Vol. 17: #49 • Tidbits Studies the Slippery Subject of EELS

Sometimes referred to as "the Devil of the Sea," eels are a type of elongated, ray-finned marine life ranging in length from several inches to the large European conger stretching to more than ten feet. Whether you regard them as repulsive snake-like creatures to be avoided or a delicacy to grace your dinner plate, we think you'll agree that they're a curious species that is certainly interesting!

STRANGE LIFE CYCLE

• The European eel has an unusual life cycle that begins and ends in the Sargasso Sea, as detailed by Patrik Svensson in “The Book of Eels.” The Sargasso Sea, named for the Portuguese word meaning seaweed, is a large area off the eastern coast of the U.S. where seaweed grows in abundance, providing food and shelter for sea life, including the eel.

• Eels hatch from eggs laid amid the seaweed. As larvae they look like a transparent willow leaf less than an inch long, and are commonly referred to as willow leaves. They drift across the Atlantic Ocean on the current of the Gulf Stream, heading towards the coasts of Europe thousands of miles away. They grow in size during the trip, which may take up to three years. By the time they reach Europe, they have transformed into the second phase of life, morphing into a transparent glass eel several inches long.

• From here each eel travels up a random brook or river, turning into a freshwater fish over the next six months as it chooses a pond, lake, swamp, or ditch as its new home. Now the eel enters its third phase of life, becoming a yellow eel which is muscular and strong. It grows up to a couple feet in length and weighs up to several pounds.

• Once the eel picks out its new freshwater home, it will stay within a few hundred yards of that spot, sometimes for decades, even up to 50 years. If caught and released elsewhere, it will quickly return back to that same place.

HOMECOMING JOURNEY

• Eventually, usually after 30 or 40 years, the yellow eel decides it’s time to procreate. It starts an epic journey back to the ocean, returning to the eastern coast of the U.S. and the Sargasso Sea. Along the way, it transforms once again for the fourth and final time, turning into a silver eel.

• Up to this point, the eel has never had any reproductive organs. There was no way to tell a male from a female. But as it crosses the ocean, the eel’s digestive system shuts down, sexual organs grow, and it develops reserves of either roe (eggs) or milt (sperm).

• At the rate of up to 30 miles per day while swimming at a depth of up to 3,000 feet deep, it takes at least six months for the eel to make it back to the spawning ground of the Sargasso Sea. Once there, it mates, and dies.

REPRODUCTION MYSTERIES

• No one has ever seen an eel mating. No one has ever seen an adult eel in the Sargasso Sea, either alive or dead.

• Marine scientists gained interest in the eel because of its lack of reproductive organs. An article in a German newspaper in the mid-1800s offered a reward for anyone who found roe in an eel. Many eels were mailed to the research facility but no roe was found.

• In 1876, a researcher sent one of his assistants to Trieste, Italy, to spend an entire month doing nothing but buying eels at the fish market and dissecting them in a search for sexual organs. That research assistant was Sigmund Freud, who was 19 years old at the time. He never found what he was looking for. It wasn’t until 20 years later that a sexually mature male eel was found in the Mediterranean Sea near Sicily.

• In 1904, Danish marine biologist Johannes Schmidt set out to discover where the eel went to procreate. He sailed around the Atlantic, dipping nets into the water and examining the catch looking for willow leaf eels. He deduced that if he could find them in smaller and smaller sizes, he would find the place where they were hatching. He paid trans-Atlantic freighters to dip their nets in the water, mark the location, and deliver the catch to him. He collected the data and closed in on the Sargasso Sea as their point of origin. Then he made a discovery.

• There are two species of eel which are virtually identical. Anguilla anguilla is the species that ends up in European waterways. Anguilla rostrata (with ‘rostrata’ meaning ‘curved’) is the species that ends up in American waterways. In the middle of the ocean, there were equal numbers of each species in its willow leaf phase. But the farther west he went, the more American species there were. He concluded that both species were breeding in the same spot.

• He found that the American eels grew faster than the European eels, becoming strong enough to break out of the ocean currents that sweep towards Europe. American eels undergo their first metamorphosis, turning into glass eels, after a single year, whereas European eels take three years to do so. The American eels are able to fight their way out of the prevailing currents and head west, going up rivers in the U.S., while the European eels drift with the current which carries them 4,000 miles east.

• In 1914, Johannes Schmidt found the smallest willow leaf eels ever discovered, measuring just a third of an inch long, and this led him to conclude that eels breed in the Sargasso Sea. He published his findings in 1923, summing up 20 years of research. In 1930 he was awarded the Darwin Medal.

EEL LONGEVITY

• In 1859, an eight-year-old boy caught an eel near his home in Brantevik, Sweden. He dropped it into a 15-foot well on his property, and covered the well with a stone slab. There was no way for the eel to get out, and no way for other eels to get in. Periodically he moved the slab aside to see if the eel was still there.

Brantevik is located on the southern tip of Sweden.

• When the man eventually died of old age, the eel was still alive. His children, grandchildren, and great-grandchildren monitored the eel too. The eel became famous, known as the Brantevik Eel. In 2009, a Swedish TV show documented that the eel was still alive, pulling it out to measure and photograph it, and putting it back. In 2014, the owner of the well found the eel dead, after having lived an astounding 155 years.

• In 1980, a research expedition caught a load of silver eels off the coast of Ireland that were on their way to the Sargasso Sea to breed. The scientists wanted to know how old they were. Researchers can tell the age of a fish by counting the number of rings on its otolith, which is an organ in its inner ear. They were surprised to find that the eels ranged in age from 8 to 57, yet they were all in the same developmental stage. No one knows what triggers this transformation or why it varies so much. We know they always leave their freshwater homes in the autumn, between October and December, and that the freshly hatched willow leaf larvae appear in the Sargasso Sea the following spring.

• In 2016, over 700 silver eels were tagged with transmitters and released all over Europe. Data was retrieved from 87 trackers. It showed that eels can swim without stopping or eating for six months. Some eels managed 30 miles a day, while others swam only two. They followed wildly different routes. But of all of them, not a single one could be documented as having made it all the way back to the Sargasso Sea.

• The study of these unique creatures continues to challenge scientists as they attempt to unravel the mysteries of their unusual life cycles. The answers they find may some day open even broader areas in the study of marine animals. □

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• Eel blood is toxic to humans and other mammals, but both cooking and the digestive process destroy the toxins. The toxin derived from eel blood serum was used by Charles Richet in his Nobel winning research which discovered anaphylaxis.

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