What are Swamp Eels?

When describing swamp eels, it’s often easier to start by explaining what they are not. First things first, they are not eels! Rather, they are elongate percomorphs – a type of ray-finned fish that has a body shape much longer than it is deep. Secondly, they do not have any visible fins, only highly reduced fin folds, giving them a distinctly serpentine appearance. While they can be confused with the native American eel, swamp eels never travel to the ocean to spawn and the American eel has visible fins. Swamp eels are primarily freshwater species, yet they can breathe air and survive on land for extended periods. They can also be distinguished by a unique, V-shaped gill opening located as a narrow slit on their lower throat. Within the family Synbranchidae, there are 28 species and 7 genera of swamp eel, with the four genera of large species being Monopterus, Ophichthys, Ophisternon, and Synbranchus.   

Swamp eels are impactful and successful invaders. In addition to being air breathers, they possess serpentine locomotion, which allows them to move across land during rain and flooding events. Some species are also protogynous hermaphrodites, meaning they are born female and transition to male during their lifetime — allowing small numbers of individuals to found new populations. During droughts, swamp eels are known to burrow themselves in the mud to survive until favorable conditions return. Finally, at least one introduced population of swamp eel can tolerate brackish waters up to 16%, giving it the potential to invade freshwater ecosystems through coastal and estuarine environments. 

Research Background 

Rob Robins, Ichthyology Collections Manager at the Florida Museum of Natural History (FLMNH), has been working with Nick Trippel, Noah Peterson, Ted Lange, Mary Brown, Daniel Nelson, and Jennifer Moran to track the establishment of the Sunda swamp eel (Monopterus javanensis) and Gangetic mud eel (Ophichthys cuchia) across multiple watersheds in Central Florida.  

Swamp eels were first discovered in Florida back in the 1990s and were considered “established” in the Greater Tampa and Miami regions by 1997. Central Florida residents began reporting swamp eel sightings on citizen science platforms such as iNaturalist as early as 2022. The Florida Fish and Wildlife Conservation Commission (FWC) later confirmed the presence of the Sunda swamp eel in Howell Creek, Winter Park, in March 2023. 

To better understand how these invasive fish are spreading, Robins (FLMNH) participated in Fish Slams organized by the U.S. Geological Survey and the Florida Fish and Wildlife Conservation Commission. Other participating agencies included the U.S. Fish and Wildlife Service, the City of Winter Park, Orange County’s Environmental Protection Department, the St. Johns River Water Management District, and the U.S. Army Corps of Engineers. 

A Fish Slam is a multi‑day bioblitz designed to find and identify as many non-native species as possible within a targeted area. These collaborative surveys pool collective resources and focus on sampling waters that are rarely monitored, documenting range expansions, investigating reports of new non‑native species, collecting specimens for museum research, and contributing data to the USGS Nonindigenous Aquatic Species database. 

During one of these Fish Slams, the team found something unexpected in Lake Underhill in Orlando, FL: the Gangetic mud eel or “Cuchia” (Ophichthys cuchia). This discovery on October 24, 2023 marked the first time the species had been documented in the wild in Florida. With that collection and many more that followed, it is now confirmed that Central Florida is home to two non‑native swamp eel species — Monopterus javanensis and Ophichthys cuchia. 

Why This Research Matters 

Swamp eels are commercially important live food fishes. In the United States, they are imported from Southeast Asia and sold in live food markets. This “live food pathway,” is one of the ways non-native species have entered the United States and been introduced to the wild. Surveys of live food markets across the country have repeatedly documented the Sunda swamp eel and Gangetic mud eel being kept alive in tanks until purchase. When live animals are transported and sold this way, the chances of escape or intentional release increase significantly. 

Once released, the same attributes that allow them to survive the transport and housing conditions of the live food pathway allow them to survive in new areas. They breathe air, tolerate a wide range of water conditions, move across land during rain events, and burrow into mud during droughts.  

One possible approach to reducing future introductions is point of sale education. This would involve informing vendors and consumers about the ecological risks associated with releasing live food fish into local waterways. The challenge with this approach is that it is still unclear which agency would be responsible for creating and funding this type of outreach. The Florida Fish and Wildlife Conservation Commission, the U.S. Fish and Wildlife Service, and other partners all play roles in invasive species management, but no single group currently oversees education specific to the live food trade. 

Regulation is another important piece to consider. Under the Lacey Act, species can be listed as injurious if they pose risks to ecosystems, native wildlife, or human health. To make that determination, agencies consider factors such as the likelihood of escape, the ability to survive transport, the potential to establish and spread, and the difficulty of control. Swamp eels meet many of these criteria. They are air breathing, capable of moving across land, known to feed on native species, and may carry parasites known to cause disease in humans. Despite this, they continue to appear in live food markets, which means the risk of new introduction and spread remains. 

This is why ongoing research and monitoring are essential. Understanding how these fish arrive, where they spread, and what impacts they have provides the foundation for public education efforts, potential regulatory oversight or legislation, and species management strategies. 

 

Information from:  

Claverie T, Wainwright PC. A morphospace for reef fishes: elongation is the dominant axis of body shape evolution. PLoS One. 2014 Nov 19;9(11):e112732. doi: 10.1371/journal.pone.0112732. PMID: 25409027; PMCID: PMC4237352.

Florida Museum of Natural History, National Geographic, NatureServe  

Sakaris, P.C., Galvez, J., Callier, T.P., IV and Brown, A. (2019), Ontogenetic and Temporal Diet Shifts of the Invasive Asian Swamp Eel in South Florida. North Am J Fish Manage, 39: 1288-1300. https://doi.org/10.1002/nafm.10362 

United States Fish and Wildlife Service, United States Geological Survey