Taxonomy & naming
Mylesinus schomburgkii was described by the French ichthyologist Achille Valenciennes in 1850. It sits in the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, a family that has undergone extensive revision as the Catalog of Fishes (Eschmeyer, California Academy of Sciences) has worked through decades of tangled generic assignments among the silver dollars; several close relatives once lumped into the broad genus Myleus have since been split out into Myloplus, Tometes and Ossubtus. Mylesinus itself has remained a small, distinct genus, and M. schomburgkii is its best-known species.
The species epithet honours Sir Robert Hermann Schomburgk, the German-British explorer and naturalist who surveyed British Guiana (now Guyana) in the 1830s and 1840s and collected extensively along the rivers that still carry the species today. In the aquarium trade the fish is sometimes still labelled under the older combination "Myleus schomburgkii", a holdover from before some of the recent generic splitting in the family; the Catalog of Fishes places it in Mylesinus, which this article follows.
Morphology
Like other silver dollars, Mylesinus schomburgkii has a strongly laterally compressed, disc-shaped body built for efficient schooling and quick turns rather than pursuit. The body is bright, mirror-silver, and the fish is most easily identified by a broad, dark vertical bar running down the flank — collectors and the trade distinguish "wide-bar" and "thin-bar" colour morphs based on how pronounced this marking is. FishBase and hobby sources give a maximum standard length of about 4.5 in, making it a mid-sized silver dollar, larger than the diminutive Metynnis species but well short of the biggest pacus in the family.
The mouth carries small, close-set teeth adapted for grazing plant material rather than the blade-like, interlocking teeth of true piranhas — a useful reminder that within Serrasalmidae, dentition and diet diverge sharply between the predatory and herbivorous lineages despite the shared body plan.
Habitat
Mylesinus schomburgkii is native to northeastern South America, recorded from the Essequibo River basin (Guyana) and the Orinoco River basin, with occurrence data spanning Venezuela, Peru and Brazil. Within these systems it inhabits river channels and associated floodplain habitats typical of lowland Guiana Shield and Orinoco drainages, where seasonal flooding brings it into contact with submerged and marginal vegetation that forms much of its diet.
Water conditions across its range are warm and on the soft, mildly acidic-to-neutral side: aquarium sources give a temperature range of roughly 73–81 °F, pH 6.0–7.0 and hardness up to about 10°H, consistent with the general profile of Guiana Shield and Orinoco tributary waters rather than the extreme blackwater conditions found elsewhere in the Amazon basin.
Feeding
Mylesinus schomburgkii is predominantly herbivorous, grazing on aquatic and submerged vegetation, algae and likely fallen fruit and plant matter in the wild, in keeping with the general feeding pattern of silver dollars in Serrasalmidae. This is the single most important fact for a keeper: unlike the piranhas in the same family, this species has no interest in hunting fish, but it will happily strip a planted tank down to bare stems given the chance.
In the aquarium it should be offered a vegetable-forward diet — blanched spinach or courgette, algae wafers, spirulina-based flake or pellet — supplemented with some protein such as quality community pellets or occasional live or frozen foods. A diet too heavy in protein and too light in greens is a common keeping mistake with silver dollars generally.
Mating
Documented information on courtship behaviour specific to Mylesinus schomburgkii is sparse; no reliable account of pairing behaviour, external sexual dimorphism, or spawning triggers for this species was found in the cached sources. As with many silver dollars, spawning in the wild is likely associated with the rise of seasonal floodwaters, but this has not been confirmed for this species specifically, so it is reported here as data sparse rather than assumed.
Breeding
Mylesinus schomburgkii is an egg-scatterer, consistent with the silver-dollar pattern across the family: a female releases eggs — sources describe clutches in the range of 1,500–2,000 — over the substrate or hard scape rather than depositing them in a nest, and the eggs receive no parental care from either parent. Eggs are reported to hatch within about three days, with fry becoming free-swimming roughly a week after spawning.
Confirmed aquarium spawning reports for this species are scarce to date, and it should not be assumed to be a routinely home-bred fish; most individuals in the trade are understood to be wild-collected or farmed rather than hobbyist-bred. Keepers attempting to breed the species should expect the same conditions that trigger spawning in related silver dollars — soft, mildly acidic water and a well-conditioned group — but should treat successful captive breeding as the exception rather than the rule.
In the aquarium
Mylesinus schomburgkii is a genuinely peaceful schooling fish that should always be kept in a real group — hobby sources recommend five or more individuals — in a spacious tank; a commonly cited minimum is around 120 x 37.5 x 15 in, scaling up further for a full group of adults. Kept singly or in too small a number, silver dollars become skittish and stressed, the opposite problem from the piranhas that share their family.
The defining care consideration is that this is a committed plant-shredder: a lushly planted aquascape will not survive contact with a group of these fish for long, so the tank should be hard-scaped with driftwood and rockwork, with any plants limited to tough, fast-growing or floating species the keeper is willing to sacrifice. Juveniles enter the trade small, around 2.5–3.5 inches, but grow steadily toward their roughly 4.5 in adult size, so the tank should be sized for the adult, not the juvenile on offer at the store.
Conservation
Mylesinus schomburgkii has not been evaluated by the IUCN Red List, so it carries a status of Not Evaluated rather than an assessed conservation category. Its range across the Essequibo and Orinoco basins spans several countries with limited fine-grained monitoring of individual serrasalmid species, and no specific population trend or major threat was identified in the cached sources.
As with many characins outside the handful of species reared commercially at scale, the aquarium trade for this species likely still relies at least partly on wild collection from its native rivers, which makes habitat pressures in the Essequibo and Orinoco systems — such as mining-linked sedimentation and deforestation — long-term watch items even in the absence of a formal assessment.