Taxonomy & naming
Cyphocharax mestomyllon was described by the American ichthyologist Richard P. Vari in 1992 from the Rio Marauiá, a tributary of the Rio Negro in the Brazilian Amazon; the holotype (catalogued MZUSP 41755) measures 1.5 in standard length. It belongs to the genus Cyphocharax within the family Curimatidae — the "toothless characins," a large Neotropical family of small to medium detritivorous and algivorous fishes that lack functional jaw teeth, distinguishing them from the predatory piranhas (Serrasalmidae) and the migratory Prochilodus/Semaprochilodus flannelmouths that share this atlas section.
The species epithet mestomyllon derives from Greek roots meaning "full" (mestós) and "lip" (mýllon), referring to the fish's fleshy, well-developed upper lip — a feature typical of curimatids, which use their mouths to scrape biofilm and organic sediment from submerged surfaces. Cyphocharax is a large and widespread genus across South America's Amazon, Orinoco and Paraná basins, and the Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognizes C. mestomyllon as a valid, currently accepted species with no history of reclassification.
Morphology
Cyphocharax mestomyllon is a small, slender-bodied curimatid. The holotype measured just 1.5 in standard length, and available records indicate males reach a maximum of roughly 1.5 in total length — this is a genuinely tiny fish, smaller than most aquarium tetras, and nothing like the large-bodied pacus or predatory piranhas that dominate this atlas section's public image. As a curimatid it lacks functional jaw dentition, relying instead on a fleshy, muscular upper lip (the source of its species name) to graze algae, biofilm and organic detritus from submerged wood, rock and sediment.
Detailed published descriptions of its colouration and fin proportions are sparse in the available literature; like most Cyphocharax species it is presumed to carry the silvery, laterally compressed, fusiform body typical of the genus, built for holding station and grazing in current rather than for speed or predation. Data on this specific species' external field marks remains limited.
Habitat
The species is known from the Rio Marauiá, a tributary of the Rio Negro in the Brazilian Amazon. The Rio Negro is one of the world's largest blackwater rivers — its dark, tannin-stained water is the product of extensive decaying leaf litter and organic matter draining from surrounding forest, and it runs notably soft and acidic compared to the sediment-rich "whitewater" rivers of the Amazon basin such as the Solimões.
Specific temperature, pH and hardness measurements for the type locality are not documented in the sources available for this account. Given the well-characterized blackwater chemistry of the Rio Negro system generally, soft and acidic conditions are a reasonable inference, but no site-specific water-chemistry data has been confirmed for C. mestomyllon and none is asserted here.
Feeding
As a curimatid, Cyphocharax mestomyllon is presumed to feed the way the genus does broadly: grazing algae, biofilm and fine organic detritus from submerged surfaces using its fleshy, well-developed upper lip rather than jaw teeth, which the family lacks. This detritivorous/omnivorous grazing strategy is well documented across Cyphocharax and the wider Curimatidae, though no diet study specific to C. mestomyllon itself is available in the sources consulted for this account.
In a hypothetical aquarium setting, fish of this feeding guild are typically maintained on a base of algae wafers, blanched vegetable matter and fine sinking foods that mimic biofilm grazing, supplemented with small amounts of protein — but this is inference from the genus, not documented husbandry for this species, which has no recorded aquarium history.
Mating
No courtship or spawning behaviour has been documented for Cyphocharax mestomyllon in the sources available for this account. The species has no recorded aquarium or aquaculture history, and field observations of its reproductive behaviour in the Rio Marauiá have not been located. Data is sparse to the point that nothing specific can be stated here beyond what is inferred from its family.
Breeding
Breeding biology for Cyphocharax mestomyllon specifically is undocumented — this is not a species kept or bred in the aquarium hobby, and no scientific account of its reproduction was found. More broadly, curimatids as a family are understood to be egg-scatterers with no parental care, consistent with most small-bodied Neotropical characiforms, but this should be read as a family-level generalization rather than a confirmed fact about this species. No breeding mode is asserted here with confidence, and this account deliberately avoids inventing detail that has not been observed or published.
In the aquarium
Cyphocharax mestomyllon has essentially no aquarium history: there is no SeriouslyFish profile, no established common name, and no documented husbandry record for the species. It is not part of the ornamental trade in any known capacity, and anyone encountering it would almost certainly be dealing with a scientific collection or an unintentional mixed wild-caught import rather than a deliberately traded fish.
Given its tiny adult size (around 1.5 in total length) and its position as a peaceful, toothless grazer, it would in principle be an undemanding, non-predatory community fish for soft, acidic blackwater setups alongside other small Rio Negro species — but this is inference from its morphology and family, not documented care advice, and no specific temperature, pH or tank-size guidance can be honestly given for this species today.
Conservation
Cyphocharax mestomyllon has not been evaluated by the IUCN Red List — its status is Not Evaluated (NE), reflecting the limited attention given to small, non-commercial, single-basin characins relative to widely fished or heavily traded species. No population trend, threat assessment or abundance data is available.
Its known range center, the Rio Negro basin, faces broader long-term pressures common to Amazonian blackwater systems — deforestation-linked changes to leaf-litter input and water chemistry, and increasing development pressure — but no threat has been specifically documented for this species, and its conservation status should be read as genuinely unknown rather than presumed safe or presumed at risk.