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In the scientific community, the longest and probably the most interesting mystery has always been the reproduction of eels. The species were considered to be devoid of testicles, which meant they couldn’t create offspring.
And despite uncountable dissections, centuries of work, and different individually designed stories of their origin by well-known scholars, no one could actually find out their reproductive organs or produce a well-grounded answer.
But if eels lack balls, then how, even after centuries, are they still alive? It wasn’t discovered until recently when scientists made their way through some more complex and untouched aspects of eel reproduction and showed the world how eels have been surviving against all odds.
But before jumping onto that, let’s learn what eels are, where they come from, how they reproduce, and how the fancy inventions of different equipment in the scientific world made it easier for scholars to solve this riddle. Let’s start our journey to answer the question of how eels reproduce by asking our first question…
What Are Eels?
While there’s an odd similarity in the appearance of a snake and an eel, eels are actually fish that don’t even remotely resemble any other fish species you have ever known.
These impressively ugly yet slightly adorable species belong to the order Anguilliformes, which consists of 8 suborders and almost 800 species.
Despite being different from one another, eels share some common characteristics within their species. They feature a continuous dorsal fin that is linked with the caudal and anal fin. Some eels have small scales, while others have embedded or no scales at all.
Take, for example, European freshwater eels. They lack scales and breathe through their skin to survive. They are highly capable of surviving out of water for almost 48 hours. Not only that, they have the ability to bury themselves in mud in the winter season for hibernation – this is also helpful in summers when the ponds they live in happen to dry out.
Eels are found across Europe, Japan, and other countries. They live in both freshwater and saltwater and produce slime. The set sharp teeth fixed in their mouths make them horror creatures.
Where Do They Come From?
From ancient Greece to the late century, many well-known scholars and philosophers dedicated a remarkable period of their time to discovering the exact origin of eels. Many of them were in search of eel testicles so that eel reproduction could make sense. And the first one among them was probably Aristotle.
According to him, eels spontaneously came into existence when rainwater mixed with mud. Since no eye had ever seen eels mate or no brain could produce a solid answer that could justify the mysterious reproduction of eels, he suggested that eels didn’t have any reproductive organs. Instead, they came into being out of nothing.
This might sound illogical now since science has progressed a great deal, but the monumental influence of Aristotle didn’t allow any alternations in his theory for at least 2000 years.
And like Aristotle, Pliny the Edler also came up with his theory of eel reproduction. He said that through the shredded skin of other eels, new eels were born. This was possible when eels rubbed themselves against hard objects like rocks. The subsequent scrapping, as a result, came into being.
Some people were convinced that eels were born from sea foam or when sunlight hit the water in a specific way, while others thought that eels hatched on rooftops or came from the gills of fish. Some people even linked their birth to the bodies or beetles and when the hairs of a horse tail fell into the river.
While all these stories don’t make sense now, they were widely accepted during those simpler times.
Despite many efforts, it was still impossible to make sense of eel reproduction without the presence of testicles in them.
Like others, the famous psychoanalyst Sigmund Freud also found himself unable to stay away from finding a good set of eel testicles.
In 1876, he was a medical student at the University of Vienna when one day, he decided to travel to the city of Trieste and buy as many eels as possible. The reason behind this big investment was his never-ending interest in genitals. And his keen interest in balls helped him make at least 400 dissections within a month.
Well, his efforts didn’t go to waste, and he would finally locate gonads buried deep in the abdominal cavity.
Prior to this, no dissections of eels had what he was looking for. The reason is simple: he needed to understand how eel reproduction worked, and their lifecycle worked.
How Do They Reproduce?
How eels reproduce has always been a tough question for many researchers to answer. Even today, when we have discovered so much about the mysterious ways of eel reproduction, there are still so many unknown aspects to it.
But before we dive into that, I want to let you know that there are five stages of an eel’s lifecycle, with each having a striking difference from the other.
5 Stages Of Their Life Cycle
The lifecycle of an eel is divided into 5 distinct stages. Each stage plays an important part in helping you discover how eels mate and at which stage they develop sex organs. They are:
- Glass Eel
- Yellow Eel
- Silver Eel
You can see an illustration of the life cycle in the photo below (Wikipedia photo source)
The first stage of an eel’s lifecycle is Leptocephalus. In this phase, baby eel changes to larval, in which they develop bigger bodies than other fish larvae.
Seeing through them is easier since they have radically compressed bodies and largely reduced muscles and organs. They feature a simple stripe running down the middle and display their pane-like bodies with a clear gel. During metamorphosis, the compound GAG present in the gel changes into adult tissue.
They need marine snow and organic matter for proper growth.
