How Do Eels Reproduce? The Mystery Of Eel Sex Explained

After traveling thousands of miles from freshwater streams and rivers, freshwater eels spawn in the Sargasso Sea. External fertilization is thought to be how they reproduce. Millions of eggs are released into the water by the females, which are fertilized by the male’s sperm. They die as a result of this.

The reproduction of eels has long been a mystery. For years, scientists were perplexed about how and where these animals reproduce. Eels have been studied since antiquity by researchers, and they are still being researched today.

Freshwater eels may be found in rivers throughout Europe, Japan, and other nations. They had never seen them migrate to their spawning areas and had never seen them mate. During the dissections, researchers were unable to locate reproductive organs or eggs. So, how do eels reproduce?

Scientists have recently made significant advances in understanding eel reproduction and migration behaviors. Tracking and keeping the animals captive has allowed us to make significant progress.

The reproductive process of eels is still a mystery, even today.

What is An Eel?

Eels are not always eels, as some species are labeled. The electric eel, for example, isn’t really an eel. The moray eel, on the other hand, is an eel. Therefore, how can you distinguish between an eel and a snake?

The animal must have a set of particular characteristics in order to be classified as an eel.

Many fish have distinct fins over different body parts. With one dorsal fin, eels have a long body. The caudel and anal fins are joined by a continuous dorsal fin.

Eels may have microscopic scales implanted in their skin, which are used to measure their size. They might be completely scales-free as well. Eels make slime when they are frightened, in addition to lacking scales.

Eels that live in freshwater are known as “skin breathers.” Capillaries are underneath the skin’s surface and are used to receive oxygen. Their teeth are also very pointed.

The location where a freshwater eel lives versus breeds is one of the most distinguishing features. Eels develop in freshwater as youngsters and adults, but breed in saltwater. Saltwater species, such as the moray eel, may spend their whole lives in salty water.

Since it lays its eggs in freshwater, an electric eel isn’t a true eel. You must migrate from freshwater to saltwater to become a freshwater eel. Electric eels also lack teeth and a dorsal fin, in addition to that.

Types of Eels: Common Species

1. Common European or American Eel (Anguilla anguilla)


Size: Up to 52 inches (133 centimeters)

Color: The color of the fish is silvery. They turn green, yellow, and brown as they fully develop.

Lifespan: Up to 85 years


Habitat: As they travel to or from the Sargasso Sea, juvenile and spawning adult eels can be found in marine waters. They may be found in freshwater and brackish water environments such as streams and rivers for the remainder of their lives. They may be found at depths ranging from 328 feet (100 meters) to 328 feet (100 meters).

Range: The Mediterranean Sea’s coasts and the English Channel. From Iceland to Mauritania, they can be found in the northern Atlantic Ocean.


The life stage of the American Eel determines their diet. During the leptocephali stage, the diet is unknown. Insect larvae, tiny crustaceans, and dead fish are among the insects they eat as glass eels. Adults will eat a wide range of aquatic invertebrates, as well as whatever they can find.


Larger eels, fish, cormorants, and herons are among the natural predators.

2. Shortfin & Longfin Eels: New Zealand Eels (Anguilla australis & Anguilla dieffenbachii)

The Shortfin eel and the Longfin eel are two of the most common varieties of eels in New Zealand.


Size: 3.2 feet/1 meter (shortfin), 3.2-6.5 feet (1-2 meters) (longfin)

Color: Light brown to olive (shortfin), dark brown or black (longfin)

Lifespan: 18-23 years (shortfin), 35-52 years (longfin)


Habitat: Longfin eels live in a broad range of altitudes, from extreme heights, while shortfin eels prefer lowland areas.

Range: New Zealand, eastern Australia, and the Pacific Islands are all home to the Shortfin eels. Only in New Zealand can you find longfin eels.


Insect larvae, snails, and worms will be eaten by juveniles. Fish will be the mainstay of the adults’ diets, but crayfish and tiny birds will be included.


Perch and rats are two natural predators.

