Callichthys is the type genus of family Callichthyidae, the armored catfish family that also holds Corydoras and its relatives. The atlas's five species range across Colombia, Brazil, Venezuela, Ecuador, Peru, Argentina, Bolivia, and French Guiana, one of the broader distributions among South American catfish genera, and grow 3–8.5 in. Like other callichthyids, they carry two rows of bony armor plates along the body and can gulp atmospheric air, absorbing oxygen through a highly vascularized hindgut that lets them survive in stagnant, oxygen-poor water lethal to most fish.
Callichthys callichthys, the cascarudo, is the genus's best-known member - a hardy, adaptable catfish valued less for bright color than for its unusual biology and its habit of building a bubble nest at the water's surface to guard its eggs, a distinctive behavior among aquarium catfish. Conservation is mixed rather than uniformly reassuring - two Least Concern, but one Vulnerable and one Near Threatened among the five atlas species - a reminder that even a widespread-seeming genus can hold locally restricted species under real pressure.