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One of the most fascinating mysteries of the underwater community is the reproductive processes of both marine and freshwater eels. Specifically, the European eel (Anguilla anguilla) has confounded philosophers, naturalists, and scientists for decades. It was long believed that male eels lacked reproductive organs and thus couldn’t create offspring. The truth of eel reproduction is much more complex than this, though.
Thousands of eels were dissected, tracked, and studied to understand their reproductive and lifecycle processes, but the evidence was scarce and only theories remained. If eels lacked reproductive organs since the time of their evolution, how did they survive as a species? It wasn’t until the 20th century that these simple eel questions began to be answered.
Before deep-diving into how eels continue to live and reproduce, we need to first understand what eels are, where they come from, and eventually, how they reproduce.
What Are Eels?
If you’re not familiar at all with eels, then they’ll initially look otherworldly. Are they fish? Are they snakes? Or are they something new altogether?
Don’t be fooled, these are actually thin, almost finless fish that belong to the Anguilliformes order. Within this scientific order are 8 suborders and almost 800 species! Both saltwater and freshwater eels belong to this scientific order, though not every fish that’s labeled as an ‘eel’ is a true eel!
If you are familiar with eels at all, then you may have heard of the electric eel (Electrophorus spp.) which is capable of delivering a deadly shock. While these fish certainly look like true eels, they are actually more closely related to knife fish as members of the Gymnotiformes order.
Although freshwater eels and saltwater eels look very different from each other and their related genera, there are a few defining features that make them true eels of the Anguilliformes order.
Anguilliformes are ray-finned fish. But unlike other fish, eels lack pelvic fins entirely and most species don’t have pectoral fins. Eels also have a conjoined dorsal and anal fin that makes a singular, long fin that extends across the latter half of their body. Interestingly, this evolution also allows these fish to swim backward. Another defining feature of eels is that some species have small scales while others have incorporated scales into their skin. Some may lack scales entirely.
Anglers and other eel-handlers especially note their ability to produce slime, which is a defense mechanism against predators and pathogens. Eels have sharp teeth fixed in their mouths, with some species having a second jaw known as the pharyngeal jaw. The first set of jaws is thought to specialize in catching the prey while the second set processes the food and moves it along to the digestive tract.
True eels can be found in freshwater and marine ecosystems around the world. Some species are catadromous which means that they migrate from one body of water to another. As we’ll see, some species of eel, especially those in the Anguillidae family, make extensive journeys from freshwater ecosystems to purely saltwater conditions.
An example of a true eel is the European freshwater eel. This species of freshwater eel lacks scales and breathes through its skin. This unique ability makes them able to survive out of water for up to 48 hours! European freshwater eels can also bury themselves and hibernate in the mud over the winter months; this is also needed when their natural habitats dry out over the summer.
Where Do They Come From?
Fish have been around for a long time which means that they’ve been studied for a long time. So much so that even ancient Greek philosophers devoted their entire lives to understanding their origins, especially the beginnings of eels and their reproductive processes. Surprisingly, the great Aristotle contributed much thought to these lifecycles.
According to Aristotle, eels materialized from mud and were actually a type of earthworm. This became a popular theory as no one had ever seen eels reproduce and there was no other reasonable explanation; this also eliminated the need to explain their apparent absence of reproductive organs.
It’s hard to believe that this exact explanation for eel reproduction persisted for over 2,000 years, but few dared to challenge the findings of Aristotle.
It wasn’t until Pliny the Edler, a Roman naturalist and natural philosopher, theorized another answer to eel reproduction. He believed that new eels were born from the old shed skins of parent eels that scratched themselves on hard surfaces, like on the seafloor and on rocks. Each scraping would turn into a new eel.
Other popular beliefs included eels being born from sea foam or from when sunlight reflected off the surface of the water in just the right way. Some even thought that eels hatched on the rooftops of great buildings or derived from the gills of other fish. The more absurd theories mentioned the involvement of beetles and hair from horses falling into the river.
Many centuries later, famous psychoanalyst Sigmund Freud took a stab at uncovering the mystery of eel reproduction. In 1876, while a medical student at the University of Vienna, Freud bought as many eels as possible. After 400 dissections, Freud finally located the male eel’s gonads deep within the abdominal cavity, dispelling any previous theories!
