Taxonomy & naming
Moenkhausia margitae was described by Axel Zarske and Jacques Géry in 2001. The valid name, as recorded in Eschmeyer's Catalog of Fishes, is Moenkhausia margitae Zarske & Géry, 2001; because the species was described within Moenkhausia and has never been moved to another genus, no parenthetical authority is used.
Moenkhausia is a large and heterogeneous genus within the family Characidae, currently comprising well over 70 species across the lowland river systems of tropical South America. The genus is united primarily by the presence of a complete lateral line and scales on the caudal-fin base but lacks a single strong synapomorphy, and several revisions in recent years have reassigned individual species to neighbouring genera. M. margitae has not been subject to a published generic transfer and sits in Moenkhausia as of the current Catalog of Fishes. Its closest relatives within the genus have not been fully resolved by phylogenetic analysis available in the literature.
Morphology
Males reach approximately 2.5 in standard length according to FishBase data. Like most Moenkhausia species, M. margitae is a laterally compressed, moderately deep-bodied fish with the compact form and large eyes characteristic of the genus. The caudal fin is forked, the dorsal fin short-based, and the lateral line complete — features that are diagnostic for Moenkhausia at the genus level.
Detailed external colour descriptions from the scientific literature are not available in the current data set. The genus encompasses species ranging from nearly transparent to those with bold dark lateral bands, caudal-spot markings, or reflective silver flanks. Data on M. margitae's specific livery are sparse; aquarists encountering the species should cross-reference photographs from the original description or authoritative ichthyological databases.
Habitat
Moenkhausia margitae is distributed in the Río Ucayali drainage of Peru, which forms the primary headwater tributary of the upper Amazon system. FishBase classifies it as benthopelagic, occurring in freshwater tropical environments, with recorded habitat characteristics including slow-flowing to still waters over substrates of gravel and mud.
The Ucayali basin spans a range of Amazonian habitats from Andean foothills to broad floodplain várzea. Slow-flowing and still-water microhabitats in such systems typically feature soft, slightly acidic to neutral water, alluvial sediment, and abundant riparian vegetation that shades the banks and contributes organic material. These conditions produce the warm, moderately soft, slightly acidic to neutral water chemistry typical of upper Amazonian lowland fish communities.
Feeding
FishBase records a trophic level of 3.3 ± 0.5 for Moenkhausia margitae, placing it as a low-level carnivore that feeds on smaller organisms — a category that in practice for small Amazonian tetras includes small aquatic invertebrates, insect larvae, microcrustaceans, and opportunistically taken plant matter or organic detritus.
In the aquarium, Moenkhausia species of comparable size typically accept a broad range of prepared foods, including quality flake and small granules, as well as frozen and live foods such as Daphnia, brine shrimp, and bloodworm. A varied diet supports better condition and colouration. Specific feeding observations for M. margitae under aquarium conditions are not documented in available sources, but the trophic level and genus-level biology suggest it would be an undemanding omnivore.
Mating
No published mating observations for Moenkhausia margitae are available. Members of the genus Moenkhausia are egg-scattering open spawners: males court females with lateral displays and chasing, driving them into fine-leaved vegetation or open water, where both sexes scatter adhesive or semi-adhesive eggs without parental guarding.
Courtship in the genus is typically brief and non-elaborate. Several males may pursue a single receptive female simultaneously. Spawning activity in related species is often most intense in the early morning hours, and the presence of multiple males in a group tends to stimulate competitive display. These are general genus-level patterns; specific courtship details for M. margitae have not been described.
Breeding
No aquarium breeding account for Moenkhausia margitae is documented in available sources. Breeding biology can be inferred from the Moenkhausia genus template: egg-scattering spawner depositing small adhesive eggs among fine-leaved plants or over open substrate; no parental care is provided, and adults will consume eggs and fry if not separated.
For related Moenkhausia species, breeders typically condition adults on live and frozen foods, set up a separate breeding tank with fine-leaved plants or spawning mops and a layer of marbles to protect eggs from the parents, trigger spawning with a water change to slightly cooler, softer water, and remove adults after eggs are laid. Eggs hatch in roughly 24–48 hours at tropical temperatures, and fry are free-swimming within a few days, requiring infusoria or commercial liquid fry food as a first food before graduating to newly hatched brine shrimp.
In the aquarium
Moenkhausia margitae is not commonly encountered in the mainstream ornamental-fish trade, and virtually no specific husbandry literature exists for the species. Based on its habitat profile — slow-flowing to still, warm Amazonian water over gravel and mud — a setup with temperatures in the mid-to-upper 20s Celsius, neutral to slightly acidic pH, and soft to moderately soft water would be appropriate.
As with most small Moenkhausia tetras, it would be expected to do best in a group of six or more individuals, in a well-planted aquarium with gentle filtration that does not produce strong current. Tankmates of similar size and temperament — other small tetras, peaceful rasboras, small corydoras — would be suitable. The species is likely active and sociable in the midwater. Because the species is rare in captivity, prospective keepers should source specimens from specialist breeders or importers with verified provenance, and treat early husbandry observations as potentially novel contributions to the knowledge base for this species.
Conservation
Moenkhausia margitae is assessed as Near Threatened (NT) on the IUCN Red List, based on a 2014 assessment. The Near Threatened category indicates that the species does not currently meet the threshold criteria for a threatened category but is close to qualifying, or is likely to qualify in the near future if pressures continue.
The Ucayali drainage, where M. margitae occurs, is subject to a range of anthropogenic pressures including deforestation, agricultural runoff, and increasing commercial fishing in Peru's Amazon region. Slow-flowing and still-water habitats in floodplain systems are among the most vulnerable to sedimentation and water-quality changes from catchment modification. The 2014 assessment date means the species is due for reassessment, and current conditions in the Ucayali basin may warrant a revised evaluation.