Taxonomy & naming
Metynnis guaporensis was described by Carl H. Eigenmann in 1915, in the same era that produced most of the genus's currently recognised species, based on material from the Guaporé (Iténez) River. It belongs to the family Serrasalmidae — piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Metynnis, one of several silver-dollar lineages (alongside Myloplus, Myleus and their relatives) that Serrasalmidae taxonomy has repeatedly split and reshuffled as species have been re-examined against type material. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the currently valid combination, and this account follows the cache and current usage in keeping the species in Metynnis rather than assuming an older or newer combination.
Silver dollars as a group are frequently confused with one another in the aquarium trade, where fish are often sold simply as "silver dollar" with no species-level identification; Metynnis guaporensis is a comparatively little-documented member of the genus, and much of what circulates about its care is inferred from better-known congeners rather than from species-specific study.
Morphology
Metynnis guaporensis has the classic silver-dollar build: a strongly laterally compressed, disc-shaped body with a bright, mirror-like silver flank, a small head, and a long-based anal fin typical of the genus. FishBase records a maximum size of 6.5 in standard length and a maximum recorded weight of about 180 g, making it a modest-sized member of the family compared with its much larger pacu relatives.
As with other Metynnis species, coloration is dominated by reflective silver scales that can show a faint olive or golden cast depending on light and water conditions, with fins that may carry a light dusky or reddish tinge in some individuals. Species-level distinguishing features within Metynnis rely on fine details of fin-ray counts and body proportions that are not well documented for this species outside the technical taxonomic literature; the cache does not provide further morphological detail beyond size.
Habitat
The species is native to the Guaporé (Iténez) River basin, a major tributary system straddling the Brazil–Bolivia border in the southwestern Amazon, and GBIF records place its type locality and known occurrences within that basin. FishBase describes it as a freshwater, pelagic species, consistent with the open-water schooling habit typical of silver dollars, which tend to move in loose to tight groups over open stretches of river and floodplain rather than hugging cover the way many piranhas do.
Detailed water-chemistry data specific to Guaporé basin populations was not found in the cache; the general guidance for the genus is warm, soft water, discussed further under "In the aquarium." As with much of the southwestern Amazon, the Guaporé system experiences a pronounced seasonal flood pulse, and silver dollars generally track that cycle by moving between main channels and flooded margins.
Feeding
Metynnis guaporensis is a herbivore, in keeping with the rest of its genus. Wild silver dollars graze on soft aquatic and marginal vegetation, algae, and fallen fruit and seed material from the flooded forest, and they are well known among fish that browse plant matter rather than hunt. In the aquarium, silver dollars take vegetable-based foods readily — blanched spinach, lettuce and cucumber, algae wafers and spirulina-based flake or pellet — alongside occasional protein such as frozen or live foods in smaller amounts.
This strongly plant-leaning diet has a direct practical consequence: a tank housing this species should not be planted with anything the keeper wants to keep intact, since soft-leaved aquarium plants will be grazed down quickly by a group of hungry silver dollars.
Mating
Species-specific courtship behaviour for Metynnis guaporensis was not found in the cache. Silver dollars as a genus do not show strong, reliably documented external sexual dimorphism, and pairs or spawning groups are generally identified by behaviour and condition — females becoming noticeably fuller-bodied when gravid — rather than by fixed colour or fin differences. Conditioning for spawning in the hobby typically follows a rise in temperature and softer, more acidic water, mirroring the onset of the flood season in their native range, but this has not been confirmed specifically for M. guaporensis and should be treated as inference from the wider genus rather than documented fact for this species.
Breeding
Silver dollars in the genus Metynnis are egg-scatterers over fine-leaved plants, and Metynnis guaporensis follows that pattern: no nest is built and no parental care is provided. In the hobby, spawning is occasionally achieved in soft, acidic water, with adults released over a mass of fine-leaved or feathery plants (or a spawning mop substitute, since live plants are otherwise likely to be eaten) where the scattered eggs fall and adhere. Eggs reportedly hatch within a few days at warm tropical temperatures, and free-swimming fry begin taking fine foods shortly after absorbing their yolk sac.
Because the adults show no protective behaviour and will readily eat their own eggs given the chance, successful home breeding of silver dollars typically requires removing the adults after spawning or using a separate spawning setup with generous plant cover. Species-specific breeding accounts for M. guaporensis in particular are sparse, and this description reflects the general pattern documented for the genus.
In the aquarium
Metynnis guaporensis stays modest by serrasalmid standards — a maximum of about 6.5 in — but it is still an active, fast-swimming schooling fish that needs both company and room. A group of five or more in a tank of at least 80 US gal with a long swimming axis suits the genus well; keeping fewer fish, or keeping them cramped, tends to produce a nervous, skittish shoal rather than the confident, free-ranging group seen in a well-set-up tank.
The single most important care note for any silver dollar is that it is a committed plant-shredder: soft, delicate aquarium plants will be grazed to the stem in short order, so the tank should be hard-scaped with driftwood and hardy, tough-leaved or artificial plants rather than set up as a conventional aquascape. Water should be kept warm and on the soft side; general genus guidance calls for roughly 79–82 °F and soft water (around 8 dGH or below), though these figures come from general silver-dollar care sources rather than data specific to M. guaporensis. Feed a vegetable-forward diet as described above, and choose tankmates that can handle a lively, sometimes food-competitive shoal.
Conservation
The IUCN Red List assesses Metynnis guaporensis as Least Concern, with an assessment recorded in the cache as dated December 2020. This reflects the absence of documented major threats to the species within its Guaporé basin range, though as with many localized South American characins, detailed population trend data is limited. The species does not appear to be a significant target of the aquarium trade compared with the handful of better-known Metynnis species more commonly sold as generic "silver dollars," and no specific conservation concerns or habitat pressures were found in the cache beyond the general, long-term watch items facing Amazonian freshwater systems (damming, deforestation-linked sedimentation and water-quality change).