Taxonomy & naming
Tometes trilobatus was described by the French zoologist Achille Valenciennes in 1850 and is the type species of the genus Tometes, within the family Serrasalmidae (piranhas, pacus and silver dollars) in the order Characiformes. Historically the species was conflated with Myleus setiger — the type species of the separate genus Myleus — because early descriptions and museum material for the two were muddled together; more recent taxonomic review has clarified that T. trilobatus is a distinct species and that Tometes is a valid genus in its own right, allied with Mylesinus, Ossubtus and Myleus sensu stricto within the broader "silver dollar" lineage of the family.
The Serrasalmidae has undergone heavy revision in recent decades, with the old, broadly defined Myleus split across several narrower genera. Tometes trilobatus should not be "corrected" back to Myleus — the Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises Tometes as the valid genus for this species, and it is treated that way here. No English common name is in wide, settled use for this species; it is generally referred to by its scientific name even in the specialist literature that discusses it.
Morphology
Like other silver dollars, Tometes trilobatus has the classic deep-bodied, laterally compressed, coin-like silhouette of the Myleus/Tometes lineage, with a small terminal mouth and a row of close-set, multicuspid teeth adapted for scraping and cropping vegetation rather than the blade-like cutting teeth of true piranhas. FishBase records a maximum published length of 17 in standard length for the species, which is large for a silver dollar and reflects a substantial-bodied fish by the time it reaches adulthood.
As a rheophilic species — one adapted to living in fast-flowing water — its body plan and fin structure are consistent with holding position in current and rapids habitat, a trait shared with other rapids-specialist serrasalmids. Detailed published descriptions of live colouration are sparse; general silver dollar colouration (a bright, silvery flank, sometimes with faint dark speckling) is the reasonable expectation but should not be stated as confirmed for this species specifically.
Habitat
Tometes trilobatus is documented as effectively endemic to the Oyapock River system, on the French Guiana–Guyana border in the Guiana Shield of northeastern South America. It is described in the literature as rheophilic — a rapids specialist restricted to fast-flowing, well-oxygenated stretches of river rather than the sluggish floodplain lakes and backwaters many other serrasalmids use.
Within those rapids it is closely tied to beds of Podostemaceae, a family of unusual aquatic plants that anchor directly to submerged rock in fast current ("river-weed"), which forms both its primary food source and, presumably, cover and structure. The cache does not report specific temperature or pH figures for the Oyapock; as a Guiana Shield blackwater-influenced system, soft, acidic-to-neutral water is a reasonable general expectation for the region, but exact figures for this species are not confirmed and should be treated as data sparse rather than asserted.
Feeding
Tometes trilobatus is phytophagous — stomach-content studies cited in the literature show its diet dominated by Podostemaceae, the rock-anchored river-weed that grows in the rapids it inhabits, making it a specialist grazer rather than a generalist omnivore. This puts it at the herbivorous end of the serrasalmid spectrum, alongside other silver dollars, and a considerable distance from the fish- and fruit-eating diet of the true piranhas in the same family.
In captivity, a diet built around vegetable matter — blanched leafy greens, spirulina-based flake or pellet, algae wafers — with only occasional protein would follow directly from its documented wild diet. Any keeper should expect this species, like other silver dollars, to treat live aquarium plants as food rather than décor.
Mating
No species-specific courtship or spawning behaviour for Tometes trilobatus is documented in the available sources. General silver dollar accounts describe pairing or grouping behaviour triggered by warm, soft water, with fish becoming more active and less food-focused ahead of spawning, but this cannot be confirmed for T. trilobatus specifically and should be treated as inferred from the broader silver dollar group rather than established fact for this species.
Breeding
As a silver dollar, Tometes trilobatus would be expected to follow the group's typical mode: an egg-scatterer that releases eggs over fine-leaved plants or vegetation in soft, acidic water, with no parental care of eggs or fry afterward. This species has not been documented as bred in the aquarium hobby — its extreme habitat specialisation (fast-flowing rapids tied to Podostemaceae beds) and its very restricted, remote native range make it an unlikely candidate for home breeding, and no hobby breeding reports exist in the sources reviewed. Prospective keepers should not expect to replicate a rapids-rearing environment casually.
In the aquarium
Tometes trilobatus is not an established aquarium fish — it is essentially unknown in the hobby, reflecting both its narrow, remote native range and its demanding rheophilic habits. A fish adapted to fast, well-oxygenated rapids and a diet centred on a specific rock-anchored river-weed is a poor match for a typical planted community tank: it would need strong, sustained water flow, excellent oxygenation, and open swimming space, not calm, heavily planted water. Given its documented 17 in SL maximum size and its phytophagous habits, any tank housing it would need to be large and hardscaped — rockwork and driftwood rather than a plant collection, since it will graze vegetation as readily as any other silver dollar.
Silver dollars in general are schooling fish that do best in a real group rather than alone or in pairs, and there is no reason to expect T. trilobatus to differ. Given how little is documented about this species in captivity, anyone considering it should treat husbandry requirements as inferred from its wild ecology rather than proven aquarium practice.
Conservation
The IUCN Red List currently assesses Tometes trilobatus as Least Concern. The species remains a narrow endemic of the Oyapock River, and a 2024 genomic monitoring study out of French Guiana lists it among exploited freshwater fish species being tracked for management and conservation in that river system, but it is not currently considered at elevated extinction risk.
A species restricted to a single river system and its rapids habitat still carries some inherent exposure to localised pressures — fishing pressure, damming or flow alteration, and habitat degradation in and around the Oyapock — even under a Least Concern assessment, since it cannot range elsewhere if conditions there decline. The available sources do not report captive-breeding or ex-situ conservation programmes for this species; its conservation outlook rests on continued monitoring of the wild Oyapock population and its rapids habitat.