Taxonomy & naming
Myleus pacu was described by the Scottish naturalist William Jardine in 1841. It sits in the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and in the genus Myleus, one of several silver-dollar-type genera in the family alongside Myloplus, Metynnis and Tometes.
Serrasalmidae taxonomy has been revised substantially in recent decades, with a number of species historically placed in Myleus moved into segregate genera such as Myloplus, Tometes and Ossubtus as the family's phylogeny has been resolved with molecular data. Myleus pacu itself has remained in Myleus, but the genus-level churn around it is a reminder that identifications in the trade — where "silver dollar" is used loosely for several genera — should not be taken at face value without checking the current Catalog of Fishes (Eschmeyer, California Academy of Sciences) placement.
Morphology
Myleus pacu has the classic silver-dollar body plan: a strongly laterally compressed, disc-shaped body that is nearly as tall as it is long, giving the fish its coin-like silhouette and its common name. FishBase records a maximum published length of 8 in total length, a genuinely large size for a silver dollar and one that keepers routinely underestimate when buying juveniles at a few centimetres.
The body is bright silver overall with a fine, mirror-like sheen, and the dorsal and anal fins typically carry darker, sometimes reddish or grey-black markings that contrast against the flanks. Like other serrasalmids it has small, close-set, somewhat molariform teeth suited to grazing plant material rather than the blade-like cutting teeth of true piranhas.
Habitat
The species is native to the Essequibo River system in Guyana, South America, where it occupies tropical freshwater river habitat. Data on its precise habitat preferences within the Essequibo are sparse in the available literature, but FishBase lists a tropical temperature range of roughly 72–82 °F, consistent with lowland Guianan river conditions.
As a plant-feeding serrasalmid it is closely tied to submerged and marginal vegetation, including aquatic plants of the family Podostemaceae, which grow attached to rocks in flowing water — a habitat cue that points to at least seasonally clear, moving water rather than still blackwater swamp.
Feeding
Myleus pacu is a committed herbivore. FishBase records its diet as feeding on aquatic plants, with a particular note of Podostemaceae (river-weed), plus seeds, placing it firmly among the plant-grazing serrasalmids rather than the omnivorous or predatory ones.
In the aquarium this translates directly: it is an enthusiastic and rapid plant-stripper, readily consuming most stem and leaf plants down to bare stalks. Keepers should offer a vegetable-forward diet — blanched vegetables such as courgette, spinach and peas, spirulina-based flakes or pellets, and algae wafers — supplemented with only occasional protein. A tank stocked with real plants should be expected to lose them; many keepers furnish the tank with driftwood, rock and robust or plastic decor instead.
Mating
Detailed courtship behaviour for Myleus pacu is not well documented in the available literature. What is recorded is that the species migrates to small creeks during the rainy season to spawn, a pattern shared with several other Amazonian and Guianan serrasalmids whose reproduction is tied to seasonal flood-pulse hydrology rather than to conditions that are easily replicated in a home aquarium.
No reliable external sexual dimorphism is documented for this species; data is sparse and no consistent colour or fin difference between males and females has been reported.
Breeding
Myleus pacu is an egg-scatterer, not a nest-builder or mouthbrooder. In the wild, adults migrate to small creeks during the rainy season to spawn, releasing eggs that are not guarded by either parent.
Breeding this species in the home aquarium is reported as extremely difficult to essentially impossible; the seasonal migratory trigger that governs its spawning is very hard to replicate in tank conditions, and any commercial-scale or deliberate breeding is understood to require hormone induction or very large outdoor pond systems rather than typical hobbyist setups. In practice, virtually all specimens in the trade are wild-collected or come from large-scale pond operations rather than home breeding.
In the aquarium
Myleus pacu reaches 8 in and is a schooling fish, so it needs both a genuinely large aquarium and the company of its own kind — a lone individual in a small tank is both undersized housing and social deprivation. Plan for a long, wide tank with open swimming space well beyond what its disc shape might suggest, sized for a group rather than a single specimen.
The defining care consideration is its appetite for plants: this is not a fish for a planted aquascape. Robust hardscape — driftwood, rockwork — stands up far better than live vegetation, which a settled group will reduce to stems in short order. It is described as peaceful by nature relative to its piranha relatives and generally safe with similarly sized, non-plant tankmates, but its size and schooling requirement make it a fish for an experienced keeper with tank space to spare, not a casual community addition.
Conservation
The IUCN Red List assesses Myleus pacu as Near Threatened (2022 assessment). This places it in a more precarious position than many widespread serrasalmids, consistent with a comparatively restricted native range centred on the Essequibo River system in Guyana rather than the vast, multi-country basins occupied by species such as the pacu Colossoma macropomum or the red-bellied piranha.
Data on the specific pressures behind the Near Threatened listing — habitat change, collection for the aquarium trade, or other factors — is sparse in the sources available here. Given its restricted range, any habitat degradation within the Essequibo system is a plausible long-term concern for the species, though this article does not assert specifics beyond the IUCN status itself.