Nannostomus digrammus (FOWLER, 1913)

Twostripe Pencilfish

SynonymsTop ↑

Poeciliobrycon digrammus Fowler, 1913

Etymology

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

Classification

Order: Characiformes Family: Lebiasinidae

Distribution

Native to the central Amazon basin with records existing from the Madeira, Branco, Trombetas, and Tapajós river drainages, plus the Rupununi basin in Guyana.

Although an affluent of the rio Essequibo and thus not officially part of the Amazon basin, the latter is connected to the rio Takutu, a tributary of the upper rio Branco, during the annual wet season.

Type locality is given by Fowler as ‘Rio Madeira about 200 miles east of W. Long. 62°20′, Brazil’.

In recent years very similar-looking fish with an additional thin dark stripe in the lower portion of the body (see ‘Notes’) have been exported with Colombia given as origin, and it may also occur in Peru.

Habitat

Inhabits sluggish tributaries, small rivers and swampy areas, particularly in areas with dense growth of aquatic vegetation or submerged woody structures and leaf litter.

Maximum Standard Length

25 – 30 mm.

Aquarium SizeTop ↑

Base dimensions of at least 45 ∗ 30 cm or equivalent are required.

Maintenance

Should ideally be kept in a heavily-planted set-up, preferably with a dark substrate.

The broken lines of sight that exist in such a display allow it to display natural behaviour as well as helping to reduce skittishness.

Floating plants are a useful addition as are driftwood branches and dried leaf litter, 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.

Water Conditions

Temperature: 22 – 28 °C

pH: 5.0 – 7.0

Hardness: 18 – 179 ppm

Diet

A 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.

Though gregarious by nature it’s a shoaling rather than schooling species with rival males sparring on a regular basis during daylight hours.

It’s not uncommon to see nipped fins within a group though this behaviour does not normally extend to tankmates.

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.

Sexual Dimorphism

Adult males possess a highly-modified, extended anal-fin, are more intensely-coloured, and usually slimmer than females.

Reproduction

Unreported.

NotesTop ↑

This species is rare in the aquarium hobby and it most commonly arrives as bycatch among shipments of other fishes.

Among congeners it’s most similar to N. bifasciatus but can be distinguished by presence (vs. absence) of an adipose-fin.

Diagnostic characters include: presence of two dark lateral body stripes, one along midbody and the other, often less well-developed, in the upper portion of body; non-permanent noctural patterning of oblique blotches; adipose-fin always present; snout fits 1.1-1.3 times in eye diameter; total vertebrae 34-35; adult male with elongate anal-fin, reachingh caudal-fin when adpressed; 19 caudal-fin rays, 10 in upper and 9 in lower lobe.

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.

References

1. Fowler, H. W., 1913 - Proceedings of the Academy of Natural Sciences of Philadelphia 65: 517-179
   Fishes from the Madeira River, Brazil.
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. 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.
4. 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).