3. Japanese Eel (Anuilla japonica)


Color: Dark brown to black


Range: Waters between Japan and China


Fish, crustaceans, and insects make up the adult diet.


4. The African Longfin Eel: South African Eel (Anguilla mossambica)


Size: Up to 59 inches (150 centimeters)

Color: Olive to gray-black with a light underside


Habitat: Quiet, flowing waters

Range: From Kenya to Cape Agulhas, and Madagascar, this is the western Indian Ocean.


They eat mostly fish and crabs, but they may also consume carrion.

How Do Eels Reproduce

The lifecycle of freshwater eels is quite complicated, and it wasn’t fully comprehended for a long time. Even today, there is a lot of controversy about how much scientists understand about eel reproduction. We don’t seem to know much about the reproduction process itself, despite the fact that the life cycle of freshwater eels is fairly well-known.

Saltwater Eels vs. Freshwater Eels

Freshwater eels are distinguished from marine species by the fact that they spend some of their lives in saltwater. Freshwater eels are born in saltwater and die there, although the majority of their lives are spent in freshwater.

How do they manage to achieve such results? Because of the difficulty of their cells, most fish are unable to transition between fresh and saltwater. How does an eel adapt to the change?

The eel’s kidneys alter to permit them to store more salt in order to compensate for the lack of salt. The salinity levels in the blood are maintained when the kidneys hold extra salt.

Freshwater eels may be found in streams and lakes throughout their adult lives. They will all swim downstream at the same time when it is mating season, arriving at the water as a group.

Where Do Eels Breed?

Eels are extremely motivated to reach their breeding location in the Sargasso Sea when it comes time to mate. All freshwater eels mate in the Sargasso Sea, a two-million-square-mile stretch of water.

Bermuda, the Azores, and the West Indies are all home to the Sargasso Sea. Even though most eels live far away from this location, they are not afraid to go thousands of miles/kilometers to get here.

Life Cycle

The eggs will float to the surface and hatch when the eels have placed them. The larvae are transparent and shaped like leaves. They are very tiny. Larvae eels are known as “leptocephali” at this stage.

The leptocephali will then migrate to a freshwater habitat, where they will spend the rest of their lives. This is a very long voyage. Over the next year, the leptocephali will drift over the ocean. They’ll have changed shape again by the time they arrive.

As the eels move back up into the rivers, they reach this subsequent stage, known as the “glass eel.” Because they are transparent, glass eels are referred to as such. Glass eels, on the other hand, are roughly 2-3 inches (5-7.6 centimeters) long and indistinguishable from fully grown eels in appearance. Their fins have grown in, and they possess the slender, elongated form of an adult eel.

They’re eels that want freshwater to live in, like glass eels. These eels are similarly tenacious in their search for a freshwater habitat, as mating is. They’ll frequently move from ponds and lakes to rivers in search of a home. How do they accomplish this? Like a snake, they slither across the ground.

Their hue changes from clear to black as they proceed up the river. They’ve now reached the size of a 4-inch (10-centimeter) “elver,” at this point. The juvenile eels are simply miniature copies of the adult. They are still moving up streams and rivers at this point.

The eels will grow more immature as they get older. Freshwater eels spend the majority of their lives at this age. Eels are in this age for around 20 years, and they are still regarded as juveniles.

Eels that grow up in mostly freshwater environments, according to researchers, may live longer and expand. The rate at which eels develop in brackish regions near the sea is much faster.

Eel Breeding: How It Works

Since they lack juvenile eels, researchers aren’t sure when eels acquire their sexual organs. In eels, however, when they were approximately 11 inches (26 centimeters) long in their late juvenile stage, they photographed gonads.

Environmental factors are thought to influence when eels reach sexual maturity, despite the fact that this is not known. It’s likely that the temperature and salinity levels play a role. Low eel populations might even produce more females as a result.

It is time for the elvers’ lives to come to an end when they reach adulthood. They are extremely persistent in their desire to complete the life cycle once they reach this point. Males average 15.7 inches (40 centimeters) long when it’s time to reproduce, while females may reach 47.2 inches (120 centimeters).

