Why these slender South American fish are named after a common writing tool

Nature · Published 3 October 2026 Swipe more like this →

In short: Pencil fish, or Nannostomus, are small, elegant characins named for their slender, pencil-like bodies. While they have been aquarium staples since the early 20th century, their taxonomy was debated for nearly a hundred years before a major 1975 revision finally unified the genus into the twenty species recognized today.

The genus name Nannostomus derives from the Greek word for small and the Latin term for mouth. While initially applied to only two species in the 1920s, the name eventually expanded to cover the entire group by the late 1950s. These fish are native to a vast range across South America, spanning from Colombia and Venezuela down to the southern Amazon basin and Bolivia. Because of this wide distribution, many species exhibit significant color variations, which historically led researchers to misidentify local populations as distinct species.

The taxonomy of these fish remained contentious for decades, with various authors splitting the group into different genera like Poecilobrycon and Nannobrycon. This confusion was largely resolved in 1975 by Dr. Stanley Howard Weitzman and Dr. J. Stanley Cobb, who unified the species under the Nannostomus genus. Weitzman also described several newer species, including N. marilynae and N. anduzei. Today, twenty species are recognized, though researchers continue to encounter unidentified specimens imported as bycatch alongside other small characins.

Physically, most species are slender, reaching lengths between one and two inches. However, some, like N. marginatus, have blockier, stub-like shapes. While most species swim horizontally, N. unifasciatus and N. eques are notable for their unique, oblique snout-up posture. Their behavior also varies; while most are peaceful, males of certain species like N. mortenthaleri and N. trifasciatus can be aggressive toward one another if their aquarium environment lacks sufficient space or plant cover.

Despite their popularity, only two species, N. beckfordi and N. harrisoni, are commercially raised in fisheries. Most others found in home aquaria are wild-caught. When kept in suitable conditions—soft, acidic water between 22 and 28 degrees Celsius—these hardy fish can live for five or more years. They are known for being avid biofilm grazers, and their diet often includes algae, a staple that is frequently under-reported in their care requirements.

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