Catfish (order Siluriformes) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores (species that eat dead material on the bottom), and even to a tiny parasitic species commonly called the candiru, Vandellia cirrhosa. There are armour-plated types and there are also naked types, neither having scales. Despite their name, not all catfish have prominent barbel. Members of the Siluriformes order are defined by features of the skull and swimbladder. Catfish are of considerable commercial importance; many of the larger species are farmed or fished for food. Many of the smaller species, particularly the genus Corydoras, are important in the aquarium hobby. Many catfish are nocturnal, but others (many Auchenipteridae) are crepuscular or diurnal (most Loricariidae or Callichthyidae for example).
Catfish have inhabited all continents at one time or another. Catfish are most diverse in tropical South America, Asia and Africa with one family native to North America and one family in Europe. More than half of all catfish species live in the Americas. They are the only ostariophysans that have entered freshwater habitats in Madagascar, Australia, and New Guinea.
In the United States, catfish species may be known by a variety of slang names, such as "mud cat", "polliwogs", or "chuckleheads". These nicknames are not standardized, so one area may call a bullhead catfish by the nickname "chucklehead", while in another state or region, that nickname refers to the blue catfish.
Most catfish are bottom feeders. In general, they are negatively buoyant, which means that they will usually sink rather than float due to a reduced gas bladder and a heavy, bony head. Catfish have a variety of body shapes, though most have a cylindrical body with a flattened ventrum to allow for benthic feeding. A flattened head allows for digging through the substrate as well as perhaps serving as a hydrofoil. Some have a mouth that can expand to a large size and contains no incisiform teeth; catfish generally feed through suction or gulping rather than biting and cutting prey. Catfish also have a maxilla reduced to a support for barbels; this means that they are unable to protrude their mouths as other fish such as carp.Catfish have one of the greatest ranges in size within a single order of bony fish. Many catfish have a maximum length of under 12 cm. Some of the smallest species of Aspredinidae and Trichomycteridae reach sexual maturity at only 1 centimetre (0.39 in). The wels catfish, Silurus glanis, is the only native catfish species of Europe, besides the much smaller related Aristotle's catfish found in Greece. Mythology and literature record wels catfish of astounding proportions, yet to be proven scientifically. The average size of the species is about 1.2–1.6 m (3.9–5.2 ft), and fish more than 2 metres (6.6 ft) are very rare. The largest specimens on record measure more than 2.5 metres (8.2 ft) in length and sometimes exceeded 100 kilograms (220 lb). In July 2009, a catfish weighing 88 kilograms (194 lb) was caught in the River Ebro, Spain, by an 11-year-old British schoolgirl. In North America the largest Ictalurus furcatus (Blue catfish) caught in the Missouri River on 20 July 2010, weighed 130 pounds (59 kg). The largest flathead catfish, Pylodictis olivaris, ever caught was in Independence, Kansas, weighing 56 kilograms (123 lb). These records pale in comparison to a giant Mekong catfish caught in northern Thailand on 1 May 2005 and reported to the press almost 2 months later that weighed 293 kilograms (646 lb). This is the largest giant Mekong catfish caught since Thai officials started keeping records in 1981. This species is not well studied since it lives in developing countries and it is quite possible it can grow even larger. Also in Asia, Jeremy Wade caught a 75.5 kilograms (166.4 lb) Goonch following three fatal attacks on humans in the Kali River on the India-Nepal border. Wade was of the opinion that the offending fish must even have been significantly larger to have taken an 18-year-old boy as well as a water buffalo.