Taxonomy & naming
Metynnis longipinnis was described by Zarske & Géry in 2008 in a revision of the genus published in Vertebrate Zoology, based on material from the río Casiquiare drainage in southern Venezuela. The type series is separated from its closest relative, Metynnis hypsauchen, by a higher serrae count along the ventral keel (36–37 versus 27–32 in M. hypsauchen), a scale count of 91–93, a uniformly silver body without the blotching sometimes seen in congeners, and notably elongated dorsal and anal fins — the feature behind the species epithet.
The genus Metynnis sits within the family Serrasalmidae — the piranhas, pacus and silver dollars — in the order Characiformes. Serrasalmidae has been reorganised substantially in recent decades, with several silver dollar lineages moved out of the old catch-all genus Myleus into Myloplus, Tometes and Ossubtus, but Metynnis itself has remained a stable, recognised genus throughout, and M. longipinnis has not been affected by those reclassifications. As with many recently described, geographically narrow Metynnis species, it is easily confused with M. hypsauchen and other silver dollars in the aquarium trade, where fish are rarely sold under a confirmed species identification.
Morphology
Metynnis longipinnis has the classic silver dollar shape: a strongly laterally compressed, disc-like body that is almost as tall as it is long, giving the schooling, coin-like silhouette the group is named for. The holotype measured 2.5 in standard length (roughly 2.5 in), but that figure reflects a single collected specimen rather than the species' full adult size — closely related congener Metynnis hypsauchen reaches 6–8 in as an adult in the aquarium, and general Metynnis literature gives a genus-wide range of roughly 3–7 in, so a mature M. longipinnis should be expected in that same range rather than at the holotype's size.
Colouration is uniformly silver across the flanks, without the dark blotching or barring seen in some other silver dollars, giving the fish a bright, mirror-like sheen typical of open-water schoolers. Its most distinctive feature relative to M. hypsauchen is fin shape: both the dorsal and anal fins are extended and elongated, a trait reflected directly in the name longipinnis ("long fin"). Sexes are not reliably distinguished by external features in this species; see "Mating" below.
Habitat
The type locality for Metynnis longipinnis lies in the río Casiquiare drainage of southern Venezuela — the unusual natural channel that links the upper Orinoco to the Rio Negro and, through it, the Amazon basin. Fish from this region typically inhabit slow-flowing river channels, backwaters and flooded-forest margins, moving with the seasonal rise and fall of Amazonian and Orinoco hydrology, though detailed habitat data specific to this species has not been published beyond the original description.
Species-specific water chemistry data for M. longipinnis is not available in the literature reviewed here; general Metynnis care guidance describes warm water in the roughly 75–84 °F range with soft to moderately hard, mildly acidic to neutral chemistry around pH 6–7.5, consistent with the blackwater-influenced conditions typical of the Casiquiare and broader upper Orinoco/Rio Negro region. Given the lack of direct data for this species, these figures should be treated as a genus-level guide rather than a confirmed species account.
Feeding
Metynnis species are herbivore-leaning omnivores, and M. longipinnis should be assumed to follow the same pattern documented for its close relative M. hypsauchen and the genus generally: a diet dominated by plant material, algae, and fallen fruit and seeds in the wild, with some incidental insects and other small invertebrates. This is not a fish that hunts — its blunt, incisor-like teeth are built for grazing vegetation, not for the blade-like slicing bite of a true piranha.
In the aquarium, silver dollars are voracious plant-eaters and should be fed a diet weighted heavily toward vegetable matter — blanched spinach, courgette, cucumber and other greens, spirulina-based flake or pellet, and algae wafers — supplemented with a smaller portion of protein such as quality omnivore pellets or frozen foods. Underfeeding vegetable matter is the single most common cause of these fish turning their attention to any live plants in the tank.
Mating
No species-specific courtship or spawning behaviour has been documented for Metynnis longipinnis; the record for this species is essentially limited to its 2008 taxonomic description. What is known for the genus, and presumed to apply here, is that silver dollars pair or group-spawn over fine-leaved vegetation, typically triggered by softer, more acidic water and a rise in temperature that mimics the onset of seasonal flooding.
External sexual dimorphism is not documented for this species. In related Metynnis, sexes are difficult to distinguish outside spawning condition, when gravid females may appear noticeably rounder-bodied; there is no reliable colour or fin difference reported between males and females.
Breeding
As with other silver dollars in the genus, Metynnis longipinnis is presumed to be an egg-scatterer: adults release eggs over fine-leaved plants or plant-like structure rather than building a nest or guarding a clutch. Congener M. hypsauchen is documented spawning this way in soft, acidic water, with females producing large numbers of adhesive eggs that are left unattended once laid — there is no parental care, and the eggs and fry receive no guarding or fanning from either parent.
There are no confirmed reports of Metynnis longipinnis being bred in the aquarium hobby; the species has been in the trade only since its 2008 description and has never become a widely distributed staple. Breeders working with the genus generally condition a group in soft, warm, slightly acidic water and provide fine-leaved or artificial spawning mops, then remove the adults once eggs are noticed to prevent them being eaten. Treat any spawning of this particular species as effectively undocumented rather than routine.
In the aquarium
Size the tank to a realistic adult length in the 6–8 in class shared with its close relative M. hypsauchen, not to the smaller holotype measurement — a mature group needs a long, wide aquarium with substantial open swimming space, generally cited at 80 US gal upward for an adult group of silver dollars. This is a dedicated schooling fish: keep at least five or six individuals so the group feels secure, since a lone or thinly grouped silver dollar tends to be skittish and prone to startling, glass-surfing behaviour.
The defining care warning for this genus is that it is a committed plant-shredder — Metynnis will strip soft-leaved aquatic plants to bare stems within days, so this is a fish for a robust, hard-scaped tank with driftwood, rockwork and tough or floating plants, not a planted aquascape. Keep water warm and soft to neutral, filtration strong given the group's bioload and heavy grazing-driven waste, and tankmates limited to similarly sized, peaceful, fast-water-tolerant species; silver dollars are not aggressive but can be nervous, so avoid boisterous or fin-nipping companions.
Conservation
Metynnis longipinnis has not been formally assessed by the IUCN Red List and carries no conservation status; as a species described only in 2008 from a single drainage, it falls into the large and unremarkable category of recently described Neotropical fishes that simply have not yet been evaluated, rather than one known to be at risk.
No species-specific threats, population trend, or fishery/trade pressure data has been published for M. longipinnis. Its type locality in the río Casiquiare drainage lies within a relatively remote and lightly developed part of southern Venezuela, but broader Amazon/Orinoco basin pressures — deforestation-linked sedimentation, water-quality change and localised damming — are the general long-term watch items relevant to freshwater fish across the region, even though none has been specifically linked to this species.