Arriving at the mouths of northern Europe's cold rivers, minute eel larvae are on the verge of a major life transition. Jens De Meyer from Ghent University, Belgium, explains that when the tiny creatures leave the sea, they transform into transparent glass eels ready to embark on the tough journey upstream, where they spend the next 3–12 years and develop into sub-adult yellow eels. However, by the time they are fully grown and ready to begin the gruelling return journey to the Sargasso Sea, some of the eels have developed long narrow heads, while others have broad heads. ‘It is very interesting to find out how and when this dimorphism develops during the eels’ life cycle', says De Meyer. Speculating that the animals' head shapes could be influenced by their diet, De Meyer and Joachim Christiaens headed to sluice gates at the mouth of the Leopold Canal in Belgium to capture glass eels in order to find out how diet affects the animals' heads.
Dividing the eels into three groups, the duo put each on a strict diet; one received hard food, the second had a soft diet and the third group received a mixture of the two. ‘Selecting the diet was the most difficult part’, recalls De Meyer, explaining that it had to be varied enough to give the animals on the hard diet a good workout whereas the eels on the soft diet would consume the same nutrients while having an easier time. After searching the literature, De Meyer and Dominique Adriaens selected several species for the animals to dine on: ‘Then we observed their feeding behaviour’, De Meyer recalls, adding, ‘When they required biting or spinning behaviour, we considered it to be “hard food”; however, when the prey could be sucked in, we considered it “soft prey”’. Finally, they photographed the glass eels' heads once a month over 5 months and analysed the growth to see whether diet altered head development.
Amazingly, by the end of the test period, the team could see clear differences in the small eels' head shapes. The animals on the hard diet had wider heads that could accommodate larger jaw muscles for biting hard, while the eels that had been fed a soft diet had slender heads. ‘We did not expect such a rapid and pronounced diversification in head shape’, says De Meyer, adding that the differences had been thought to emerge after the animals had transformed into growing yellow eels. And when De Meyer investigated the eels that had been provided with a mixed diet, some of the animals had broad heads while others had narrow ones. ‘This suggests that when eels are given the choice, some prefer hard prey and others soft. This difference in prey preference could decrease the competition for food in eels and thus increase their survival’ he says.
However, De Meyer adds that there are concerns for the welfare of eel populations in Europe's rivers and he warns that the broad-headed animals may be at greater risk from pollution than the narrow-headed animals, because they are further up the food chain and consume a diet that has already accumulated more pollutants. He says, ‘Our study shows that this difference in the position in the food chain can be present very early in the eels’ life cycle', and he hopes that a better understanding of how diet affects these animals could be vital in re-establishing eel populations in Europe's depleted rivers.
