Taxonomy & naming
Mastacembelus erythrotaenia was described by the Dutch ichthyologist Pieter Bleeker in 1850 from the Indonesian archipelago. The Catalog of Fishes (Eschmeyer, the authority for valid names) records it in the genus Mastacembelus, the type genus of the spiny-eel family Mastacembelidae, within the order Synbranchiformes — the same order that contains the swamp eels. This placement is the key fact about the fish: it is an "eel" only by shape and trade name. The true eels are the Anguilliformes; the spiny eels are not closely related to them at all, and their eel-like body is a case of convergence on a burrowing, elongate way of life.
The genus Mastacembelus is large and spans both Africa and tropical Asia, and its African members were for many years split into separate genera such as Aethiomastacembelus and Caecomastacembelus before being folded back into Mastacembelus. The fire eel itself has always sat comfortably in Mastacembelus and is not part of that taxonomic churn. Its closest aquarium relatives are the other big Asian spiny eels — the tire-track eel (M. favus) and the zig-zag eel (M. armatus) — with which it is sometimes confused when small.
Morphology
The fire eel is the largest of the commonly-kept spiny eels, reaching about a metre in length in the wild, though aquarium fish more often top out around 23.5–29.5 in. The body is greatly elongate and laterally compressed toward the rear, scaleless to the eye and covered instead in tiny embedded scales and a heavy coat of mucus. The snout is drawn out into a fleshy, trunk-like proboscis tipped with tubular nostrils, used to probe the substrate for food. Along the back runs a row of short, isolated dorsal spines — the "spiny" of spiny eel — ahead of the soft dorsal fin, which is continuous with the anal fin around the tapering tail.
The ground colour is dark, from deep brown to near-black, marked with several wavy longitudinal red to orange-red stripes and rows of spots down the flanks, and red edging on the fins. The intensity of the red increases with age and good condition, so young fish are duller and far less striking than adults. There is little obvious external difference between the sexes, though ripe females become noticeably fuller-bodied.
Habitat
Mastacembelus erythrotaenia is native to the lowland fresh waters of Southeast Asia, including the Mekong and Chao Phraya basins of Thailand, Laos, Cambodia and Vietnam, and the great rivers of Sumatra and Borneo. It is a fish of slow to moderate, often turbid lowland rivers, floodplains, canals and lakes, where it lives over soft beds of sand and mud into which it burrows, frequently leaving only the head exposed.
It favours warm, soft to neutral water — broadly the high twenties Celsius, with a pH around neutral — and is most active at night and at dusk. By day it shelters in the substrate, among submerged roots and leaf litter, or in crevices, emerging after dark to forage across the bottom. Because it migrates onto seasonally flooded ground with the monsoon, it tolerates a range of conditions, but it is intolerant of the cool, hard or polluted water that some of its hardier relatives accept.
Feeding
The fire eel is a nocturnal carnivore. In the wild it works the river bed after dark, using its sensitive fleshy snout to root out benthic invertebrates — insect larvae, worms, crustaceans and molluscs — and to take small fishes. The down-turned, probing mouth and the long body are the tools of an animal built to hunt along and within the substrate rather than in open water.
In the aquarium it should be fed meaty foods in the evening: earthworms and bloodworm are particular favourites, alongside other frozen and live invertebrates, and many specimens eventually learn to take sinking pellets. Newly imported fish are often shy and may refuse to feed in daylight or while watched, so feeding after lights-out helps them settle. It is a slow, deliberate feeder and should not be left to compete with fast mid-water fishes at mealtimes.
Mating
Spiny eels are egg-scattering spawners that give no parental care, and the fire eel is no exception. It is not a true eel and does not undertake the marine spawning migration of Anguilla; it breeds in fresh water. Spawning is thought to be triggered by the rising, warming water of the wet season, when adults move onto flooded margins, and in the limited captive accounts a courting pair circle and entwine among vegetation before scattering adhesive eggs.
There is no nest and no pair bond beyond spawning, and the adults do not tend the eggs. Reliable wild breeding observations are scarce because the fish is large, nocturnal and lives in turbid water, so much of what is known comes from a small number of aquarium and farm spawnings rather than from field study.
Breeding
The fire eel is rarely bred in the home aquarium, and most fish in the trade are wild-caught or pond-raised on a commercial scale rather than tank-bred by hobbyists. Its great adult size, the need for a large, mature, soft-substrate set-up, and the difficulty of sexing and conditioning a compatible pair all stand in the way. Where captive spawnings have been reported, they have typically followed large water changes with cooler, softer water that mimics the onset of the rains, with the pair scattering a large number of adhesive eggs among fine plants or roots.
The eggs hatch in a few days and the tiny larvae are extremely small and demanding, requiring infusoria-grade first foods before they can take newly hatched brine shrimp. Because successful rearing is so uncommon outside specialist and commercial operations, the fire eel remains a fish bought as a wild import rather than as captive-bred stock — one reason to buy it knowingly and house it for the decade or more it can live.
In the aquarium
The fire eel is a spectacular but demanding aquarium fish that is very frequently bought far too small for the tank it will need. It reaches well over half a metre and is long-lived, so an adult requires a large aquarium — on the order of two metres of length — with a deep bed of soft sand it can burrow into, plenty of caves, pipes and shaded retreats, and excellent filtration to cope with a big, messy carnivore. Like the other spiny eels it is scaleless and sensitive to copper-based medications and to poor water, so a stable, mature system and careful treatment are essential.
Above all it is an escape artist: spiny eels climb and squeeze through the smallest gaps, breathe at the surface, and will leave an open or loosely-covered tank and die on the floor. A tight-fitting, fully sealed lid is non-negotiable. It is best kept singly or with robust, peaceful tankmates too large to be swallowed, since it will eat small fishes at night and can be territorial toward its own kind. Given space, a sand bed, dim light and meaty evening feeds, it becomes a remarkably interactive fish that many keepers come to recognise their owners.
Conservation
Mastacembelus erythrotaenia is assessed as Least Concern on the IUCN Red List. It is a widespread species across the major river systems of mainland and insular Southeast Asia, it tolerates turbid and modified lowland waters, and it is also farmed and harvested as a food and ornamental fish, so there is no evidence of a range-wide decline at present.
That said, the lowland rivers it depends on face mounting pressure from damming, sand extraction, pollution and habitat conversion across Southeast Asia, and large quantities of wild fish enter the aquarium and food trades. The species is secure for now by virtue of its wide range, but it is a large, slow-maturing predator, and local populations can be depleted where collection and habitat loss combine. Buying a fish that may live well over a decade and reach a metre is a long commitment that the trade too often glosses over.