Taxonomy & naming
Mylesinus paucisquamatus was described by Michel Jégu and Ronaldo Souza Santos in 1988, one of a small number of species placed in the genus Mylesinus within the family Serrasalmidae — the piranhas, pacus and silver dollars, part of the order Characiformes. The genus Mylesinus is one of several silver-dollar lineages carved out of the old, broadly-defined Myleus over the course of taxonomic revisions to the family; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the currently valid combination, and this article keeps to Mylesinus paucisquamatus as the accepted name rather than any older or informal combination that may still circulate in hobby literature.
The species epithet paucisquamatus is a straightforward Latin descriptor — paucus (few) plus squama (scale) — alluding to a comparatively sparse or reduced scale count relative to its relatives, a morphological detail used to distinguish it from other Mylesinus and Myleus-group species. It is endemic to the rio Tocantins basin, and its rarity in collections means it is far less documented, in the ichthyological literature and in the aquarium trade alike, than the widespread Amazonian silver dollars in genera such as Metynnis and Myloplus.
Morphology
Like other silver-dollar serrasalmids, Mylesinus paucisquamatus has the classic laterally compressed, disc-shaped body of the group, built for manoeuvring in current rather than for open-water cruising. FishBase records a maximum size of 13 in standard length in males and 12 in SL in females — a large fish by silver-dollar standards, and considerably bigger than the small Metynnis species most commonly sold as "silver dollars" in the trade. Aquarium-trade accounts describe imported juveniles at a modest 2–3 in and note that most specimens seen in the hobby are well short of the documented wild maximum, commonly cited around 8–10 in.
Detailed published descriptions of adult colouration are sparse in the sources available here; hobby accounts describe the species as an attractively marked silver dollar, consistent with the mirror-bright flanks typical of the group, but this article does not assert specific fin-colour or pattern detail beyond what is documented.
Habitat
Mylesinus paucisquamatus is endemic to the rio Tocantins basin in central Brazil, and unlike many silver dollars that inhabit calm floodplain lakes and backwaters, it is a rheophilic specialist — a fish adapted to fast-flowing, rocky rapids. Field ecology work on the species places it specifically in and around the confluence zones of Tocantins tributaries, including the rios Paranã and Cana Brava, where rapids expose rock surfaces to strong current.
That rock-and-current habitat is inseparable from the species' feeding ecology: the rapids' submerged rocks carry a coating of periphytic bryophytes (mosses) and Podostemaceae — a family of flowering plants specialised to grow attached to rock in fast water — which forms the bulk of the species' diet (see Feeding). This rheophilic, current-dependent lifestyle is also what makes the species acutely vulnerable to river damming, which drowns rapids under reservoir pools and eliminates the specific rock-and-current habitat the fish and its food plants depend on.
Feeding
Mylesinus paucisquamatus is herbivorous and notably specialised even by silver-dollar standards: published ecological work on the species found periphytic bryophytes made up roughly 68% of stomach-content volume, with Podostemaceae — the rock-adapted rapids plants described above — forming most of the remainder. This is a fish built to graze a specific kind of algae-and-moss film off submerged rock in fast water, rather than a generalist browser of soft aquatic vegetation.
In the aquarium, this specialised wild diet translates to a straightforward practical rule: offer vegetable-forward foods — blanched greens, spirulina-based flakes or pellets, and algae wafers — supplemented with some protein, and expect the fish to graze on any algae growth and soft-leaved plants in the tank. As with other silver dollars, it should be expected to shred ordinary aquarium plants rather than coexist with them (see In the aquarium).
Mating
The available sources for this species are limited to ecological field data rather than aquarium breeding accounts, and no description of courtship behaviour, pairing, or external sexual dimorphism was found in the cache used for this article. What is documented is seasonal timing: a field study of the species in the rio Tocantins found reproductive activity increased toward the early dry season, in the April–September window, a pattern consistent with many Neotropical riverine fish that time spawning to seasonal water-level and flow changes rather than a fixed calendar. Beyond that seasonal signal, courtship detail is data sparse and this article does not speculate further.
Breeding
As a silver dollar, Mylesinus paucisquamatus fits the group's general reproductive pattern — an egg-scatterer over fine-leaved vegetation, without nest-building or parental care of eggs or fry — but this is inferred from the well-documented pattern in related Myleus-group and Metynnis silver dollars rather than from a species-specific breeding account, since no aquarium spawning report for this particular species was found in the sources used here. Reproductive activity in the wild is documented to increase in the early dry season (April–September) in its native rio Tocantins.
Honesty is warranted here: Mylesinus paucisquamatus is described in trade literature as not commonly imported and not commonly bred in the hobby. Its combination of large adult size, rheophilic habitat requirements (strong current, well-oxygenated water) and specialised herbivorous diet make it a poor candidate for casual home breeding, and prospective keepers should not expect to replicate its life cycle without deliberately recreating fast-flowing, well-aerated conditions.
In the aquarium
Mylesinus paucisquamatus is a large silver dollar — up to 13 in SL in the wild, and even trade-reported aquarium adults reaching 8–10 in — so it needs a genuinely large, long aquarium with strong water movement and open swimming space, not a small planted community tank. As a rheophilic species from Tocantins rapids, it will do best with robust filtration and current, well-oxygenated water rather than the sluggish, still conditions tolerated by floodplain-dwelling characins.
Like other silver dollars, it is a committed plant-shredder: its wild diet of bryophytes and rock-plants translates directly to a taste for soft aquarium vegetation, and it will strip a planted tank rather than coexist peacefully with it. Keep it in a robust, hard-scaped setup — driftwood, rock, hardy or artificial plants — and feed a vegetable-forward diet to satisfy its grazing instinct before it turns to the decor. It is a peaceful schooling fish by disposition and, as with most silver dollars, is best kept in a group rather than alone. It is not commonly imported or bred, so availability in the trade is limited and inconsistent.
Conservation
The IUCN Red List assesses Mylesinus paucisquamatus globally as Least Concern. That global assessment sits alongside a separate domestic listing: Brazil's ICMBio (Instituto Chico Mendes de Conservação da Biodiversidade) classifies the species as threatened within Brazil, and a dedicated ecological field study likewise frames it as an endangered fish species from the rio Tocantins basin — reflecting the narrow endemic range and specialised rapids habitat documented for this fish domestically, even though its global status is currently assessed as more secure.
The species' core vulnerability, wherever it is assessed, is tied to river damming: because it depends on fast-flowing rapids and the specific periphyton and Podostemaceae growth those rapids support, impoundment and habitat fragmentation from dam construction on the Tocantins and its tributaries can eliminate the only habitat this fish can use. That dependence is worth watching even under a Least Concern global assessment, given how localised and habitat-specific the species is.