This was a major breakthrough in understanding eel reproduction and their lifecycle.
You might be saying to yourself that you’ve never actually seen an eel in the wild. This isn’t because eels aren’t abundant, but rather because they’re reclusive and nocturnal.
When not migrating thousands of miles, these fish like to stay as elusive as possible, cramming themselves into tight caves and rock formations, burying themselves in the substrate, or even living together in communities known as eel pits.
In freshwater ecosystems, they can be found on nearly every continent excluding Antarctica in rivers, lakes, streams, and even ponds. Keep in mind that some eels are able to move across land, allowing them to enter isolated areas of water. Marine eels have a much larger range and can be found in most regions of the ocean at varying depths.
How Do They Reproduce?
The long-asked and long-evaded question: how do eels reproduce? Even today, after thousands of theories and anatomical dissections, there are many unknowns about the lives of eels.
To help understand how eels reproduce, we need to first look at the 5 incredibly complex stages of an eel’s life cycle.
5 Stages Of Their Life Cycle
The lifecycle of an eel is divided into 5 distinct stages. Each one marks different biological and sexual maturity levels of the eel. Most notably, sex organs develop in the later stages.
- 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 the larval stage, individually known as leptocephalus. In this phase, the baby eel emerges from its fertilized egg in or near its species’ breeding grounds.
Leptocephalus are largely transparent. They have radically compressed bodies with minimal muscles and organs. One of their most defining features is a simple stripe that runs down the middle of their pane-like body. During the next metamorphosis period from larva to glass eel, the transparent gel changes into solid adult tissue.
Interestingly, leptocephali feed on marine snow and other organic matter to facilitate growth. They are largely planktonic and depend on ocean currents to get them to where they need to go, mostly estuaries and other safe coastal ecosystems. Once in an intertidal region, they begin their transformation into glass eels.
The transformation from leptocephalus to glass eel is both a geographical and physical change. As leptocephali, eel larvae live in the ocean. As they begin their metamorphosis into glass eels, they must make their way into protected estuaries where freshwater and saltwater ecosystems mix. This is a challenging move and many eels do not survive due to their lack of control over their navigation.
Physically, leptocephali start to change into more recognizable glass eels. One of the major changes during this life stage is the development of pigmentation which takes over the previous clear gel. But underneath their skin, there is another major change happening.
A regular fish’s body needs to osmotically adapt to changes in salinity, like the differences between freshwater and saltwater conditions; typically, when marine fish enter brackish water, their cells begin to swell as there is a difference in osmotic pressure. Eels have evolved to combat this problem with specialized kidneys that are able to retain more salt to better match their internal salinity to their external salinity.
The next stage of an eel’s life cycle is the elver stage. This is the pre-adult stage that occurs once the eel is about 2 to 3 years old. The elver will have dark yellow pigmentation and reach up to 8 inches in length. If they’ve not already made their way upstream to freshwater rivers and streams, then young elvers will begin their journey.
At this point, the eels are omnivores and will try to eat anything they can fit into their mouth. This largely includes crustaceans, worms, and insects. Elvers and the next lifecycle, yellow eels, can stay in these freshwater upstream regions of rivers for up to 20 years until they are mature enough to migrate back out to the ocean. Many eel species return to the Sargasso Sea to spawn, though this varies by species.
Sadly, elvers are a very popular dish that has caused issues with eel sustainability in the past.
Before becoming a silver eel, or an adult eel, elvers must first become a yellow eel. This is when the pigmentation turns even darker and their true adult colors start to form, including brown, olive green, yellow, and black. These colors will largely depend on environmental factors, such as temperature and turbidity. At this point, juvenile eels can measure over 30 inches in length.
Yellow eels will continue to live in and move from one freshwater ecosystem to the next until they’re ready to transform into their final life stage, the silver eel.
Silver or Adult
Believe it or not, not much is known about adult eels, also known as silver eels. Scientists aren’t sure what determines when eels transition into their final form. This is even more complicated by being unable to tell how old an eel is by its appearance alone at this point. One thing is understood though, eels aren’t considered fully mature until they develop sex organs in the last stage of their life cycle as an adult eel.
