Taxonomy & naming
Metynnis melanogrammus was formally described in 2016 by Ota, Rapp Py-Daniel and Jégu, part of a wave of taxonomic work that has split the historically messy Serrasalmidae — the family uniting piranhas, pacus and silver dollars within the order Characiformes — into a more accurate set of genera and species. The genus Metynnis itself holds the classic small-to-medium, laterally compressed, herbivorous silver dollars, distinguished from the larger pacu genera (Colossoma, Piaractus, Mylossoma) and from the true predatory piranhas (Pygocentrus, Serrasalmus) by their disc-like body shape, small mouth and multicuspid, leaf-shredding dentition.
The species epithet melanogrammus is Greek-derived, from melas (black) and gramma (a line or mark), referring to a dark line or marking used to distinguish it from its close congeners. The type locality is the rio Parauari near Maués, Brazil, and the description places the holotype at 5.5 in standard length, with paratypes reaching up to 7 in SL. As with many recently split Serrasalmidae, older aquarium literature and trade listings may still lump this fish under a broader "Metynnis sp." or a related species name; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the current valid combination, Metynnis melanogrammus.
Morphology
Metynnis melanogrammus shows the unmistakable silver dollar silhouette: a strongly laterally compressed, near-circular disc of a body, a small terminal mouth suited to grazing rather than predation, and bright, mirror-silver flanks that flash as the shoal turns in unison. The original description records a holotype of 5.5 in SL, with the largest paratypes reaching 7 in SL — meaning adult fish in the wild run to roughly 6–7 in standard length, consistent with other mid-sized Metynnis species.
Sexual dimorphism becomes visible only in breeding condition: males develop a patch of blood-red coloration near the supracleithrum (the shoulder region behind the gill cover), and the anal fin's front lobe is anteriorly rounded with a distinct concavity, differing from the female's fin shape. Outside of breeding condition the sexes are difficult to distinguish externally, as is typical across the genus.
Habitat
The species is documented from the rio Negro basin and its tributaries — the Uatumã, Trombetas, Parauari and Sucunduri rivers — in northwestern Brazil, with the known range extending into southern Venezuela. This places it squarely in the dark-stained-water heartland of the Amazon: the original description notes it from black-water and clear-water systems, including lakes and rivers with tannin-stained, dark waters, and specifically records no occurrences in milky (whitewater/sediment-laden) river systems, unlike some of its relatives.
These are still, slow-flowing to lentic floodplain and channel habitats typical of the rio Negro drainage, generally soft and acidic, low in dissolved minerals, and warm year-round. Submerged wood, root tangles and marginal or aquatic vegetation likely provide both feeding grounds and shoal cover, consistent with the grazing, plant-associated ecology documented for the genus generally.
Feeding
Metynnis melanogrammus is a herbivore in the mould of other silver dollars: general Metynnis biology from FishBase describes a diet built around grazing on soft aquatic vegetation and algae, supplemented with fallen fruit, seeds and some incidental animal matter such as insects. In the wild this grazing pressure on submerged and marginal plants is a normal part of floodplain and river-margin ecology.
In the aquarium, silver dollars in this genus accept dried flake and pelleted foods readily but do best on a diet weighted toward vegetable matter — blanched courgette, cucumber, peas, spinach and spirulina-based foods — with occasional protein such as bloodworm, daphnia or brine shrimp offered sparingly. Keepers should expect the fish to treat any live aquarium plant as food first and decoration a distant second; this is normal behaviour, not a sign of an unbalanced diet.
Mating
No species-specific courtship account for Metynnis melanogrammus exists in the available literature, but the 2016 description does record clear breeding-condition dimorphism: mature males develop a patch of blood-red colour near the supracleithrum and a distinctively shaped, concave anal-fin lobe, both presumably functioning in courtship display or mate recognition, as is typical of sexually dimorphic serrasalmids.
Across Metynnis generally, conditioning for spawning follows the seasonal-flood cue common to Amazonian fishes — a period of softer, more acidic water and a temperature rise thought to trigger reproductive readiness — but detailed pairing and courtship behaviour has not been documented for this particular species, and this account does not extend beyond what the source material states.
Breeding
Like other Metynnis silver dollars, Metynnis melanogrammus is understood to be an egg-scatterer: FishBase's general genus account describes Metynnis species spawning over fine-leaved aquatic plants, broadcasting adhesive eggs with no nest construction and no parental care afterward. This is a fundamentally different strategy from the true piranhas in the same family, which are substrate nest-guarders — Metynnis parents do not defend the eggs or fry.
Breeding of this specific species has not been documented in the hobby; it remains a recently described, little-kept fish, and any successful spawning would likely require soft, acidic water conditions similar to its native black-water habitat, generous plant cover for the eggs to adhere to, and separation of adults from the clutch soon after spawning to prevent the eggs being eaten. Data on captive breeding of Metynnis melanogrammus specifically is sparse.
In the aquarium
A prospective keeper should plan around the real adult size documented in the description — up to roughly 7 in SL — and around the species' absolute requirement for company: silver dollars are nervous, skittish fish in isolation or small numbers, and settle only in a real shoal, typically five or more individuals, in a long aquarium with open swimming room. A group of this size needs a large tank, commonly cited at 80 US gal or more for adult silver dollars generally, with strong filtration for the bioload and mess that comes with a mostly vegetable diet.
The single most important warning for this fish is that it will shred live aquatic plants; a tank intended for M. melanogrammus should be hardscaped with wood and rock rather than aquascaped with the soft, fine-leaved plants it evolved to eat, or the plants should be treated as an expected, replaceable food source. Water should be kept soft and on the acidic side, echoing its black-water rio Negro origins, with stable warm tropical temperatures. As a recently described and still poorly known species in the trade, stock offered as "Metynnis melanogrammus" should be treated with some caution, since silver dollars are frequently mislabelled to species level.
Conservation
Metynnis melanogrammus has not been assessed by the IUCN Red List and currently carries no conservation status — it is Not Evaluated. This is unsurprising for a species described as recently as 2016 from a defined set of rio Negro basin tributaries; formal assessment work has simply not yet reached it.
Its documented range across the Uatumã, Trombetas, Parauari and Sucunduri rivers sits within a part of the Amazon basin that faces the general pressures common to the region — damming, mining-linked sediment and water-quality change, and deforestation — but no population data exists to say whether the species is currently affected by any of them. In the absence of an assessment or trade data specific to this species, its conservation status should be treated as genuinely unknown rather than assumed safe.