单线铅笔鱼 · One-lined Pencilfish

Overview

The head-up swimming angle of Nannostomus unifasciatus is often seen around floating vegetation. Although Nannostomus eques shares the posture, this fish lacks dark brown spots on individual body scales and usually has an eye-like mark in the upper tail lobe.

Distribution & habitat

Native records involve the upper Amazon in Brazil, Bolivia, and possibly Colombia, the upper Orinoco in Colombia and Venezuela, and the Essequibo and Demerara in Guyana. The Trinidad and Tobago population is introduced.

It inhabits slow tributaries, small rivers, swamps, blackwater flooded forest, and floodplain lakes rich in vegetation, woody debris, and leaves. At one Apure blackwater stream in Venezuela, rainy-season flooding spreads fish through adjacent forest and grassland, while falling dry-season water returns them to the channel and margins. This species uses slow, flooded vegetation with abundant palm and other plant debris.

Aquarium setup

Provide a footprint of at least about 75 × 30 cm. Dense planting and dark substrate work well. Retain space around floating plants, where fish gather with their bodies angled upward. Branches and dry leaves add cover, and leaf-associated microorganisms can supplement fry food.

Use gentle filtration, normally an air-driven sponge unit, although some current is tolerated.

Diet & feeding

Tiny invertebrates and other zooplankton form the natural diet. Offer appropriate-sized preparations with daily Artemia nauplii, small water fleas, grindal worms, and similar live or frozen foods.

Behaviour & compatibility

Small size and timidity limit community choices despite its peaceful nature. Avoid forceful or much larger companions; similarly sized peaceful characins and smaller bottom-dwelling catfishes are better candidates. Middle-to-upper activity and lack of active fry hunting can make it useful with suitable dwarf cichlids, without guaranteeing no predation.

Maintain at least ten where possible for confidence and natural group behavior. Males seldom spar or injure one another as some relatives do.

Sexual dimorphism

Adult males are generally more colorful and slimmer, with a larger anal fin. Females are fuller-bodied. The extent of male anal-fin modification varies among localities.

Breeding

Reliable captive breeding records for this species are lacking. Earlier reports appear to concern Nannostomus eques, so that relative's leaf-underside spawning should not be presented as confirmed here.

Notes

A strong middle flank stripe is present, with the upper and lower stripes absent. Most populations show a tail ocellus, and the lack of dark spots on body scales helps distinguish them from N. eques. These characters are more useful than oblique posture alone.

Historical names may place it in Poecilobrycon or Nannobrycon. A more cryptic appearance after dark need not indicate a problem, and color changes alone do not establish nocturnal behavior.