As adult eels, males can reach 2 feet in length while females can grow up to 4 or more feet. They change drastically in color from dark natural colors to steel silver. Only as adults is when eels are mature enough to make the trip to the Sargasso Sea or other spawning grounds depending on the species.
How Do They Mate In The Wild
When talking about eel reproduction, you’ll often hear about the Sargasso Sea. The Sargasso Sea encompasses Bermuda (including the infamous Bermuda Triangle), the Azores, and the West Indies. This area of the Atlantic Ocean is a 2-million-square-mile span of saltwater that is home to the breeding grounds of millions of eels. Most notably, freshwater European and American eels make this thousand-mile journey.
When male and female eels arrive at their specific breeding grounds, it is believed that eels reproduce through external fertilization. This means that the females release millions of eggs that then get fertilized by the males’ sperm. Sadly, the adult eels die soon after. The eggs then get fertilized and repeat the previous lifecycles, making their way back to North America and Europe respectively. Other species of eel have different, yet still specific breeding grounds.
Remember, there are 800 species of known eel around the world! Imagine how many there are undiscovered if scientists couldn’t even identify their sexual organs for thousands of years.
The truth is that a lot is still unknown about eels and new species are still being discovered. Even the species that we have documented are not fully understood.
Here are some of the most well-known species of freshwater eel.
The European eel is a common type of freshwater eel that can live to be 85 years old in the wild and 55 years old in captivity! Unfortunately, they are listed as a critically endangered species due to overfishing, parasites, and other human activity.
These fish feature a pair of small pectoral fins and have the typical snake-like body. They grow to between 2 to 3 feet in length. As catadromous fish, European eels spend their adult lives in freshwater rivers and streams throughout much of Europe and migrate to saltwater conditions to reproduce. This particular species travels to the Sargasso Sea in the Atlantic Ocean.
Interestingly, European eels don’t stop traveling upstream until they’re sexually mature. They’ve been found in seasonal ecosystems that dry up over hot months and in isolated lakes and ponds, confirming their ability to travel over land and to survive for extended periods buried in the substrate.
Many breeding programs have been installed to help declining numbers of the European eel with slight success using hormones and controlled conditions.
Shortfin and Longfin
Two more common types of eels originate from New Zealand: the longfin eel (Anguilla dieffenbachii) and the shortfin eel (Anguilla australis). The longfin eel is endangered while the shortfin eel is near threatened.
Like European eels, longfin and shortfin eels can live for a considerably long time. They are found in freshwater streams and lakes and make their way to regions in the Pacific Ocean for spawning. Longfin eels are found further inland in New Zealand than shortfin eels, but shortfin eels can also be found in several other countries across the South Pacific.
These two fish also have different breeding grounds. Longfin eels travel to areas near Tonga while shortfin eels migrate to deep waters around New Caledonia. Every female can produce up to 20 million eggs!
Longfin eels are aptly named for their lengthy fins. Their dorsal fin is almost two-thirds the length of their body. Shortfin eels have a similar appearance but have a fin that starts farther away from the head. Another difference is that when a longfin eel bends, wrinkles form on its skin; a shortfin eel’s skin will remain smooth. Females are notably larger and live longer than males.
Japanese eels (Anguilla japonica) are mainly found in Japan, Korea, China, and Vietnam. Like the other types of eel on this list, Japanese eels also spawn in the sea but spend a large majority of their lives in freshwater.
When the time comes, Japanese eels travel to the North Equatorial Current in the western North Pacific. Throughout this long journey, Japanese eels do not eat. Instead, they collect all the nutrients they need through a specialized oil before making the trip.
These fish feature a dull grey, brown, and greenish dorsal on top of a white underbelly, though these colors depend on environmental factors.
Sadly, Japanese eel populations are decreasing due to changing ocean temperatures, habitat loss, and fluctuating salinities. Japenese eels are also a popular culinary dish, popularly known as unagi, though regulations have been implemented to reduce overharvesting and to encourage supplementation. As a result, they are listed as an endangered species.
