Taxonomy & naming
Myloplus asterias was described by Johannes Müller and Franz Hermann Troschel in 1844, with a type locality on the Essequibo River in Guyana. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes.
The genus Myloplus is one of several silver-dollar lineages carved out of the old, catch-all genus Myleus during the taxonomic revisions of Serrasalmidae; species once lumped under Myleus have since been redistributed among Myloplus, Tometes, Ossubtus and other genera as the family was worked out more carefully. Older aquarium literature and some trade labels may still list this fish under Myleus asterias, but the Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises Myloplus asterias as the valid combination, and that is the name used here.
Morphology
Myloplus asterias has the classic silver-dollar profile: a deep, laterally compressed, near-circular body plan built for turning and holding position among vegetation and current, not for sprinting. The flanks are bright silver with a metallic sheen, and the common name "redgill pacu" points to a characteristic reddish tinge around the gill covers seen in many individuals.
FishBase records a maximum published length of 10 in total length for the species — a genuinely large-bodied characin, not the small "dollar coin" size associated with some smaller Metynnis species sold under the same generic silver-dollar label. Like other serrasalmids it carries molariform, plant-crushing dentition suited to grazing rather than the blade-like cutting teeth of true piranhas.
Habitat
The species is native to the Amazon basin, the Orinoco basin and Guiana Shield rivers, with records spanning Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Suriname and Venezuela — a broad Neotropical range typical of widespread Amazonian serrasalmids.
FishBase describes it as benthopelagic and a mid-depth schooler, typically encountered in shallow, vegetated water to around 0–10 ft depth, among roots, submerged wood and decaying organic matter along river margins and flooded habitat. Water across this range is tropical, generally warm, in the region of 73–84 °F; the species shows high resilience and a fast population-doubling time in the wild, consistent with a hardy, adaptable river fish rather than a habitat specialist.
Feeding
Myloplus asterias is herbivorous, grazing on aquatic vegetation, algae, and presumably fallen fruit, seeds and other plant material typical of the flooded-forest diet documented in related Serrasalmidae. FishBase and hobby sources agree the species is flatly incompatible with live aquarium plants — it is a dedicated plant-shredder, not an occasional nibbler.
In the aquarium, feed a vegetable-forward diet: blanched vegetables such as courgette, cucumber and spinach, spirulina-based flake or pellet, and algae wafers, supplemented with occasional protein (quality pellet or the odd piece of prawn or bloodworm). A diet that is too protein-heavy for a herbivorous serrasalmid is a common husbandry mistake worth avoiding.
Mating
Detailed courtship behaviour for Myloplus asterias has not been documented in the sources available for this article. As with other silver dollars, spawning in the wild is believed to be tied to the seasonal flood cycle and a shift toward softer, more acidic water, which is the trigger typically used to condition Serrasalmidae egg-scatterers in captivity, but this has not been specifically confirmed for this species.
No reliable external sexual dimorphism is documented for this species — data is sparse, and keepers generally cannot sex individuals by appearance alone.
Breeding
Myloplus asterias is an egg-scatterer, spawning over fine-leaved plants in the manner typical of silver dollars in the family Serrasalmidae. The eggs and resulting fry receive no parental care — adults do not guard the spawning site and will consume eggs or fry left unprotected, so breeders who want to raise young typically remove the parents or use a mesh-bottomed spawning setup to let eggs fall out of reach.
Successful reproduction is associated with soft, acidic water, consistent with the conditions used to trigger spawning in other Neotropical egg-scattering characins. The species is only occasionally bred in the hobby, and detailed accounts of home breeding are scarce; keepers attempting it should expect to condition a well-fed group and mimic a soft-water, warm-season shift rather than follow a well-established aquarium protocol.
In the aquarium
Myloplus asterias is a true schooling fish and should be kept in a real group — a minimum of eight individuals is recommended — in a large aquarium, with sources citing roughly 130 US gal and a tank frontage of at least 59 in as the practical minimum for an adult group. Undersized groups or solitary specimens tend to be shy, skittish and more prone to stress than the same fish kept properly.
Because it reaches around 10 in and will strip a planted tank to bare stems, this is a fish for a robust, hard-scaped setup — driftwood, rockwork and tough, inedible decor — rather than an aquascape; treat any live plants in the tank as temporary. It is a peaceful community fish by serrasalmid standards and can be housed with other large, non-aggressive tankmates that can handle a big tank and a hearty bioload; strong filtration and generous water changes are essential given the fish's size and mostly vegetable diet.
Conservation
The IUCN Red List assesses Myloplus asterias as Least Concern, an assessment dated December 2020, with no immediate documented conservation threats. This reflects its wide distribution across the Amazon, Orinoco and Guiana Shield river systems and, per FishBase, a high resilience with a fast population-doubling time.
No specific range-wide threats are flagged in the sources reviewed for this article; as with other widespread Amazonian and Guiana Shield fishes, general pressures on Neotropical freshwater habitat — deforestation, damming and water-quality change — are long-term watch items rather than documented immediate risks to this particular species.