The newly hatched larvae are usually transported on the ocean currents from the spawning grounds to intertidal areas, where they transform into glass eels.
For the first 17 months, eels have to travel toward estuaries. But in the 18th month, they change into an underdeveloped Glass eel. Once the transformation takes place, the juvenile eels have to enter river bays and grapple with swimming difficulties in the open ocean. This typically happens because of the flood on the coastline.
Glass eels can be defined as developing from Leptocephalus metamorphosis to fully pigmented eel. Upon arriving at the coastal area, they travel up rivers and streams. And as I mentioned earlier, they have to go through many natural challenges by sometimes climbing over hurdles.
Apart from physical transition, they go through internal changes in order to survive. Marine fish, when entered into brackish water, can explode since their cells begin to swell. But eels can overcome this difficulty because their kidneys are known to retain more salt to keep their blood’s salinity level intact.
Elver is the pre-adult stage of an eel’s life when they start featuring a dark yellow pigmentation on their bodies and stretch themselves up to 7.8 inches in length. Compared to adult eels, they are a little shorter. They are 2 to 3 years old in this phase.
These omnivores grow bigger and devour anything that can fit in their mouth.
They develop pigmentation in freshwater and turn into elvers. Here they feed on crustaceans, worms, and insects and live upstream for 10 to 14 years until they are fully mature and able to migrate to the Sargasso Sea. The typical size of an elver is 60 to 80 cm.
They push themselves over wet grass and dig through wet sand in order to reach upstream ponds. Since they are in millions, you can say that they actually colonize the continent.
Before turning into silver eels, the freshwater eel takes on a darker yellow shade. European eels can remain in these habitats for up to 20 years and feed on invertebrates and other fish. You can also see diversity in their body coloration, including brown, olive green, yellow and black colors.
Silver or Adult
Eels don’t grow sex organs until the last stage of their life cycle. The time that they spend in the watery lands of Europe is mainly a part of eel adolescence.
Silver eel is the final stage, in which they grow up to 31 inches in size.
They change their body color from yellow to silver which is actually a sign of sexual maturity. Once they turn silver, they leave freshwater and travel towards the tropical sea.
How Do They Reproduce In The Wild
The Sargasso Sea can be found in Bermuda, The Azores, and the West Indies. The Sargasso Sea is a two-million-square-mile span of the ocean that functions as a breeding site for millions of eels.
Most eels inhabit areas that are thousands of miles away from their spawning grounds. But when it comes to mating, they are determined to travel thousands of kilometers to the Sargasso Sea.
Eel species spawn through external fertilization when the females release millions of eggs and get them fertilized by the males’ sperm. After this, they die. The eggs then get fertilized and go through the early stages I talked about earlier in the article. Then they drift out of the Sargasso Sea and chalk out their trip to North America and the European continents.
There are different types of eels that are found around the world.
European eel or freshwater eel is a common type of eel that lives up to 85 years in the wild and almost 55 years in captivity.
They feature a pair of small pectoral fins and snake-like bodies. Since they are catadromous fish, they spend their adult lives in freshwater rivers and streams. Once they’re old, they travel to reach the Sargasso Sea.
As soon as they enter freshwater streams, European eels go through a transformation. They gain pigmentation and become elvers. And after spending 5 to 20 years, they gain weight and develop yellow undersides, known as the yellow-eel stage.
European eels don’t stop traveling upstream until they become sexually mature.
Shortfin and Longfin
In New Zealand, you can find two common types of eels: The Longfin eel and the Shortfin eel.
Like European eels, longfin and shortfin eels have longer lifespans, and they can be found in freshwater streams and lakes.
Longfin eels have lengthy fins. The dorsal fin is almost two-thirds the length of the body and begins towards the head than the bottom. In shortfin eels, you can see similar-sized fins. But there’s a difference between these two types. When a longfin eel bends, it gets wrinkles inside each band, whereas the skin of the shortfin eel remains smooth.
Females are larger and live longer than males. Longfin eels breed only once in their lives by traveling thousands of miles from New Zealand to their spawning grounds. Every female can produce 1 to 20 million eggs, making the eel population stronger.
Like European eel species, Longfin eels are also omnivores and can attach their prey without missing a Chance.
Japanese eels are mainly found in Japan, Korea, China, and Vietnam. Similar to other types of eel, Japanese eels also spawn in the sea and spend parts of their lives in freshwater.
When it is time to breed, these eels leave their freshwater habitats and travel to the ocean near the North Equatorial Current in the western North Pacific.
The unique trait of Japanese eels is that they can cover thousands of miles without nutrients. They have the ability to collect oils in their bodies before migrating out.
These eels feature a dull grey, brown, and greenish top and a white underbelly. The color of their body primarily depends on the environment.
