Taxonomy & naming
Colossoma macropomum was described by the French naturalist Georges Cuvier in 1816. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes. The genus Colossoma is small; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the valid taxonomy of this heavily revised family, in which the old, broad "pacu" groupings have been split hard over the past few decades, with many former Myleus and related species moved into genera such as Myloplus, Tometes and Ossubtus. Colossoma macropomum itself has kept its genus placement and remains the single largest-bodied member of the pacu lineage.
The common name "tambaqui" is the term most widely used in Brazil and across much of its native range, while "black pacu" and "gamitana" (Peru) and "cachama" (Colombia/Venezuela) are used regionally; "pacu" more broadly covers the whole fruit-eating lineage within Serrasalmidae, of which C. macropomum is the best-known and largest representative. It is sometimes confused in the trade with smaller, superficially similar silver dollars, but its adult proportions, dentition and eventual size set it apart entirely.
Morphology
The tambaqui has a deep, laterally compressed, disc-like body typical of the pacu lineage, but on a far larger scale than the silver dollars it is sometimes mistaken for as a juvenile. FishBase records a maximum length of 42.5 in total length, with published weights exceeding 40 kg in large wild individuals; most sources agree adults commonly exceed 35.5–39.5 in and 20–30 kg. Juveniles are a plain silvery colour, often with scattered dark spots, and are frequently sold under the generic "pacu" label at only a few centimetres in length — giving no visual hint of the size the fish will eventually reach.
The most distinctive feature of the species is its dentition: unlike the piranhas in the same family, tambaqui have a single row of blunt, broad, molar-like teeth that interlock to crush hard-shelled seeds and nuts rather than flesh. Adults darken with age, developing a dusky, often near-black colouration on the lower body and fins that gives rise to the "black pacu" name, while the upper body remains a duller olive-grey to silver.
Habitat
Colossoma macropomum is native to the Amazon and Orinoco river basins of South America, where it inhabits large rivers, floodplain lakes (várzea) and seasonally flooded forest. Its ecology is tightly bound to the Amazon's seasonal flood pulse: during high water it moves into inundated forest to feed heavily on fallen fruit and seeds, then retreats to the main river channels and larger lakes as the floodwater recedes. This migratory, flood-pulse-driven life cycle underpins both its role as a keystone seed disperser in the flooded forest and its long-distance spawning migrations (see Mating and Breeding).
Water across its range is warm, tropical and generally soft to moderately hard with a mildly acidic to neutral pH; Seriously Fish gives a temperature range of roughly 72–82 °F and pH 5.5–7.5 with hardness of 1–15°H, reflecting the variety of blackwater, whitewater and clearwater habitats it moves through as it follows the flood cycle rather than being confined to one narrow water type.
Feeding
Tambaqui are primarily frugivorous and omnivorous. In the wild their molar-like teeth are adapted specifically for crushing fruit, nuts and seeds that fall into flooded forest from the surrounding canopy during high water, supplemented opportunistically by insects, zooplankton and other small animal matter. This diet makes the species an important seed disperser for many Amazonian tree species, since seeds pass through the fish and are deposited, often at some distance from the parent tree, as floodwaters recede.
In captivity — where the species is realistically only suitable for very large systems or ponds — tambaqui take a varied diet readily: quality pellets, chopped fruit and vegetables such as apple, courgette and leafy greens, alongside occasional protein. Overfeeding fatty or protein-heavy foods is best avoided given the fish's naturally plant- and seed-based diet and its capacity for very rapid growth.
Mating
Colossoma macropomum is a long-distance migratory spawner. Mature adults undertake substantial upstream migrations timed to the Amazon basin's flood pulse, gathering to spawn in open water rather than pairing off at a fixed nest site the way true piranhas do. This migratory reproductive strategy is characteristic of several of the basin's larger commercially important characins and is fundamentally different from the substrate-nesting behaviour seen in Pygocentrus and Serrasalmus.
Because natural spawning depends on long migratory runs and specific flood-triggered environmental cues that cannot be replicated in a home aquarium, courtship and pairing behaviour of the kind documented for nest-building piranhas has essentially no equivalent in captive tambaqui; virtually everything known about inducing spawning in this species comes from commercial aquaculture rather than aquarium observation.
Breeding
Colossoma macropomum is not a fish that is bred in the home aquarium. It is a migratory food-fish species that, across much of South America and increasingly in aquaculture operations elsewhere, is spawned commercially at scale using hormone induction — mimicking the natural flood-pulse migration trigger — in large outdoor ponds and hatchery systems. This farmed, hormone-induced production is what supplies both the food-fish market and the small aquarium fish sold to hobbyists; wild spawning migrations continue in native rivers but have never been replicated in a home tank.
Given its eventual size, its migratory spawning biology and its total reliance on flood-pulse cues, breeding this species outside commercial or research aquaculture facilities is not a realistic goal for hobbyists, and no home-aquarium breeding reports exist in the sources consulted here.
In the aquarium
The tambaqui is the size trap in its purest form: it is commonly sold at around 2 in as an attractive, fast-swimming juvenile, and within a few years can reach or approach its documented maximum of roughly 42.5 in and tens of kilograms. It is not suited to a home aquarium of any conventional size — sources recommend nothing smaller than roughly 3,210 US gal as a realistic minimum for an adult, and in practice only public aquariums, aquaculture ponds or very large custom systems are appropriate for a fully grown individual. Anyone acquiring a juvenile pacu needs a concrete plan — a large pond, an aquaculture partner, or a public aquarium — well before the fish outgrows a display tank.
Juveniles are described as shoaling and generally peaceable, and the species overall is not aggressive toward tankmates in the way piranhas can be, though crowded or undersized juveniles may nip fins. Its molar dentition means it is not a fin- or scale-eating threat in the way some other characins are, but its sheer eventual bulk and waste output demand very heavy filtration regardless of temperament. Because pacus are hardy, fast-growing and have established feral populations on multiple continents after aquarium releases, keepers must never release a tambaqui — outgrown or otherwise — into any open waterway.
Conservation
The IUCN Red List currently assesses Colossoma macropomum as Near Threatened. It remains widespread across the Amazon and Orinoco basins and is heavily supported by aquaculture, but wild populations face documented pressure from overfishing, habitat degradation and pollution across parts of its native range — the tambaqui is among the most heavily fished food species in the basin, with large adults particularly targeted. Sustainable aquaculture is increasingly emphasised as a way to reduce collection pressure on wild stock while meeting demand for the species as a major South American food fish.
The species' outsized ecological role as a seed disperser in flooded Amazonian forest adds a conservation dimension beyond the fish itself: sustained overfishing of large adult tambaqui in some rivers has been linked to broader concerns about seed dispersal and forest regeneration in the flood-pulse ecosystems it inhabits. Outside its native range, established feral populations arising from aquarium releases are a separate, human-caused conservation and invasive-species issue, underscoring why the species should never be released.