Nannostomus unifasciatus, commonly known as the one-lined pencilfish, is a freshwater species of fish belonging to the genus Nannostomus in the characin family Lebiasinidae. N. unifasciatus, is broadly distributed throughout the Amazon basin, in Brazil, the Guiana Shield, Colombia, Venezuela, and northern Bolivia. With its long, pencil-shaped profile, its single jet black stripe reminiscent of the graphite core of a pencil, and its eraser-colored caudal fin, this is the species for which the popular name, ‘pencilfish’, was first coined in the 1920s, later to be applied to all the species of the genus Nannostomus. N. unifasciatus commonly inhabits slow-moving tributaries, small rivers, and swampy areas throughout its substantial range. Dense aquatic vegetation and/or submerged branches and leaf litter are most often present. They also congregate beneath floating islands. Water parameters invariably range from slightly acidic (pH 6.5) to strongly acidic (pH 4.0) with negligible hardness. Their congener, N. eques, is often found associated with them nearby, along with numerous other species of small characins.
Though the one-lined pencilfish is a small fish, with adults not reaching more than 2.75 inches in length, it is nevertheless one of the largest species of Nannostomus. It is a long slender species with a small terminal mouth, and an adipose fin. It has one black longitudinal stripe that runs the length of the body and onto the caudal fin. The single black stripe is bordered above by a thin metallic gold band. The caudal is colored variously depending on the geographic population. Most all forms have red or orange pigment of varying intensity in the lower quadrant of the caudal, ventral to the black stripe. In some especially colorful populations, the red pigment extends above the black stripe as well, as pictured above. The ventral and anal fins are usually tipped with an enamel blue-white.
Sexual dimorphism in this species is typical of many species in the genus Nannostomus. The anal fin of males is modified in shape in most populations, the male’s fin being slightly elongated. The white tips of the ventral and anal fins are invariably brighter and more prominent on males. Males are also usually slimmer in profile.
N. unifasciatus is omnivorous and feeds primarily on insects and small crustaceans. It is also an avid periphyton grazer, ingesting microorganisms as well as algae.
It is an appropriate community aquarium species, if tankmates are of similar size and demeanor, and will thrive if kept in soft, mildly acidic water, low nitrate levels, and at temperatures between 72F and 82F. Baby brine shrimp and other small-sized foods are preferred. Males will establish and defend small territories. Unlike its congener, N. eques, which prefers the upper strata of the aquarium, N. unifasciatus tends to congregate in the mid to lower reaches.