Taxonomy & naming
Monopterus albus was described in 1793 (as Muraena alba) and is placed by the Catalog of Fishes (Eschmeyer, the authority for valid names) in the genus Monopterus, family Synbranchidae, order Synbranchiformes — the swamp eels. Like the spiny eels, the swamp eels are not true eels: their eel shape is convergent, and their nearest relatives are the spiny eels and the labyrinth fishes, not the marine and catadromous Anguilliformes. A number of nominal genera, including Amphipnous and Ophichthys, have been folded into Monopterus over the years.
The "Asian swamp eel" is now understood to be not a single species but a complex of several genetically distinct, cryptic lineages spread across its huge Asian range, which look almost identical but differ in DNA and, sometimes, chromosome number. Taxonomists continue to disentangle these, so the name Monopterus albus as used here covers a group of closely related forms rather than one uniform fish — a caution worth remembering for any account of its biology.
Morphology
The Asian swamp eel has a long, cylindrical, tapering body reaching up to about a metre, though most are far smaller. It is scaleless and almost finless: the paired fins are absent entirely, and the dorsal, caudal and anal fins are reduced to a low fold of skin around the pointed tail. The two gill openings are confluent into a single V-shaped slit beneath the head, an arrangement that gives the order its name (Synbranchiformes, "fused gills"). The eyes are small and the body is coated in heavy mucus.
Colour is plain and variable — olive, grey-brown or yellowish above, paler below, sometimes finely speckled or with a faint dark mid-line. There are no bright markings. Because the species is a protogynous hermaphrodite, size and sex are linked: the largest individuals are males, while smaller fish are female, and external sexing is otherwise difficult. The gill apparatus is much reduced, for this is a fish that gets much of its oxygen from the air.
Habitat
Monopterus albus is native to a vast swathe of East, South and Southeast Asia, from India and Myanmar across southern China to Japan and south through Indochina and the Malay archipelago. It is a fish of still and sluggish fresh waters — rice paddies, marshes, swamps, ditches, ponds and the muddy margins of slow rivers — rather than of clear, flowing streams.
Its defining trait is air-breathing: the lining of the throat and gut is richly supplied with blood vessels and takes up atmospheric oxygen, so the fish thrives in warm, stagnant, oxygen-poor water that would suffocate most others, and it can survive out of water for long periods if kept moist. In the dry season it burrows deep into damp mud and aestivates, waiting out the drought until the rains return. This combination of air-breathing, drought tolerance and a taste for warm muddy water is exactly what makes it so successful — and so invasive where it escapes.
Feeding
The swamp eel is a nocturnal, opportunistic carnivore and scavenger. It feeds on a wide range of animal prey — insects and their larvae, worms, crustaceans, molluscs, frogs, tadpoles and small fishes — taken from the bottom and from among vegetation, and it readily takes carrion and detritus. Its small eyes and the sensory pores of its head suit a fish that hunts by touch and smell in muddy water and after dark.
In captivity, where it is kept far less often than the spiny eels, it accepts earthworms, prawn, fish pieces and other meaty foods. Its broad, undemanding appetite is part of why it does so much damage as an invader: in the Florida Everglades it preys heavily on native fishes, crayfish and amphibians, and its impact on those ecosystems is a subject of active study and concern.
Mating
The reproductive biology of the swamp eel is one of the most distinctive among aquarium-relevant fishes, and is nothing like that of either the spiny eels or the true eels. The species is a protogynous hermaphrodite: individuals mature first as females and, as they grow larger and older, change sex to become males. A population therefore tends to have small females and large males, with a transitional phase in between.
Breeding takes place in fresh water in the warm season. A male excavates and occupies a burrow in the bank or mud and builds a floating nest of mucus bubbles at its mouth, into which eggs are spawned. There is genuine parental care here — the male guards the bubble-nest and the developing eggs and larvae — a striking contrast to the egg-scattering spiny eels and to the true eels, which give no care at all and spawn only once, far out at sea.
Breeding
Monopterus albus is bred on a large scale in aquaculture across Asia, where it is an important food fish, but it is seldom deliberately bred by aquarium hobbyists. In farm and field settings, males establish bank burrows in shallow, warm, vegetated water and construct the characteristic bubble-nest; females deposit their relatively large, buoyant eggs there, and the guarding male tends them until after the larvae hatch.
The sex-changing life history complicates any breeding effort: a working group needs the right mix of smaller females and a dominant male, and the slow protogynous transition means sex ratios shift over time. The eggs and the air-breathing young are robust by the standards of swamp-dwelling fishes, an adaptation to the unstable, deoxygenated waters in which they develop. For the hobbyist the species is more a curiosity and a cautionary tale than a breeding project — and in many places it is illegal to keep or release because of its record as an invasive.
In the aquarium
The Asian swamp eel is kept only occasionally, and it is a fish for the specialist rather than the community tank. It grows large, is a determined predator that will eat any tankmate it can swallow, and is a supreme escape artist — an air-breathing, snake-bodied animal that climbs, squeezes through tiny gaps and can travel over damp ground, so an absolutely sealed lid is essential. Being scaleless, it is sensitive to copper-based medications and to dissolved pollutants, despite its tolerance of low oxygen.
It needs a large tank with a soft substrate it can burrow into, secure hiding places, and a tight cover, with warm water and gentle filtration. Crucially, it must never be released or allowed to escape into the wild: this is one of the world's more damaging invasive freshwater fishes, established in the southeastern United States and elsewhere, and its keeping is restricted or banned in many jurisdictions. Anyone tempted by its extraordinary biology should check local law first and commit to keeping it contained for life.
Conservation
As currently recognised, Monopterus albus is assessed as Least Concern on the IUCN Red List. It is abundant and extremely widespread across its native Asian range, tolerant of degraded and modified habitats, and supported rather than threatened by the aquaculture and fisheries built around it, so it is in no danger as a species.
The real conservation story runs the other way: this is a fish that threatens other ecosystems. As an introduced predator in Florida and beyond it preys on native fishes, crustaceans and amphibians and is extremely difficult to eradicate once established, thanks to the very air-breathing and drought tolerance that make it so hardy. There is also a taxonomic caveat — because "M. albus" is a complex of cryptic lineages, some local forms may have far smaller ranges than the blanket Least Concern listing implies, and their status cannot be judged until the group is properly resolved.