Taxonomy & naming
Metynnis otuquensis was described by the German ichthyologist Ernst Ahl in 1923, its name referencing the Otuquis River, a tributary of the Paraguay River basin near the Bolivia–Paraguay border where the type specimens were collected. It sits in the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and in the genus Metynnis, one of several silver-dollar lineages (alongside Myloplus and others) that Serrasalmidae taxonomy has repeatedly split and reshuffled as the Catalog of Fishes (Eschmeyer, California Academy of Sciences) has refined the family's internal relationships.
Metynnis species are notoriously difficult to distinguish from one another and from related silver dollars without close examination of dentition and fin-ray counts, and Metynnis otuquensis is no exception — it is frequently mixed up in the aquarium trade with other Metynnis species sold under the generic label "silver dollar." The genus name Metynnis and the species epithet otuquensis are discussed further below.
Morphology
Like other silver dollars, Metynnis otuquensis has a strongly laterally compressed, near-circular body — the source of the common trade name — with a small head, blunt snout and a flattened profile that gives it a coin- or disc-like silhouette. The body is covered in bright, reflective silver scales that flash as the fish moves, and the fins are typically clear to lightly dusky, sometimes with a faint reddish or orange tinge in the anal fin depending on individual and mood.
FishBase records a maximum size of 3.5 in for this species, making it one of the more modestly sized members of a genus whose other species can run somewhat larger. The dentition is the key functional difference from its piranha relatives: rather than the single row of interlocking, blade-like teeth used for biting flesh, Metynnis carries teeth adapted for shearing plant material, consistent with its herbivorous diet.
Habitat
Metynnis otuquensis is known from the Otuquis River and the upper Paraguay River basin, spanning the Bolivia–Paraguay border region and extending into Mato Grosso, Brazil. Within this range it occupies freshwater, pelagic habitat — open or lightly vegetated water in river channels and associated floodplain habitat rather than tight cover, consistent with its schooling, open-water lifestyle.
Water in this part of the upper Paraguay basin is generally soft to neutral and mildly acidic; keepers extrapolating from related Metynnis species commonly cite a temperature range around 75–82 °F and a pH of roughly 6.0–7.0. Detailed, species-specific water-chemistry data for Metynnis otuquensis is sparse in the available literature, so these figures should be treated as a reasonable starting range drawn from the wider genus rather than as measurements taken at the type locality.
Feeding
Metynnis otuquensis is a herbivorous plant-eater, grazing on soft aquatic vegetation and algae, with occasional scavenging on other food sources when the opportunity arises. This diet places it at the opposite end of Serrasalmidae from its piranha relatives despite the shared family — the same deep-bodied, schooling characin lineage produced both flesh-biting predators and dedicated plant-grazers.
In the aquarium it accepts a wide range of foods: flake and green flake, algae wafers, blanched vegetables such as spinach and lettuce, fine-leaved plants like Cabomba, and occasional frozen foods such as mosquito larvae and brine shrimp for variety. Vegetable matter should form the bulk of the diet — a fish fed only protein-rich foods is more likely to turn its appetite on tankmates' plants and its own fins.
Mating
Documented breeding accounts for Metynnis otuquensis describe group spawning with side-by-side courtship among a conditioned shoal, a pattern consistent with other Metynnis silver dollars. There is no reported external sexual dimorphism that reliably distinguishes males from females outside of spawning condition, and hobbyists generally identify breeding-ready fish by behaviour and body fullness rather than colour or fin shape.
Spawning is triggered by warmer, softer, more acidic water conditions, and a well-fed group given fine-leaved plants such as Cabomba as a spawning substrate is the standard approach described for this and related Metynnis species.
Breeding
Metynnis otuquensis is an egg-scatterer, not a nest-builder or mouthbrooder. A prolific spawner among fine-leaved plants, it scatters eggs during group spawning events rather than depositing them at a single guarded site, and there is no parental care of eggs or fry afterward — once the eggs are released and fertilised, the adults take no further part.
Eggs hatch in roughly three days, and the free-swimming fry begin taking crushed spirulina flakes or other fine foods shortly after. The species is occasionally bred in the hobby, generally by conditioning a group in soft, slightly acidic water and providing dense plant cover for the eggs to fall into and be sheltered from adult attention, since neither parent guards the clutch.
In the aquarium
Metynnis otuquensis is a schooling fish that should be kept in a proper group rather than alone or in pairs — a lone or thinly-kept silver dollar is a nervous, skittish fish. It needs open swimming space to match its active, mid-water habits, and a tank sized generously enough to accommodate a settled group of a fish that, while modest at 3.5 in, is an active and fairly robust swimmer.
The single most important care note is that this is a committed plant-shredder: a group of silver dollars will strip soft-leaved aquarium plants to the stems in short order, so a tank intended to house them should be hard-scaped with driftwood and rockwork rather than set up as a delicate aquascape, or should rely on tough, unpalatable plants like Anubias and Java fern lashed to hardscape rather than planted in substrate. Water should be kept warm and soft to neutral, mildly acidic, with good filtration for an active schooling fish. It is a peaceful, community-friendly species toward other fish too large to be seen as food, and its silver-flashing schooling behaviour is one of its main draws in a large, open aquarium.
Conservation
Metynnis otuquensis is assessed by the IUCN Red List as Least Concern, an assessment dated 27 October 2020. This reflects a species with a defined but not obviously threatened range in the upper Paraguay River basin along the Bolivia–Paraguay border and into Mato Grosso, Brazil, without the widespread habitat pressure or targeted exploitation that would justify a higher threat category.
As with many range-restricted upper Paraguay basin fish, ongoing habitat pressures in the region — including land-use change and water development in the Pantanal and its tributaries — are long-term watch items for the basin generally, though the species is not currently flagged as threatened. Detailed population trend data specific to Metynnis otuquensis is limited in the available literature.