Taxonomy & naming
Moenkhausia robertsi was described by Jacques Géry in 1964, with the type locality given as the upper Amazon region surrounding Iquitos, in Loreto Department, Peru. The holotype (USNM 200427, 2 in SL) is deposited at the Smithsonian National Museum of Natural History. Eschmeyer's Catalog of Fishes recognises the valid name as Moenkhausia robertsi Géry, 1964; because it was described in Moenkhausia and remains there, no parenthetical authority is used.
The genus Moenkhausia belongs to the family Characidae and is one of the largest and most heterogeneous characid genera, spanning dozens of species distributed widely across tropical South America. Subfamily placement within the broad Characidae has varied across classifications; the cache source notes placement in Pristellinae for this genus, though molecular and morphological revisions continue to reshape subfamily boundaries within the family. M. robertsi is a member of a diverse assemblage of medium-small, laterally compressed characins that share the general Moenkhausia body plan.
Morphology
Moenkhausia robertsi is a small, slender-bodied characid reaching a maximum recorded standard length of 2 in according to FishBase. Like most members of the genus it is laterally compressed, with a relatively deep body, a short head, and the characteristic adipose fin of the Characidae positioned between the dorsal and caudal fins.
Detailed colour and meristic data for this species are sparse in the accessible literature. The genus Moenkhausia encompasses species ranging from nearly translucent silver to those with bold body spots or tail-base ocelli, and without specific published colour notes for M. robertsi, precise adult coloration cannot be stated with confidence. The type specimen at 2 in SL falls near the species' recorded maximum size, suggesting the holotype was an adult individual.
Habitat
Moenkhausia robertsi is recorded from the upper Amazon River basin in South America, with the type locality in the Iquitos region of Loreto, Peru — a lowland Amazonian area characterised by white-water and clear-water tributaries, flooded forest (várzea), and extensive oxbow lake systems. FishBase classifies the species as benthopelagic, indicating it occupies the lower-to-mid water column rather than being a strict surface or bottom fish.
The broader Amazon system around Iquitos experiences warm, soft, slightly acidic to near-neutral water conditions typical of western Amazonian lowlands. Seasonal flooding cycles dramatically alter available habitat in this region, and many characids exploit flooded forest margins and leaf-litter beds. Specific microhabitat associations for M. robertsi have not been widely documented, and the species is described as a freshwater obligate with no known tolerance for brackish conditions.
Feeding
FishBase assigns Moenkhausia robertsi a trophic level of 3.3, consistent with an omnivorous or generalist invertivore diet. This places it in the broad middle of the food web, consuming small aquatic invertebrates, insect larvae, and likely plant material and detritus opportunistically, as is typical for small Amazonian characids occupying similar ecological niches.
In the aquarium, members of the genus Moenkhausia are generally undemanding feeders that accept a wide range of prepared foods — quality flake, small pellets, and frozen or live offerings such as Daphnia, brine shrimp, and bloodworm. A varied diet that includes live or frozen invertebrate foods supports optimal condition. Feeding little and often suits these active mid-water fish better than single large meals.
Mating
Moenkhausia robertsi has not been specifically documented in the published breeding literature accessible from available sources, but as a member of Moenkhausia it is expected to follow the egg-scattering reproductive strategy characteristic of the genus. Males of Moenkhausia species typically become noticeably more colourful and active when in spawning condition, pursuing females through fine-leaved vegetation or open water.
The closely related Moenkhausia pittieri and other genus members are known free-spawners with no pair bond and no parental care, spawning in soft, acidic water. Males chase receptive females among vegetation, and spawning takes place with brief, vigorous embraces during which small numbers of eggs are scattered. Conditioning groups of adults on varied live and frozen foods is the usual approach for bringing fish into reproductive readiness.
Breeding
Detailed breeding accounts specific to Moenkhausia robertsi are absent from the available literature. Based on genus-wide patterns documented for Moenkhausia, breeding is expected to follow the standard characid egg-scattering mode: adhesive or semi-adhesive eggs deposited among fine-leaved aquatic plants or spawning mops, with no parental guarding and a tendency for adults to consume eggs if left in the spawning tank.
Typical Moenkhausia breeding parameters drawn from related species call for soft, acidic water (pH around 5.5–6.5, hardness 1–5 dGH), slightly elevated temperature in the range of roughly 79–84 °F, and subdued lighting. Eggs typically hatch within 24–36 hours at breeding temperature, and the small fry initially require very fine first foods such as infusoria or commercial liquid fry preparations before accepting newly hatched brine shrimp. Until specific records for M. robertsi emerge, these genus-level parameters represent the best available guide.
In the aquarium
Moenkhausia robertsi is not among the most commonly traded Moenkhausia in the hobby, and detailed husbandry records are limited. As a small Amazonian characid of under 2 in, it is suitable for a modest-sized aquarium — a tank of 23.5–31.5 in length would accommodate a school of six or more comfortably. Soft, slightly acidic to near-neutral water matching western Amazonian conditions is appropriate: pH in the range of 6.0–7.2, low to moderate hardness, and temperatures in the 75–82 °F range.
Moenkhausia in general are active, peaceful schooling fish that do best in groups and make compatible tankmates for other small Amazonian species — pencilfish, small corydoras, dwarf cichlids, and other tetras of similar temperament. A well-planted tank with darker substrate and some open swimming space replicates lowland Amazonian habitat and reduces the stress that encourages washed-out colour in small characids. Filtration should maintain good water quality without generating excessive current. Feeding is straightforward, as noted above.
Conservation
Moenkhausia robertsi is listed as Data Deficient (DD) on the IUCN Red List, a status reflecting insufficient survey data to assess the species' population trend or threat level rather than any confirmed decline. The assessment date corresponds to the 2022.2 release of the IUCN database. The species' known range is centred on the Iquitos area of the Peruvian Amazon, a region that has experienced habitat pressure from agriculture, urban expansion, and commercial fishing, though how these pressures specifically affect M. robertsi is unknown.
A Data Deficient listing is a call for more fieldwork rather than a statement of safety. Many small Amazonian characids remain poorly surveyed, and a lack of records can reflect under-collection as much as genuine rarity. Until targeted surveys establish population estimates and range boundaries, the species' conservation needs cannot be properly evaluated. Aquarists keeping and breeding documented Moenkhausia species contribute to ex-situ knowledge of the genus, which may support future conservation planning for rarer congeners.