Taxonomy & naming
Prosomyleus rhomboidalis was described by Georges Cuvier in 1818 and belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes. It was originally placed in the genus Myleus, and older hobby and scientific literature still refers to it as Myleus rhomboidalis; the Catalog of Fishes recognises the modern combination in Myloplus following the hard taxonomic split of the old, broadly defined Myleus into several narrower genera, among them Myloplus, Tometes and Ossubtus.
The species is one of a large group of silver dollars that share a similar disc-shaped body plan and are frequently confused with one another, and with true Myleus and Metynnis species, in the aquarium trade — precise identification generally rests on fin-ray counts and dentition rather than colour pattern alone. The vernacular name "silver dollar" describes the group's flattened, coin-like silhouette and reflective scales rather than being specific to this species.
This fish was long listed as Myloplus rhomboidalis; Eschmeyer's Catalog of Fishes places the species in Prosomyleus, and the current combination Prosomyleus rhomboidalis is followed here.
Morphology
Like other silver dollars, Prosomyleus rhomboidalis has a strongly laterally compressed, rhomboid (diamond-shaped) body — the source of its species epithet — with a small head, a blunt snout and a mouth armed with rows of strong, molar-like teeth built for crushing seeds and shredding plant matter rather than for predation. The body is covered in bright, reflective silver scales that give the fish its mirror-like sheen, and fins are typically clear to lightly dusky.
FishBase records a maximum published length of 16 in and a weight of up to 5.0 kg, making this one of the larger silver dollars kept in the hobby — considerably bigger than the more commonly traded Metynnis species. Aquarium specimens rarely approach that wild maximum, but keepers should still plan for a genuinely large adult fish, not a small tetra-sized schooler.
Habitat
Prosomyleus rhomboidalis is native to the Amazon River basin and river systems draining the Guiana Shield, with occurrence records concentrated in the Orinoco basin of Venezuela, the Brazilian Amazon and Suriname; FishBase notes Suriname as part of its type area. Within this range it is found along calm river banks and in the vicinity of rapids, in freshwater, benthopelagic, tropical habitat typical of Amazonian and Guianan lowland rivers.
As in many Amazon basin fish, its habitat and behaviour shift with the seasonal flood pulse: during high water it disperses into flooded forest and marginal vegetation to feed, and during the dry season it concentrates in the main river channels and remaining pools. The cache does not report specific pH or temperature figures for the species; keepers should follow the general soft, mildly acidic, warm-water profile typical of its Amazon and Guiana Shield range.
Feeding
Prosomyleus rhomboidalis is a herbivore with a markedly seasonal diet. During the rainy season it feeds heavily on fruit and seeds — particularly palm fruit — that fall from the flooded forest canopy into the water, using its strong jaws and molar-like teeth to crack tough seed coats. As the floodwaters recede in the dry season, its diet shifts toward aquatic plants, along with gastropods (snails) and small fish.
In the aquarium this translates directly into a plant-eating fish that needs vegetable-forward feeding: blanched greens, spirulina and algae wafers should form the bulk of the diet, supplemented with some protein such as bloodworms or brine shrimp. Keepers should expect the fish to treat live aquarium plants as food rather than decor — this is a direct extension of its wild feeding ecology, not a behavioural quirk.
Mating
Detailed courtship behaviour specific to Prosomyleus rhomboidalis is not well documented in the available sources. FishBase notes a distinct pairing during copulation, consistent with the egg-scattering reproductive mode reported generally across Myloplus and other silver dollars, but does not describe courtship displays, colour changes or triggers in detail for this species.
No reliable external sexual dimorphism is documented for this species in the cache; silver dollars in general are difficult to sex visually, and hobbyists typically rely on relative body depth or behaviour around spawning rather than fixed colour or fin differences.
Breeding
Prosomyleus rhomboidalis is an egg-scatterer, consistent with the reproductive mode reported across the Myloplus/silver-dollar group: a pair (or group) sheds adhesive eggs over fine-leaved plants rather than building a nest or guarding a fixed site. Breeding in captivity is documented but uncommon, reportedly most successful in soft, acidic water that mimics natural rainy-season conditions.
Parental care is absent — the eggs and resulting fry are not guarded once released, and adults will consume their own eggs if given the chance, as is typical for egg-scattering characins. Home breeders who succeed generally remove or protect the eggs from the adults promptly after spawning; commercial supply for the aquarium trade otherwise relies on wild-collected stock.
In the aquarium
Prosomyleus rhomboidalis is a peaceful, schooling silver dollar that should always be kept in a group of six or more — a lone or lightly-stocked group is a stressed, skittish fish that spends more time hiding than swimming. Given its published maximum of 16 in, adults need a genuinely large aquarium with plenty of open swimming space; hobby guides for the silver-dollar group generally recommend a minimum of around six feet in length once a group reaches adult size.
The single most important care note is that this is a dedicated plant-shredder, not a fish for a planted aquascape: it will strip soft-leaved aquatic plants to bare stems, mirroring its wild dry-season diet of aquatic vegetation. A robust, hard-scaped tank with driftwood and rockwork — plus a vegetable-forward diet of blanched greens, spirulina and algae wafers alongside some protein — suits it far better than live plants. It is peaceful toward similarly sized tankmates and only a threat to plants, not to other fish.
Conservation
Prosomyleus rhomboidalis is assessed by the IUCN Red List as Least Concern, reflecting its wide distribution across the Amazon basin and Guiana Shield river systems and the absence of significant range-wide population pressure. It is not a species of particular conservation concern at present.
The cache does not report specific localized threats or fishery pressure for this species; as with many wide-ranging Amazon basin fish, general long-term watch items such as damming, deforestation-linked sedimentation and water-quality change across its native rivers are relevant even though the species itself is not currently threatened.