Taxonomy & naming
Acnodon normani was described by William A. Gosline in 1951 and placed in the small genus Acnodon, part of the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises Acnodon as a valid genus of herbivorous serrasalmids distinct from the true piranha genera Serrasalmus and Pygocentrus, and distinct also from the larger silver-dollar genera such as Myloplus and Metynnis with which it shares its disc-shaped body plan and plant-based diet.
Serrasalmidae has undergone substantial revision in recent decades, with older "catch-all" genera such as Myleus broken apart and their species redistributed among Myloplus, Tometes, Ossubtus and related genera; Acnodon has remained a comparatively stable, narrowly defined genus through these changes. The species epithet honours J. R. Norman, a British ichthyologist associated with the Natural History Museum, London, who worked extensively on Neotropical freshwater fishes.
Morphology
Like other silver dollars, Acnodon normani has a strongly laterally compressed, disc-shaped body that gives it an efficient, mirror-bright profile in open water. FishBase records a maximum published size of 8 in total length, making it a modestly sized serrasalmid well within the range of a large home aquarium, unlike its pacu relatives that can approach a metre. The mouth and dentition are built for grazing rather than predation — small, closely packed teeth suited to cropping algae and soft plant tissue rather than the blade-like, interlocking teeth of a true piranha.
Colouration is typically a bright, uniform silver with a faint olive or golden cast along the back, giving the fish the reflective, mirror-like appearance characteristic of silver dollars in general. There is no documented strong sexual dimorphism in colour pattern for this species.
Habitat
Acnodon normani is native to the lower Amazon basin in Brazil, with records from the Xingu and Tocantins watersheds and a type locality at the Rio Santa Teresa in the upper Rio Tocantins, Goiás State. It has also been recorded from Mato Grosso and Pará states, indicating a range across several lower-Amazon tributary systems rather than a single isolated river.
SeriouslyFish describes its native waters as crystalline (clearwater) habitats typical of the lower Amazon's rocky and sandy-bottomed tributaries, generally warm and only mildly acidic to neutral rather than the deeply tannin-stained blackwater associated with some other Amazonian characins. These waters tend to carry moderate current and good oxygenation, conditions the species appears to favour.
Feeding
Acnodon normani is herbivorous to frugivorous in the wild, grazing algae and soft aquatic vegetation, and it is also documented picking scales from larger fish on occasion — an unusual but recorded feeding behaviour for a fish otherwise built around a plant-based diet. In the aquarium it takes sinking pellets, frozen foods and spirulina-based preparations readily, and keepers report it grazing on any live plants present.
Because of its plant-shredding tendency, keepers should plan on offering blanched vegetables, algae wafers and spirulina flake as dietary staples alongside occasional protein such as frozen bloodworm or brine shrimp, rather than relying on tank vegetation to meet its needs — the vegetation will simply be eaten.
Mating
No courtship behaviour has been documented for Acnodon normani in the aquarium or in the wild-biology literature available. As with most silver dollars, sexing is not straightforward from external features, and the cached sources give no reliable account of dimorphism or pairing behaviour for this species. Prospective breeders should expect to work largely from general silver-dollar husbandry knowledge rather than species-specific accounts.
Breeding
Breeding has not been documented in aquaria for Acnodon normani. Based on its placement among the silver dollars, the expected mode — by analogy with better-known genera such as Metynnis and Myloplus — is egg-scattering over fine-leaved plants in soft, acidic water, with no parental care of eggs or fry. This has not been confirmed for this particular species, and hobbyists should treat any breeding attempt as exploratory rather than following an established protocol.
Given the total absence of documented spawning accounts, this is a species best approached with realistic expectations: it is kept and appreciated as a peaceful schooling fish, not a species with an established captive-breeding record.
In the aquarium
SeriouslyFish recommends a minimum tank size of roughly 150×17.5 in (around 80 US gal) for Acnodon normani, with strong filtration turning over four to five times the tank volume per hour to replicate the well-oxygenated, moderate-current water it favours in nature. Home Aquarium sources suggest a group of four to six individuals as a practical minimum, in at least 55 US gal — this is a schooling fish that should never be kept alone or in pairs, and a settled group is calmer and less prone to stress-driven scale-nipping at tankmates.
The single most important planning point for keepers is that live plants will be consumed — this species is a committed grazer and will strip a planted aquascape rather than coexist with it, so robust hardscape (driftwood, rock) rather than a delicate planted layout is the appropriate setup. Its occasional scale-picking habit also makes it a poor choice for community tanks with slow-moving or thin-scaled tankmates; it is best kept with other robust, similarly sized fish or as a species group on its own.
Conservation
The IUCN Red List assesses Acnodon normani as Least Concern, consistent with a species distributed across multiple tributary systems of the lower Amazon basin (Xingu, Tocantins and surrounding drainages) rather than confined to a single vulnerable water body. It is not a significant target of commercial fisheries and appears only occasionally in the aquarium trade, limiting collection pressure.
As with other lower-Amazon endemics, the longer-term watch items are the general pressures facing Brazilian freshwater systems — damming (notably extensive hydroelectric development in the Xingu and Tocantins basins), deforestation-linked sedimentation and water-quality change — rather than any specific threat currently documented for this species.