Taxonomy & naming
Nannostomus nigrotaeniatus was described by D. Zarski in 2013, in a paper published in Vertebrate Zoology 63(2): 125–137 (29 August 2013). The type locality is Venezuela, and the species belongs to the family Lebiasinidae — the pencil fishes and their relatives. Eschmeyer's Catalog of Fishes (Catalog of Fishes, CAS) recognises the valid name as Nannostomus nigrotaeniatus Zarski, 2013.
The genus Nannostomus comprises the pencilfish of the neotropics, a group of slender, often strikingly marked characoid fishes widespread across the drainages of the Orinoco and Amazon systems and the Guianas. N. nigrotaeniatus is one of the more recently described members, and its specific epithet — from the Latin niger (black) and the Greek taenia (band) — refers to the prominent dark lateral stripe that runs the length of the body, a character shared by many pencilfish though combined here with the distinctive fin colouration that separates the species from its congeners.
Although N. nigrotaeniatus has entered the ornamental trade as the Redmouth Pencilfish, the hobbyist literature available before and shortly after its formal description sometimes treated similar Venezuelan forms under different labels, and its boundaries within the complex of Venezuelan Nannostomus remain an active area of interest as more material is studied.
Morphology
Nannostomus nigrotaeniatus is a slender, torpedo-shaped pencilfish reaching a maximum recorded length of 1.5 in standard length (male or unsexed), making it among the smaller members of the genus. Like all Nannostomus it lacks an adipose fin and carries the characteristically elongated, fusiform body with a small, slightly upturned mouth well suited to surface and mid-water feeding.
The most useful distinguishing characters are chromatic. The species shows a broad, intense dark lateral stripe running from the snout tip through the eye to the caudal peduncle, with a pale zone above and a reddish-bronze to gold ventral field below. Males display sexual dichromatism: the dorsal, anal, and ventral fins, and the base of both caudal lobes, carry ruby-red to scarlet spots or blushes, and the mouth and lower head region show the reddish flush that gives the fish its common name. Females have largely hyaline (clear) fins with little or none of the red pigmentation, making sexing straightforward in adult fish.
Like other pencilfish, Nannostomus nigrotaeniatus shows a nocturnal colour change: at night the lateral stripe breaks into a pattern of blotches and the overall contrast softens, reverting to the full-stripe daytime pattern as light returns.
Habitat
Nannostomus nigrotaeniatus is native to Venezuela in South America. The broader Nannostomus genus is characteristic of slow-moving to still blackwater environments — shallow tributaries, creeks, swamps, oxbow lakes, and densely vegetated stream margins — where water is warm, soft, and acidic, stained brown by humic compounds released from leaf litter and decomposing organic matter. Submerged roots, floating plants, and fine-leaved aquatic vegetation provide the structure these small fish rely on for cover and as a matrix for their invertebrate prey.
Water parameters reported for the care of this species — which broadly reflect Venezuelan blackwater habitats — span 72–82 °F with a pH of 6.0–7.5, aligned with the characteristically soft, slightly acidic conditions found in Orinoco-drainage tributaries. The fish occupies the surface to mid-water zone and, consistent with other pencilfish, shows a preference for gentle flow and densely planted surroundings over open, fast-moving water.
As is common for recently described Venezuelan ornamentals, detailed field data on the precise localities, population structure, and microhabitat preferences of N. nigrotaeniatus remain sparse, and the IUCN assessment reflects this information gap.
Feeding
In the wild, Nannostomus nigrotaeniatus is a micro-predator, feeding primarily on small invertebrates available in the water column and at the surface. Across the genus, the natural diet consists of small crustaceans, insect larvae, zooplankton, and other minute aquatic organisms; the upturned mouth is well suited to picking prey from the surface film.
