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Nannostomus unifasciatus STEINDACHNER, 1876

One-lined Pencilfish

SynonymsTop ↑

Poecilobrycon ocellatus Eigenmann, 1909


Nannostomus: from the Latin nannus, meaning ‘small‘, and Greek stoma, meaning ‘mouth’, in reference to the small mouthparts of member species.

unifasciatus: from the Latin uni, meaning ‘one, single’, and fasciatus, meaning ‘banded’, in reference to this species’ colour pattern.


Order: Characiformes Family: Lebiasinidae


Type locality is given as ‘near mouth of Rio Negro, Brazil’, but this species is currently accepted to range throughout much of the upper Amazon basin in Brazil, Bolivia, and probably Colombia, plus the upper Orinoco system in Colombia and Venezuela, Essequibo and Demerera rivers in Guyana.

An introduced population exists in Trinidad and Tobago.

Records exist from various tributary drainages including the rios Negro, Jutaí,  in the Amazon, and the Casiquiare, Guaviare, Inírida, and Vita in the Orinoco basin.


Inhabits sluggish tributaries, small rivers, and swampy zones, especially in areas where aquatic vegetation grows densely or with submerged woody structures and leaf litter.

It’s often found in areas of flooded forest and floodplain lakes in blackwater regions.

Caño La Guardia is a blackwater, floodplain morichal stream in the llanos of Apure state, southwestern Venezuela.

It has a forested riparian zone but open grassland dominates much of its drainage basin.

During the annual wet season (May to October) the riparian forest and adjacent savanna are flooded, and fish disperse throughout the floodplain.

The dry season is associated with falling water levels, forcing them back into the main channel and associated littoral habitats.

N. unifasciatus occurs here alongside numerous other fish species, particularly in flooded, vegetated habitats with slow current and largely covered by woody debris derived from morichito palms and other riparian vegetation, grass, and leaf litter.

Other fishes recorded in, and displaying a preference for, this habitat-type included the congener Nannostomus eques plus Brittanichthys sp., Charax condeiGnathocharax steindachneri, Hemigrammus analis, H. barrigonae, H. elegans, H. rhodostomus, H. stictus, H. vorderwinkleriIguanodectes spilurus, Characidium sp., Elachocharax pulcherMicrocharacidium gnomus, Hoplias malabaricusHoplerythrinus unitaeniatusCarnegiella marthae, Copella metae, C. nattereriPyrrhulina lugubrisFluviphylax obscurusMicrosternarchus bilineatusAcaronia vultuosaApistogramma hoigneiApistogramma sp., Crenicichla aff. wallacii, Heros aff. severusMesonauta insignisMikrogeophagus ramireziSatanoperca mapiritensiCorydoras sp., Amblydoras affinis, A. gonzaleziPhysopyxis ananasScorpiodoras heckeliiPanaque maccusParotocinclus sp., Microglanis iheringi, and Synbranchus marmoratus.

Maximum Standard Length

35 – 40 mm.

Aquarium SizeTop ↑

Base dimensions of at least 75 ∗ 30 cm or equivalent are required.


Should ideally be kept in a heavily-planted set-up, preferably with a dark substrate and some patches of floating vegetation around which the fish will tend to congregate, bodies angled towards the surface.

Driftwood branches and dried leaf litter can also be added, the latter in particular driving establishment of microbe colonies as decomposition occurs.

Such microorganisms can provide a valuable secondary food source for fry, whilst the tannins and other chemicals released by the decaying leaves are also thought beneficial.

Use gentle filtration; an air-powered sponge-style unit should prove adequate in most cases though a degree of flow is acceptable.

Water Conditions

Temperature23 – 28 °C

pH4.0 – 7.0

Hardness18 – 179 ppm


micropredator feeding on tiny invertebrates and other zooplankton in nature.

In the aquarium it will accept dried foods of a suitable size but should also be offered daily meals of small live and frozen fare such as Artemia nauplii, Moina, grindal worm, etc.

Behaviour and CompatibilityTop ↑

Very peaceful but does not make an ideal community fish due to its small size and rather timid nature.

In a community it’s best kept with similarly-sized, peaceful characids and smaller callichthyid or loricariid catfishes.

It also makes an ideal dither fish for Apistogramma spp. and other dwarf cichlids since  it tends to inhabit the middle-to-upper regions of the tank, and does not actively predate fry.

It’s very much a gregarious animal so buy as many as possible, ideally 10 or more, as when kept in larger groups any aggression is spread between individuals plus the fish are bolder and exhibit more natural behaviour.

Unlike some relatives rival males rarely spar or do each other harm in any way.

Sexual Dimorphism

Adult males are more colourful and usually slimmer than females, and also possess an enlarged anal-fin.


