Taxonomy & naming
Anguilla bengalensis was described by the British naturalist John Edward Gray in 1831 from material collected in India, making it one of the earlier-described members of its genus. The Catalog of Fishes (Eschmeyer, CAS) — the authority for valid names used throughout this site — records the species in the genus Anguilla Garsault, 1764, family Anguillidae, order Anguilliformes. Two subspecies have been recognised: the nominotypical A. bengalensis bengalensis from the Indian subcontinent and A. bengalensis labiata from East Africa and Madagascar, though the validity and boundaries of these subspecies remain a matter of ongoing study.
The genus Anguilla comprises the true catadromous eels: all some 20 valid species grow in fresh or brackish water and migrate to the open ocean to spawn once and die. Anguilla bengalensis belongs to the Indo-Pacific radiation of the genus, which also includes A. marmorata (giant mottled eel), A. mossambica (Mozambique long-fin eel), and the East Asian A. japonica and A. bicolor. These Indo-Pacific species are morphologically cryptic — they overlap extensively in external appearance and distribution — and DNA barcoding using cytochrome oxidase I (COI) and other mitochondrial markers is now considered necessary for reliable identification wherever two or more species occur sympatrically. The old name Anguilla bengalensis is stable, but field workers and aquarists alike should treat any unsequenced "A. bengalensis" from overlapping range areas with caution.
Morphology
Like all Anguilla, A. bengalensis is a long, muscular, serpentine fish with a continuous dorsal-caudal-anal fin that runs most of the body length and a small but distinct pectoral fin behind the gill opening. The body appears scaleless to the naked eye but is in fact covered in tiny, deeply embedded cycloid scales set in a thick coat of mucus — a characteristic that makes these fish as slippery as they are and that confers sensitivity to copper-based medications and other chemicals. The jaws are well-developed and the mouth fairly broad, suited to seizing prey rather than probing the substrate in the manner of a spiny eel.
Adults reach 23.5–39.5 in or more in standard length under natural conditions; FishBase records maximum reported sizes of 39.5 in and above. Females grow considerably larger than males, a pattern widespread across the Anguillidae and linked to the different energetic demands of producing eggs at sea versus investing in smaller gonads. The "mottled" in the common name refers to a pattern of irregular brown, yellowish-brown or olive-grey marbling on the dorsal and lateral surfaces, though live colour varies with habitat, condition, and age, and overlaps with the marbling of A. marmorata — one more reason species identification by eye is unreliable.
Habitat
Anguilla bengalensis is a freshwater and brackish-water resident during its continental growth phase. It occupies a wide range of lotic and lentic environments: lowland and hill rivers, lakes, irrigation channels, rice paddies, estuaries, and coastal lagoons across the Indian subcontinent and into Bangladesh, Myanmar, and Southeast Asia. The species tolerates a broad range of conditions during this growth phase — temperatures in the tropical to subtropical range, roughly 75–86 °F (75–86 °F) in its core native range, and salinities from fresh through brackish — allowing it to exploit the full river-to-estuary gradient.
Like other Anguilla, it tends to be nocturnal and secretive, sheltering by day in burrows, under rock overhangs, or in dense root masses and emerging at night to forage. The estuaries and coastal lagoons of the Indian Ocean coastline — from India and Sri Lanka east through Myanmar and the Andaman region — form the critical transition zone where juvenile eels recruit from the sea and where maturing adults eventually begin their terminal migration outward.
Feeding
Anguilla bengalensis is a generalist carnivore. In fresh and brackish water it feeds on fish, invertebrates, and crustaceans — benthic prey probed from the substrate and ambushed in crevices or vegetation at night. The muscular, moderately pointed snout and broad gape are well adapted to seizing live prey, and large adults are capable of taking surprisingly large fish and crabs. Worms, insect larvae, shrimp, and small frogs are also recorded in gut-content studies from related species and are consistent with field accounts for A. bengalensis.
In aquarium or pond culture (grow-out operations use wild-caught glass eels as stock), the species accepts formulated feeds alongside natural prey items, though wild eels always require an acclimation period before recognising unfamiliar food forms.
