Taxonomy & naming
Metynnis luna was described by the American palaeontologist and ichthyologist Edward Drinker Cope in 1878. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Metynnis, one of several silver-dollar lineages in the family alongside Myloplus, Myleus and Mylossoma. GBIF's taxonomic backbone confirms Metynnis luna Cope, 1878 as a currently valid binomial.
Serrasalmidae has been split hard by recent taxonomic revision, with several silver-dollar species moved out of older, broader genus concepts such as Myleus into narrower ones. Metynnis itself has remained a reasonably stable genus for the small, laterally compressed, herbivorous "dollar" species, distinguished from the piranhas proper (Pygocentrus, Serrasalmus) by their blunt, non-predatory dentition and from the larger pacus (Colossoma, Piaractus) by their much smaller adult size. The common name "moon dollar" reflects both the coin-like silhouette shared by the group and the species epithet luna.
Morphology
Metynnis luna has the classic silver-dollar body plan: a strongly laterally compressed, disc-shaped body that is nearly as deep as it is long, giving the fish its coin-like profile and its common name. The flanks are a bright, mirror-like silver that catches and scatters light as the shoal moves, with fins that are typically clear to lightly dusky rather than boldly marked.
FishBase records a maximum size of 5 in standard length and a maximum recorded weight of about 102 g, making it a comparatively modest member of Serrasalmidae next to the piranhas and pacus in the same family. The mouth is small and set with blunt, close-packed teeth suited to grazing plant material rather than the blade-like, interlocking teeth of a true piranha — a dental difference that underlines how different the diet and temperament of this fish are from its more famous relatives.
Habitat
Metynnis luna is native to the Amazon River basin in South America, where FishBase and GBIF occurrence records place it as a pelagic freshwater species moving through open water in the main river channel and connected habitats rather than clinging to cover. Like many Amazonian characins it likely follows the basin's strong seasonal flood pulse, using flooded forest and marginal vegetation during high water when submerged plant growth is most available to graze.
Detailed water-chemistry data specific to this species is sparse in the cache, but as an Amazon basin fish it is consistent with the warm, generally soft and mildly acidic conditions typical of the region. In the aquarium, hobby sources for silver dollars broadly recommend a temperature around 79–82 °F and a pH in the 6.0–7.0 range, with soft water preferred but tolerance for somewhat harder conditions.
Feeding
Metynnis luna is a strict herbivore, and this is the single most important fact for anyone considering it for a planted tank. FishBase and hobby sources agree: this is a harmless, plant-grazing schooler that will strip soft-leaved aquatic vegetation to bare stems if given the opportunity, not a threat to tankmates. In the wild its diet likely includes leaves, algae, and fallen fruit and seeds from the flooded forest canopy, in keeping with the broader feeding ecology documented for silver dollars in the family.
In the aquarium, keepers feed a vegetable-forward diet — flake and pellet foods formulated for herbivores, algae wafers, and blanched greens such as spinach, lettuce and watercress, along with occasional cucumber. A diet too heavy in protein is not appropriate long-term for a fish built to process plant matter, and keepers should expect the fish to sample any live plants in the tank regardless of how well it is otherwise fed.
Mating
Specific courtship behaviour for Metynnis luna is not well documented in the sources available, but silver dollars in general are described in hobby and aquaculture accounts as pairing off within a well-conditioned group ahead of spawning, with warmer, softer and more acidic water acting as a trigger that mimics the onset of the flood season. There is no reliable external sexual dimorphism reported for this species — males and females are not readily distinguished by colour or fin shape, and breeders typically rely on conditioning a full group and watching for pairing behaviour rather than selecting sexed individuals in advance.
Data on this specific species' pairing and spawning behaviour is sparse; the account here follows the general pattern documented for the Metynnis/silver-dollar group rather than species-specific observation.
Breeding
Silver dollars including Metynnis luna are egg-scatterers, not nest-builders or mouthbrooders. A conditioned group spawns over fine-leaved plants or plant-like spawning mops, with females releasing large numbers of eggs — cited in hobby accounts for the group as ranging into the thousands — that are scattered rather than deposited in a guarded site. Neither parent provides any care after spawning, and adults will readily eat their own eggs and fry if not separated, so breeders typically remove the adults or use a mesh or marble substrate that lets eggs fall out of reach.
Hobby sources describe eggs hatching within about three days in warm, soft, acidic water, with fry becoming free-swimming roughly a week later and starting on infusoria before moving to finely crushed foods and brine shrimp nauplii. Breeding is occasional in the hobby rather than routine, and most fish in the trade are wild-caught or farm-raised in Southeast Asian and South American aquaculture operations rather than home-bred.
In the aquarium
Metynnis luna is a peaceful, fin-safe schooling fish that should always be kept in a real group — hobby sources recommend six to eight or more individuals — to settle its natural shoaling behaviour and reduce stress. At a modest adult size of around 5 in it does not need the enormous tanks that pacus or large piranhas demand, but a group still needs a long tank with open swimming space, since this is an active, pelagic fish rather than a cover-hugging one.
The defining care consideration is its appetite for plants: this species will shred soft-leaved aquarium vegetation, so a tank intended for a planted aquascape is the wrong setting for it. Keepers instead furnish a robust, hard-scaped tank — driftwood, rock, and tough or artificial plants — and rely on vegetable-based foods to satisfy its grazing instinct. It does not nip fins or scales and is considered a safe, non-aggressive tankmate for other peaceful, similarly sized community fish, a useful contrast to the piranhas that share its family.
Conservation
Metynnis luna is assessed by the IUCN Red List as Least Concern, with the assessment dated December 2020 reflecting its distribution across the Amazon River basin and no evidence of significant population decline. As a widespread, harmless Amazon basin species with no major commercial fishery pressure specific to it beyond ornamental collection, it does not carry the conservation concerns seen in more range-restricted or heavily fished South American fish.
The aquarium trade sources silver dollars including this species partly from wild collection in the Amazon and partly from aquaculture, though species-specific trade data for Metynnis luna is not available in the cache. No major threats specific to this species are documented; broader Amazon basin pressures such as deforestation-linked habitat change remain long-term watch items for the region's freshwater fish generally.