Taxonomy & naming
Metynnis cuiaba was described in 2009 by Pavanelli, Ota and Petry, with its type locality in the Rio Cuiabá, a major tributary of the Rio Paraguai in the Pantanal region of Mato Grosso, Brazil. It sits in the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and in the genus Metynnis, which the Catalog of Fishes (Eschmeyer, California Academy of Sciences) continues to recognise as a distinct silver-dollar lineage; unlike some other former "Myleus" silver dollars that were split into Myloplus, Tometes and Ossubtus, Metynnis cuiaba has not been reassigned to another genus.
The species epithet "cuiaba" is a straightforward toponym, referencing the Rio Cuiabá where the type specimens were collected. As with many silver dollars, field identification within the genus Metynnis can be difficult — species are separated on fin-ray counts, gill-raker number and other meristic detail rather than obvious external colour differences, and Metynnis cuiaba is best confirmed by its Rio Paraguai/Pantanal provenance alongside these technical characters.
Morphology
Metynnis cuiaba has the classic silver-dollar body plan: a strongly laterally compressed, almost circular disc-shaped profile, a small head, and a bright, mirror-silver flank that gives the genus its common name. FishBase records a maximum size of 5.5 in standard length, a modest figure compared to some other serrasalmids, keeping this species firmly in the medium-sized end of the silver-dollar group. Meristic counts recorded for the species include 22–24 gill rakers, 16–18 dorsal soft rays, 35–39 anal soft rays and 38 vertebrae — details used by ichthyologists to separate it from closely related Metynnis species rather than field marks a hobbyist would use.
Like other Metynnis, the body is deep and thin from side to side, with a long-based anal fin and a small adipose fin typical of characins. Colouration is a plain, high-gloss silver overall, without the bold banding or spotting seen in some other serrasalmids, and the fish is easily mistaken for other Metynnis species without close examination.
Habitat
Metynnis cuiaba is known from the Rio Paraguai basin, including the Rio Cuiabá and the associated baías — floodplain lakes — of the Pantanal wetland in Mato Grosso, Brazil. The Pantanal is one of the world's largest freshwater wetland systems, and its hydrology is dominated by a strong seasonal flood pulse: vast areas of forest and grassland flood during the wet season and the water recedes into channels and residual lakes during the dry season, a rhythm that shapes the movement and schooling behaviour of the fish living there.
As a South American endemic confined to the Paraguay basin, Metynnis cuiaba is never found outside the Neotropics — it has no presence in Africa or Asia. Within its range it is associated with the vegetated margins and open floodplain lakes typical of Pantanal habitat, where schools of silver dollars graze on submerged and marginal plant growth.
Feeding
Metynnis cuiaba is a herbivore, and diet is one of the most important practical facts for anyone considering the species. Wild Metynnis graze algae and soft aquatic and marginal vegetation, supplementing this with fallen fruit, seeds and other plant material that enters the water during the Pantanal flood pulse, in the same pattern documented for other silver dollars in the genus.
In the aquarium, Metynnis cuiaba should be offered a diet built around vegetable matter — blanched courgette, cucumber and other greens, spirulina-based flake or pellet, and algae wafers — with only occasional protein such as frozen or freeze-dried foods. Keepers should expect this species to treat any live aquarium plants as food rather than decoration; see "In the aquarium" for how that shapes tank planning.
Mating
Specific courtship behaviour for Metynnis cuiaba has not been documented in the sources available, and the cached record for this species does not describe pairing or spawning triggers in detail. Based on the broader pattern for the genus Metynnis, sexual maturity in silver dollars is generally reached at a modest size (well under maximum adult length) and spawning is associated with soft, warm, low-light water — conditions that mimic the onset of the flood season in their native Pantanal and Paraguay basin habitat.
External sexual dimorphism has not been reported for this species; data is sparse enough here that keepers should not expect a reliable way to visually sex individuals.
Breeding
Metynnis cuiaba is an egg-scatterer, in keeping with the breeding pattern documented for the genus Metynnis as a whole: there is no nest-building and no parental care. Spawning silver dollars scatter adhesive eggs over fine-leaved plants or plant-like spawning mops, typically in soft, acidic, warm water, and once released the eggs and any resulting fry receive no protection from the adults.
Breeding this species in the home aquarium is occasionally reported for the genus but is considered moderately difficult, generally requiring a dedicated soft-water breeding setup separate from a display tank, since adult Metynnis will treat both the eggs and the fine-leaved plants used as a spawning substrate as incidental food if left in a communal tank. No commercial or hobby breeding reports specific to Metynnis cuiaba were found in the sources consulted; treat this species as rarely, if ever, bred at home.
In the aquarium
Metynnis cuiaba should be kept as a genuine school — a group of six or more is the standard recommendation for the genus — since a lone or thinly-grouped silver dollar becomes skittish and stressed. Fish sources for the genus recommend a minimum tank size in the 55–75 gallon range, with a long, roughly six-foot footprint favoured for an active adult group given how much these fish appreciate open swimming space in the water column's middle and upper reaches. Given this species' 5.5 in maximum size, it sits at the smaller end of what a home aquarium needs to accommodate compared to larger Metynnis and Myloplus species, but the schooling requirement still demands real floor space.
The single most important planning decision is around plants: Metynnis are notorious, committed plant-shredders, and a heavily planted aquascape will be reduced to stems within days. Keepers who want greenery in the tank should rely on tough, unpalatable species such as Anubias or Java fern, or use artificial plants and driftwood instead. Water should be kept warm, in the 75–82 °F (75–82 °F) range typical for the genus, with a pH around 5–7, and strong filtration is essential since these fish are messy, waste-producing feeders. Metynnis cuiaba is peaceful and can be housed with medium-to-large, non-nippy tankmates such as larger cichlids, bristlenose plecos and robust Corydoras, but should be kept away from small or delicate fish that could be outcompeted at feeding time.
Conservation
Metynnis cuiaba is assessed by the IUCN Red List as Least Concern, an assessment dated 18 December 2020. This reflects a species restricted to, but not obviously threatened within, its native range in the Rio Paraguai basin and the Pantanal floodplain of Mato Grosso, Brazil, where extensive wetland habitat remains intact relative to more heavily developed parts of South America.
The Pantanal as a whole faces long-term pressures from agricultural expansion, altered fire regimes and hydrological change, all of which are relevant watch items for the wetland's fish fauna generally, though no species-specific threat has been documented for Metynnis cuiaba in the sources reviewed here. There is no indication in the available sources of significant aquarium-trade collection pressure on this particular species.