Taxonomy & naming
Tometes siderocarajensis was described in 2017 by Andrade, Machado, Jégu, Farias and Giarrizzo in PLOS ONE, based on a holotype of 13.5 in standard length and a paratype series ranging 3.5–14 in SL collected from the Itacaiúnas River at Serra dos Carajás, Parauapebas, Pará State, Brazil. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Tometes, one of several silver-dollar lineages (alongside Myloplus and Ossubtus) that the Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises as split out of the older, broader concept of Myleus following the family's ongoing taxonomic revision.
The species epithet siderocarajensis references the Serra dos Carajás, the iron-ore-rich highland region of its type locality (sidero- from the Greek for iron, echoing the Carajás mining district), tying the name directly to the small stretch of the Itacaiúnas River basin where it is currently known to occur. As a fish described only in 2017, it has not yet been formally reassessed by the IUCN Red List, and comprehensive distribution and life-history data beyond the type description remain limited.
Morphology
Tometes siderocarajensis has the classic silver-dollar build: a deep, laterally compressed, disc-shaped body typical of rheophilic serrasalmids adapted to holding position and grazing in rapids. The original description reports a maximum size of 14 in standard length among the type series, with the holotype itself measuring 13.5 in SL — a solidly large silver dollar by aquarium standards, though the aquarium trade markets adult specimens as reaching roughly 11 in (11+ inches).
Colouration is the fish's calling card in the trade: a body that darkens toward black with age, overlaid with vivid red-to-orange splotches that intensify in breeding condition, giving rise to the trade name "red and black sailfin pacu." As in other Tometes species, males of this species carry a diagnostic additional lobe on the anal fin along with curved hooks on the anal-fin rays — a sexually dimorphic trait used to distinguish males from females and from closely related congeners.
Habitat
The species is known from the Itacaiúnas River and the broader Tocantins-Araguaia basin in Pará State, Brazil, specifically rapids habitat with rocky outcroppings in the Serra dos Carajás region. This is fast, well-oxygenated, rock-strewn river water rather than the slow floodplain backwaters associated with many other serrasalmids, and it is the kind of habitat where rheophilic (current-loving) Podostemaceae — the aquatic "riverweed" family that roots directly onto submerged rock in rapids — can establish the dense patches that make up the bulk of this fish's diet.
Habitat loss is a live concern for this narrowly distributed species: the type description notes pressure from mining, agriculture and dam construction in the region, including the Tucuruí Hydroelectric Reservoir, which submerged rapids habitat in the lower Tocantins and altered the rheophilic plant communities that species like T. siderocarajensis depend on. Detailed water chemistry data (pH, hardness) for the type locality were not captured in the available sources; keepers should treat this as a moderately warm, well-oxygenated, flowing-water species pending firmer figures.
Feeding
Tometes siderocarajensis is a herbivore, and the type description is explicit about it: stomach contents of the type series were full of Podostemaceae leaves and flowers, together with fruits from Fabaceae (legumes) and only occasional insect material. This tracks with the fish's rapids habitat, where dense algae-fouled Podostemaceae mats growing on submerged rock are the dominant food resource, and it explains the species' anal-fin dentition and grazing-adapted jaw structure.
In the aquarium this translates directly: the fish needs a vegetable-forward diet — blanched greens (courgette, spinach, cucumber), spirulina-based flake or pellet, and algae wafers — supplemented with some protein and occasional fruit. Keepers should expect the fish to treat live aquarium plants exactly as it treats riverweed in the wild: as food, not decoration.
Mating
No courtship or spawning behaviour has been documented for Tometes siderocarajensis in the literature located for this account; as a species described only in 2017 from wild-caught material, its reproductive biology has not been studied in the field or reported from aquarium breeding. What is documented is the sexual dimorphism used to distinguish the sexes: males carry an additional lobe on the anal fin and curved hooks on the anal-fin rays, a trait shared with other Tometes species and typical of serrasalmid silver dollars, where males use modified anal-fin structures during spawning to clasp against the female.
Because data are sparse, this account does not speculate further on pairing behaviour, spawning triggers or timing for this species specifically.
Breeding
Like other silver dollars in the family Serrasalmidae, Tometes siderocarajensis is presumed to be an egg-scatterer that broadcasts adhesive eggs over fine-leaved vegetation or, in its natural rapids habitat, over dense Podostemaceae mats, with no parental care of eggs or fry. This mode is consistent with the fish's anal-fin clasping structures in males, which are typical of silver-dollar spawning across the group, but no direct observation of spawning in the wild or in captivity has been reported in the sources available for this account.
Given how recently the species was described and how little aquarium-breeding history exists for it, it should be treated as essentially unbred in the hobby at this time — soft, acidic water and a well-conditioned group are the general conditions that trigger spawning in related silver dollars, but this has not been confirmed for T. siderocarajensis specifically.
In the aquarium
Tometes siderocarajensis reaches a substantial size for a silver dollar — up to 14 in SL by the type description, with trade sources citing adults around 11 in (11+ inches) — and needs a correspondingly large aquarium, with commercial sources recommending 120+ US gallons (roughly 450+ litres) and plenty of open swimming space to accommodate its active, current-loving nature. As a schooling silver dollar it should be kept in a proper group rather than alone or in pairs, mirroring the shoaling behaviour typical of the genus.
The single most important care note is that this is a dedicated plant-shredder: any live aquarium plants will be treated as food, so the tank should be hardscaped with rock and driftwood rather than aquascaped. Strong filtration and good water movement help replicate the rapids conditions the species is adapted to. Because it is a genuinely new addition to the hobby, availability is limited and the trade currently relies on wild-collected or small-scale commercially imported stock rather than any established captive-breeding supply.
Conservation
Tometes siderocarajensis has not yet been formally assessed by the IUCN Red List; described only in 2017, it currently carries no official conservation status (Not Evaluated). The species is known only from a narrow stretch of the Itacaiúnas River and the surrounding Tocantins-Araguaia basin in the Serra dos Carajás region of Pará State, Brazil — one of Brazil's most heavily mined landscapes for iron ore — which makes its true range and population status a genuine open question rather than a documented certainty.
The type description flags real, specific pressures on this habitat: mining activity, agricultural expansion and dam construction, including the Tucuruí Hydroelectric Reservoir, which has already submerged rapids habitat in the lower Tocantins River and altered the rheophilic plant communities the species depends on for food. Given its narrow known range and reliance on a specialised rapids habitat, this is a species worth watching even in the absence of a formal Red List assessment.