Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Catfish are named for their prominent barbels, which resemble a cat's whiskers, though not all catfish have prominent barbels. All siluriformes lack scales, instead possessing either smooth skin or armour-plated bodies. This order of fish are defined by features of the skull and swimbladder. Catfish range in size and behavior from the three largest species alive, the Mekong giant catfish from Southeast Asia, the wels catfish of Eurasia, and the piraíba of South America, to detritivorous and scavenging bottom feeders, down to the tiny ectoparasitic species known as the candiru. In the Southern United States, catfish may be known by a variety of slang names, such as "mud cat", "polliwogs", or "chuckleheads". Such names are regional and unstandardized. For instance, "chucklehead" in one region may refer to a bullhead catfish but indicate blue catfish elsewhere. Catfish as a group are of considerable commercial importance; many of the larger species are farmed or fished for food, such as the Pangasius (a shark catfish) and Clarias (like the walking catfish). Many of the smaller species, such as members of the genus Corydoras, are important in the aquarium hobby.