The African Longfin
The African longfin eel (Anguilla mossambica) is another common type of eel, though not much is known about them. They have olive, gray, and black bodies with a light underside. They prefer fast currents in the western Indian Ocean off of Africa.
They eat a variety of fish and invertebrates, but will also scavenge.
Difference Between Freshwater and Saltwater
There are freshwater eels and there are saltwater eels, but most freshwater eels spend some time in saltwater conditions. Where marine eels live in the ocean can vary, with some preferring coastal lagoons and others enjoying the open sea. Some species even dig into the mud and sand.
The major difference between freshwater and saltwater eels is body texture and appearance. Freshwater eels have a firmer texture, while saltwater eels are softer and leaner. Most saltwater eels are also much more colorful than freshwater eels, making them a very popular addition to the marine aquarium.
For the adventurous eel connoisseur, freshwater and marine species also have different tastes.
How Do They Survive Migration?
You might be wondering how eels survive for thousands of miles as they transition from freshwater to saltwater. What do they eat? When do they sleep? How do they know where to go?
In fact, most eels don’t eat on their way to their final destination. In most cases, their stomach deteriorates. The blood vessels around the swim bladder increase for additional support while swimming. Their eyes also double in size, which increases sensitivity to blue waves of light, increasing their ability to see in the dark. Their kidneys also adapt to hold more salt to increase internal salinity levels to compensate for their surrounding salinity changes.
European eel migration is one of the longest migrations documented by any marine creature. They typically start their migration in autumn and arrive at the Sargasso Sea in late winter and spring. This journey is approximately 3,000 miles long and can range from 80 to 170 days.
It is believed that eels use every sense they have to navigate their journey. It’s also strongly believed that they rely on lunar cycles and wait for the perfect conditions in regard to temperature, salinity, tides, and currents, to make their move.
What About Captivity?
Because of their complex lifecycles, eels have not been bred in the home aquarium. Many researchers and hobbyists have attempted artificial breeding, but nothing can replicate the years spent in freshwater, transition to saltwater, and external fertilization.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.
That isn’t to say that researchers aren’t close, though. In fact, some species of eel have been successfully bred in closely monitored systems with the help of hormones and fluctuating parameters. However, the full lifecycle of an eel has never been achieved in captivity.Many people tried different artificial ways to breed mature eels in a home aquarium but to no avail.
Eels are very hard to kill. They are built to withstand extreme differences in environmental conditions and have a very long lifespan. Their blood is poisonous and they have been known to swim for hours on end even with their head cut off.
However, any traces of toxic ammonia, nitrite, or nitrate in the aquarium can cause your freshwater or saltwater eel to succumb to unfavorable environmental factors.
Do They Have A Reproductive System?
This exact question puzzled philosophers and scientists for hundreds of years. Even after thousands of dissections, it was unclear how eels could reproduce if young males lacked sexual organs. However, it isn’t until males and females reach full sexual maturity that they develop these organs deep within their abdominal cavities.
So, yes, eels have a reproductive system but it doesn’t develop or become evident until much later in their lives.
What About In Captivity?
Eels have not been fully bred in captivity, though some life stages have been achieved. It is too difficult to replicate external fertilization where a female releases millions of eggs and has them fertilized by a male. It is also impossible to recreate the incredible growth and migration these fish make from freshwater to saltwater to their spawning grounds over the course of decades.
Do They Lay Eggs Or Give Live Birth?
Yes, female eels lay eggs which males then fertilize through external fertilization. As soon as they are done spawning, the mature eels die. A fun fact is that electric eels (not true eels) reproduce during the dry season by the female depositing eggs into a saliva nest made by the male.
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.
Eels are some of the most complex aquatic creatures but you wouldn’t know that from their simple snake-like appearance. For years, it was impossible to know the complex life stages these fish go through while transitioning from freshwater to saltwater. While we’re still not exactly sure how eels go through these stages and make their migrations, we will make those dissections over time.
Do you have any theories about how eels reproduce? Let us know your thoughts below! Let us know your thoughts below! Subscribe to our YouTube channel for more crazy videos.
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