In aquarium conditions the species accepts small live and frozen foods readily — brine shrimp nauplii, Cyclops, Daphnia, and tubifex worms are all reported. Its small adult size means that prey particle size matters: anything too large will be ignored or rejected. Small micro-pellets and fine-grade prepared foods are accepted by acclimated individuals, but the fish thrives and shows its best colour when live or frozen foods make up a meaningful part of the ration. Feeding small amounts two or three times daily suits its continuous, active foraging style.
Mating
Like other Nannostomus, N. nigrotaeniatus is an egg-scattering spawner with no parental care. Males are the more intensely coloured sex and take an active role in courtship, displaying their ruby fin-spots and red facial colouration to females and rival males. The species is reported to be somewhat more assertive or aggressive toward conspecifics than many other pencilfish, and groups should be large enough — a minimum of eight individuals is commonly recommended — to distribute any aggression and allow subdominant fish to feed and behave normally.
Courting males pursue females into fine-leaved vegetation or among root tangles, where eggs are deposited in small numbers over spawning runs rather than in a single massed clutch. Adhesive eggs are scattered into plant material or fine substrate, and the adults take no interest in them once laid.
Breeding
Conditioning N. nigrotaeniatus for spawning follows the standard approach for blackwater pencilfish: soft, slightly acidic water (pH around 6.0–6.5, very low hardness), warm temperatures in the mid-20s Celsius, subdued lighting, and dense fine-leaved planting or spawning mops to receive the eggs. A small, lightly filtered tank with gentle air-driven filtration helps maintain water quality without creating the current these fish dislike.
Eggs are small and adhesive, deposited singly or in small groups among the vegetation. Because adults will eat eggs given the opportunity, breeders typically remove adults after a day or two of spawning, or harvest the spawning medium daily. Incubation takes several days at breeding temperature. Fry are tiny at hatching and require the finest first foods — infusoria, rotifers, or commercial liquid fry preparations — before graduating to newly hatched brine shrimp. The species is not as commonly bred in captivity as some other pencilfish, and detailed aquarist breeding accounts remain limited.
In the aquarium
Nannostomus nigrotaeniatus is a small, peaceful fish suited to nano and species aquaria, though it requires a thoughtful setup to look its best and remain healthy. Its preference for slow or still water, warm temperatures (72–82 °F), and soft, acidic to neutral conditions (pH 6.0–7.5) should guide tank design: a dark substrate, dense plantings of fine-leaved species, floating plants to reduce surface light intensity, and gentle filtration are all beneficial. The combination of dark substrate and tannin-stained water — achieved with Indian almond leaves or peat filtration — suppresses stress and encourages the best colouration.
The species is reported to be somewhat more aggressive than typical pencilfish within a group, so keeping a proper shoal of eight or more individuals is important to buffer inter-fish tension. It should not be combined with large or boisterous tankmates; small, similarly peaceful nano fish — other dwarf tetras, pygmy corydoras, small rasboras — make suitable companions in a community setup. Its small adult size (under 1.5 in) and preference for the upper water column make it easy to overlook at feeding time in a mixed tank, so ensuring it receives adequate food — particularly live or frozen micro-foods — is worthwhile.
Conservation
Nannostomus nigrotaeniatus is listed as Data Deficient (DD) on the IUCN Red List, assessed on 16 November 2020. A Data Deficient designation does not imply that the species is necessarily secure; it means that insufficient information was available at the time of assessment to determine its status against the IUCN criteria. For a species described as recently as 2013 from Venezuelan waters, baseline data on population size, distribution extent, and habitat trends are inevitably limited.
Venezuela's freshwater fish diversity faces ongoing pressure from habitat degradation, watershed disturbance, and ornamental collection in some drainages. As a small, attractive pencilfish that has entered the ornamental trade, N. nigrotaeniatus is subject to collection pressure, though the scale is unknown. Until systematic surveys establish its distribution and population status, the Data Deficient assessment will remain the most honest summary of what is known. Aquarists interested in the species are encouraged to source tank-bred individuals where available, reducing any demand on wild populations.