page_title: Pencil fish
section_title: Description
hierarchical_section_title: Pencil fish / Description
caption_attribution_description: English: Nannostomus eques
context_page_description: Nannostomus, is a genus of fish belonging to the characin family Lebiasinidae. All of the species in this genus are known as pencil fish or pencilfish, a popular name that was first only applied to two species in the 1920s, Nannostomus unifasciatus and Nannostomus eques, but by the late 1950s would come to be applied to all members of the genus. Several of the species have become popular aquarium fish due to their attractive coloration, unique shape, and interesting demeanor.
context_section_description: Most species are slender, pencil-shaped fish ranging in size from under 1 to about 2 in long. N. marginatus, N. rubrocaudatus, and N. mortenthaleri possess shortened, blockier outlines reminiscent of pencil stubs. All but one species, Nannostomus espei, possess one to five horizontal black or brown stripes with gold or silver iridescence appearing dorsal to the primary stripe. Most also display red, orange, or maroon highlights in their fins, and many have flashes of these colors on their flanks, as well. The recently described N. mortenthaleri and N. rubrocaudatus are especially vividly colored. For N. espei alone, horizontal stripes are only weakly present and  are supplanted by five dark comma-shaped blotches. This pattern is assumed by other species at night, but only N. espei displays the pattern permanently and in daylight.  The adipose fin is present in some species and absent in others, while in certain species, N. eques for example, it may be present or absent within the species. All swim in a horizontal attitude except N. unifasciatus and N. eques, which assume an oblique, 'snout-up' posture. A range of sexual dimorphism occurs in the genus, with males being more brilliantly colored in some species, especially with regard to color present in the fins, and far less evident in other species. However, a reliable indicator of gender for most of the species rests in the anal fin of adult males which is enlarged and elongated (as in N. espei, N. eques, etc.) and/or the anal fin of males is more colorful (as in N. harrisoni, N. marginatus etc.). The popular aquarium species, N. trifasciatus, is an exception in this regard.
caption: N. eques
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