How Do Eels Reproduce? – The Reproduction Mystery of Eel

Eels in freshwater aquarium

Eels and how they reproduce have always been a big mystery in the ocean. Many people have been curious and have done lots of studies to find out more.

Because eels look different and have secret parts of their lives, lots of scientists have tried to figure out how they have babies.

But, even with all our modern science, we still don’t fully understand how eels mate. This article will tell you what we know and what we’re still trying to find out about eel babies. Let’s dive in!

What Is an Eel?

Eel in clean freshwater

An eel is a long, slim fish from a large family known as Anguilliformes. Eels have a distinct look: they’re notably slippery, lack the common pelvic fins, and sport a continuous dorsal fin that traces along their backs. 

However, eels are more than just long, slender fish; they’re creatures known for their adaptability and unique features.

For instance, they inhabit different bodies of water, from the gentle currents of freshwater rivers to the depths of the open sea. This showcases their impressive ability to adapt and survive in varied environments.

Additionally, eels are not just diverse in their habitats but also in their species. With a staggering count of over 800 species, each eel brings its different flavor to the aquatic ecosystems they inhabit. 

Some eels, like the electric eel, have developed astonishing abilities to generate electric charges, used both for hunting and self-defense. 

Others, like the moray eel, are known for their impressive jaw mechanics, enabling them to secure prey in the crevices of coral reefs.

Where Do Eels Come From?

Eels originate from various locations worldwide, with their birthplaces often being quite distant from where they eventually settle. 

For example, the European Eel, despite being found across Europe, is born in the Sargasso Sea, located in the Atlantic Ocean. After birth, they migrate, traveling thousands of miles to freshwaters, where they grow. 

Similarly, while prevalent in various bodies of water across the United States and parts of Canada, the American Eel spawns in the distant Sargasso Sea, similar to its European counterpart.

After hatching from their eggs, baby eels, called leptocephali, start a long trip that covers thousands of miles. They travel to the fresh and slightly salty waters on the east coast of North America.

Here, they live for most of their lives. When it’s time, they head back to the Sargasso Sea to have babies and finish their life’s journey.

Here are a few known spawning locations for different species of eels:

  • Sargasso Sea: As mentioned, it’s the spawning site for both European eels (Anguilla anguilla) and American eels (Anguilla rostrata).
  • Coral Sea: The short-finned eel (Anguilla australis) is believed to spawn in the Coral Sea, northeast of Australia.
  • Western North Pacific Ocean: Japanese eels (Anguilla japonica) are known to spawn in the western North Pacific Ocean near the West Mariana Ridge.
  • Celebes Sea: The Indonesian shortfin eel (Anguilla bicolor bicolor) is thought to spawn in the Celebes Sea.
  • Tonga: The spawning area of the giant mottled eel (Anguilla marmorata) is believed to be near Tonga in the Pacific Ocean.

It’s crucial to note that the exact spawning locations for many eel species are still not definitively known and are often inferred from the locations of their leptocephali. 

Further research and technological advancements in tracking methods may be needed to pinpoint exactly where different eel species come from.

In short, eels start their lives in specific spawning locations and then go on amazing trips through different locations to grow up.

After several years, they navigate back to their birthplaces, completing a fascinating life cycle by reproducing in the same locations where their journeys began.

Keep in mind, though, that while many eel species exhibit the catadromous migration pattern (moving from freshwater to spawn in the sea), not all eels strictly adhere to this.

How Do Eels Reproduce?

Life cycle of eels

Eels are believed to reproduce by external fertilization: females release eggs, and males release sperm into the ocean, where fertilization occurs. 

However, the exact details of this process are still unknown because it often happens in far away and hard-to-reach places like the Sargasso Sea.

When it comes to the events that lead up to fertilization, though, scientists know quite a bit. For starters, eels live an incredible life full of extensive journeys. 

Once they become adults, they travel thousands of miles from their freshwater homes to specific ocean areas to breed. 

After laying eggs, adult eels die, and the new eggs hatch into a leaf-shaped larva, which then travels back, riding ocean currents to fresh or brackish water, where they grow into adults, and the cycle restarts.

Despite what we know, many aspects of how eels mate remain a mystery. Questions about how they navigate, their exact spawning behaviors, and why we’ve never observed these processes directly are still unanswered.

