Information Last Reviewed Spring 2007
Significant adverse impacts have yet to be documented, but due to their predatory nature, Asian swamp eels appear to have the potential for adverse environmental impacts in North America. There is concern, for example, that they may disrupt the Everglades National Park ecosystem.
Swamp eels are not true eels (order Anguilliformes). True eels have scales, dorsal, anal and caudal fins, and functioning pectoral fins. True eels have two gill openings and the gill membranes are not united. An obvious difference between the two groups is that anguilliform eels have distinctive, planktonic larvae that are thin, transparent, and can be quite large and leaf-like in appearance. As such, the Anguilliformes is a group of fish with largely ancestral characteristics and placed in the teleost infradivision Elopomorpha. Swamp eels have mostly derived characteristics and are placed in the infradivision Eutelostei and are percomorphs.
Scaleless, elongated body with a tapering tail and blunt snout
Teeth appear like bristles
One V-shaped gill located beneath the head
May be mistaken for native American eel (Anguilla rostrata), except that American eel have pectoral fins, rayed dorsal, anal and caudal fins, and scales (although embedded).
May be mistaken for lamprey, but these latter fish have no jaws and have ovoid mouths; lampreys also have distinct dorsal and caudal fins and seven gill openings on each side. (Fig. 1)
Body is cylindrical with the tail compressed and tapering to a point (Fig. 2)
Snout bluntly rounded (Fig. 3)
Thick upper jaw may slightly overlap lower jaw (Fig. 4) Small eyes are covered with a layer of skin (Fig. 4)
Lateral line is well defined
Slate brown or greenish in color; ventral portion is lighter in color (often white, orange, or light brown) with darker spots along the sides
Dorsal, caudal, and anal fins are diminished to skin fold in adults
Length of up to 100 cm in US waters (average 25-40 cm), larger in native Asian range
Males typically larger than females
Gill opening is reduced to a single V-shaped slit located on the underside of the head, which enables air bubbles to be easily trapped in the gill chamber
Nocturnal; rarely seen by humans
Able to burrow to a depth of 1.5 m
Capable of moving over dry land for short distances (Fig. 5)
Able to relocate due to ability to travel in water or on land
Broad range of prey, including fish, shrimp, crayfish, frogs, turtle eggs, and aquatic invertebrates (e.g., worms and insects)
The life cycle of the Asian swamp eel takes place solely in freshwater.
All young are females
Begin as mature females
Some females develop into males; males can change back to females if female densities are low; change from one sex to another can take up to a year
Reproduction can occur throughout the year
Eggs are laid in bubble nests located in shallow waters
Bubble nests float at water's surface and are not attached to aquatic vegetation
Up to 1,000 eggs per female per spawning event
Wide variety of freshwater habitats: shallow wetlands, stagnant waters, marshes, streams, rivers, ditches, canals, lakes, reservoirs, and ponds
Depths of <3 m
Appear to tolerate cold temperatures well (successfully established in areas where temperatures fall below freezing)
Able to tolerate wide range of water oxygen levels; if not using gills underwater, can obtain up to 25% of oxygen from air cutaneously
Prefer freshwater habitats, but can tolerate brackish and saline conditions
Asia, from northern India and Burma to China
North American Distribution
See Fig. 6
Probable Means of Introduction
Stocking as a food source
Escape from fish farms during flooding events
Although the ecological impacts in North American waters are relatively unknown, the following impacts are documented in other regions of the world where they have become established:
Competition may displace native aquatic species
Accelerates the drying of shallow waterbodies (when the species is abundant) during periods of drought via their extensive burrow system, thereby causing additional risk to other organisms
Used as food source in many Asian cultures
Electroshocking devices used for collection (Fig. 7)
Combination of electrical barriers (to deter movement), vegetation removal, and trapping may assist in limiting dispersal and are currently being evaluated
Prohibit release of bait into waterbody or transportation of bait from one waterbody to another (it is suspected that juveniles have been used as bait material)
Prohibit the release of aquarium fish into local watersources
Prohibit intentional stocking
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Sherpa Guide to the Asian Swamp Eel
United States Geological Survey Nonindigenous Aquatic Species
United States Geological Survey Press Release
This report was prepared by Danielle M. Crosier and Daniel P. Molloy (New York State Museum) with assistance from Robert A. Daniels (New York State Museum).