Asian Swamp Eel (Monopterus albus) — identification, habitat & look-alikes

The Asian swamp eel is an air-breathing freshwater fish from Asia that has become invasive in parts of the U.S., including Florida, Georgia, and Hawaii. Where it has spread, it can sharply cut the numbers of native crayfish and small fish. Despite the name and shape, it is not a true eel.

Appearance

Appearance: Usually 10–16 in (25–40 cm), sometimes up to about 3 ft (1 m). Smooth and scaleless, with no pectoral or pelvic fins and a single V-shaped gill opening under the throat. Most are olive-brown with small dark flecks and a yellowish belly; Florida also has orange, pink, or calico ones, likely from the aquarium trade.

Look-alikes

American EelAnguilla rostrata

American eels have pectoral fins just behind the head; swamp eels have none.

Why they get mixed upBoth are long, slimy, eel-shaped fish in Florida waters.

SirensSirenidae

Sirens are salamanders with small front legs (their cousins the amphiumas have tiny front and back legs). Swamp eels have no limbs at all.

Why they get mixed upThey share the same muddy southeastern swamps and look eel-like.

Gangetic Swamp Eel (Cuchia)Ophichthys cuchia

Cuchia have bigger brown spots, about 1/8 in (2–3 mm) or more across; Asian swamp eels have tiny flecks under 1 mm. Other differences (tiny tail scales, air sacs, tooth rows) need the fish in hand.

Why they get mixed upAnother introduced swamp eel found in Florida.

Where it lives

Typical habitats: canals
ponds
marshes
slow streams
Range: Native to East and Southeast Asia, from India and Burma east to China, Korea, and Japan. Established in the U.S. in Florida (since the late 1990s, including canals near Miami, the eastern Everglades, and the Tampa area), Georgia (ponds and marshes near Atlanta along the Chattahoochee since about 1990), and Hawaii (Oahu, probably brought by Asian immigrants as a food fish in the 1800s). It was also found in New Jersey in 2008 and 2012, status unknown. U.S. populations most likely came from aquarium releases, fish-farm escapes, or the live food-fish trade.
Preferred conditions: Slow-moving fresh water with soft bottoms. Florida populations can handle somewhat salty water, so coastal marshes may help it spread.

Behavior

Diet: Fish, crayfish and shrimp, insects, frogs, and worms. In one Florida study, fish turned up in over half of stomachs.
Activity: Nocturnal. It burrows into mud or hides in crevices by day, and gulps air at the surface, so it can live in water with very little oxygen.

Life cycle

Nest: Depends on the lineage: in Chinese and Japanese forms the male builds a floating bubble nest and guards the eggs, while the most widespread form lays eggs among floating plant roots with no care.
Time to maturity: Every swamp eel starts life as a female, and some later turn into males. South Florida populations are 90–98% female.

Cautions

Cautions: It can carry Gnathostoma roundworms; eating raw or undercooked swamp eel is a leading cause of human infection in parts of Southeast Asia, such as Thailand and Vietnam.

Name and classification

About the name: What is called Monopterus albus may be several look-alike species. U.S. populations came from at least three different parts of Asia — Atlanta's likely from Japan or Korea, Tampa's and North Miami's from southern China, and Homestead's from Southeast Asia.