Taxonomy & naming
Nannostomus erythrurus was described by the American ichthyologist Carl H. Eigenmann in 1909. Eigenmann was among the most prolific describers of South American freshwater fishes of his era, and many of his Lebiasinidae descriptions were based on material collected from the rivers of British Guiana (now Guyana) and adjacent drainages. The valid name as recognised by Eschmeyer's Catalog of Fishes (Catalog of Fishes, California Academy of Sciences) is Nannostomus erythrurus Eigenmann, 1909.
The genus Nannostomus Günther, 1872 — the pencilfish — is placed within the family Lebiasinidae and comprises a group of small, elongate characiforms characterised by their pencil-like body profile, often striking horizontal colour stripes, and, in some species, a distinctive oblique resting posture. The pencilfish are distributed across the Amazon, Orinoco, and Guiana Shield river systems of South America. Nannostomus erythrurus is one of the smaller members of the genus.
Morphology
Nannostomus erythrurus is a small, slender fish, with FishBase recording a maximum standard length of 1 in. The body is pencil-shaped and laterally compressed, with a pointed snout and a small, upturned mouth suited to surface and mid-water feeding. Like other pencilfish, it lacks an adipose fin, a useful diagnostic character that separates Lebiasinidae from many other small characins.
The species takes its name from a reddish flush concentrated at the caudal peduncle and the base of the tail — erythrurus from the Greek erythros (red) and oura (tail). The rest of the body typically shows the horizontal banding pattern common to the genus: a dark mid-lateral stripe runs from the snout to the tail, bordered above and sometimes below by lighter or golden bands. Overall colouration is subdued but precise, and intensity varies with the fish's mood, lighting, and water conditions. Sexual differences are modest; ripe females appear somewhat fuller-bodied than males.
Habitat
Pencilfish of the genus Nannostomus are characteristically inhabitants of shallow, slow-moving, or standing marginal waters: flooded forest edges, stream backwaters, and densely vegetated stretches where drifting leaf litter accumulates and submerged roots and stems provide cover. Water chemistry in these habitats tends toward the soft and acidic end of the spectrum, particularly in blackwater systems influenced by tannins leached from decomposing organic matter.
Nannostomus erythrurus is recorded from the northern drainages of South America within the range described for Eigenmann's 1909 material, consistent with the Guiana Shield and adjacent Amazon tributaries. In these environments, the water is typically warm, very soft, and acidic to near-neutral, with low conductivity and stained amber to brown by humic acids. The fish lives in the upper and middle water column among fine-leaved aquatic vegetation and overhanging bank vegetation, where its small size and cryptic patterning offer concealment from predators.
Feeding
Nannostomus erythrurus is a micropredator and opportunistic omnivore, feeding on tiny invertebrates, zooplankton, insect larvae, and small crustaceans in the wild. Its small, upturned mouth is suited to picking prey from the water surface and from the undersides of leaves and plant surfaces in the mid-water zone.
In the aquarium the species accepts suitably sized prepared foods — micro-pellets and finely crushed flake — but thrives and shows best colour when live and frozen small foods form a significant part of the diet. Micro-worms, baby brine shrimp, Cyclops, Daphnia, and similar items are ideal. Feeding frequency matters for this small species: multiple small meals per day suit it better than a single large feed, as the fish has a limited stomach volume and burns energy quickly in active schooling.
Mating
Pencilfish of the genus Nannostomus are egg-scattering open spawners. Males court females with fin displays and body posturing, often showing heightened colour intensity during courtship. Spawning takes place among fine-leaved aquatic plants, algae mats, or synthetic spawning mops, where the pair deposit small, adhesive eggs that cling to plant surfaces.
There is no parental care; adults do not attend the eggs and will consume them if given the opportunity. Spawning in natural habitats likely coincides with seasonal flooding and the associated flush of small invertebrate prey that allows fry to grow. In captivity, mating behaviour can be triggered by regular water changes with slightly cooler, very soft water, mimicking the conditions of seasonal rain.
Breeding
Nannostomus erythrurus can be bred in captivity, though its small size and preference for very soft, acidic water call for some care. A dedicated breeding setup with pH in the acidic range, very low hardness, and a temperature in the mid-to-upper 20s Celsius suits the species. Fine-leaved plants such as Java moss or dense synthetic spawning mops should be provided as egg-deposition sites.
Eggs are small and adhesive and should be separated from the adults promptly to prevent predation. The incubation period at typical breeding temperatures is several days. The newly hatched fry are very small and require infusoria, rotifers, or proprietary liquid fry food as first foods before graduating to newly hatched brine shrimp nauplii. A mature, aged aquarium with naturally occurring microorganisms supports fry survival. Growth is slow, consistent with the species' small adult size.
In the aquarium
Nannostomus erythrurus is best suited to a specialised small-fish aquarium rather than a community tank with larger or more boisterous species. At under 1 in it is easily outcompeted for food and can be stressed or harmed by active tankmates. A small, heavily planted aquarium of 10–15 US gal or more, dimly lit or shaded by floating plants, and filled with very soft, slightly acidic water is the ideal setting.
The species is a schooling fish and should be kept in groups of at least six to eight individuals; a larger group produces more natural behaviour, reduces individual shyness, and shows the full courtship and social dynamics of the genus. Suitable tankmates, if any, are other very small, gentle species from similar soft-water habitats — dwarf corydoras, small Apistogramma, or other Nannostomus species kept in a biotope arrangement. Water quality must be excellent, as small pencilfish are sensitive to dissolved wastes; a mature, well-filtered aquarium with gentle flow and regular small water changes is essential. Avoid sharp fluctuations in temperature or chemistry.
Conservation
Nannostomus erythrurus is assessed as Least Concern (LC) on the IUCN Red List, reflecting a distribution that spans river systems of northern South America without evidence of acute population-level decline at present. The genus Nannostomus as a whole occupies a wide range of habitats across the Amazon and Guiana Shield region, and while individual species have restricted ranges, Least Concern status indicates no current threshold-level threat across the assessed range.
The primary pressure on softwater pencilfish habitats is the same broad threat facing Amazonian and Guiana Shield fish fauna generally: deforestation and land-use change that alters stream hydrology, increases sedimentation, and degrades the dense marginal vegetation these fish depend on. Collection for the ornamental-fish trade has historically touched many Nannostomus species, though captive breeding has reduced reliance on wild-caught fish for most commonly kept pencilfish. Maintaining soft-water biotopes in captivity supports conservation awareness for this group.