Taxonomy & naming
Ossubtus xinguense was described by the French ichthyologist Michel Jégu in 1992, and it remains the only recognised species in the monotypic genus Ossubtus. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes. The genus sits among the serrasalmid lineages that have been split apart from the old, catch-all Myleus in recent decades as revisions clarified relationships among the silver dollars; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Ossubtus as its own distinct genus rather than a synonym or subgenus of any other silver-dollar group.
The species epithet xinguense refers directly to its home river, the Rio Xingu, a major clearwater tributary of the Amazon in the Brazilian state of Pará; the type locality lies near Altamira. In the hobby it circulates under names such as "parrot pacu" and the French "pacou du Rio Xingu," though it is a silver dollar in body plan and diet, not a true pacu of the Colossoma/Piaractus lineage, and the genus name Ossubtus should not be corrected or swapped for an older combination.
Morphology
Ossubtus xinguense has the classic silver-dollar build: a strongly laterally compressed, near-circular body with a small head and a mouth set for grazing rather than biting flesh. FishBase records a maximum published length of 10 in total length, a solid adult size for a disc-bodied characin and considerably larger than the small tetras most aquarists are used to keeping in a school. The flanks carry a bright, mirror-silver sheen typical of the group, giving the fish its reflective, coin-like appearance in open water.
As a rheophilic species — one adapted to fast, well-oxygenated flow — it shows the compact, hydrodynamically efficient profile common to fish that hold station in rapids, sheltering among rocks and boulders rather than drifting in slack water. Detailed accounts of sexual differences in colour or fin shape are not well documented for this species; data are sparse on external dimorphism.
Habitat
Ossubtus xinguense is endemic to the Rio Xingu basin in Pará, Brazil, and is not known from any other river system. Within the Xingu it favours clear, swift-flowing stretches over rocky outcrops and sandy bottoms, sheltering under and among boulders in rapids — a specialised, high-flow habitat shared with a distinctive rheophilic fish community and with beds of Podostemaceae, the river-weed family that anchors to rock in fast current and that this species grazes alongside.
Water temperatures across its range run in the tropical 72–77 °F band. This rapids-and-rock habitat is precisely the stretch of the Xingu most affected by the Belo Monte hydroelectric complex, which has altered flow and inundated large sections of historic rapids habitat — a pressure discussed further under Conservation.
Feeding
Ossubtus xinguense is a herbivore, grazing aquatic macrophytes and filamentous algae off the rocks and river-weed beds of its rapids habitat. This plant-based diet is the single most important fact for a keeper to internalise: a silver dollar of this genus is built to eat vegetation efficiently and will not distinguish between wild river-weed and cultivated aquarium plants.
In the aquarium it adapts readily to prepared foods — flake, frozen foods and larger shrimp are all accepted — but a diet skewed toward vegetable matter (blanched greens, spirulina-based flake or wafers, algae wafers) alongside modest protein keeps it closer to its natural feeding ecology and, in a mixed tank, does little to blunt its appetite for live plants.
Mating
Courtship and pair formation in Ossubtus xinguense are not well documented; the cached sources note that females can be notably aggressive, which is consistent with mate selection or territory disputes around spawning but does not establish a described courtship sequence. No reliable account of external sexual dimorphism — colour, fin shape or body depth differences between males and females — is available for this species.
Given its rheophilic habitat, it is plausible that spawning is tied to seasonal flow or water-level cues typical of Amazonian rivers, but this has not been confirmed in the literature available, and this account does not speculate further.
Breeding
As a silver dollar, Ossubtus xinguense fits the group's typical reproductive pattern: an egg-scatterer over fine-leaved vegetation, with no nest building and no parental care of eggs or fry. This is consistent with breeding in related silver-dollar genera such as Metynnis and Myloplus, though a fully described spawning account specific to O. xinguense — clutch size, egg characteristics, incubation time — is not available in the sources consulted here.
The species has occasionally been reported bred in the hobby, reportedly favouring soft, acidic water conditions that likely mimic natural spawning triggers, but it is not commercially farmed or routinely bred at scale, and most specimens in the trade are presumed to originate from wild collection in the Xingu. Data on this species' captive breeding remain sparse.
In the aquarium
Ossubtus xinguense is a schooling fish and should be kept in a group, in a spacious tank that gives it real open swimming room — it is an active, disc-bodied swimmer, not a fish suited to a cramped or heavily obstructed layout. Given its rapids-dwelling origin, strong filtration and good water movement suit it well, echoing the well-oxygenated, flowing water of its native rocky stretches of the Xingu.
The critical caveat for any prospective keeper is that this is a committed plant-eater: it will graze and shred live aquarium plants, and a soft, densely planted aquascape is the wrong environment for it. A robust, hard-scaped tank — rockwork, driftwood and tough, unpalatable or artificial planting — suits its needs far better than a show planted tank. It is a peaceful community fish toward tankmates of appropriate size, with its impact falling on vegetation rather than on other fish.
Conservation
The IUCN Red List assessed Ossubtus xinguense as Vulnerable (VU) in 2018, reflecting its narrow endemic range confined to the Rio Xingu and the scale of habitat change in that river. The PLOS ONE redescription and distribution study covering the species' habitat assessment goes further, characterising its rapids habitat as Endangered — underscoring that the specific rocky, fast-flowing stretches this species depends on are under acute pressure.
The dominant threat is the damming of the Xingu, most significantly the Belo Monte hydroelectric complex, which has altered flow regimes and inundated or dewatered large sections of the historic rapids that this rheophilic, rock-associated species and its Podostemaceae food source depend on. As with other Xingu rapids endemics, its future is tied directly to how much of that specialised habitat survives ongoing hydrological change.