Why Don’t We Know How Eels Reproduce?

The mystery of eel reproduction remains unsolved primarily due to the inaccessible and remote locations they choose for spawning, such as the deep and distant seas of the Sargasso, Coral, and Celebes, among others. 

Eels undertake challenging, vast oceanic journeys to reach these breeding grounds, navigating through complex and varied environments. So, figuring out their exact path and following it is really hard for scientists.

Even with better tools for tracking and exploring the deep ocean, we still don’t fully understand how eels reproduce, especially in certain places and depths.

Fortunately, initiatives like the Eel Project are hoping to inspire more efforts in uncovering these mysteries. 

Spearheaded by Chris Bowser in collaboration with Cornell’s Water Resource Institute, the Citizen Science Hudson River Eel Project involves community volunteers and students in monitoring eel populations in New York. 

This initiative not only aids eel conservation but also serves as an educational platform, providing insights and invoking interest in the eels’ mysterious life cycle.

Where Do Eels Reproduce?

Baby eels in sea water

Eels choose some of the most remote and inaccessible parts of the world’s oceans to reproduce. For instance, the Sargasso Sea, a vast, warm region in the Atlantic Ocean, is a well-known breeding ground for several eel species. 

These creatures embark on an extraordinary journey from their freshwater habitats, traveling thousands of miles to reach these distant spawning sites. Different eel species select specific locations to lay their eggs. 

For example, Japanese Eels spawn in the western North Pacific Ocean, near the West Mariana Ridge, while the Short-finned Eel chooses the Coral Sea, northeast of Australia. 

The Indonesian shortfin eel is thought to spawn in the Celebes Sea and the giant mottled eel near Tonga in the Pacific Ocean. 

It’s essential to note that these locations are figured out by finding young eel larvae. This is because scientists have never observed the actual act of eel spawning.

The Life Cycle of Eels

The eel life cycle, a series of remarkable metamorphoses and extensive migrations, is one of the few things we know about these mysterious creatures. 

Here are the various stages of an eel’s life:

  • Leptocephali (Larval Stage): Newly hatched eels, known as leptocephali, are flat, transparent, and leaf-like. They drift with ocean currents, often traveling vast distances from their spawning grounds.
  • Glass Eel Stage: Upon reaching coastal waters, leptocephali transforms into glass eels. At this stage, they are transparent and begin to resemble adult eels in shape.
  • Elver Stage: Glass eels become elvers as they start to gain pigmentation. They begin to migrate toward freshwater habitats, where they will spend most of their lives.
  • Yellow Eel Stage: Elvers further darken, becoming yellow eels. This stage is where they grow and mature, living in freshwater environments for several years.
  • Silver Eel Stage: As they become ready to breed, yellow eels transform into silver eels. They undertake a long migration back to their oceanic spawning grounds, where they will reproduce and conclude their life cycle.

In this cyclical journey, eels navigate through various habitats, from the open ocean to freshwater rivers, showcasing a remarkable ability to adapt and survive in dramatically different environments. 

To visualize how eels change throughout their life, watch this informative video:

Eli the eel: A mysterious migration - James Prosek

Saltwater Eels vs. Freshwater Eels

Electric eel in aquarium

Eels live in different types of water: some in saltwater and others in freshwater, each having their own living and breeding habits. 

Saltwater eels, like the giant moray and ribbon eels, live their whole lives in the ocean, swimming through reefs and wide-open waters. They stay in the same salty environment, eating sea creatures, and don’t migrate.

On the other hand, freshwater eels, like European and American eels, live in rivers and lakes but travel a long way to special ocean spots, like the Sargasso Sea, to lay eggs. 

This major migration means they have to adapt and be able to live in saltwater for a while. After the eggs hatch, the baby eels travel back to freshwater to grow up. 

This journeying life of freshwater eels is quite different from the stay-put life of saltwater eels, highlighting a key difference between the two.

How Do Eels Reproduce in Captivity vs. in the Wild?

In the wild, as we’ve explored, eels undertake monumental journeys to specific and remote oceanic breeding grounds to reproduce. 

Despite being a focal point of numerous scientific studies, this process has never been observed directly, leaving many questions unanswered.

Similarly, reproducing eels in captivity has also been challenging for scientists and aquaculturists. 

