In addition to being delicious, European eels have puzzled biologists for more than a century. They spend their adult lives in estuaries and streams, and travel to the Sargasso Sea near Bermuda to breed. The tiny transparent larvae then hitch a ride back to Europe on the warm Gulf Stream. But eel populations have been mysteriously declining, prompting drastic measures to restore them.
Now, researchers understand one danger young eels face on their journey: hungry fish. It was once thought that these young fish would be difficult for most predators to spot and catch. But a new study looking for DNA traces in the guts of fish near where eels breed suggests that at least six Marine species can quickly eat young eels.
European eels were once common, but their numbers have declined dramatically over the past 45 years. What's more, the number of young fish that end up in Europe as "glass eels" has dropped by 90 percent, leading some to wonder what they may have been through. Did some creature eat them up?
That seems unlikely. Young eels - about the size of a small willow leaf - have been detected in the intestines of other fish only once, in the late 19th century. Or it could be that once swallowed, they disappear so quickly that they leave no trace. In fact, the eel is hard to spot, "even in a basin of water," says co-author Mads Reinholdt Jensen, now a graduate student at Aarhus University in Denmark. Researchers looking for declining eel populations have examined every possibility except who is eating the young eels.
Instead of analyzing the young eels themselves, Jensen and his colleagues at the University of Copenhagen looked at DNA from 62 eels collected by a Danish team in 2014 and quickly frozen. The team looked for brooding adult eels in the Sargasso Sea, but found none. Jensen's team developed molecular tags specific to eels. These tags can "catch" any eel DNA in the fish's gut. In the end, the researchers examined the DNA of European eels in six different fish species. They report their findings in the current issue of the journal Marine Biology.
The latest findings came as a surprise to Tracey Sutton, a Marine ecologist at Nova Southeastern University in Florida who was not involved in the study. "This is contrary to the idea that these fish primarily prey on crustaceans." "The latest research shows a new food chain pathway that we didn't know about before," he explained.
