Eels are one of the most fascinating marine and freshwater creatures on the planet, and their reproductive processes have baffled naturalists and scientists for decades.
Even with today’s technology and scientific resources, we’re still unsure how eels create offspring. However, in this comprehensive guide to eel reproduction, we dive deeper into what we know about how eels reproduce.
Keep reading to learn the secrets of these slippery, snake-like animals.
Are Eels Fish or Something Else?
At first glance, eels look like snakes. however, there’s nothing reptilian about these creatures, which actually belong to the Anguilliformes order of thin, almost finless fish.
There are almost 800 species of saltwater and freshwater eels in that order, some of which are suitable for life in an aquarium.
You won’t often see an eel in the wild environment, not because they’re scarce, but rather because most species are nocturnal and very shy.
Some species of eels migrate thousands of miles, while others hide in tight caves, crevices, and rock formations, bury themselves in the substrate, or live together in communities called eel pits.
Freshwater eels are found on almost every continent, excluding Antarctica, where they live in rivers, lakes, streams, and ponds. Some eels can move across land, enabling them to enter isolated water bodies during seasons of drought. Marine eels are found in most ocean regions, ranging for hundreds of miles and at varying depths.
So How Do Eels Reproduce?
To help understand how the eel reproduces, you first need to understand the five extremely complex stages of the eel’s life cycle.
Each stage of the eel’s life cycle marks different sexual and biological maturity levels of the creature, most notably the development of the sex organs, which happens in the latest stages.
The larval stage is the first stage of the eel’s life cycle, known as leptocephalus.
During this phase, the baby eel hatches from its fertilized egg near the species’ breeding grounds.
Eel larvae are mostly transparent and flat, with minimal organs and muscles and a stripe that runs down the middle of their body. The leptocephali are largely planktonic, drifting on the ocean current within safe coastal ecosystems and estuaries until they reach an intertidal region where their transformation into glass eels begins.
Eel larvae live in the ocean, but before they can morph into glass eels, the glass eels must find their way into areas where saltwater and freshwater ecosystems mix, such as protected estuaries.
During this life stage, the clear gel of the larvae’s body starts to develop pigmentation. In addition, the eels develop specialized kidneys that can retain more salt to match their external salinity.
The elver stage of the eel’s life cycle is the pre-adult stage and typically occurs once the eel is around 2 to 3 years old.
Elvers have dark yellow pigmentation and can grow up to eight inches long. At this stage, the young eels go upstream to freshwater bodies. The young eels are omnivores that will eat pretty much whatever they can fit into their mouths, including worms, crustaceans, and insects.
The elvers and the next lifecycle, yellow eels, can remain in these freshwater upstream areas of rivers for as long as 20 years until they mature enough to migrate back downriver and out into the ocean.
Unfortunately, elvers are an extremely popular dish, and that has caused problems with eel sustainability in the past.
In the next lifecycle stage, the elvers develop darker pigmentation to become yellow eels. At this stage, the eels begin to develop their true adult colors, including olive green, brown, black, and yellow.
Interestingly, the eels’ color will largely depend on various environmental factors, including turbidity and temperature. At this stage of their development, juvenile eels can reach over 30 inches in length.
Yellow eels continue to inhabit one freshwater ecosystem after another until they are ready to enter their final life stage, the silver eel.
Silver or Adult Eels
Not much is known about silver or adult eels, and we can’t really tell how old they are by their appearance alone. However, it is accepted that an eel isn’t considered fully mature until it has developed sex organs, which happens in its final adult stage.
Male adult eels can reach two feet long, while females often grow up to 4 feet long or more. At this stage, the eel’s color changes drastically from its dark natural shade to a steely silver.
Once the eels have reached adulthood, they are mature enough to make the migratory trip to their spawning grounds.
How Do Wild Eels Reproduce?
Incredibly, most freshwater American and European eels make the thousand-mile-plus journey to the Sargasso Sea. This 2-million-square-mile area of saltwater in the Atlantic Ocean is the breeding ground for millions of eels of various species.
It’s thought that female eels release millions of eggs that the males then fertilize. unfortunately, the adult eels die soon after spawning, leaving the eggs to hatch and continue the previous lifecycle.
What Are Some Different Types of Eels?
As mentioned previously, there are 800 species of known eels throughout the world, and many new species are still being discovered.
Here’s an overview of some of the most common eels found today.
European eels are freshwater eels that have a lifespan of around 85 years old in the wild and up to 55 years old in captivity! unfortunately, this species is currently listed as critically endangered on the IUCN Red List because of overfishing, parasites, and habitat destruction.
