Taxonomy & naming
Metynnis maculatus was described by the Austrian ichthyologist Rudolf Kner in 1858, originally as Myletes maculatus, before later authors moved it into the genus Metynnis along with the rest of the smooth-bodied "silver dollar" characins. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, a family whose internal taxonomy has been extensively revised in recent decades as ichthyologists split the old, catch-all genera into narrower, better-defined lineages.
The species epithet maculatus is Latin for "spotted," referring to the dark blotches scattered over the silvery flanks, and it survives in the common name "spotted metynnis." In the aquarium trade it is more often sold simply as "silver dollar," a name shared loosely across several Metynnis and Myloplus species that look broadly similar to non-specialists; positive identification rests on spotting pattern, fin colour and origin rather than on the generic trade name alone.
Morphology
Metynnis maculatus has the classic silver dollar build: a strongly laterally compressed, near-circular body that gives the fish its common name, a small head, and a deeply forked tail built for sustained cruising rather than the sudden strikes of its piranha relatives. The flanks are a bright, mirror-like silver, usually marked with irregular dark spots or blotches (hence maculatus), and the fins can show a reddish or orange tinge, particularly in the anal fin.
SeriouslyFish records a maximum size of 6 in standard length, while FishBase lists a maximum recorded total length around 7 in — a genuinely large, dinner-plate-sized adult for a community aquarium fish. Unlike piranhas, the mouth is small and set with close-packed, incisor-like teeth adapted for grazing plant material rather than tearing flesh, and there is no elongated lower jaw or blade-like dentition.
Habitat
The species is native to the upper Rio Madeira drainage spanning Bolivia and Brazil, including the Rio Beni, Rio Mamoré and Rio Guaporé (Iténez), and it also reaches the upper Rio Paraguai in the Pantanal. Across this range it inhabits lowland river channels, floodplain habitats and vegetated backwaters typical of the wider Amazon-Madeira-Pantanal system, where seasonal flooding governs both water chemistry and the availability of the fruit, seeds and submerged vegetation that make up much of its diet.
SeriouslyFish gives a temperature range of 68–82 °F, pH 5.0–7.5 and hardness of 18–215 ppm for the species — a wide tolerance consistent with a fish that moves between open river channels and flooded vegetation across the seasons rather than a narrow blackwater specialist. FishBase corroborates a similar picture with a pH range of 6–7.2 and moderate hardness.
Feeding
Metynnis maculatus is best described as an omnivore that leans heavily herbivorous. In the wild it grazes algae, soft aquatic and marginal vegetation, and takes fallen fruit and seeds from the flooded forest canopy during high water, supplementing this plant-based diet with invertebrates. This is the same general feeding strategy shared across the silver dollars, and it is the reason the group has such a fixed reputation among aquarists as plant-shredders.
In the aquarium, silver dollars should be offered a diet centred on vegetable matter — blanched courgette, cucumber, spinach and other soft greens, spirulina flake or pellets and algae wafers — with occasional protein such as bloodworm or quality flake mixed in. Fed a diet too heavy in protein and too light in vegetable matter, this species will simply turn to any live plants in the tank instead.
Mating
SeriouslyFish describes the species in the wild as exhibiting an opportunistic reproductive strategy tied to the flood cycle, with a long breeding season and repeated small-batch spawning rather than a single synchronised event — a pattern documented across several Metynnis species in their native floodplain habitats. There is no reliable account of a distinct courtship display or pairing behaviour for this species from the cache sources available, and no documented external sexual dimorphism.
Breeding has not been reliably recorded in home aquaria for this species; SeriouslyFish notes it explicitly as unrecorded in captivity. Where silver dollars generally have been bred by hobbyists, conditioning a well-fed group in soft, slightly acidic water and allowing them to spawn in fine-leaved vegetation is the standard approach, but for Metynnis maculatus specifically the cache does not support claiming this has actually been achieved.
Breeding
Where silver dollars as a group do spawn in captivity, the mode is egg-scattering: a conditioned pair or group releases eggs over fine-leaved plants or a spawning mop in soft, acidic water, with no nest built and no parental care of eggs or fry afterward. This fits the broader Serrasalmidae pattern for the Metynnis/Myloplus silver dollars, which stands in sharp contrast to the substrate-guarding, nest-defending breeding of true piranhas in the same family.
For Metynnis maculatus specifically, however, the available sources report breeding as unrecorded in the aquarium — this is a species that is not reliably home-bred, and any fish in the trade should be assumed to be wild-caught or farm-produced by commercial operations rather than hobbyist-spawned. A keeper hoping to breed this species should expect to be working largely without a documented precedent.
In the aquarium
Metynnis maculatus is an active, midwater schooling fish that should be kept in a real group of six or more; SeriouslyFish notes it is not aggressive but can become skittish and flighty when kept alone or in too small a number, dashing at the glass and injuring itself. Given its adult size of up to roughly 6–7 in and its need for sustained swimming room, a school needs a genuinely large aquarium — a long, open tank well beyond the footprint suited to smaller community characins.
The single most important care note is dietary: this is a committed plant-eater that will graze soft-leaved aquatic plants down to bare stems. Keepers wanting a planted look should stick to tough, unpalatable species (Anubias, Java fern, Vallisneria) or accept a hardscaped, decor-only setup instead of a delicate aquascape. Water should be kept within the wide 68–82 °F, pH 5.0–7.5 range the species tolerates in the wild, with good filtration for a large-bodied, actively swimming shoal, and tankmates should be similarly robust, peaceful, open-water fish rather than small or slow-moving species that might be outcompeted at feeding time.
Conservation
The IUCN Red List assesses Metynnis maculatus as Least Concern, reflecting a wide distribution across the upper Madeira, Beni, Mamoré, Guaporé (Iténez) and upper Paraguai drainages spanning Bolivia and Brazil, with no evidence of significant population decline. The species is not the subject of any specific conservation programme and is not considered threatened.
As with many widespread Neotropical floodplain fish, the broader long-term pressures on its range — damming and hydrological alteration of the Madeira system, deforestation-linked sedimentation, and water-quality change in the Pantanal — are relevant watch items for the region generally rather than documented threats to this species specifically. The aquarium trade sources silver dollars from a mix of wild collection and commercial breeding operations, though for M. maculatus in particular home breeding is not established.