Mating
The mating behaviour of Anguilla bengalensis, like that of all Anguilla, takes place entirely at sea and has never been directly observed. What is understood comes from the biology of related species and from the capture of pre-spawning adults in oceanic waters: at some point in their adult life — triggered by cues that remain poorly characterised for this species — mature eels cease feeding, undergo physiological transformation (eyes enlarge for deep-water vision, the gut degenerates, gonads develop), and migrate from fresh or brackish water into the open Indian Ocean to locate the spawning grounds.
The precise location of A. bengalensis spawning areas is not yet confirmed. Analogues from A. japonica (spawning in the western North Pacific) and A. anguilla (Sargasso Sea) suggest that the Indian Ocean Anguilla group spawns in deep, warm oceanic water far offshore, but this has not been directly confirmed with tracked adults. The mating itself is presumed to involve mass aggregation with external fertilisation, as in other Anguilla.
Breeding
Anguilla bengalensis cannot be bred in captivity, and this is not a limitation of aquarium technique — it is a fundamental biological fact of the species' life history. The true eels are semelparous: they spawn once, at sea, and die. No individual survives to spawn a second time, and no captive facility has successfully induced or completed the full reproductive cycle in any Anguilla species. Eel "farming" across South and Southeast Asia — where A. bengalensis is an important food species — depends entirely on catching wild glass eels (post-leptocephalus juveniles arriving at the coast) and growing them out in earthen ponds or recirculating systems. There is no captive broodstock.
After spawning at sea the fertilised eggs hatch into leptocephalus larvae — transparent, laterally compressed, leaf-shaped animals that are a wholly different morphology from the adult. These larvae drift on ocean currents, potentially for a year or more, gradually approaching coastlines where they metamorphose into glass eels (small, pigmentless juveniles) and then elvers before recruiting into rivers and lagoons to begin the continental growth phase. There is no parental care at any stage.
In the aquarium
Anguilla bengalensis is not a practical aquarium fish for the vast majority of hobbyists, and it is not a species this site recommends for the home tank. Adults reach 23.5–39.5 in or more, are powerful, and are capable of forcing through surprisingly small gaps; like all Anguilla and indeed most elongate, scaleless, nocturnally active fishes, they are consummate escape artists, and a tank without a fully sealed, weighted lid will lose one overnight. The thick mucus coat that makes these eels slippery also makes them sensitive to copper-based medications and to rapid or extreme changes in water chemistry.
Where juveniles are kept for educational or research purposes, a large aquarium with deep soft substrate, caves, and dim or nocturnal lighting is appropriate. They are solitary, aggressive toward conspecifics, and will consume any fish or invertebrate they can swallow. In many parts of its range A. bengalensis has cultural significance as a food fish and is subject to fisheries regulation; hobbyists should verify the legal status of the species in their jurisdiction before acquiring one. It is not a fish suited to a community tank, and its eventual size and dietary requirements make it unsuitable for most private aquaria.
Conservation
Anguilla bengalensis is assessed as Near Threatened (NT) on the IUCN Red List — a step short of the threatened categories, but with the direction of travel downward. The Anguillidae as a family are under severe pressure globally, and several of the best-studied species (A. anguilla, Critically Endangered; A. japonica, Endangered; A. rostrata, Endangered) face population collapses driven by overfishing, glass-eel poaching, dam construction that blocks upstream migration, habitat degradation, and in the Atlantic the Anguillicola crassus nematode parasite. The pressures on A. bengalensis are broadly comparable: intensive harvesting of glass eels for aquaculture grow-out operations, widespread damming of South and Southeast Asian rivers, loss of estuarine and coastal lagoon habitat, and pollution.
The fact that reliable species identification requires DNA barcoding compounds the difficulty of setting sound stock assessments and catch limits for A. bengalensis specifically, since harvest records in range states often conflate it with A. marmorata, A. bicolor, or other sympatric congeners. Conservation organisations and fisheries researchers have flagged the data gap for Indo-Pacific Anguilla species as a priority concern. Until a formal assessment is completed, a precautionary stance is warranted.