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Southeast Ecological Science Center

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Frequently Asked Questions about
The Asian Swamp Eel

Are there other common names?
In addition to the name swamp eel, other English names used for members of this group of fish include rice eel, rice-paddy eel, and belut.

What is the scientific name?
Swamp eels belong to the family Synbranchidae. The swamp eel introduced to the United States belongs to the genus Monopterus.  There may be more than one species. Until recently scientists have used the name Monopterus albus for all U.S. populations.

What is it?
Swamp eels are fish, but they are not closely related to other living eels or snake-like marine and freshwater fishes. Unlike the native American eel (Anguilla rostrata), swamp eels do not migrate to the ocean to spawn.  The swamp eel family includes more than a dozen species.  They are native to Central and South America, Africa, Australia, and from India to eastern Asia, including much of China. In Asia, swamp eels are widespread and commonly sold live in markets as food for human consumption.

What does it look like?Asian Swamp Eel - click to enlarge
Asian swamp eels may be confused with a number of native animals, including the native American eel, as well as several snake-like amphibians (e.g., sirens and amphiuma). The swamp eel has an elongate or snake-like body with no noticeable scales or fins. The head is relatively short and the teeth are small and not easily seen. The gill opening forms a V shape on the lower throat area. The body and head are dark, sometimes dark olive or brown above, but lighter, often light orange below.  Some individuals are brightly colored with yellow, black, and gold spots over a light tan or almost-white background.  The skin produces a thick mucous layer making the eels difficult to hold.

How big do they get?
Up to nearly 1 meter (about 3 feet) long and about 0.5 kilos (about 1 pound).  Some sources from Asia suggest the swamp eel may grow to about 1.5 meters, but all eels captured in the United States have been less than 1 meter in total length.

Where are they from?
Monopterus albus is native to tropical and temperate parts of eastern and southern Asia. The eel has been found in northern Australia, but it is unclear if the eel is native to that continent.

When were they first found in the United States?
Swamp eels were first introduced to the United States in Hawaii some time around 1900. It was first identified as being present in the continental United States in 1994 based on specimens collected in ponds at the Chattahoochee Nature Center north of Atlanta, Georgia.

Where are they in Florida, and when were they first found there?
The eels were first found in Florida in 1997.  Three populations are known.  Sites include canals in the northern Miami area, a small drainage near southern Tampa Bay, and a canal system close to Homestead near Everglades National Park.

Are they in the Everglades?
The first swamp eels were found in the extreme southeast corner of Everglades National Park in 2007. More research needs to be done to determine if they are elsewhere in the park.

How did they get here?
The method of introduction is not known. The eel may have been introduced as a result of aquarium releases or a fish-farm escape.  Chinese immigrants who wanted to propagate the animal as a food fish reportedly brought the eel to Hawaii.  Similarly, it is possible that individuals who wanted to establish it as a food fish illegally released eels into open waters in Georgia or Florida.

Why should we care about the Asian swamp eel?
The Asian swamp eel has characteristics that make it very adaptable to a new environment.  It has the potential to become widespread in the U.S. and impact a variety of native aquatic and wetland species, and eventually entire ecosystems.

  • It has the potential to spread from South Florida to the southern half of the country because it can survive both in hot and cold climates.
  • It breathes air and can move across land.  This gives it the potential to spread rapidly and makes control difficult.
  • It can increase its population without detection because of its 'secretive' behavior.  It is active primarily at night; during the day it hides in small burrows, crevices or thick aquatic vegetation. 
  • It has no known predators in the United States.

What do they eat?
Asian swamp eels are predators that eat a variety of animals including crayfish, shrimp, worms, frogs, tadpoles, and other fishes.

What impact are they having on the ecosystem?
At present scientists say that it is too early to predict what the impacts of the eel will be.

What's unusual about the species?

  • It breathes air and in addition can achieve up to 25% of respiration cutaneously (through the skin).
  • It is able to survive weeks without food.
  • After spending part of their life as females, some individuals undergo sex reversal, transforming into males.  All larger individuals are males.
  • The large males construct bubble nests at the mouth of burrows and guard the eggs and young. A high degree of parental care is given probably assuring greater reproductive success.
  • Because they are air breathers, conventional eradication measures, such as ichthyocides (chemicals which deny oxygen to the gills), are ineffective.

How can the spread be prevented?
Asian Swamp Eel - click to enlarge
The USGS focus is to document the eels' geographic distribution and to learn as much as possible about its behavior and biology.  The resulting information is considered critical in helping to develop strategies aimed at containing or controlling its spread.  Meanwhile, catching and transporting for use as bait, food, or aquarium pets is highly discouraged.

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