Not thought to have been bred in captivity, with earlier reports all appearing to pertain to the congener N. eques which is known to deposit eggs on the underside of plant leaves and similar structures.

NotesTop ↑

This species has been referred to the genera Poecilobrycon and Nannobrycon in the past and in the aquarium hobby is also known as ‘oneline’ or ‘red fin’ pencilfish.

It’s a popular aquarium fish and has an interesting oblique swimming-style, a behavioural trait shared only with N. eques among congeners.

The most recent diagnosis of the species appears to be that given by Weitzman (1975) who characterised it by possession of: central (primary) lateral stripe well-developed, upper and lower (secondary and tertiary) stripes absent; nocturnal oblique bars absent in preserved specimens; adipose-fin present; 28-30 scales in lateral series; 2-5 perforated lateral line scales; gill rakers 9+14; inner tooth row of dentary absent; 6-7 teeth in outer dentary row; principal caudal-fin rays 10/9, but 8 ending in upper lobe, 11 in lower lobe; anal-fin in males slightly modified, larger in specimens from Brazil and Venezuela, smaller in those from Guyana; most populations with a caudal ocellus.

Due to the oblique swimming position it’s difficult to confuse with any species other than N. eques but is easily told apart since it lacks dark brown spots on the body scales (vs. present in N. eques) and normally possesses an ocellus in the upper caudal-fin lobe (vs. absent in N. eques).

Don’t worry if your fish look different when you switch on the aquarium lights after dark or in the morning as like most Nannostomus species it assumes a cryptic, vertically-barred colour pattern at night.

This diurnal rhythm has been show to occur in blind specimens, suggesting it’s an automatic response that the fish cannot control.

The family Lebiasinidae is included in the order Characiformes and sometimes split into the nominal subfamilies Lebiasininae and Pyrrhulininae, though there has not been a major review of the grouping in recent times.

All lebiasinid genera possess a relatively long, elongate body shape with 17-33 scales in the lateral series and laterosensory canal system absent or reduced to 7 scales or less.

Some species have an adipose fin while others do not, and the anal-fin has a relatively short base of 13 scales or less.

In the majority of members males have an enlarged or otherwise well-developed anal-fin used in courtship and spawning.

The frontal/parietal fontanelle is always absent, the cheek well-covered by the orbital and opercular bones, the supraoccipital crest is absent, and the scales of the dorsal body begin over the parietal bones.

Characiformes is among the most diverse orders of freshwater fishes currently including close to 2000 valid species distributed among 19 families.

This tremendous taxonomical and morphological diversity has historically impaired the ability of researchers to resolve their genetic relationships with many genera remaining incertae sedis.

A further limiting factor has been that in many cases exhaustive study of these on an individual basis is the only way to resolve such problems.

Modern molecular phylogenetic techniques have allowed some headway, though, and a research paper by Calcagnotto et al. published in 2005 revealed some interesting hypotheses.

Their results suggest that Lebiasinidae forms a trans-atlantic, monophyletic clade alongside the families Ctenoluciidae and Hepsetidae, this clade further forming a sister group to Alestidae.

Others such as Oliveira et al. (2011) have concluded that the family Erythrinidae is also closely-related to this grouping with Hepsetidae and Alestidae more distant.


  1. Steindachner, F., 1876 - Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften. Mathematisch-Naturwissenschaftliche Classe v. 74 (1. Abth.): 49-240
    Ichthyologische Beiträge (V).
  2. Calcagnotto, D., S. A. Schaefer, and R. DeSalle, 2005 - Molecular Phylogenetics and Evolution 36(1): 135-153
    Relationships among characiform fishes inferred from analysis of nuclear and mitochondrial gene sequences.
  3. Montaña, C. G., C. A. Layman, and D. C. Taphorn, 2008 - Neotropical Ichthyology 6(4): 577-582
    Comparison of fish assemblages in two littoral habitats in a Neotropical morichal stream in Venezuela.
  4. Oliveira, C. A., G. S. Avellino, K. T. Abe, T. C. Mariguela, R. C. Benine, G. Orti, R. P. Vari, and R. M. Corrêa e Castro, 2011 - BMC Evolutionary Biology 11(1): 275-300
    Phylogenetic relationships within the speciose family Characidae (Teleostei: Ostariophysi: Characiformes) based on multilocus analysis and extensive ingroup sampling.
  5. Weitzman, S. H., 1966 - Proceedings of the United States National Museum v. 119 (no. 3538): 1-56
    Review of South American characid fishes of subtribe Nannostomina.
  6. Weitzman, S. H. and J. S. Cobb, 1975 - Smithsonian Contributions to Zoology 186: i-iii + 1-36
    A revision of the South American fishes of the genus Nannostomus Günther (family Lebiasinidae).

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