Taxonomy & naming
Osteoglossum ferreirai was described by the ichthyologist James Ted Kanazawa in 1966. It belongs to the family Osteoglossidae within the order Osteoglossiformes, the bonytongues — a scattered, deep-time radiation found today on four continents (South America, Africa, Southeast Asia and Australasia) and generally regarded as one of the most ancient living branches of the teleost fishes. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Osteoglossum ferreirai as a valid, South American endemic species.
The genus name Osteoglossum comes from the Greek osteon ("bone") and glossa ("tongue"), referring to the bony, toothed tongue that bonytongues bite against toothed plates on the roof of the mouth, an adaptation for gripping slippery, struggling prey. The genus contains only two species — O. ferreirai and its more widespread relative O. bicirrhosum, the silver arowana — and the epithet ferreirai honours a collector surnamed Ferreira credited with supplying type material from the Manaus region. In the aquarium trade this species is distinguished from its congener by its darker, near-black adult coloration overlaid with a blue-violet iridescence, versus the brighter silvery sheen of O. bicirrhosum, and by its narrower range restricted to blackwater.
Morphology
The black arowana has the unmistakable arowana build: a long, laterally compressed, ribbon-like body, a straight dorsal profile, and long-based dorsal and anal fins set far back toward a rounded tail, giving the whole rear half of the fish the look of a single continuous fin. The head is bony and flattened on top, and the lower jaw is strongly upturned into a hinged, "drawbridge" mouth armed with two long barbels at the chin tip — sensory organs used to detect prey and disturbances at the water's surface. Large, upward-oriented eyes reflect its habit of watching the surface from just below it.
FishBase reports a maximum length of about 35.4 in total length, and this species can approach that size in captivity as well as in the wild, making it a genuinely big aquarium fish rather than a juvenile-sized novelty. Like other Osteoglossidae it has a modified, highly vascularised swim bladder that functions as an accessory air-breathing organ, letting it gulp air at the surface — a useful adaptation in the oxygen-poor blackwater it calls home. Juveniles are more silvery-grey with a blue sheen; adults darken toward the near-black coloration that gives the species its common name.
Habitat
Osteoglossum ferreirai is native to the Rio Negro basin in the Brazilian and Colombian Amazon, one of the world's great blackwater river systems: tannin-stained, acidic, and extremely soft and mineral-poor, the product of water draining ancient, nutrient-leached soils rather than young sediment-rich terrain. It has also been recorded from the upper Orinoco basin and the Branco River, though whether the Orinoco population is natural or a human-mediated introduction from the adjoining Rio Negro headwaters is disputed.
Within this blackwater system the species is largely sedentary, favouring backwaters, marginal lagoons, tributary mouths and seasonally flooded forest (igapó) rather than open river channel. During the high-water season these flooded forests become a rich hunting ground, putting insects, spiders and small vertebrates stranded on branches within jumping range of a fish built to hit the surface hard. This is a soft, acidic, blackwater specialist, distinct from its wider-ranging congener O. bicirrhosum, which occupies whitewater and floodplain habitat across much of the broader Amazon basin.
Feeding
The black arowana is a predatory surface-cruiser. In the wild it hunts insects, other fish, and small terrestrial animals — anything unlucky enough to fall, crawl, or perch within striking range of the water's surface — and it is renowned as a powerful jumper capable of leaping clear of the water to seize prey directly off overhanging branches and foliage. This aerial hunting habit is one of the signature behaviours of the genus.
In the aquarium it is a committed carnivore and a heavy feeder, taking live and frozen foods such as bloodworm readily, along with meaty items like beef heart and chicken liver once established. It is also, by FishBase's own characterisation, a high-waste producer — a large, active predator eating protein-rich meals generates a correspondingly heavy bioload, which has direct consequences for tank size and filtration (see "In the aquarium").
Mating
Osteoglossum ferreirai breeds seasonally, with spawning documented during the high-water season, roughly June to July in the Rio Negro and Orinoco systems, when flooded forest habitat expands and offers cover and food for developing young. As in its congener and in the related Scleropages arowanas of Asia and Australasia, fertilisation is external, but parental investment does not stop there.
After the eggs are laid and fertilised, the male takes them into his mouth. This is a paternal mouthbrooder: the female plays no further role once the eggs are released, and all subsequent care of the clutch falls to the male. The eggs themselves are large and comparatively few in number, consistent with a strategy that trades egg quantity for a long period of direct, sheltered parental care rather than broadcasting large numbers of unprotected eggs into open water.
Breeding
Having taken the fertilised eggs into his mouth, the male black arowana continues to brood them there through incubation and then continues to shelter the hatched, still yolk-sac-bearing fry in his mouth for a period of weeks, releasing them gradually as they grow more capable of independent swimming and feeding. This extensive paternal mouthbrooding is the standout reproductive trait of the genus and the reason arowana breeding is often described in terms of the male's mouth capacity and behaviour rather than nest-building or substrate choice.
Breeding this species is essentially unreported in home aquaria — it demands both the sheer tank volume of an adult pair (each fish approaching a metre) and precise reproduction of seasonal, soft blackwater conditions that are impractical to recreate outside large purpose-built systems. The aquarium trade in Osteoglossum ferreirai is supplied overwhelmingly by wild collection from the Rio Negro rather than by captive breeding, which is worth knowing before buying — this is a wild-caught fish, not a farmed one, and IUCN's own notes flag it as taken for both the food and aquarium trades.
In the aquarium
This is an emphatically large-tank fish and not a beginner's project. Aquarium care sources recommend a minimum footprint on the order of 96 in × 36 in × 36 in (roughly 244 × 91 × 35.8 in), commonly cited around 2,0 US gal, and larger is genuinely better for a fish that can approach 35.4 in — the size trap here is real: a black arowana bought as a manageable juvenile will, with good care, need to be rehomed into something much bigger within a couple of years, and it can live roughly two decades. Do not buy one without a firm plan for the adult tank.
Because it is a committed jumper that hunts by leaping at the surface, a heavy, tight-fitting lid is not optional — an open-topped tank all but guarantees an eventual escape and a dead fish on the floor. Water should be kept soft and acidic to mimic its Rio Negro origins, roughly pH 5.0–7.0 at 75–86 °F, and given the species' heavy bioload, strong filtration and generous water changes (25% or more as a baseline) are needed to keep pace with waste production. It is a predator that will eat any tankmate small enough to fit in that upturned mouth, so companions need to be robust, similarly large, and non-aggressive rather than small or delicate; keep it with fish of comparable size rather than in a typical community setup.
Conservation
The IUCN Red List assesses Osteoglossum ferreirai as Least Concern, an assessment recorded via FishBase as dated November 2020. It remains a common fish across its Rio Negro range and is not currently considered at elevated risk at the population level.
That said, it is taken from the wild both for food and for the aquarium trade, and — as with any species restricted to a single blackwater river system — its long-term outlook is tied to the health of that habitat: deforestation, mining-related sedimentation, and hydrological changes in the Rio Negro basin are the relevant long-term pressures, even though none currently pushes the species out of a Least Concern assessment.