Taxonomy & naming
Nannostomus rubrocaudatus was described by Zarski in 2009. The type locality is Saramirisa, a village on the Marañón River in the Loreto department of Peru, situated between the Ríos Morona and Santiago in the upper Amazon drainage. The valid name is governed by Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & van der Laan), which lists it as Nannostomus rubrocaudatus Zarski, 2009.
The genus Nannostomus belongs to the family Lebiasinidae and contains the pencilfish — slender, torpedo-bodied characoids of the tropical American lowland forest streams. The genus is concentrated in the Amazon and Orinoco basins and their tributaries, and most species are found in soft, acidic blackwater or clearwater habitats. N. rubrocaudatus is a relatively recent addition to the roughly 20 valid species currently recognised in the genus, and given how recently it was formally named, its full range, variation, and phylogenetic affinities within Nannostomus are not yet comprehensively documented.
Morphology
Nannostomus rubrocaudatus is a small, slender pencilfish reaching a maximum recorded standard length of 1 in — placing it at the miniature end of even the already small Nannostomus genus. The body is cylindrical and elongate in the classic pencilfish fashion, tapering toward the caudal peduncle.
Colouration follows the general Nannostomus plan of bold longitudinal striping: a dark lateral band running the length of the body, contrasting with pale or vivid ground colour above and below. The specific epithet rubrocaudatus — from the Latin ruber (red) and cauda (tail) — signals a distinctive reddish suffusion on the caudal fin or peduncle region. Like other Nannostomus, the species likely shows nocturnal colour change, with the bold daytime stripes giving way to a muted oblique barring pattern at night when the fish rests.
Habitat
The type locality lies on the Marañón River near Saramirisa in the Loreto region of northeastern Peru, at the confluence zone of the Ríos Morona and Santiago — a region of upper Amazon floodplain and tributary systems characteristic of lowland Amazonian forest. This is warm, soft, and typically acidic freshwater habitat, shaded by riparian and gallery forest and rich in leaf litter and submerged woody debris.
Water chemistry data for aquarium-maintained specimens indicate a temperature range of 75–82 °F and a pH of 4.0–7.0, with very low hardness (18–90 ppm). These figures are consistent with the soft, ion-poor blackwater and clearwater streams typical of the western Amazon lowlands in Loreto. FishBase classifies the species as freshwater and benthopelagic, reflecting its tendency to occupy the lower-middle water column amid leaf litter and fine vegetation in its natural environment.
Feeding
Nannostomus rubrocaudatus is a micropredator, feeding in the wild on tiny invertebrates and zooplankton — the diet common to small pencilfish species that inhabit soft-water forest streams. Its small mouth size and the fine-grained invertebrate community of blackwater habitats together shape a diet of minute prey items: small crustaceans, insect larvae, and microorganisms.
In the aquarium, providing appropriate food is one of the central challenges of keeping this species well. Standard dried flake or pellet foods are typically too large and nutritionally insufficient on their own. Seriously Fish recommends small live and frozen foods — Artemia nauplii, Moina, and grindal worms — as staples. Feeding a variety of suitably sized live and frozen foods, supplemented with high-quality micro-pellets or crushed flake, keeps specimens in good condition and may be important for bringing fish into breeding condition.
Mating
Nannostomus rubrocaudatus, like all pencilfish, is an egg-scattering open spawner that deposits adhesive eggs among fine-leaved aquatic plants or over dense plant growth, with no parental care after spawning. Males are territorial among themselves and will defend small areas of the tank, displaying and posturing at rivals — behaviour described as more pronounced than in some other Nannostomus congeners.
The species is noted as being more assertive and territorial than many pencilfish, so providing visual breaks and sufficient space reduces conflict. As with other Nannostomus, males likely display enhanced colouration during courtship interactions, intensifying the body pattern and reddish caudal pigmentation to signal condition to females and compete with rival males.
Breeding
Breeding Nannostomus rubrocaudatus in captivity is reported as rare, and the species is seldom seen in the trade. When spawning does occur, it follows the egg-scattering Nannostomus pattern: adults spawn among dense fine-leaved plants or over spawning mops, depositing small, adhesive eggs. The adults should be removed after spawning, as they will consume the eggs.
Fry typically become visible within a few days of hatching. Initial fry are very small and require infusorian-sized first foods — Paramecium or equivalent micro-organisms — before graduating to larger live foods such as Artemia nauplii as they grow. Seriously Fish notes that conditioning with appropriate live foods and providing dense plant cover are prerequisites for inducing spawning. The rarity of captive breeding records likely reflects both the scarcity of the species in the hobby and the specific requirements it places on water chemistry and diet.
In the aquarium
Nannostomus rubrocaudatus is an uncommon and somewhat challenging pencilfish best suited to experienced aquarists who can meet its specific requirements. The very small size — under 1 in — means it must be housed only with similarly tiny, equally peaceful tankmates; it will be intimidated or outcompeted by anything larger or more boisterous. A species-only setup or a carefully assembled nano community of comparable microfish is the safest approach.
Water chemistry should match the soft, acidic conditions of its native Loreto habitat: pH 4.0–7.0 and low hardness, at 75–82 °F. Heavily planted tanks with a dark substrate, dried leaf litter, and subdued lighting replicate the blackwater forest-stream aesthetic and help the fish feel secure. Groups of ten or more are recommended to spread the territorial interactions among males and ensure natural social behaviour — a small group of two or three risks one individual being persistently harassed. Diet is a particular consideration: only suitably sized live and frozen foods reliably keep the species in good health, and the absence of such foods from a keeper's routine makes maintaining this species unnecessarily difficult.
Conservation
Nannostomus rubrocaudatus was assessed by the IUCN Red List as Least Concern (LC) in an assessment dated 28 October 2020. This reflects that, at the time of assessment, the species was not considered to face threats of a magnitude sufficient to qualify it for a threatened category, though the assessment also reflects the limited data available for a species described only in 2009.
The known range is restricted to the Loreto department of Peru in the upper Amazon basin — a region that, while currently retaining large areas of intact lowland forest, faces ongoing pressures from deforestation, agricultural expansion, and hydrocarbon extraction. As a species with a limited documented range and sparse representation in the aquarium trade or scientific literature, ongoing monitoring of both its distribution and habitat condition would be valuable. Its scarcity in the hobby means collection pressure is minimal, but that same scarcity limits the establishment of captive populations that might provide a buffer if wild conditions change.