The secretive and complex nature of eel reproduction, coupled with their specific environmental needs during different life stages, makes mimicking their natural breeding conditions incredibly difficult. 

In captivity, scientists have managed to induce eel reproduction using hormonal treatments to stimulate the development of reproductive organs and egg laying. 

However, the process could be more straightforward and is still being refined to this day.

The ITS-EEL project is one notable initiative I encountered in breeding eels in captivity. This research and innovation project, which has been ongoing for over a decade, aims to establish the larval culture of the European Eel. 

The study hopes to find a way to the production of eels in culture. It has made significant advancements in technology and techniques, now being able to produce millions of fertilized eggs and hatched larvae.

The team has succeeded in continuously expanding survival with repeated achievements of larval batches feeding, growing, and living up to 55 days, representing a major breakthrough for eel hatchery development.

You can watch this video to learn more about ITS-EEL and their efforts in breeding eels in captivity:

Early life of European eel (ITS-EEL)

Besides the ITS-EEL project, there are other important projects worldwide trying to either figure out how to breed eels in captivity or learn more about how they reproduce in nature.

In Japan, for instance, researchers have been making significant progress in breeding eels and raising them to adulthood, which you can read more about in this article.

Meanwhile, various initiatives are also being set up to improve our understanding of eels and how they reproduce.

For instance, the Eel Reproduction Innovation Centre (EELRIC) serves as a collaborative platform, emphasizing innovative strategies for eel reproduction and sustainability. 

Additionally, a study on the Reproductive Ecology of Freshwater Eels has used baby eel samples to learn more about how freshwater eels reproduce in nature.

In the end, the big differences between how eels reproduce in nature versus captivity show that we still have a lot to learn about them. It just goes to show how important more research and efforts to protect them are.

Eel Reproduction FAQs

Eel swimming on the rocky part of the water

Has Anyone Seen an Eel Reproduce?

Eel reproduction has never been directly observed in the wild; however, captive breeding of eels has been observed. The reproduction of eels in captivity and in the wild are allegedly very different, though.

Do Eels Have Reproductive Organs?

Yes, eels do have reproductive organs, but they are often internal and only fully developed during the final stage of their life cycle.

Do Eels Change Gender?

Some eel species are known to change gender during their life cycle, although this is not a universal trait among all eel species.

Do Eels Lay Eggs?

Yes, eels lay eggs, but the eggs of many eel species have rarely been observed or studied in the wild.

What Are Eel Eggs Called?

Eel eggs are typically referred to simply as “eel eggs” without a specialized term.

Eel Reproduction Discovery and Unanswered Questions

While the scientific community has made countless efforts to understand eels and their life cycles, numerous questions remain unanswered.

Here are some ongoing questions that researchers and biologists still find puzzling:

  • Why do eels travel such vast distances to specific spawning grounds, and how do they navigate to these precise locations?
  • How exactly does external fertilization occur in the wild, and what factors influence its success?
  • Why have eel eggs never been found in the wild, and what are the characteristics of these eggs?
  • What are the specific mating mechanisms and behaviors exhibited by eels in their natural spawning grounds?
  • How do eels ensure the survival of their offspring, given the apparent lack of parental involvement post-spawning?
  • What triggers the physiological changes in eels as they transition through their life stages?
  • What are the primary factors contributing to the decline in eel populations, and how does their reproduction process influence this?
  • How can conservation efforts be effectively implemented when the breeding grounds and reproductive processes of eels remain largely unknown?
  • Why is breeding eels in captivity so challenging, and what can be learned from the limited successes in this area?

Exploring the mysteries of eel reproduction remains a challenge for scientists. Needless to say, the unknowns about eel breeding, migration, and life cycles fuel both curiosity and ongoing research. 

Final Thoughts

Diving into the secrets of how eels reproduce has been a fascinating adventure. In this article, we’ve uncovered some of the ocean’s mysteries, but there are still questions that even science still needs to figure out. 

All things considered, the secrets of eel reproduction continue to invite more exploration and study into the unknown underwater world.

As we continue to probe these depths, each bit of discovery brings us closer to understanding these fascinating creatures and ensuring their preservation in our waters. 

So, did this guide answer most of your eel-related questions? Or did it add more fuel to your curiosity? In any case, share with us your thoughts about eels and eel reproduction by leaving a comment below! 

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