Hemiodus is a genus of slender, silvery characiforms in the family Hemiodontidae, sometimes called halftooths for their distinctive jaw structure — teeth are reduced or absent from the upper jaw, with grazing done mainly by the lower jaw. The genus is widespread across the Amazon, Orinoco, and Guiana Shield river systems, with the atlas's records spanning Brazil, Guyana, Venezuela, Colombia, Suriname, Bolivia, Peru, and French Guiana. Fish in the genus are open-water, mid-column schoolers that graze algae and biofilm rather than hunt, and several species carry bold black markings on the tail or body that stand out against an otherwise plain silver coloring. The atlas covers 23 species, from about 3–13.5 in as adults.
Hemiodus is kept by a smaller circle of large-aquarium and South American biotope enthusiasts drawn to its elegant, torpedo-shaped form and schooling behavior in open water — species with bold caudal-fin markings, such as the barred and one-spot forms in the atlas's species list, are the ones most likely to turn up in the trade. They are peaceful, active swimmers that need more horizontal space than most tetras and do best kept in groups. The genus's wide range across several major river basins is reflected in a reassuring conservation picture, with most assessed species rated Least Concern and only a handful unevaluated or Data Deficient.