The adults will return down the river sometime between late winter and early spring. They die after traveling thousands of kilometers to their spawning location.

It’s the last thing eels do before they spawn. They don’t make it through the spawning process. Hundreds of eels congregate to create eggs, and they all perish as a result. Nonetheless, how does the spawning process work?

Females lay the eggs in the water column when the eels arrive. The sperm will then fertilize eel eggs in the water column, releasing males. A single female eel is believed to be capable of producing 2-10 million eggs.

Survival Facts About Eel

It’s also fascinating how these eels manage to get to the Sargasso Sea after surviving thousands of miles. Adults do not stop to eat, unlike juveniles traveling the same distance. Their stomachs deteriorate as a result of this.

While swimming, the eels’ blood vessels surrounding the swim bladder expand, allowing them to be supported. Their eyes also expand in size, becoming more sensitive to blue light waves and allowing them to see in the dark as a result.

How Do Eels Reproduce in Captivity Vs Eels in the Wild

At home, many people keep eels in aquariums. Despite their popularity, they are difficult to care for. What about agriculture, though?

Farming eels in captivity has been tried out in Japan, China, Taiwan, Australia, Morocco, European nations, and Scandinavian nations. The most prolific producer is Japan.

Researchers are attempting to conserve eel populations, which are in rapid decline. In Japan, where eel is a popular dish, this is especially significant.

Overfishing for the meat industry is thought to be the reason for the declining populations. Flood-deferrents construction may also be to blame. In rivers and streams, both situations keep mature eels from maturing.

Farms have been producing eel meat for almost 60% of the world’s population since they first began farming eels.

First, eels are caught in the wild and screened for diseases before being sold. They are then cultivated in ponds or specialized tanks. To avoid cannibalism and competition for food, the eels must be divided by size after six weeks.

Breeding eels in captivity has been gaining traction among researchers, something that was not possible in the recent past. Hormones are used to help the eels reach sexual maturity more quickly. The breeding process is then initiated by introducing males and femen.

Because they can adapt and tolerate a variety of conditions, eels are a good animal to farm. Being kept in huge numbers, they may also endure.

Eel Reproduction Discovery

It’s been a long time since we’ve figured out how eels reproduce. Researchers thought that the five distinct life phases were completely different animals for years. Larvae eels were not recognized as leptocephali until 1896. They couldn’t figure out where the larva came from, though.

Johannes Schmidt, a Danish oceanographer, spent 18 years trawling the Atlantic across four continents. Just beyond the Sargasso Sea, he discovered the tiniest larva of the period.

Eels were thought to be spawned in the Sargasso Sea for a long time. They thought that eels might journey from freshwater rivers to the Sargasso Sea, although they weren’t sure how. They hadn’t seen any mating or migration in action. They discovered eel eggs there, and that was all they needed to know.

After scientists opted to monitor American eels a few years ago, everything changed. 38 American Eels were successfully tracked for a long distance by several Canadian researchers off the coast of Nova Scotia using GPS trackers.

During the whole migration, one eel was able to be tracked by the researchers. After traveling 1,500 miles (2,400 kilometers) in less than 45 days, the eel swam to the northern reaches of the Sargasso Sea.

Due to these studies, we now have a better understanding of freshwater eel migration. Because of the captive breeding programs I outlined above, we also know a lot more about their breeding habits.

In such a short period of time, researchers have made significant progress. Consider how eels have remained a mystery for hundreds of years.

How Are Eels Born FAQs

Do Eels Lay Eggs?

Eels do, in fact, lay eggs. In fact, many of them. During spawning, a single freshwater eel may produce 2-10 million eggs. Now, picture hundreds or even thousands of eels spawning all at once. Millions of eggs will be created in the millions.

What Are Eel Eggs Called?

Eel eggs are simply referred to as “eggs.”

There are five phases in the life cycle of eels. The first larval stage is Leptocephali, and the second larval stage is glass eels. The yellow eels, who are sexually immature adults, follow the miniature eel stage of the elvers. Finally, the eel reaches maturity.

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