Monopterus is a genus of swamp eels (family Synbranchidae) — eel-shaped but not true eels, unrelated to the Anguilliformes despite the body plan. They are air-breathing, burrowing fish of Asian rice paddies, ditches, swamps and slow-moving fresh and brackish water, recorded in the atlas from India, Indonesia, Malaysia, Thailand, Vietnam, Bangladesh, China and Japan. Most species lack paired fins entirely and push through mud and vegetation with lateral undulation, surviving in low-oxygen water by gulping air through a modified, richly vascularized mouth and pharynx. The atlas covers 9 species, and the size range is unusually wide — from about 3.5 in up to roughly a metre in the largest species.
Monopterus albus, the Asian swamp eel, is the species most people encounter — it is farmed and eaten across much of its range, has been introduced outside Asia, and is protogynous, meaning individuals begin life as females and some later change sex to male as they grow, a trait shared across much of the genus. Aquarium interest is limited and specialist: swamp eels are secretive burrowers that need a soft substrate to tunnel in, a tight lid, since they are accomplished escape artists, and tankmates that won't be eaten, as larger individuals are opportunistic predators. Conservation status is mixed and, for several species, simply unknown — the atlas tally runs two Least Concern, three Not Evaluated, two Data Deficient, one Vulnerable and one Critically Endangered, against the abundant, farmed M. albus.