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Scientific classification
Catfish are a very diverse group of fish. They are found in freshwater environments of all kinds, with species on every continent except Antarctica. Some species from the families Ariidae and Plotosidae are also found in marine environs. They range in size and behavior from the largest freshwater fish in the world (the European wels, or Silurus glanis), to detritivores (species that eat dead material on the bottom), and even to a tiny parasitic species commonly called the candiru (Vandellia cirrhosa). At present there are 35 catfish families, although this number is in constant flux due to taxonomic work on the order.

They belong to a superorder called the Ostariophysi, which also includes the Cypriniformes, Characiformes, and Gymnotiformes (although some place Gymnotiformes as a sub-order of Siluriformes).

Catfish have no scaless. All catfish, except members of Malapteruridae (electric catfish), posses a strong, hollow, bonified leading ray on their dorsal and pectoral fins, through which a stinging protein can be delivered if the fish is irritated. In members of the family Plotosidae, and of the genus Heteropneustes, this protein is so strong it may hospitalize humans unfortunate enough to receive a sting.

Catfish are important food fish throughout the world. Ictalurids are cultivated in North America, while Clariids and Pangasiids are heavily cultured in Africa and Asia. There is also a large and growing ornamental fish trade, with catfish a popular component of many aquaria.