Nannostomus trifasciatus

Nannostomus trifasciatus, called the three-lined or three-stripe pencilfish, is a small freshwater fish in the characin family Lebiasinidae. Its name comes from Greek and Latin roots meaning "small," "mouth," and "three bands." Hobbyists value it for its small size, attractive colors, and resilience.

First described by Steindachner in 1876, it was one of the earliest four species in its genus. Because it lives across a wide area, the species shows different color forms, and some were mistakenly labeled as separate species in the past. Two such names, Poecilobrycon erythrurus and Poecilobrycon vittatus, are now considered junior synonyms.

This fish prefers slow-moving, slightly acidic water and is found in several habitat types. Its range covers much of the Amazon basin, including Brazil, Colombia, Peru, northern Bolivia, and the Guianas. In the Rio Negro, it lives in large swamps where tributaries meet the main river. During floods, it moves into flooded rainforest; in the dry season, it often ends up in isolated lakes or stays near wooded shorelines. At night, it rests near the surface; by day, it swims in the middle to upper water layers.

Adults grow to no more than 6 cm. They have a small terminal mouth, and an adipose fin may or may not be present. Like many pencilfish, this species changes color between day and night. By day, three black stripes run along the body. At night, three large dark spots appear on each side, stretching from back to belly. This shift is caused by the pineal hormone melatonin acting on pigment cells in different skin areas. The daytime pattern may warn predators or help fish recognize each other; the nighttime pattern may hide them from nocturnal hunters. Between the dark markings, the back and sides are greenish-gold, and the belly is silver. The gill cover and dorsal, pelvic, anal, and caudal fins have red blotches that vary in size by population. It is often confused with Nannostomus marginatus, another three-striped pencilfish, but that species is smaller and has a blockier body shape.

Sexual differences are subtle compared to other Nannostomus species. The male’s anal fin is not modified or more colorful. Some males have an extra row of red spots in the gold area between the middle and top stripes, but this varies even within the same population. Males may be slimmer and more colorful, but again, this is not consistent.