Catfish and their relatives account for three-quarters of all species of freshwater fish. Although they are a varied group, almost all species have a bony internal structure, known as the Weberian apparatus, which is used in sound reception and contributes to their acute hearing. They are divided into 5 groups: milkfish and relatives, cyprinids (the most widespread of which are carp and minnows), characins (including tetras, tigerfish, and piranhas), neotropical knifefish, and catfish themselves. Although mostly confined to freshwater, some species are found in brackish water or the sea.
There are about 85 families and 10,500 species of catfish and relatives.