Eels Seven Thousand Leagues under the Sea Ethology

In 1980, a century-old “Anguilla dieffenbachia” was found in New Zealand

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In 1980, a century-old “Anguilla dieffenbachia” was found in New Zealand
In 1912, the Danish researcher Johannes Schmidt started taking things seriously. Not only did he confirm previous studies proving that eels originated from a larvae called leptocephalus, but at the end of four successive campaigns, he delimitated their spawning area: the Sargasso Sea, in the tropical area of the North Atlantic.

The biologist’s discovery left many questions unanswered. How do these tiny creatures manage to cross the Atlantic to reach our shores? How do they choose the rivers where they settle, generation after generation? What is the process of the three metamorphoses completed throughout their life cycle? Why do they decide to leave one day and how do they find their spawning area again? If, in one century, some of these mysteries have been solved, “today we still have many more questions than answers,” Eric Feunteun emphasizes. Elsa Amilhat, his colleague from the University of Perpignan, also adds: “With everything that science has managed to solve, it is unbelievable to what extent this animal remains mysterious.”

So, at the beginning, a larva. A transparent flat body, a few millimeters long. A small elongated head, large eyes and long teeth. “More like a willow leaf than a fish,” as described by Eric Feunteun. Coming out in millions from each egg-laying fish, these apparently inert creatures already display complex behavior. At night, they let themselves be carried away by the current at a depth of about 25 meters. During the day, they go down to 200 meters, to the “oceanic desert.” Little light, no plankton. And yet, they feed themselves. According to recent measurements made with nitrogen and carbon isotopes, they consume marine snow, dead matter agglomerate on which microorganisms grow. “The leptocephalus assimilates what nobody wants, Eric Feunteun insists. From the very beginning, eels settle in habitats that no other creatures occupy. And they go on like this all their lives.”

But for the moment, the eels have to keep going. Certainly, there is the powerful Gulf Stream, a real highway to Europe and the larvae will let it carry them for two to three years. At least, this is what oceanographic models applied to an inert particle showed for years. Except that leptocephali swim. In 2009, the Canadians showed that, under the best conditions, the larvae could complete their migration in just over a year.

This brings us closer to the results of the otolith analyses. Located in the ear of fish, these small concretions bear the traces of time, just like tree trunks. Since 1990, the University of Pau has been the world champion in the microscopic examination of striations less than 1 micron thick, reaching a rate of one a day … almost. Or between the first feeding, ten days after hatching, and the first metamorphosis, just before reaching the European shores, a total number of 200 to 300 striations have been found. So less than one year of migration? Two to three years? The controversy continues, supported by new articles every year. “And it is unlikely to be quickly resolved,” concluded a summary article published in the Journal of Biology in 2012.

The shore is getting closer, the leptocephalus has grown, measuring 8 cm. It stops feeding. It loses 25% of its weight and one eighth of its length. Its primary teeth fall, being replaced by permanent teeth. The body becomes refined, turning the “leaf” to a “stem.” The skin, translucent until then, becomes pigmented. The animal is ready for its elver life. It is at this stage that eels are most prized by fishermen, restaurant owners and other aquaculture farms.

As for elvers, they have two priorities: to wait for the right tide and to find the river, their river. How do they choose? Undoubtedly, using their incredible sense of smell. Laboratory studies have shown that elvers trace elements with an infinitesimal concentration, of the same order as the one detected by forensic science. Traces left by the passage of their elders have opened their way. Our “experts” can now rush to streams and rivers.

[Photo caption:] When leaving the Sargasso Sea, the leptocephalus larva is 7 mm long.

Little by little, elvers grow, their skin thickens. Eighty kilometers upstream, they have already become eels. At the end of their migration, they will become yellow eels. Nothing seems to discourage them. Clear or brackish waters, swamps, ponds or springs, from Lapland to Morocco for the European eel, and on four other continents for its sixteen cousin species, an eel adapts to anything. An obstacle in its way? It comes out of the water, avoids it by crawling or even climbing over it. Back from his mission in Vanuatu, in the Pacific Ocean, Anthony Acou’s eyes are still sparkling as he describes the achievements of the Anguilla megastoma: “It climbs up waterfalls and during its downstream migration, it goes back down tail first.”

Meanwhile, it will have “chosen” a sex. Indeed, the determination takes place around the age of 3. “We know the hormonal mechanism that guides this choice and the importance of environmental factors, explains Sylvie Dufour, biologist at CNRS (National Institute for Scientific Research) and Head of the Boréa Research Unit in Paris. Density, salinity, temperature, stress… But the impact of each of these factors has yet to be demonstrated.” Observing their distribution, we can see that saline waters, close to estuaries and a high population density, favor males. Conversely, females are pioneers and go further upstream. “As if the species had understood that, if they find a hostile environment it’s better to return to the sea as soon as possible, in order to pass on the genes, explains Eric Feunteun. Or males reach maturity faster, migrating downstream earlier. On the contrary, in a more favorable environment, an eel can take the “risk” of being a female and stay in the river longer, because if it reaches its spawning area, it is sure that its genetic heritage will be passed on in the form of eggs.”

How long do they live like that, waiting for the great departure? Two to … one hundred years, depending on the sex, species and latitude. In 1980, a century-old Anguilla dieffenbachia was found in New Zealand. In Europe, eels over 40 years old have been regularly observed in the Irish lakes. On November 19, Anthony Acou examined the ten specimens that he had kept for us from the day before. The largest eel measured 76 cm and weighted 950 grams. Using a gun, he introduced a magnetic tag that would allow for monitoring its evolution in the river. The next eel was smaller, the scale showing 57.5 cm and 459 grams. But it had overall completed its last metamorphosis: the silvering. Its eyes had quadrupled in size, its pectoral fin had developed and its skin had thickened. A very distinctive line separated its black back from its white silvery abdomen. This time, no animal badge was needed: captured and tagged in 2002, it had been found again twice during scientific fishing, a few kilometers upstream from the mill. Based on calculations, Anthony Acou estimated that it was 21 years old. “An old little beast,” he says.

An adolescent, still, that would complete its puberty during the most mysterious part of its life: the return to the spawning area. What we know is already amazing. During the silvering stage, the eel’s anus closes and the eel completes its great journey without feeding. How? Thanks to an extremely economical mechanism. The oxygen consumption studies conducted in a swimming tunnel concluded that no other fish could even come near its frugality. Tags also allowed for establishing its swimming profile.

Trying to avoid light and predators, our marathon runner spends its days at a depth of 600 meters, sometimes even deeper, facing cold temperatures and especially pressures that few living beings can withstand. But every night it climbs back up 300 meters, towards warmer waters. An alternation that allows silver eels to reach their spawning area at their sexual maturity. Neither sooner, nor later.

However, this is where knowledge stops and speculations begin. How do eels find directions? Do they follow currents, a salinity front or a magnetic “compass”? Do they use light and their high-performance eye that allows them to distinguish a sunbeam in places where others remain plunged in the dark? Do they take advantage of a memory recorded on their coming route that allows them to find their way back? Just as many questions which the European Eeliad Project (from the English “eel”), launched in 2009, hoped it could answer, by equipping 120 eels with Argos tags. Very ambitious, but financially burdensome (each tag costs 3,000 Euro), the device presented a risk: could our brave travelers withstand the 30 additional grams in their backpack?

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