Taxonomy & naming
Nannostomus bifasciatus was described by the Dutch ichthyologist J.J. Hoedeman in 1954, in a paper published in Beaufortia (volume 4, number 39). The type locality is Berg en Dal on the Suriname River in Suriname; the holotype was deposited as ZMA 100513 but appears to have been subsequently lost. Eschmeyer's Catalog of Fishes confirms the name as valid, placing the species in the family Lebiasinidae, subfamily Pyrrhulininae.
The genus Nannostomus encompasses the pencil fishes — slender, small characoids long classified in Lebiasinidae, a family that also includes the splashing tetras and piranhas are not close relatives. The species sits in a group that includes N. trifasciatus (three-striped pencilfish) and N. eques (diptail pencilfish), all sharing the pencil-shaped body and lateral-stripe patterning characteristic of the genus. No subspecies of N. bifasciatus are currently recognised.
Morphology
Nannostomus bifasciatus is a compact, fusiform pencilfish reaching a maximum of about 1.5 in standard length. The body is long and narrow — earning the common name pencilfish — with a small, upturned mouth well suited to surface and mid-water feeding. The caudal fin is forked and the dorsal fin is set well back along the body.
The key diagnostic feature is the pair of dark horizontal stripes running the length of the flank. The lower stripe is the more prominent, running from the snout tip through the eye and along the lower flank to the lower lobe of the caudal fin; the upper stripe is thinner and parallels it along the mid-upper flank. Between and below these stripes the flanks show a pale gold to cream ground colour. Males are slimmer and may show a slightly brighter red pigment in the caudal fin lobes; females are deeper-bodied and rounder in the belly when in breeding condition, providing the primary external clue for sexing.
Habitat
This species is distributed across the coastal river systems of Suriname and French Guiana, with records also from Amapá state in northern Brazil. These are lowland tropical drainages characterised by soft, dark, acidic water — the tea-coloured blackwater streams and forest creeks of the Guiana Shield region, fed by tannin-laden leaf litter and humic acids.
Within these systems Nannostomus bifasciatus occupies counter-current zones and edge habitats, often in schools of a dozen or more individuals. It tends to stay in the middle and upper water column, holding position close to submerged roots, overhanging bank vegetation, and floating or sunken leaf litter. Water conditions in the natural habitat are warm, very soft, and distinctly acidic: FishBase records temperatures of 73–81 °F, pH 5.5–7.0, and hardness of 0–4 °dH.
Feeding
Nannostomus bifasciatus is an omnivore with a strong tendency toward insectivory, and FishBase records its feeding behaviour as notably crepuscular — most active at dusk and dawn. The natural diet likely centres on small invertebrates: aquatic insect larvae, tiny crustaceans, and insects that fall or float on the surface film, supplemented by small amounts of algae and plant material.
In the aquarium it accepts quality micro-pellets and finely crushed flake, but colour and condition improve markedly on a diet that includes small live or frozen invertebrates: micro-worms, freshly hatched brine shrimp nauplii, Daphnia, and Cyclops all suit its small mouth. Because feeding tends to intensify around the low-light periods of morning and evening, supplementing with small evening feeds can help ensure this species gets its share in a community tank.
Mating
Like all Nannostomus species, N. bifasciatus is an egg-scattering open spawner with no parental care. Males court receptive females with brief chases and side-by-side displaying, the male pressing close to the female to orient her toward spawning vegetation. The courtship is subtle compared with larger characins — brief, low-key pursuits among fine-leaved plants rather than dramatic displays.
Spawning takes place among fine-leaved aquatic plants or their equivalent, where the pair release small numbers of adhesive eggs that stick to plant surfaces or settle into the substrate. Because pencilfish are continuous or near-continuous spawners rather than seasonal ones, well-conditioned adults in a stable planted aquarium may spawn intermittently throughout the year without specific manipulation. Both adults and tankmates will consume eggs given the opportunity, so targeted breeding requires separating adults from the spawning site.
Breeding
Breeding Nannostomus bifasciatus follows the standard pencilfish approach: a well-planted species tank or a tank set up with synthetic spawning mops, very soft and acidic water matching the natural range (pH 5.5–6.5, hardness near zero, temperature in the mid-to-upper range of 75–81 °F), and a conditioning diet of live and frozen small invertebrates. A small group rather than a single pair spreads spawning activity and reduces stress on females.
Eggs are adhesive and attach to fine-leaved plants or settle into the base of spawning mops. Adults will eat the eggs without hesitation, so the eggs or spawning medium should be moved to a separate, bare-bottomed rearing container as soon as spawning is observed. Incubation is typically 48–72 hours, after which the tiny fry hang near the surface before becoming free-swimming. First foods must be appropriately small: infusoria, rotifers, or commercial liquid fry food are standard before graduating to freshly hatched brine shrimp nauplii. Fry growth is slow, and several months are required before juveniles show the full adult stripe pattern.
In the aquarium
Nannostomus bifasciatus is well suited to the smaller, mature planted aquarium and works best when kept in a group of at least eight to twelve individuals. Schooling together, the fish are far more confident and natural in their behaviour than isolated specimens, and the twin stripes of a moving school are visually compelling in a well-planted setup. Given its small size and crepuscular tendencies, it pairs well with other diminutive Amazonian and Guianan species — small rasboras, pygmy corydoras, or other Nannostomus species — rather than with larger or more boisterous tankmates that may outcompete it at feeding.
Water chemistry should lean soft and slightly acidic to match the natural habitat: pH 5.5–7.0, hardness 0–4 °dH, temperature 73–81 °F. It is sensitive to ammonia and nitrite and does best in a well-established, well-filtered aquarium with gentle flow. Tannin-stained water from botanicals or Indian almond leaves is not required but is appreciated. Because the species feeds most actively at low light, care at feeding time — either with a dedicated feeding spot at dusk or by supplementing with small frozen offerings — ensures it receives adequate nutrition in a mixed community.
Conservation
Nannostomus bifasciatus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment confirmed in November 2020. The species has a moderately broad distribution across the Guiana Shield coastal drainages of Suriname, French Guiana, and Amapá (Brazil), and there is no current evidence of major population declines or widespread habitat loss across its range.
Nonetheless, the coastal lowland forests and stream systems of the Guiana Shield are subject to ongoing pressures from deforestation, agricultural runoff, and small-scale gold mining, all of which degrade the soft, acidic water chemistry on which this and related species depend. The species appears regularly in the ornamental trade but is not among the most heavily collected pencil fishes. Captive breeding is straightforward enough that hobbyists can readily source tank-bred stock, reducing any demand-driven pressure on wild populations.