Asian swamp eel

What it is

The Asian swamp eel is a scaleless, eel-like fish with a tapering tail and blunt snout. It lacks pectoral and pelvic fins, while its dorsal, anal, and caudal fins are rudimentary or absent. The body is generally olive or brown with dark flecks, though Florida populations often show darker coloring with light orange bellies. This species belongs to the family Synbranchidae, not true eels. It possesses fused gill membranes and a unique V-shaped gill structure that prevents reverse water flow. Adults typically measure between 25 and 40 centimetres, reaching a maximum length of one metre.

Where it lives

This fish is native to tropical and subtropical regions of East and Southeast Asia, extending from northern India and Burma through China, Japan, and the Indo-Malayan Archipelago. Populations in Japan and Taiwan appear to be introduced. In the United States, the species was introduced near Atlanta, Georgia, around 1990, and to Florida by 1997. Large populations now inhabit canal systems in southern Florida, including areas near Miami and Everglades National Park. These American populations likely originated from aquarium releases or intentional stocking for food.

How it lives

The Asian swamp eel is nocturnal and feeds on fish, shrimp, crayfish, frogs, turtle eggs, and aquatic invertebrates. It thrives in shallow, muddy environments such as rice paddies, ditches, ponds, and canals, preferring depths under three metres. The species can survive in low-oxygen water by breathing air directly through its skin, absorbing up to twenty-five percent of its oxygen this way. Contrary to older claims, it does not move over dry land. It faces predation from larger aquatic animals and serves as a host for the nematode Gnathostoma spinigerum.

Breeding and life cycle

All young swamp eels begin life as females, with some maturing into males later in their lives. This sequential hermaphroditism helps maintain population balance when females are scarce. Spawning occurs year-round. Some Japanese and Chinese forms build floating bubble nests at burrow entrances, where males guard the eggs and larvae. Females produce up to 1,000 eggs per event. Other widespread clades do not create nests but lay eggs among plant roots without parental care, relying on larvae using pectoral fins to increase oxygen intake.

Fish and people

The fish is a vital protein source in Thailand and is farmed extensively in Vietnam. In Indonesia, Cambodia, China, Laos, Myanmar, and Thailand, it is raised in polyculture rice fields alongside crops. In Bali, dried eels known as lindung are sold for Hindu offerings. In Japan, it is called ta-unagi and is rarely eaten. In Thailand, releasing these eels into natural waters is a religious practice intended to generate merit and end suffering. However, eating raw or undercooked swamp eel in Southeast Asia is a primary cause of gnathostomiasis, a parasitic disease.

Status and threats

In the United States, the Asian swamp eel is an invasive species threatening native biodiversity. Studies in the Everglades indicate that crayfish and other fish populations have declined significantly following its introduction. Control efforts by the US Geological Survey include restricting water management structures to limit dispersal. Standard piscicides are ineffective because the eels breathe air, bypassing gill-targeted toxins. Authorities discourage catching and transporting the species to prevent further spread.

Relatives and look-alikes

Monopterus cuchia is a closely related species also introduced to the USA. It differs from M. albus by having scales, suprapharyngeal air chambers, and a single row of teeth. While M. albus expands its throat ventrally when breathing, M. cuchia expands the sides of its head. Taxonomists suggest M. albus may comprise multiple cryptic species based on genetic diversity and distinct brooding behaviours across different geographic regions.

Questions people ask about Asian swamp eel

Good reading and sources

• Asian swamp eel on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data