Freshwater eels spawn in the Sargasso Sea after migrating thousands of miles from freshwater streams and rivers. Researchers believe they spawn via external fertilization. The females release millions of eggs into the water where they are fertilized by the male’s sperm. After this, they die.
The reproduction of eels has long been a mystery. Scientists were baffled for years about how and where these creatures mate. Researchers have been studying eels since ancient Greece, and they are still being studied today.
We knew that freshwater eels were found in rivers across Europe, Japan, and other countries. Still, nobody had ever seen them mate, and they’d never seen them migrate to their spawning areas. Researchers couldn’t even find reproductive organs or eggs during their dissections. So, how do eels reproduce?
Recently, scientists have been making progress learning about the breeding and migration patterns of eels. We’ve made a lot of headway thanks to tracking and housing the animals in captivity.
Even still, there is still so much about the reproductive process of eels that remains a mystery.
Some fish that are labeled as eels are not really eels. For example, the electric eel is not really eels at all. However, the moray eel is a true eel. So, how do you know what an eel really is?
To be considered an eel, the animal must have a set of specific characteristics.
Most fish have separate fins on different areas of their body. Eels have an elongated body with one dorsal fin. The dorsal fin is continuous and combines with the caudel and anal fin.
When it comes to scales, eels may have very small scales that are embedded in the skin. They may also have no scales at all. Along with an absence of scales, eels will produce slime when they feel threatened.
Freshwater eels are called “skin breathers”. They have capillaries under the surface of the skin that they use to absorb oxygen. They also have very sharp teeth.
One significant characteristic that differentiates a freshwater eel is where it lives versus where it breeds. Eels live in freshwater as juveniles and adults, but they migrate to saltwater to breed. However, there are saltwater species, like the moray eel, that spend their entire lives in saltwater.
An electric eel is not a true eel because it lays its eggs in freshwater. To be a freshwater eel, you must travel from freshwater to lay your eggs in saltwater. In addition to that, electric eels have no teeth and no dorsal fin.
Types of Eels: Common Species
1. Common European or American Eel (Anguilla anguilla)
Size: Up to 52 inches (133 centimeters)
Color: Silvery in coloration. As they fully mature, they change in color to greens, yellows, and browns.
Lifespan: Up to 85 years
Habitat: Juvenile and spawning adult eels are found in marine waters as they travel to or from the Sargasso Sea. The rest of their lives, they are found in freshwater or brackish water bodies like streams and rivers. They are found at depths up to 328 feet (100 meters).
Range: The English Channel and coasts along the Mediterranean Sea. They can also be found in the northern Atlantic Ocean from Iceland to Mauritania.
The diet of the American Eel depends on their life stage. The diet is unknown during the leptocephali stage. As glass eels, they eat insect larvae, small crustaceans, and dead fish. Adults will eat primarily aquatic invertebrates, but they will eat just about anything they can find.
Natural predators include larger eels, fish, cormorants, and herons.
2. Shortfin & Longfin Eels: New Zealand Eels (Anguilla australis & Anguilla dieffenbachii)
There are two common types of eels in New Zealand: the Shortfin eel and the Longfin eel.
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: Shortfin eels prefer lowland areas while Longfin eels live in a wide range of altitudes including extreme elevations.
Range: The Shortfin eels are found across New Zealand, eastern Australia, and the Pacific Islands. Longfin eels are found only in New Zealand.
Juveniles will feed on a diet of insect larvae, snails, and worms. The adults will primarily eat fish, but will also eat crayfish and small birds.
Natural predators include perch and rats.
3. Japanese Eel (Anuilla japonica)
Color: Dark brown to black
Range: Japanese and Chinese waters
The adult diet consists of fish, crustaceans, and insects.
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 and fast-flowing waters
Range: The western Indian Ocean of Africa from Kenya to Cape Agulhas, and Madagascar.
Their diet consists mainly of fish and crabs, but they may eat carrion as well.
Freshwater eels really have a complex life cycle, and it wasn’t well understood for the longest time. Even today, there is quite the debate about how much researchers really know about the reproduction of eels. While the life cycle of freshwater eels seems to be understood fairly well, we don’t seem to know much about the reproduction process itself.
The thing that sets freshwater eels apart from marine species is that they also spend part of their life in saltwater. Freshwater eels will spend the majority of their lives in freshwater, but they are birthed in saltwater, and that’s also where they die.
How are they able to do this? Most fish are unable to travel between fresh and saltwater because their cells can’t handle it. How do eels handle the shift?
To accommodate the lack of salt, the eel’s kidneys change so that they’re able to hold more salt. When the kidneys hold extra salt, it maintains the salinity levels in the blood.
During their adult lives, freshwater eels can be found spending time in streams and lakes. When it’s time to mate, they will all swim downstream at once, making their way as a group toward the ocean.
When it’s time to mate, eels are very determined to make it to their breeding site at the Sargasso Sea. The Sargasso Sea, a two-million-square-mile span of ocean, is the site in which all freshwater eels mate.
The Sargasso Sea can be found at Bermuda, the Azores, and the West Indies. Although most eels live extraordinarily far from here, they are not afraid to travel thousands of miles/kilometers to reach it.
After the eels have laid their eggs, the eggs will float to the surface where they hatch. The larvae are very small, shaped like a leaf, and are transparent. At this stage, the larval eels are called “leptocephali”.
The leptocephali will then travel to the freshwater location where they’ll spend the majority of their lives. This is an incredibly long journey. The leptocephali will drift across the ocean for about a year. By the time they make it, they will have transformed again.
