Taxonomy & naming
Arapaima agassizii was described by the French ichthyologist Achille Valenciennes in 1847, from a specimen collected in the Brazilian Amazon, as part of the monumental Histoire naturelle des poissons he co-authored with Georges Cuvier. It belongs to the order Osteoglossiformes — the bonytongues, one of the oldest surviving lineages of teleost (bony) fishes, so named because a toothed bone on the floor of the mouth works against toothed plates on the roof to grip and crush prey. Osteoglossiformes is a scattered, deep-time radiation found today across four continents — South American arapaima and arowanas, African butterflyfish, Asian arowanas, and Australasian saratoga among them — and Arapaima agassizii sits within the family most commonly recognised as Arapaimidae alongside the other South American giants of the group.
The genus Arapaima has an unusually tangled taxonomic history. For most of the twentieth century it was treated as a single, wide-ranging species, Arapaima gigas, with populations across the Amazon lumped together. Compounding matters, the holotype of A. agassizii itself was destroyed during World War II bombing, leaving taxonomists without a physical reference specimen for decades. More recent taxonomic review of the genus recognises several valid species — among them A. agassizii alongside A. gigas and others — distinguished by fine differences in body proportions and geography, though the precise boundaries between them remain debated in the literature. This account follows the Catalog of Fishes and the available source material rather than asserting a fully settled classification.
The genus name Arapaima is of Tupian origin from indigenous South American usage, closely associated with the Brazilian vernacular name pirarucu, though its exact etymological translation is debated. The species epithet agassizii is a Latinized eponym honouring the Swiss-American naturalist Jean Louis Rodolphe Agassiz, one of the towering figures of nineteenth-century natural history.
Morphology
Arapaima agassizii shares the unmistakable body plan of its genus: an elongated, torpedo-shaped fish covered in large, thick, overlapping bony scales that form a genuine suit of armour, tough enough that arapaima scales have drawn interest from materials scientists studying natural, bite-resistant armour. The dorsal and anal fins are set far back toward a rounded tail, and the head is broad and somewhat flattened. Colouration runs dark olive-grey to blackish along the back and flanks, with mature fish typically developing a striking flush of coppery-red to crimson scales toward the tail and rear body — brighter in fish approaching breeding condition.
The single most important anatomical feature is internal rather than external: like every arapaima, A. agassizii carries a modified, lung-like, heavily vascularised swim bladder that functions as an accessory breathing organ. This makes it an obligate air-breather that must periodically rise and gulp air at the surface regardless of how much oxygen is dissolved in the water below; denied access to the surface, it will drown.
Species-specific maximum size for A. agassizii is poorly documented — the loss of its type specimen has not helped — but it is reported as smaller than the giant A. gigas. Congeners across the genus have historically been recorded at anywhere from roughly 78.7–177.2 in total length depending on species, and by any reasonable estimate arapaima remain among the largest freshwater fishes alive today.
Habitat
Arapaima agassizii is a South American fish, confirmed from the Amazon basin of Brazil, where it inhabits floodplain lakes (known locally as várzea) and adjoining slow-flowing river channels. It tends to favour overgrown, vegetated margins with soft currents, and appears to prefer clear-to-turbid, nutrient-rich floodplain habitat over the tannin-stained blackwater typical of rivers such as the Rio Negro.
Amazon floodplain lakes swing dramatically between high-water and low-water seasons, and dissolved oxygen in these still, vegetation-choked waters can fall very low, especially as levels drop in the dry season. This is exactly the pressure that favours an obligate air-breather: while gill-only fish struggle in oxygen-poor várzea pools, arapaima simply rise to the surface. Water is warm, in the region of 77–84 °F ideally (with a much wider reported tolerance, up to roughly 102 °F in extreme conditions), and mildly acidic and soft, around pH 6.0–6.5.
Feeding
Arapaima agassizii is carnivorous, taking live fish and other meaty prey as its primary diet, in keeping with the ambush-predator habits of the genus. It hunts by cruising through cover before striking with a fast, powerful lunge, and — as in its relatives — juveniles are thought to take a broader diet of aquatic invertebrates before shifting toward the near-exclusively piscivorous habits of the adult.
Where arapaima are raised commercially in Brazil, fish are grown on meaty and formulated feeds rather than exclusively live prey, but the species' natural inclination is unmistakably that of a large predatory fish rather than a grazer or omnivore.
Mating
As in the rest of the genus, breeding in Arapaima agassizii is tied to the seasonal rhythm of the Amazon floodplain. During the low-water season, a male excavates a shallow nest depression directly in the substrate of a floodplain lake or river margin, rather than building a bubble nest or relying on open-water broadcast spawning.
A pair spawns over this prepared nest. It is a site-guarding strategy built around defending a fixed nest location — a marked contrast to most ambush-predator fish of comparable size, which invest little to nothing in their eggs after spawning.
Breeding
After spawning, the male takes on the great majority of parental care. He guards the nest and, once the eggs hatch, continues guarding the fry, which do not scatter but instead gather into a tight, ball-like school that shoals closely around the parent's head. The young are reported to orient to — and apparently feed on — a whitish secretion produced from the guarding parent's head during this vulnerable early period, an unusual degree of sustained parental investment for a fish of this size.
This combination of substrate nesting and intensive, largely male-led guarding is characteristic of the genus and is one reason Arapaima species can be bred productively in aquaculture settings: farms across the Brazilian Amazon raise arapaima at scale for the food market, a practice that both eases pressure on wild populations and supplies most of the arapaima that reach the aquarium trade as small juveniles.
In the aquarium
Be blunt about this one: Arapaima agassizii is not a home-aquarium fish. It is a member of one of the largest freshwater fish genera on Earth, an obligate air-breather that must have unimpeded access to the surface at all times, and a species whose adult size — even at the more modest end of the genus's reported range — demands a pool-scale system, not a display tank. Juveniles are occasionally sold small in the trade and grow quickly, which has produced a well-known cautionary pattern: a fish bought at a modest size can outgrow almost any private setup within a year or two.
Arapaima genuinely belong to public aquariums, research institutions and, above all, the commercial aquaculture ponds of the Brazilian Amazon, where they are raised as a major food fish. Anyone encountering a juvenile arapaima for sale should treat the eventual adult size and the absolute need for surface access as the central facts of the purchase decision, not an afterthought.
Conservation
The IUCN Red List currently assesses Arapaima agassizii as Data Deficient (2022.2 assessment) — a reflection of genuine scientific uncertainty rather than reassurance. The species is known from limited historical Brazilian Amazon material, its original type specimen was destroyed during World War II, and the genus-wide taxonomic revision that recognises A. agassizii as distinct from A. gigas and other congeners means population-level data have not caught up with current species boundaries; there is simply not enough species-specific data to assign a firmer threat category.
The wider genus provides useful context: historical overfishing sharply reduced wild arapaima populations across parts of the Amazon during the twentieth century, and the genus Arapaima is now listed on CITES Appendix II, requiring non-detriment findings before commercial trade in wild-origin specimens can proceed, with Brazil among the range states subject to this requirement. That history, combined with the group's slow growth to enormous size, is why extensive aquaculture — now a substantial industry in Brazil — matters for conservation as well as commerce: farmed supply reduces the incentive to target wild fish.