Taxonomy & naming
Myloplus levis was described by Carl H. Eigenmann and William L. McAtee in 1907, one of many silver-dollar species originally placed in the old, unwieldy genus Myleus. Serrasalmidae taxonomy has been split hard since then: the Catalog of Fishes (Eschmeyer, California Academy of Sciences) now recognises the true silver-dollar lineage that includes this species under the genus Myloplus, separate from the pacu genera Colossoma, Piaractus and Mylossoma and from the other reassigned silver-dollar genera Tometes and Ossubtus. Older literature and some aquarium-trade labels may still carry the species under "Myleus levis" or simply lump it in with the generic "silver dollar" trade name shared by several genera; the valid combination is Myloplus levis.
The family Serrasalmidae — piranhas, pacus and silver dollars — sits within the order Characiformes, and Myloplus levis belongs to its herbivorous, deep-bodied branch. FishBase lists "moon pacu" and "silver dollar" as vernacular names for the species, though "silver dollar" is applied loosely across the trade to several similarly shaped genera, so scientific-name verification matters for anyone stocking a species-specific tank.
Morphology
Myloplus levis has the classic silver-dollar profile: a strongly laterally compressed, near-circular body that gives the fish its common name, a small terminal mouth set for grazing rather than biting, and a bright, mirror-silver flank that flashes as the fish turns in open water. FishBase records a maximum standard length of 8 in, a solidly mid-sized silver dollar — smaller than some of the larger pacu-adjacent Serrasalmidae, but still a big-bodied, deep-tank fish rather than a nano species.
Colouration is understated compared with the flashier trade silver dollars: an overall silvery sheen, without the bold black blotches or red fin flashes seen in some Metynnis or Myloplus relatives, though fin edges can carry a subtle amber or dusky tint depending on individual and water conditions. The body depth and rounded outline, combined with the small grazing mouth, are the most reliable field marks distinguishing it from young pacus, which share the same disc shape as juveniles but grow into a heavier, more elongate adult form.
Habitat
Myloplus levis is native to the Paraguay-Paraná River basin of South America, a large subtropical drainage spanning parts of Brazil, Paraguay, Bolivia and Argentina. Within that system it occupies tropical to subtropical freshwater river channels, consistent with FishBase's classification of the species as freshwater and benthopelagic — spending time both over the bottom and in open water rather than tied to a single microhabitat.
Because the Paraguay-Paraná basin runs subtropical rather than equatorial, water conditions and seasonal temperature swings there are typically cooler and more variable than in the core Amazon; the cache does not report exact pH or hardness figures for this species specifically, so aquarists should treat the moderate, slightly soft-to-neutral conditions typical of Paraná-basin characins as a reasonable starting point rather than an extreme blackwater target.
Feeding
Myloplus levis is a herbivore, and its genus is documented in the literature as feeding on plant material, algae, aquatic vegetation, fruit and seeds — floodplain forest inputs that drop into the water are a plausible part of the wild diet, in line with what is known for herbivorous Serrasalmidae generally. The small, forward mouth and dentition of silver dollars are built for grazing rather than for the shearing, scale-and-fin-nipping bite of true piranhas.
In the aquarium this translates directly into a vegetable-forward feeding regime: blanched greens (courgette, cucumber, spinach, kale), spirulina-based flake or pellet, and algae wafers should form the bulk of the diet, supplemented with only modest amounts of protein. Keepers should expect the fish to test every live plant in the tank as a food source — this is normal grazing behaviour, not a sign of an unbalanced diet, and it is the central planning fact for anyone considering this species.
Mating
Detailed courtship behaviour specific to Myloplus levis is not well documented, but silver dollars in general are reported to pair or loosely group off from the shoal ahead of spawning, with activity most often triggered by soft, warm, slightly acidic water — conditions that mimic a seasonal rise in river levels. As a schooling species, courtship interactions typically play out within a maintained group rather than between an isolated pair, and keepers report increased chasing and close following between ripe individuals in the days before spawning.
External sexual dimorphism is not established for this species; the cache and general silver-dollar literature do not describe a reliable colour or fin difference between males and females, and breeders typically rely on conditioning a full group and watching behaviour rather than sexing individual fish by eye.
Breeding
Myloplus levis is an egg-scatterer, not a nest-builder or mouthbrooder. Breeding accounts for silver dollars describe a female releasing large numbers of eggs — reported in the low thousands for the group generally — broadcast among fine-leaved aquatic plants, with the male following to fertilise them as they fall. There is no nest, no defended site and no parental care: once spawning is finished, the adults do not guard the eggs and will readily eat them if given the chance, so eggs and plants are typically moved to a separate rearing container or the adults removed.
Spawning is reported to occur in soft, warm, slightly acidic water, and eggs hatch quickly — on the order of a few days — with the tiny fry needing infusoria-sized foods before graduating to brine shrimp nauplii and crushed flake. Silver dollars are occasionally bred in the hobby under these conditions, but it is not a routine, frequently repeated event the way it is for many smaller characins, and most fish in the trade are wild-caught or farm-raised rather than home-bred.
In the aquarium
Myloplus levis needs to be sized and planned around two facts: it is a genuine shoaler that wants a group and open swimming room, and it is a determined plant-eater that will shred a soft-leaved aquascape. A group of five or more in a large, long aquarium — several hundred litres at minimum for adults reaching 8 in SL — with strong water movement and open swimming lanes suits its active, schooling nature far better than a small or heavily decorated tank. Hardscape (driftwood, rock, robust hardy plants like Anubias or Java fern that resist grazing) is a far safer choice than a soft-leaved planted layout, which this species will steadily dismantle.
It is a peaceful fish by Serrasalmidae standards — unlike its piranha relatives it poses no bite risk to tankmates or keepers — and it mixes well with other robust, similarly sized South American community fish that can handle a large, well-filtered tank. The main husbandry pitfalls are keeping too few individuals (which stresses a schooling fish and can trigger fin-nipping within the group), underestimating the adult footprint, and stocking it into a display built around live plants.
Conservation
The IUCN Red List assesses Myloplus levis as Least Concern, with the most recent assessment dated December 2020 and no major documented threats to the species. Its native Paraguay-Paraná basin is a large, still relatively intact subtropical river system, and the species is described as widely available in the aquarium hobby trade, which is consistent with a stable, unthreatened population rather than one under heavy collection pressure.
As with many characins that are both wild-ranging and hobby-popular, ongoing pressures on South American river systems generally — damming, water-quality change and habitat modification across the Paraguay-Paraná basin — remain long-term watch items for the region's fish communities even though this particular species is not currently classified as threatened.