluded that the eels do in fact breed like other fish, but do so far from Europe. This box extension also discusses the latest efforts to confirm this hypothesis by direct observation.

Aristotle believed in spontaneous generation. In arguing his point, he used European eels as one of his prime examples. Inasmuch as the eels had never been seen to breed in Europe, and yet young eels appeared each year in coastal marshes, he concluded that young eels originate from marsh mud. The key to solving the mystery did not come until the 1890s, when two Italian marine biologists discovered that the larvae of European eels are creatures that have absolutely no resemblance to the adult eels. The larvae had long been known to science, but they had been classified as an entirely different species of fish. Knowing what the larvae looked like, Johannes Schmidt (1877–1933), a Danish fisheries scientist, painstakingly established in the years before and after World War I that eel larvae could be found throughout the eastern Atlantic Ocean. Moreover, as he looked farther and farther west from Europe, he found smaller and smaller larvae. In this way, Schmidt tentatively solved the eel problem. He argued that adult European eels swim to waters in the western Atlantic near Bermuda to breed (explaining why Aristotle never observed breeding). Then the larvae make their way back to Europe, growing as they get closer.

Metabolic rate measured on each day of a 6-month, 5500-km simulated migration Nine eels (Anguilla anguilla), 0.7 m in average body length, swam for 173 days in a water tunnel. They were compared with 6 resting eels. The eels were not fed. The temperature was 19°C. (After van Ginneken et al. 2005.)

To this day, no one has observed the migration of adults or larvae directly. Thus there remains a small chance that Schmidt’s hypothesis is wrong. In 2009, a group of biologists applied a method similar to that used for studies of migrating ospreys and godwits, earlier discussed. They successfully put satellite-monitored transmitters on 14 adult eels on the shores of Ireland. Most of these eels started swimming in the general direction of Bermuda. However, the transmitters failed one by one, or eels were eaten, and none of the transmitters lasted for more than a quarter of the way to Bermuda. Thus, to this day, no European eel has been directly observed to swim to Bermuda from Europe to spawn.

References

Van Ginneken, V., E. Antonissen, U. K. Müller, R. Booms, E. Eding, J. Verreth, and G. van den Thillart. 2005. Eel migration to the Sargasso: remarkably high swimming efficiency and low energy costs. J. Exp. Biol. 208: 1329–1335.