These eels can grow up to 3 feet in length, spending their adult lives in freshwater streams and rivers throughout much of Europe, and migrating to saltwater to reproduce, typically the Sargasso Sea in the Atlantic Ocean.
Unlike other species, European eels continue traveling upstream until they reach sexual maturity. These creatures have been found in isolated lakes and ponds, as well as in seasonal ecosystems that are prone to drying out during drought seasons, which confirms the eels’ ability to travel overland and survive for long periods buried in the substrate.
To help the European eels declining numbers, successful breeding programs have been put in place.
Shortfin and Longfin Eels
Shortfin (Anguilla australis) and Longfin (Anguilla dieffenbachii) eels originate from New Zealand. Unfortunately, these eel species are considered near threatened and endangered, respectively.
Like their European cousins, both Shortfin and Longfin eels have a long lifespan. However, these eels live in freshwater lakes and streams and migrate to different regions of the Pacific Ocean to spawn.
Japanese eels (Anguilla japonica), as their name suggests, come from Japan, Vietnam, China, and Korea, spending much of their lives in freshwater but migrating to the sea to spawn, specifically the North Equatorial Current of the Western North Pacific Ocean.
Unfortunately, Japanese eel populations are declining due to habitat loss, changing ocean temperatures, and fluctuating salinity, and are regarded as endangered. These creatures are also regularly seen on restaurant menus as Unagi, Although regulations are now in place to prevent over-harvesting.
What’s the Difference Between Freshwater and Saltwater Eels?
Although freshwater eels spend their adult lives in freshwater habitats, they must migrate to saltwater to reproduce. In contrast, marine eels spend their entire adult lives in saltwater, with some preferring coastal Lagoon habitats and others, the open ocean.
The main difference between the two types of eels is their body texture and appearance. Freshwater eels are typically firmer in texture, whereas saltwater eels are more lean and softer. In addition, saltwater eels tend to be more colorful than their freshwater cousins, which is why you often see them in a marine aquarium.
How Do Eels Survive Migration?
To reproduce, eels must migrate for thousands of miles, transitioning from freshwater to saltwater. But what do the eels eat during their migration? And how do they know where to go?
Well, most eels don’t eat at all during their migration. In most cases, the creature’s stomach deteriorates, and the blood vessels around the swim bladder increase to give the animal additional support while it’s swimming.
The eels’ kidneys adapt to hold more salt, increasing internal salinity levels to compensate for changing external saline levels. In addition, visual adaptations are made to enable the eel to navigate in deep water and in the nocturnal environment.
The European eel migration holds the record for one of the longest migrations by any marine creature. The eels begin their 3,000-mile migration in the autumn, arriving at the Sargasso Sea in late winter and early spring following a grueling 80 to 170-day journey.
It’s thought that eels use all their senses to navigate their journey, relying on lunar cycles and waiting for the perfect conditions regarding salinity, temperature, current, and tides before making their move.
How Do Eels Reproduce in Captivity?
Thanks to their incredibly complex life cycles, eels are not bred in the home aquarium, so the eels you find for sale are largely wild-caught or commercially raised on eel farms.
In fact, the good news for endangered eel species is that rather than taking wild specimens for food markets, large-scale breeding operations have sprung up around the world.
Here are the answers to a few of the most commonly asked questions about eel reproduction.
Q: Do eels have a reproductive system?
A: Yes, both sexes of eels do have a reproductive system but it doesn’t develop until the eel reaches the final stage of its life cycle when the creature becomes sexually mature and develops reproductive organs deep within its abdominal cavity.
Q: Can eels reproduce in captivity?
A: Because of the eel’s complicated migration and the many years it spends living in different environments, it is not possible to breed them in your home aquarium. However, there are now many lucrative commercial eel breeding operations around the world that produce eels for the food market.
Q: Are eels egg-layers or livebearers?
A: Eels are egg-layers, with the female depositing eggs that are then fertilized by the male through external fertilization. Once spawning is complete, the adult eels die.
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Eels have an incredibly complex reproductive cycle, going through five separate life stages before reaching sexual maturity as adults. During that time, the eels migrate from freshwater to saltwater environments to spawn, sometimes traveling for thousands of miles, depending on the species.
Once the eels have spawned, they die, leaving the eggs to hatch and continue the creature’s complicated development.
Unfortunately, many species of eels are now considered to be critically endangered or at risk, largely due to human activity, such as overfishing and habitat destruction. Fortunately, many commercial eel breeding operations have sprung up in recent years, reducing the burden on wild populations.