European eels, besides being delicious, have mystified biologists for more than a century. They spend their adult lives in estuaries and rivers, and head to the Sargasso Sea near Bermuda to reproduce. Their tiny transparent larvae then hitch a ride back to Europe on the Gulf Stream. But eel populations have been mysteriously dropping, prompting desperate measures to replenish their numbers.
Now, researchers have a clue about one peril young eels face during their journey: hungry fish. The larvae were once considered too difficult for most predators to spot and catch, but a new study that looks at DNA traces in the guts of fish near eel-breeding waters suggests at least six marine species can make quick work of baby eels.
“The study shows that although eel larvae are likely difficult for predators to see, they do contribute to ocean food webs as prey for other species,” says Michael Miller, an eel expert at Nihon University in Fujisawa, Japan, who was not involved with the work.
The European eel (Anguilla anguilla) was once quite common, but its numbers have declined precipitously in the past 45 years. Moreover, the number of larvae that finally make it to Europe as “glass eels” has dropped by 90%, leaving some to wonder what might be happening to the larvae. Was something eating them up?
That didn’t seem likely. Eel larvae—which are about the size of a small willow leaf—had been detected only once, in the late 1800s, in the guts of other fish. It could also be that, once swallowed, they decayed so quickly that they disappeared without a trace. Intact, the eels are still difficult to find, “even in a tray of water,” says study co-author Mads Reinholdt Jensen, now a graduate student at Aarhus University in Denmark. As a result, researchers studying eel declines have looked at everything—everything, says Jensen, except who eats the eel larvae.
Instead of searching for the larvae themselves, Jensen and his colleagues at the University of Copenhagen looked for their DNA in 62 fish collected and quickly frozen in 2014 by a Danish team that had searched in vain for spawning adult eels in the Sargasso Sea. Jensen’s team had to develop special eel-specific molecular tags that would latch on to any eel DNA in a fish’s gut. Ultimately, the researchers verified European eel DNA in six of the fish, each of them a different species, they report in the August issue of Marine Biology.
That was a surprise to Tracey Sutton, a marine ecologist at Nova Southeastern University in Fort Lauderdale, Florida, who was not involved with the study. “It goes against the dogma that these fishes prey primarily on crustaceans,” he explains. “It shows a new [food web] pathway we didn’t have before.”
The discovery is a “novel preliminary finding” that could help reveal the “true diets” of these predatory fish, and their role within the oceanic food web, says Ryan Saunders, a marine ecologist with the British Antarctic Survey, in Cambridge, who was not involved in the study. The Sargasso Sea is notoriously low in nutrients, he notes, and not much is known about the few fish that live there.
The DNA techniques used in the new study could “revolutionize the way we study fish diets and food webs,” Saunders says. But Jensen notes they still won’t help the researchers estimate how many larvae the fish were eating or what proportion of the predators’ diet the eels represented. Calculating those numbers will be important to concluding anything about the role the fish are playing in the eel’s decline, Miller says. And Sutton is skeptical: “Predation on larvae is a normal thing,” he says. “The dramatic decline is almost surely human-induced.”