The story of the eel was a mystery for a long time and much of still is today. Aristotle did the first known research on eels, stating that they are born of "earth worms", which emerged from the mud with no fertilization needed—just from the "guts of wet soil". For a long time, nobody could prove that Aristotle was wrong. Later scientists believed that the eelpout Zoarces viviparus was the "Mother of Eels" (German name "Aalmutter").
In 1777 the Italian Mondini found the gonads and showed that eels are fish.
Until 1893 a transparent, leaflike two-inch ocean creature was named as a separate species: Leptocephalus breviroshis (from the Greek leptocephalus meaning "thin- or flat-head"). But then, in the Mediterranean Sea, the Italian zoologist Giovanni Batista Grassi observed the transformation of a "Leptocephalus" into a round glasseel, and in Roscoff the French zoologist Yves Delage proved in a laboratory that both are the same species. The name for the very unusual larval form, however, is still used today. The search for a location of spawning sites remains fruitless, and even today it is still a mystery—we know more about other planets than the first days of eel life!
A Dane, Professor Johannes Schmidt, from 1904 on directed many expeditions, largely financed by the Carlsberg Foundation, reaching out into the Mediterranean Sea and the North Atlantic. He postulated from the similarity of all leptocephali he found, that they all must originate from the same parent species. The further into the Atlantic Ocean he propelled research ships, the smaller the leptocephali he caught. Finally he ended up south of the Bermudas in 1922, where he succeeded in catching the smallest eel-larvae ever seen, in the Sargasso Sea, the part of the Atlantic which has long been one of the most mysterious places on the planet and still is—over 5000 metres deep. But the spawning itself has never been observed, nor has a ready-to-spawn adult ever been found in nature. From the size distribution, Schmidt formulated this part of the life history of the eel:
The larvae of European eels travel with the Gulf Stream across the ocean and, after three years, reach England at a size of 45 mm. The most famous place for large-scale collection of glasseels (for deli-food and stocking) is Epney at the Severn in England. They migrate up rivers, crossing all kinds of natural challenges, sometimes by piling up their bodies by the tens of thousands to reach even the smallest of creeks. They can wind themselves over wet grass and dig through wet sand underground to reach upstream headwaters and ponds, colonising the continent. In fresh water they develop pigmentation, turn into elvers and feed on creatures like small crustaceans, worms and insects. They grow up in 10 or 14 years to a length of 60 to 80 cm. They are now called yellow eels because of their golden pigmentation.
Short movie of migrating glasseels:
But then in July their instinct drives them back towards the seas, crossing even wet grasslands during the nights to reach into their rivers. Eel migration out of the Baltic Sea through the Danish belts was the basis of traditional fisheries with characteristic trapnets (Bundgarn).
Whether the adults can ever make the 6,000 km (4,000 mile) open ocean journey back to their spawning grounds north of the Antilles, Haiti and Puerto Rico remained unknown. By the time they leave the continent their gut dissolves, so they have to rely on stored energy alone. The body undergoes other dramatic changes as well: the eyes start to grow, the eye pigments change for optimal vision in dim blue clear ocean light, and the sides of their bodies turn silvery, best suited to be as invisible as possible during the long open ocean cruise ahead and past many waiting predators. Many now call these migrating eels "Silver Eel" or "Big Eyes".
The German fisheries biologist Tesch, one of the best recognised eel experts and author of the book "The Eel", equipped many expeditions with high-tech instrumentation to follow eel migration, first down the Baltic, then along the coasts of Norway and England, but finally lost the transmitter signals at the continental shelf when the batteries ran out. He—like Schmidt—kept on trying to persuade sponsors to give again. His proposal was to release 50 Silver Eels from Danish waters with probes that will detach from the eels each second day, float up and broadcast position, depth and temperature to satellite receivers, possibly jointly with an equivalent release experiment from the countries of the western coast of the Atlantic. So today our knowledge on the fate of the eels once they leave the continental shelf is based on three eels found in the stomachs of deep sea fish, a whale caught off Ireland and off the Azores and some experiments on fife eels.
Nobody knows why, but beginning in the mid-1980s, the leptocephalus and glasseel arrival in the spring dropped drastically, in Germany to 10%, and in France to 14%, of their previous levels - as reported even by conservative opinions. Also data from Maine and other North American coasts show declines - not as drastic yet. In New Jersey there is a project to monitor the glasseel migration with the most modern IT systems, an online in situ microscope. As soon as the communication lines become fast enough to carry video it will be possible to log into the system at the LEO sites.
In 1997 in Europe, the demand for eels could not be met for the first time ever, and dealers from Asia bought all they could grab. The traditional European stocking programs could not compete any longer: each week the price for a kg of glasseel went up another 30 dollars. Even before the '97 generation hit the coasts of Europa dealers from China alone placed advance orders for more than 250,000 kg, some bidding more than $1100 per kg.
The demand for adult eels has been exploding for the last three years, as of 2003. Germany imported more than 50 million dollars worth of eels last year. In Europe 25,000,000 kg are consumed each year, but in Japan alone more than 100,000,000 kg in 1996. New eel aquaculture plants pop up every month in Asia, high tech factories, and the capacity of the Japanese eel Anguilla japonica is long overrun by orders of magnitude. No eel could ever be raised artificially, investments of Japan government for artificial propagation just spawned 7 million dollars to 4 private companies. Eggs from artificially spawned eels (treated with hormones) have a diameter of about 1 mm, each female can produce 2 to 10 million eggs.
There are strong concerns, that the European population might be devastated by a new threat: by the recent infection with Anguillicola crassus, a foreign parasitic nematode of the swimbladder of eels. This parasite from East Asia (original host: Anguilla japonica) appeared in European eel populations in the early 1980s. Since 1995 it also appeared in the United States (Texas and South Carolina), most likely by uncontrolled aquiculture eel shipments. In Europe, eel populations are infected already by 30% to 100% with the nematode. Recently it was shown that this parasite inhibits the function of the swimbladder as a hydrostatic organ (Wuertz et al. 1996). For an open ocean voyager without the carrying capacity of the swim bladder (ca. 3 - 6 % of the bodyweight), there seems to be little hope to ever cross the ocean on stored energy alone.
There is another Atlantic Eel species: the American eel, Anguilla rostrata. First it was believed they were of the same origin, so close is their appearance and behavior, but some years ago genetic work distinguished two species. The spawning grounds must be very close together, rostrata probably more to the west, maybe some even within the Gulf of Mexico. These leptocephali exit the Gulf Stream earlier and immigrate the eastcoast states at an age of one year already between February to late April at a length of about 60 mm. They are now the goal of world wide operating dealers and since 1995 the cause of a "gold rush" revival, this time a "Wild East", but with real pistols and prices 5 times as high as silver.
Six years ago students and teachers and professors raised funding to build "eel ladders" in dams at creeks where the eels get stuck on their migration from the open ocean inlands. Many millions of glasseel could immigrate over these simple devices into the central USA.
One nice eel story claims that both eel families once upon a time lived on the peaceful shores of Atlantis, and even when America started to move westwards (Alfred Wegener), they gathered again once a year in Atlantis for some joint fun. So over many, many years each spring they gained a little bitty more skills in travelling, and in navigation as well, to overcome the four centimeters America drew away that past winter. Today they still meet in Atlantis, every year, make often jokes about that species of Homo sapiens, polluting their rivers and migration routes, which in the eel language have become coined Homo leptocephalus