This Air-Breathing Swamp Eel Is Quietly Taking Over the Florida Everglades

Deep in the Florida Everglades, the latest invasive threat to wildlife lies hidden in the marsh, slithering through the water in search of prey and, at times, burying itself in the mud. As a species native to places like India, Burma, and China, this species is a new kind of predator that Florida’s native fish and wildlife aren’t used to competing with.

If this animal sounds like a Burmese python, that’s because the serpent-like fish — known as Asian swamp eels — have a lot in common with the big, invasive constrictors. According to some experts in South Florida, the invading eels could pose a similar threat to native food webs and restoration efforts there.

“In a sense, this is an even worse invader than the python. We’re talking about a species that is fundamentally altering wetland production,” says Dr. Nathan Dorn, an associate professor at Florida International University who specializes in freshwater ecology and predator-prey interactions in the Everglades. “Don’t get me wrong. I think both of them are devastating predators. But we’re talking about the same level of concern.”

These concerns have taken root over the last six to 10 years as swamp eels have expanded their range into the freshwater systems of the Everglades. They feed primarily on crayfish, as well as other, smaller forage fish that many native species rely on, too. Studies led by Dr. Dorn and other researchers have shown that in some wetland areas where swamp eels are proliferating, crayfish populations have declined by 90 percent or more. 

This is bad news for the species that eat crayfish — a long list that includes most wading birds and small mammals, along with alligators, turtles, bass, gar, and other gamefish. One 2025 study that Dorn co-authored looked specifically at how this reduction in prey biomass has affected nesting wading birds, which are important indicators of ecological health.  

“This is an observational data set, so you have to rule out all the other possible hypotheses on what’s happening [with those crayfish populations],” Dorn tells Outdoor Life. “But there really isn’t a great explanation for these dramatic declines other than the invasion of this eel, which we know eats crayfish.”

Adding to these concerns are all the things biologists have yet to learn about Asian swamp eels, a grouping that includes multiple subspecies in the Synbranchidae family. And unlike Burmese pythons, which live near water but can be tracked and hunted by humans on dry land, Dorn says there are no established methods for controlling or eradicating swamp eels. 

Beady-Eyed, Air-Breathing Hermaphrodites 

We do know that Asian swamp eels were first introduced to the freshwater canals around Miami in the late 1990s. A popular food source in countries like Indonesia and China, they were brought to Florida and other states as part of the live-food trade and were dumped into urban canals. From there, the eels moved south- and westward into the Everglades.

Dorn explains that by 2007, swamp eels were present in the far southern portions of the Everglades, and by 2012, they had spread into the larger rivers and drainages within the National Park. As of 2021, they’ve moved northward into Broward and Palm Beach Counties, and the eels can now be found in most wetland environments throughout the region.

Part of the reason they’ve adapted so well is because they evolved in the rice fields of Southeast Asia, where they’re also called rice eels. They have what Dorn calls a modified breathing system “that is somewhere between a gill and a lung,” which allows them to gulp air and breathe water. They can also burrow into the mud and sediment as water levels fall and lie dormant during droughts. One of the only ways researchers are finding the eels is through electro-fishing surveys.

“How far down they can go and what their tolerance is for drought is hard to tell,” Dorn says. “But what is clear from our captures is that you can find them in a wetland that has gone dry, and they’ll be there two months after it re-floods when there are basically no other fish present.”

Dorn thinks this resilience amid drought is one of the reasons they’ve had such devastating effects on crayfish populations. Unlike the native predators that are knocked back by drought, swamp eels are already lying in wait as water levels rise, prompting crayfish and other forage fish to repopulate the wetlands. 

“You can see the effects of these animals establishing themselves. Because we had a long record of time where we had no eels in the system, and we can basically model and predict what crayfish densities should be given water depths today,” Dorn says. “Some of those same [crayfish] populations are down between 85 and 99 percent.”  

Asian swamp eels are novel hunters, too. Unlike an ibis or a largemouth bass, which mostly feeds during the day and uses its large eyes to hunt, swamp eels have tiny eyes and are more active at night.

“They appear to be ambush nocturnal predators, slowly slithering along the bottom or along vegetation until they detect something,” Dorn explains. “We can’t tell if [they’re] using mechanoreception or chemical reception or even electro-reception to identify prey. But it appears that they’ll get within 10 centimeters and then just suck [the prey] right in.

“So, this is a very different kind of predator,” Dorn continues. “And for small animals in the Everglades, I don’t even know exactly what the correct response would be … or what kind of prey traits would allow them to coexist with this animal. We’ve puzzled over that one.”

As if the beady-eyed eels were strange enough, they are also sequential hermaphrodites. That means all swamp eels are born females, and some become males.

An Unchecked Invasion with No Control in Sight

The biggest Asian swamp eels can stretch up to three feet long, making it harder for other critters to feed on them. Dorn says bigger birds like blue herons will eat them, as will alligators and larger fish. But it’s not enough to make a noticeable dent in their populations. 

“In terms of natural control, I don’t think we’re seeing anything like that yet,” Dorn says. “Their abundances are either flat in some of these wetlands where they’ve been present for years, or they’re still growing. And when I say they’re abundances are flat, I mean they’re still dominating the assemblages of larger fishes.”

As for humans controlling their populations, we still have more to learn about swamp eels before we can figure out how to remove them. And with so many other invasives now eating their way through the Everglades — species like Burmese pythons, Nile monitors, green iguanas, and cane toads — federal and state wildlife managers haven’t always paid enough attention to the impacts that Asian swamp eels are having. Dorn says that in some cases, they’ve also underestimated the eels, which have smaller mouths and seem less threatening than a 200-pound python that can swallow a deer whole.

Read Next: What It’s Like to Get Dragged Through a Florida Swamp by a 200-Pound Python

But by driving declines in crayfish, which underpin the wetland food web, swamp eels have the potential to disrupt the entire system. Dorn prefaces this concern by noting that, so far, they’ve only been able to measure these impacts in a small part of the Everglades.   

“If this is an indicator of what the eels are going to do … and I’m speaking cautiously as a scientist. But if they do have that kind of effect, it could lead to a collapse, or a partial collapse, of the ecosystem function.”

The post This Air-Breathing Swamp Eel Is Quietly Taking Over the Florida Everglades appeared first on Outdoor Life.