Taxonomy & naming
Heterotis niloticus was described by the French naturalist Georges Cuvier in 1829, and the genus Heterotis is monotypic — this is the only living species in it. It sits in the order Osteoglossiformes, the bonytongues, one of the most ancient surviving lineages of teleost (ray-finned bony) fish, with a fossil record reaching back into the Cretaceous and a present-day distribution scattered, relict-like, across South America, Africa, Southeast Asia and Australasia. Within that order it is usually placed in the family Arapaimidae alongside the giant South American arapaima, its closest living relatives, rather than with the true arowanas of the genus Osteoglossum and Scleropages.
The common name "African arowana" is a trade convenience rather than a close taxonomic tie to the New World and Asian arowanas — the resemblance is a family-level one (elongate bonytongue body plan, large scales, single continuous dorsal-fin base) rather than a sign of shared recent ancestry or shared feeding habits. "Bonytongue" itself refers to the toothed basihyal bone on the floor of the mouth that bites against toothed dermopalatine plates on the roof, a structure shared by the whole order and used in classification.
The genus name Heterotis is generally read as Greek heteros, "different", combined with ous/otos, "ear" — usually explained as a reference to a distinctive bony feature of the skull or ear region that Cuvier used to separate this fish from its relatives. The species epithet niloticus is Latin for "of the Nile", after the river system in which the species was first characterised, though its true range extends across most of the Sahelo-Sudanian belt of Africa, well beyond the Nile itself.
Morphology
The African arowana has a heavy, elongate, sub-cylindrical body quite unlike the laterally compressed, upturned-jawed silhouette of the true arowanas. The dorsal and anal fins are both long-based and set far back along the body, running close to a rounded caudal fin, and the fish is covered in large, tough cycloid scales. Colouration is a plain silvery-grey to olive on the body, often with a pinkish or reddish cast to the fins, and juveniles can show a more mottled, olive-brown pattern that fades with age.
The mouth is comparatively small and terminal rather than the large, upward-gaping mouth of a piscivorous arowana — a direct clue to its very different feeding strategy (see "Feeding"). Reported maximum size is around 39.4 in total length and up to roughly 10 kg, making it one of the larger Osteoglossiformes an aquarist might encounter, second only to the arapaima among the group's African and South American members.
Like other members of its order it is an obligate air-breather, using a highly vascularised, lung-like modification of the swim bladder to gulp atmospheric air at the surface — an adaptation that lets it survive in the warm, oxygen-depleted floodplain waters that would suffocate most other fish.
Habitat
Heterotis niloticus is native to a broad swath of Sahelo-Sudanian West and Central Africa: the Senegal, Gambia, Volta, Niger and Chad basins and the Nile system, with records also from Lake Turkana. It has additionally been widely translocated and farmed well beyond this native range for aquaculture, so its practical distribution across the African continent today is considerably larger than its original one.
It lives in floodplain rivers, swamps, oxbow lakes and seasonally inundated grassland rather than open, flowing river channels — habitats that are often warm, turbid and low in dissolved oxygen, conditions its air-breathing physiology is built to handle. FishBase reports a tolerated temperature range of roughly 77–86 °F, and aquarium-trade sources give a narrower comfortable band around 79–82 °F with a pH near 6.5–7.5, consistent with warm, soft-to-moderate, tropical African floodplain water.
Feeding
Heterotis niloticus is the odd one out among the bonytongues: rather than hunting fish, insects or fruit at the surface like an arowana, it is a microphagous filter-feeder. Fine gill rakers strain plankton and organic detritus from the water, and the material is concentrated by a coiled, spiral epibranchial organ formed from modified upper gill-arch elements — a structure unique to this species within the Osteoglossiformes and a genuinely unusual feeding apparatus for a fish this large.
In practice the diet is dominated by phytoplankton, zooplankton and fine organic detritus, though juveniles readily take live foods; aquarium-trade sources note that young fish around 11.8–15.7 in will eagerly eat red mosquito larvae and similar live foods. In captivity it accepts flake, small sinking pellets and finely divided frozen or live foods, and unlike the piscivorous arowanas it does not need a steady supply of feeder fish — a genuine point in its favour for anyone able to house an adult.
Mating
Breeding is tied to the West African rainy season, roughly May to October, timed to coincide with the seasonal flooding of the rivers and grasslands the species inhabits. As floodwaters rise and inundate marginal vegetation, pairs move into shallow water — generally less than about two metres deep — among stands of aquatic plants to begin nest construction.
Both parents build a circular nest woven from aquatic plants and mud, a task reported to take four to eight days of joint effort. This is a fundamentally different reproductive strategy from the mouthbrooding seen in the true arowanas and many other bonytongues: Heterotis niloticus does not carry its eggs in the mouth at all. Sexual maturity is reached comparatively late, at around four to five years of age.
Breeding
Spawning takes place directly into the completed nest, with external fertilisation and adhesive eggs that stick to the woven plant material rather than being picked up and brooded. Both parents then guard the nest aggressively, defending the eggs and subsequent larvae against intruders through the vulnerable early stages, and are reported to shepherd the fry to safety within the nest site if the brood is disturbed.
This is nest-building, biparental substrate-guarding — not mouthbrooding — and it is a strategy that has made the species highly amenable to pond-based aquaculture across West and Central Africa, where it is farmed as a food fish partly because natural spawning and parental care can be reproduced in earthen ponds with floating or planted nest sites. Home-aquarium breeding is essentially unheard of, both because of the size and pond-scale space the behaviour requires and because captive breeding of this species is really an aquaculture practice rather than a hobbyist one.
In the aquarium
Heterotis niloticus is a genuine size trap. Small juveniles are occasionally sold in the trade, but this is a fish that grows to roughly a metre and up to about 10 kg — a scale that puts a full-grown adult well beyond any conventional home aquarium and firmly into large-pond or public-aquarium territory. Anyone buying a juvenile should plan for that endpoint from day one, not discover it later.
On the positive side, aquarium-trade sources describe it as calmer and more manageable than the piscivorous South American and Asian arowanas — it is not chasing surface prey, so it tends to be steadier in a large tank, and its plankton/detritus-leaning diet means it does not demand a constant supply of feeder fish. As an obligate air-breather it must have reliable access to the water's surface to gulp air, so tank design needs to preserve that, and as with any large Osteoglossiform a secure, well-fitted cover is sound practice. Water should stay warm (roughly 77–86 °F) with a pH in the neutral-to-slightly-acidic range typical of its native floodplains; given its adult size, this is a species for keepers with very large systems or outdoor ponds in a suitably warm climate, not a standard community or even standard large-tank fish.
Conservation
A formal IUCN Red List assessment record for Heterotis niloticus appears to exist, but the specific threat category could not be confirmed from the sources consulted here, so no status is asserted beyond that uncertainty. What is well established is that the species is an important African food fish, long exploited by traditional floodplain fisheries and now widely farmed in aquaculture across West and Central Africa — a role that has driven deliberate introductions well outside its native river basins.
Those translocations are worth flagging for a different reason than overharvest: introducing a large, air-breathing, floodplain-adapted predator-of-plankton into new African river systems carries the same general ecological-risk profile as any non-native fish introduction, independent of whatever the species' formal conservation status turns out to be. Within its native range, the general pressures on Sahelo-Sudanian floodplain fish — wetland drainage, damming and altered flood regimes, and fishing pressure — are the relevant long-term concerns for a species whose whole life cycle is built around the seasonal flood.