Taxonomy & naming
Cyphocharax signatus was described by the American ichthyologist Richard P. Vari in 1992, as part of his monographic revision of the neotropical characiform genus Cyphocharax Fowler, published in Smithsonian Contributions to Zoology. The type specimen, 1.5 in standard length, was collected from the Rio Vermelho, a tributary of the Rio Araguaia within the larger Tocantins basin of central Brazil. The species belongs to the family Curimatidae — the curimatas — a large group of small-to-medium, toothless-jawed Characiformes that graze detritus and algae, and it sits within the broader Characiformes alongside the pacus, piranhas and silver dollars covered elsewhere in this atlas, though the curimatas are ecologically and behaviourally a world apart from those groups.
The species epithet signatus is Latin for "marked" or "stamped," referring to the dark, lozenge-shaped blotch on the caudal peduncle that helps distinguish it from other small Cyphocharax species sharing the Araguaia-Tocantins drainage. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference point for the valid combination Cyphocharax signatus, and this account keeps that genus placement; no alternate or superseded combination is indicated in the sources consulted.
Morphology
Cyphocharax signatus is a genuinely small fish: FishBase records a maximum length of just 1.5 in standard length for male or unsexed individuals, making it one of the smaller members of a genus that already runs modest by characin standards. Like other curimatids it has the characteristic toothless, downturned mouth of a substrate grazer rather than the cutting dentition of a predatory characin, an adaptation shared broadly across the family for rasping biofilm and organic material from surfaces.
The most distinctive external feature — and the one behind its scientific name — is a dark, lozenge-shaped (rhomboid) blotch on the caudal peduncle, just ahead of the tail fin. Beyond that marking and its diminutive size, detailed morphological description in the sources available is limited; the species is otherwise a slender, silvery, unremarkable-looking curimatid typical of its genus.
Habitat
Cyphocharax signatus is known only from its type locality, the Rio Vermelho, a tributary of the Rio Araguaia within the Tocantins river basin of central Brazil — a single-river endemic in the sources reviewed, with no broader distribution documented. This narrow known range likely reflects limited collecting effort as much as true rarity, a common pattern among small curimatids, but it should not be assumed to extend beyond what has actually been recorded.
FishBase characterises the species as a freshwater, benthopelagic fish of a tropical climate, consistent with life spent near the bottom of flowing water grazing surfaces rather than schooling in open midwater. No specific temperature or pH data for this species is available in the sources consulted; given its Araguaia-Tocantins origin, warm tropical conditions typical of central Brazilian rivers are a reasonable expectation, but this account does not assert numbers beyond what is documented.
Feeding
Diet has not been specifically documented for Cyphocharax signatus in the sources available, but the genus Cyphocharax as a whole is characterised as omnivorous detritivores, grazing algae, biofilm and organic detritus off submerged surfaces and substrate, supplemented opportunistically with small invertebrates and crustaceans. The toothless, downturned mouth typical of curimatids is built for this kind of continuous grazing rather than for hunting or plant-shredding.
In aquarium accounts of the genus more broadly, captive Cyphocharax thrive on a varied diet of sinking pellets and wafers alongside live or frozen foods such as Mysis shrimp, daphnia and bloodworms, with grazing opportunities on biofilm and algae in a mature, well-established tank rounding out the natural feeding behaviour. Given the very small adult size of this particular species, any aquarium diet would need to be scaled accordingly — fine, sinking foods rather than anything sized for larger characins.
Mating
No courtship or pairing behaviour specific to Cyphocharax signatus is documented in the sources consulted. General aquarium accounts of the genus describe egg-scattering behaviour typical of many small curimatids and characins, but nothing pair-bond-specific or courtship-specific has been recorded for this species, and this account does not extend genus-level generalisation into invented specifics for signatus itself.
Breeding
Breeding has not been documented for Cyphocharax signatus specifically — data is sparse to the point of absence in the sources reviewed. General hobby accounts of the wider Cyphocharax genus describe an egg-scattering mode typical of many small curimatids: eggs are released over fine-leaved plants or a similar substrate in soft, acidic, well-vegetated water, often assisted by spawning mops in aquaculture settings, with reported hatching in roughly 24 to 36 hours and no parental care beyond that. Whether C. signatus itself follows this pattern has not been confirmed by any source used here, and this account does not assert it as established fact for the species.
In practice, this is a fish that has essentially no aquarium-breeding track record on record; anyone attempting to spawn it would be working from genus-level inference rather than species-specific precedent, and any successful account would be a genuine first for the hobby literature as far as these sources show.
In the aquarium
Cyphocharax signatus is not an established aquarium fish — it does not appear to be in general trade, and this account should be read as a species profile rather than a care guide with a track record behind it. Its extremely small adult size, around 1.5 in standard length, would in principle suit a nano or small planted community setup rather than anything larger, and as a benthopelagic grazer it would likely appreciate a mature tank with biofilm and algae available to browse alongside sinking foods.
As with any small, quiet grazing characin, tankmates would need to be equally diminutive and non-aggressive; larger or boisterous fish would be an obvious mismatch for a 3-cm species. Because there is no meaningful husbandry record for this species specifically, any keeper working with it would be extrapolating from the broader Cyphocharax and Curimatidae literature rather than following established species-specific guidance — and should treat aquarium requirements as provisional until better documented.
Conservation
The IUCN Red List assesses Cyphocharax signatus as Least Concern. Given that the species is known only from a single tributary system — the Rio Vermelho within the Araguaia-Tocantins basin — an LC assessment likely reflects a lack of documented threats or population decline rather than confirmed widespread abundance; single-river endemics with limited survey coverage are common among small curimatids, and the true extent of the species' range may simply be under-recorded rather than genuinely restricted.
No aquarium trade pressure is documented for this species, and it does not appear to be a significant target of collection. The general pressures facing Tocantins-Araguaia basin freshwater fish — damming, deforestation-linked sedimentation and agricultural runoff — are relevant long-term watch items for the basin as a whole, though none are specifically documented as affecting this species in the sources reviewed.