This next stage is the “glass eel” stage when the eels are traveling back up into the rivers. Glass eels are called as such because they are transparent. However, glass eels are also about 2-3 inches (5-7.6 centimeters) long and will look almost like a fully developed eel. They have the slender, elongated shape of an adult eel, and their fins have grown in as well.
As glass eels, they are searching for freshwater to make their home. Similar to mating, these eels are very determined to find a freshwater location to make their home. Oftentimes, they will migrate from ponds or lakes until they can find a river. How do they do this? They slither over land like a snake.
As they travel further up the river, their color transforms from transparent to black. At this point, they have reached the “elver” stage where they are about 4 inches (10 centimeters) long. Elvers are basically smaller versions of the adult eel. At this point, they are still migrating further up streams and rivers.
As they age further, the eels will become immature yellow eels. Interestingly, this is the stage in which freshwater eels will live most of their lives. For about 20 years, eels are in this stage, and they are still considered juveniles.
Researchers have found that eels that live in predominantly freshwater locations will live longer and grow larger. Eels that live in brackish areas closer to the ocean mature much faster.
Researchers also aren’t certain when eels develop their sexual organs because they are lacking in juvenile eels. However, they have recorded gonads in eels in their late juvenile stage when they were about 11 inches (26 centimeters) long.
Although they aren’t certain, researchers believe that environmental factors likely determine when eels reach sexual maturity. The temperature and salinity levels are likely a determining factor. It’s even possible that low eel populations will produce more females.
When the elvers finally become adults, it is time for their life to end. Once they hit this stage, they are very determined to continue the life cycle. When it is time to reproduce, males are usually about 15.7 inches (40 centimeters) long, while females can be up to 47.2 inches (120 centimeters).
Sometime between late winter and early spring, the adults will head back down the river. They will travel thousands of miles/kilometers to their spawning site, and then they die.
When eels spawn, it is the last thing they do. They don’t survive the spawning process. Hundreds of eels will gather together to produce eggs, and then they die en masse. But how does the spawning process actually work?
When the eels arrive, the females lay the eggs into the water column. The males will then release their sperm into the water column, creating fertilized eel eggs. It is thought that a single female eel can produce between 2-10 million eggs.
How these eels survive the thousands of miles to the Sargasso Sea is also very interesting. Unlike the 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.
How Do Eels Reproduce in Captivity Vs Eels in the Wild
Many people house eels in their aquariums at home. They’re a popular pet, although a bit difficult to care for. But, what about farming?
Recently Japan, China, Taiwan, Australia, Morocco, European countries, and Scandinavian countries have been experimenting with farming eels in captivity. Japan is the single largest producer.
Eel populations are in rapid decline, so researchers are looking for ways to save them. This is particularly important in Japan where eel is a beloved meal.
The declining populations are thought to be caused by overfishing for the meat industry. It may also be due to the construction of flood-deferrents. Both scenarios prevent mature eels from reaching maturity in rivers and streams.
So far, farms are successfully farming eels, and about 60% of the worldwide eel meat is produced from these farms.
First, eels are caught from the wild and are quarantined for pests and disease. Then, they are grown in ponds or specialized tanks. After 6 weeks, the eels must be separated by size to prevent cannibalism and competition for food.
Researchers have been making progress in breeding eels in captivity, something they weren’t able to do in the recent past. The eels are treated with hormones to help them become sexually mature more quickly. Then, males and females are introduced to instigate the breeding process.
Eels are a good animal to farm because they can adapt and tolerate numerous conditions. They can also survive being housed in large numbers.
The process of figuring out eel reproduction has been a long one. For years, researchers believed that the five different life stages were different animals altogether. It wasn’t until 1896 that leptocephali were recognized as larval eels. Still, they didn’t know where the larva came from.
For 18 years, Danish oceanographer Johannes Schmidt trawled the Atlantic ocean across four continents. He finally found the smallest larva of the time just at the edge of the Sargasso Sea.
For the longest time, researchers believed that eels spawned in the Sargasso Sea. They believed that eels would travel thousands of miles/kilometers from freshwater rivers to reach the Sargasso Sea, but weren’t sure how. They hadn’t observed any actual breeding or migration. They just knew that they found eel eggs there.
A few years ago, this all changed after researchers decided to track American eels. Some Canadian researchers off the coast of Nova Scotia attached GPS trackers to 38 American Eels. 28 of the eels were successfully tracked for a significant distance.
There was one eel that the researchers were able to track for the entire migration. The eel swam to the northern parts of the Sargasso Sea after it migrated 1,500 miles (2,400 kilometers) in just 45 days.
Today, we know a lot more about freshwater eel migration due to these studies. We also know so much more about their breeding habits thanks to the captive breeding programs I described above.
Researchers have come a long way in such a short time. Short, when you consider that eels have been a mystery for hundreds of years.
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How Are Eels Born FAQs
Do Eels Lay Eggs?
Yes, eels do lay eggs. Many of them, in fact. A single freshwater eel can lay as many as 2-10 million eggs during spawning. Now, imagine that hundreds, or even thousands of eels are all spawning at the same time. Millions upon millions of eggs will be produced.
What Are Eel Eggs Called?
Simply, eel eggs are simply called “eggs”.
Eels have a total of five life stages. Leptocephali, the first larval stage, followed by glass eels which are the second larval stage. Then comes the miniature eel stage of the elvers, followed by the yellow eels who are sexually immature adults. Finally, comes the adult eel stage.