Japanese eel populations are considered an endangered species by IUCN because their population is constantly decreasing.
The African Longfin
The African eel is another common type of eel that features olive, gray, and black body color with a light underside.
They are known to inhabit areas with a fast current and can be found in the western Indian Ocean of Africa. They feed on fish, crabs, and carrion.
Difference Between Freshwater and Saltwater
Eels can live in both freshwater and saltwater. The environment they live in determines the texture of their body, size, and color. Some eels are found at sea, while some can live in shallow waters. You can also find eels inhabiting sandy areas where they dig through mud or live amongst rocks.
The difference between saltwater and freshwater eels is the texture as I mentioned above. Eels living in freshwater tend to have a firmer texture, while saltwater eels are softer and leaner.
They also tend to be different in taste.
How Do They Survive Migration?
How these eels survive the thousands of miles to the Sargasso Sea is also very interesting. Unlike juveniles traveling the same distance, adults do not stop to eat. To accommodate this, their stomachs degenerate.
The blood vessels around the swim bladder also increase so that the eels are supported while they swim. Along with that, their eyes double in size, becoming more sensitive to blue waves of light which allows them to see in the dark.
European eel migration is one of the longest migrations of any marine creature. They spend a larger part of their lives in rivers and lakes but head back to the sea to spawn once in their lives.
They usually start migrating in the autumn and travel to the Atlantic ocean to breed. As per research, eels cover 3000 miles or 4,800 kilometers to reach the Sargasso Sea.
European eels have the longest migration compared to other eel species. Since most freshwater fish species can’t survive these environmental changes, you might wonder how on earth eels travel so far and still stay healthy.
Well, unlike the baby eel, the adult eel doesn’t need to stop and fill its stomach during its journey. Their stomachs degenerate. Their eyes also get bigger to help them see in the dark while the blood vessels around their swim bladder also increase so that they can swim freely.
And since they have to travel through saltwater and freshwater, their kidneys change in order to hold more salt and maintain salinity in the blood. Because if their kidneys remain the same, their cells won’t be able to handle these extreme changes.
How Do They Reproduce In Captivity?
No one has ever seen captive eels breeding in a home aquarium. They have a complex life cycle, and their sexual organs only develop at the last stage. Also, they spend months and years traveling to the spawning ground present in the Sargasso Sea, where once eel eggs and sperm are realized, the fish die. Those free-floating eggs get externally fertilized.
Many people tried different artificial ways to breed mature eels in a home aquarium but to no avail.
Eels are extremely hard to kill. They have a high capacity to survive profound environmental changes and can live up to 100 years.
They are poisonous and can swim for hours even if you cut off their heads.
The stubborn nervous system of an eel, as well as its body’s ability to adapt to changes, help it survive anything that they come across.
Do They Have A Reproductive System?
The reproduction of eels has always been mysterious. Many researchers, despite countless dissections, couldn’t find any reproductive organs and eggs in the fish. Eels lack testicles when they are young. Once they reach the last stage of their life cycle, they grow testicles and release sperm. They have to travel a long distance from their freshwater habitats to the Sargasso Sea to spawn.
How Do They Reproduce In Captivity?
Eel reproduction in captivity is not possible yet. Instead of mating using sexual organs, the females release millions of eggs and get them externally fertilized in the Sargasso Sea where their breeding ground is present. It takes them months to cover the distance from their habitat to the Atlantic Ocean and then to the Sea.
Do They Lay Eggs Or Give Live Birth?
European eels spend almost all their lives living in freshwater rivers and only return to the ocean when they are sexually mature. They lay their eggs there and fertilize them through external fertilization.
As soon as they have done spawning, the mature will die.
Unlike real eels, electric eels reproduce during the dry season. The females deposit the eggs in a hidden nest made of saliva by the males.
Who knew that eels could be so fascinating? For centuries, humans have been wondering how these creatures reproduce since they don’t seem to have any balls (literally). Now we know that their life cycle is quite complex and involves spending most of their lives in freshwater before migrating to the ocean to mate. We’re still not sure exactly how they do it, but at least we have a better understanding now. Do you have any theories about how eels reproduce? Let us know your thoughts below! Subscribe to our YouTube channel for more crazy videos.
Mark is the founder of Aquarium Store Depot. He started in the aquarium hobby at the age of 11 and along the way worked at local fish stores. He has kept freshwater tanks, ponds, and reef tanks for over 25 years. His site was created to share his knowledge and unique teaching style on a larger scale. He has worked on making aquarium and pond keeping approachable. Mark has been featured in two books about aquarium keeping – both best sellers on Amazon. Each year, he continues to help his readers and clients with knowledge, professional builds, and troubleshooting.