For centuries, eel reproduction was one of the greatest mysteries in the world of biology. Scientists of the time knew that eels lived in freshwater rivers and oceans, but no one could determine their origin or the manner in which they reproduced. Unlike other types of fish, the reproductive organs of eels have never been seen in freshwater habitats. Furthermore, there were no instances of eels being observed during their mating activities. For thousands of years, scientists of the time proposed various theories regarding the origin of these eels.
One of the first individuals to investigate the biology of the eel was the Greek philosopher Aristotle. However, because Aristotle never observed eel eggs or young eels being born, he theorized incorrectly that eels developed from mud in the rivers. This notion was accepted by others for centuries until scientists began to investigate the topic of eel reproduction more closely.
The mystery behind the reproduction of eels is primarily due to the life cycle of these creatures. Most freshwater eel species are catadromous, which means they live in freshwater habitats but travel to the ocean to reproduce. Examples of these species include the European eel and the American eel. Scientists have only ever seen adult eels living in freshwater habitats. Because juvenile eels do not possess the reproductive organs until the final stage of their lives, scientists have difficulty classifying these species. Furthermore, since both male and female reproductive organs are underdeveloped while the eels live in rivers, scientists could not determine the sex of these eels.
However, scientists began to uncover the mystery of how and where eels reproduce in the late 1800s and early 100s. Danish biologist Johannes Schmidt began to investigate the life of the baby eel species found in the Atlantic Ocean. During his investigation, he discovered that the youngest species of baby eels lived in the Sargasso Sea. This discovery allowed other scientists to determine that the European and American eel species traveled thousands of kilometers to reproduce in the Sargasso Sea.
Prior to reproduction, the freshwater eels undergo a transformation into silver eels. These silver eels develop larger eyes to view the ocean, their skin changes color to metallic tones, and their digestive systems shut down as they do not eat during their journey to the ocean. Instead, they rely upon fat reserves in their bodies. Additionally, their reproductive organs mature during this transformation.
These silver eels travel great distances to reach their spawning grounds. The European eel species can travel over 5,000 kilometers from their habitats to the Sargasso Sea. The American eel species also make this migration from the eastern coast of the United States to the Sargasso Sea. These eels use information from ocean currents, magnetic fields, temperature changes in the water, and chemical signals to navigate these distances. Scientists are still investigating how the eels perform these migrations.
When they reach the Sargasso Sea, the silver eels release their eggs and sperm outside of their bodies. This process is known as external fertilization and is performed by many fish species. Furthermore, the adult eels die after reproduction; they do not reproduce multiple times during their lives. This phenomenon is referred to as semelparity.
The fertilized eggs grow into transparent, leaf-shaped larvae, called leptocephali. Unlike the adult eel species, these young eels have a completely different physical appearance. This led scientists to believe that these larval eels belonged to other species than the adult eel. These leptocephali eel babies travel to continental areas over periods of months.
Once they reach continental areas, the eel larvae change into juvenile eels, referred to as glass eels. These young eels are see-through and small in size. After maturing into elvers, they move into freshwater rivers and lakes. Once they grow into yellow eels, they reach the adult stage of the eel species.
The mystery of the reproduction of the eel species is due to scientists never having seen the adult eel species reproduce in their natural habitats. Additionally, the reproductive activities of these eels occur in deep oceans, thus making it hard for scientists to view these activities in their natural habitat.
However, scientists have gained new insights into the reproductive habits of eels with the improvement of technology. For example, scientists use satellite tracking devices to follow the migration of eels. Furthermore, DNA analysis is used to classify eel species. Additionally, researchers use underwater sensors to monitor these eel habitats.
Scientists have also learned that eel populations are rapidly declining. Overfishing, habitat destruction, pollution of the ecosystem, dams in the rivers, and the effects of climate change are all threats to the eel species. Dams specifically prevent the movement of eel populations from freshwater habitats to the ocean.
The European eel species has drastically declined in population, leading to its classification as critically endangered. Various conservation efforts are being made to protect these eels and their habitats. Such efforts include the construction of dams to allow the passage of eels by dams, limiting fishing of these species, and the restoration of wetland areas where eels live.
Overall, the life cycle of the eel species is still not entirely understood by scientists today. However, researchers continue to investigate the behavior of these eels to determine how they perform their migrations, how they are affected by changes in the environment, and where the spawning areas are located in the Sargasso Sea. The mystery of the eel species is one of the many mysteries of the life of the planet’s natural world.
In addition, reproduction in eels remains one of the greatest mysteries in the field of biology. The reproduction of eels occurs very far from their homes in the freshwater habitats. Scientists have never been able to find evidence of reproduction of eels or their reproductive organs. Hence, the theories of scientists about the reproduction of the species were all incorrect. However, researchers such as Johannes Schmidt have uncovered information about where and how the species reproduce. It was also discovered that these eels travel enormous distances to the Sargasso Sea. Today, scientists recognize the eel species as one of the most mysterious and remarkable species in the animal kingdom.
“Nature’s Most Mysterious Fish: Facts about Fascinating Freshwater Eels.” World Wildlife Fund, 2019, www.worldwildlife.org/resources/explainers/natures-most-mysterious-fish-facts-about-fascinating-freshwater-eels/.
Briggs, Helen. “Ancient Eel Migration Mystery Unravelled.” BBC News, 15 Oct. 2022, www.bbc.com/news/science-environment-63259738.
Ellis, Sophie. “It Lasted Thousands of Years and Puzzled Aristotle and Freud – Is This the Biggest Animal Mystery of All Time?” Discover Wildlife, 20 Feb. 2026, www.discoverwildlife.com/animal-facts/fish/eel-reproduction-mystery.