Taxonomy & naming
Metynnis polystictus was described by Zarske and Géry in 2008 in a revision of the genus Metynnis published in Vertebrate Zoology, which described two new species and clarified the status of Metynnis goeldii. The species was described from the Xingu River system in Brazil and was identified in that revision as closely related to Metynnis anisurus. It belongs to the genus Metynnis within the family Serrasalmidae — the piranhas, pacus and silver dollars — a family that has undergone substantial taxonomic revision in recent decades, with several silver dollar lineages split out of the old genus Myleus into Metynnis, Myloplus, Tometes and other genera.
The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the currently valid combination, and this article keeps the species in Metynnis as described. Because Metynnis species are visually similar disc-shaped silver dollars distinguished mainly by fin-ray counts, scale counts and subtle proportional differences, field identification of M. polystictus against other Xingu-basin Metynnis is a specialist task best confirmed against the original description rather than by eye.
Morphology
Metynnis polystictus has the classic silver dollar body plan: strongly laterally compressed, deep and disc-shaped, with a small head and a mouth suited to grazing rather than seizing prey. FishBase records a maximum size of 6.5 in standard length, a size in line with other mid-sized Metynnis species. The body is bright silver with a reflective, almost mirrored sheen, a trait shared across the genus and the origin of the common "silver dollar" name applied to the group as a whole.
As with most Metynnis, close identification to species level relies on technical characters — fin-ray counts, scale-row counts and body proportions — set out in the original 2008 description, rather than on obvious external field marks visible to an aquarist. No distinctive colour pattern beyond the genus-typical silver body is documented for this species in the sources available.
Habitat
Metynnis polystictus is known from the Xingu River system, a major clearwater tributary of the Amazon in Brazil, and its distribution beyond the type locality is not well documented in the sources available here. Like other Amazonian Metynnis, it is presumed to inhabit river channels and associated floodplain habitats within the Xingu system, though detailed habitat data specific to this species is sparse.
Water conditions across Metynnis habitats in general run soft and slightly acidic, and aquarium care guidance for the genus specifies a pH of roughly 6.0–7.0 and a temperature of 75–82 °F. Data specific to M. polystictus is limited, so these genus-level figures should be treated as the best available guide rather than as a per-population measurement.
Feeding
Metynnis polystictus is herbivorous, in keeping with the rest of its genus — a grazer of plant material and algae rather than a predator. Aquarium care sources for Metynnis broadly describe a diet centred on vegetable matter: spirulina-based flakes and pellets, algae wafers, and blanched vegetables such as courgette, spinach or cucumber, supplemented with some protein from flakes, pellets or frozen foods.
The genus's herbivorous habit is not incidental — Metynnis are notorious in the aquarium hobby for shredding live plants down to bare stems if not fed adequate vegetable matter, and a hungry group will target soft-leaved species first. Keepers should treat vegetable feeding as the backbone of the diet rather than a supplement.
Mating
No species-specific courtship behaviour for Metynnis polystictus is documented in the sources available. Aquarium accounts of Metynnis breeding in general describe group spawning rather than fixed pairing: a conditioned shoal, kept together in soft, slightly acidic water, will spawn as a group over fine-leaved plants without the courtship rituals seen in substrate-guarding characins such as piranhas.
External sexual dimorphism is not documented for this species; data is sparse enough that no reliable way to distinguish males from females by appearance can be stated with confidence.
Breeding
Metynnis polystictus is presumed, on the basis of documented Metynnis breeding biology, to be an egg-scatterer: a group of adults releases eggs over fine-leaved plants, with no nest built and no parental care given afterward. Genus-level aquarium accounts describe eggs hatching in roughly three days at an elevated temperature around 82 °F, after which fry disperse to fend for themselves.
Species-specific breeding reports for M. polystictus are not available in the sources used here, and this species should be treated as only occasionally bred in the hobby, following the pattern documented for its relatives, rather than as a fish with an established, reliable home-breeding protocol. The eggs and fry are not guarded by the adults, so a breeding setup needs dense fine-leaved planting or a mesh layer to keep eggs away from adults that will otherwise eat them.
In the aquarium
Metynnis polystictus reaches a modest maximum of about 6.5 in standard length, smaller than many silver dollars, but it is still a fast, active, open-water schooler that needs real swimming room and the company of its own kind — aquarium guidance for the genus recommends groups of five or more to keep the fish settled and to spread out any nervous energy that would otherwise turn into fin-nipping. A long, well-filtered tank with strong water movement suits the genus's active swimming habit.
The single most important practical warning for this species, as for the rest of Metynnis, is that it is a dedicated plant-shredder: soft-leaved aquatic plants will not survive alongside a hungry group unless the fish are kept extremely well fed on vegetable matter, and even then losses are likely. This is a fish for a robust, hard-scaped tank with driftwood and rockwork rather than a delicate aquascape. Water should be kept soft and slightly acidic, in the range of pH 6.0–7.0 and 75–82 °F, following genus-level guidance in the absence of species-specific water chemistry data.
Conservation
Metynnis polystictus is assessed by the IUCN Red List as Least Concern, an assessment dated 7 November 2018. No major species-specific threats are documented in the sources available here.
As a Xingu-basin species, M. polystictus shares its range with a system that has faced significant, well-documented pressure from large-scale hydroelectric development, most notably the Belo Monte Dam, which has altered flow regimes and habitat across parts of the Xingu. Whether that development affects this particular species is not established in the sources used for this article, and no claim beyond the current Least Concern assessment is made here.