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Family: Lebiasinidae, Subfamily Pyrrhulininae

Common Name: One Line Pencilfish

Origin and Habitat: South America. Distributed throughout the upper Amazon in Bolivia, Brazil and likely Colombia, upper Orinoco basin in Venezuela, and in Guyana. Inhabits sluggish streams and flooded forest, around vegetation and sunken branches.

Compatibility/Temperament: Peaceful shoaling fish that must be kept in a group of at least six but preferably more. Males will challenge each other regularly, and although very serious, damage is not usually done. Males of other pencilfish species will also be harassed when they encroach the fish's territory. A good community fish but only with very peaceful and sedate fish; should not be kept with lively nor aggressive fish.

One Lined Pencilfish Diet

Naturally feeds on very small worms, crustaceans, insect larvae and insects. In the aquarium it will accept small foods like flake, mini pellet; live or frozen daphnia, artemia, bloodworms and blackworms. This species (in the writer's experience) remains close to the substrate and feeds on foods picked from wood, plant leaves and the substrate, including prepared foods as they fall from above.


Attains 4 cm or 1.5 inches.

Minimum Tank Suggestion

20 to 24 inches in length, such as a 10, 15 or 20 gallon aquarium.

Water parameters for One Lined Pencilfish

Very soft to soft (< 10 dGH, preferably no higher than 6 dGH), acidic (pH 4 to 7), temperature 23-28C/73-82F. All fish will be wild caught unless acquired from a local breeder.


This pencilfish closely resembles Nannostomus eques but can be easily distinguished by the more defined dark lateral stripe and the absence of dark spots in the scales; a caudal ocellus may be present (Weitzman, 1975). Both species swim at an oblique angle, unique among the pencilfishes.

Although sources indicate that this fish, like N. eques, remains in the upper half of the aquarium, the group of fish in the writer's aquarium remain in the lower half, at times very close to the substrate, spending the day browsing sunken wood, plant leaves and the substrate for food bits; the mouth is forward (terminal) and always open, and designed such that the fish can easily pick up minuscule prey from these surfaces.

A quiet, sedate fish well suited to a heavily-planted aquarium of gentle fishes; when housed with active fish, this species may be too frightened to eat. Floating plants will increase the fish's sense of security and also decrease the light which is very beneficial. The flow from the filter should be very subdued; in small tanks a single sponge filter is ideal, and in larger aquaria ensure there is minimal flow plus some very still areas.

In the subject species, the adipose fin--which may or may not be present in N. eques--is always present (Weitzman, 1975). In common with all pencilfish, except for N. espei, this fish has a diurnal colour pattern: during darkness, the horizontal lines break up into a series of dashes. This has been observed in blind fish, showing that it is an automatic response and not controlled by the fish.

Males are more slender than females, and the anal fin is black, red and white whereas the female's is black. This species will spawn but only in very soft, acidic water. The pair select a leaf and the eggs are deposited on the underside.

All pencilfish are found in the tribe Nannostomini in the subfamily Pyrrhulininae. Three different genera (Nannostomus, Nannobrycon and Poecilobrycon) were used at various times prior to Weitzman & Weitzman (2003). The subject fish was originally described as Nannostomus unifasciatus by F. Steindachner in 1876, using specimens from the mouth of the Rio ***** in Brazil; the genus had been erected in 1872 by Gunther with N. beckfordi as the type species. C.H. Eigenmann (1909) described a species Poecilobrycon ocellatus, but this was determined by Weitzman (1966) to be conspecific so the name is now a synonym.

In 1910, C.H. Eigenmann assigned the present species along with N. eques to the genus Poecilobrycon which he had erected that year for the type species P. harrisoni [now Nannostomus harrisoni]. In 1950, Hoedeman proposed placing P. eques in his newly-erected genus Nannobrycon; the basis he used was the oblique swimming position and a peculiarity of the caudal fin, both traits unique to this species and N. unifasciatus [Hoedeman considered N. unifasciatus to be a synonym for N. eques]. He also alluded to a different shape of the swim bladder; Weitzman (1966) suggested that extensive anatomical study of all species in the tribe would clarify this point, but he opinioned that the oblique swimming position does suggest this.

In spite of the fore-going, Weitzman (1966) decided to place this species in Nannostomus, noting that future study might justify the two species N. eques and N. unifasciatus being moved to their own genus. Gery (1977) actually did this, but added no new evidence beyond what Hoedeman and Weitzman had mentioned. Weitzman & Weitzman (2003) listed all known species, including these two, in Nannostomus, and to date this has remained the accepted classification.

The name Nannostomus comes from the Greek meaning "small mouth," and Nannobrycon comes from the Greek meaning "small biter;" both names reference the small mouth of the pencilfishes. The species epithet unifasciatus refers to the single lateral stripe. Eigenmann's epithet ocellatus was a reference to the caudal ocellus [a dark eyespot found on the caudal (tail) fin] that Weitzman (1975) notes is present in some populations of this species.


Gery, Jacques (1977) Characoids of the World, TFH Books.

Reis, R.E., S.O. Kullander & C.J. Ferraris Jr. (2003), Check list of the freshwater fishes of South and Central America.

Weitzman, Stanley H. (1966), "Review of South American Characid Fishes of Subtribe Nannostomina," Proceedings of the United States National Museum (Smithsonian Institution), Volume 119, Number 3538.

Weitzman, Stanley H. & J.S. Cobb (1975), "A revision of the South American fishes of the genus Nannostomus Gunther (family Lebiasinidae)," Smithsonian Contributions to Zoology No. 186.

Weitzman, M. and S.H. Weitzman (2003), "Lebiasinidae (Pencil fishes)," pp. 241-251. In R.E. Reis, S.O. Kullander and C.J. Ferraris, Jr. (eds.), Checklist of the Freshwater Fishes of South and Central America.

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