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. Most 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 may have up to four pairs of barbels: nasal, maxillary (on each side of mouth), and two pairs of chin barbels, although pairs of barbels may be absent, depending on the species. Because their barbels are more important in detecting food, the eyes on catfish are generally small. Like other ostariophysans, they are characterized by the presence of a Weberian apparatus. Their well-developed Weberian apparatus and reduced gas bladder allow for improved hearing as well as sound production.
Catfish have no scales; their bodies are often naked. In some species, the mucus-covered skin is used in cutaneous respiration, where the fish breathes through its skin. In some catfish, the skin is covered in bony plates called scutes; some form of body armor appears in various ways within the order.
All catfish, except members of Malapteruridae (electric catfish), possess a strong, hollow, bonified leading spine-like ray on their dorsal and pectoral fins. As a defense, these spines may be locked into place so that they stick outwards, which can inflict severe wounds.
Juvenile catfish, like most fish, have relatively large heads, eyes and posterior median fins in comparison to larger, more mature individuals. These juveniles can be readily placed in their families, particularly those with highly derived fin or body shapes; in some cases identification of the genus is possible. As far as known for most catfish, features that are often characteristic of species such as mouth and fin positions, fin shapes, and barbel lengths show little difference between juveniles and adults.
The largest Ictalurus furcatus, caught in the Mississippi River on May 22, 2005, weighed 124 pounds (56 kg). The largest flathead catfish, Pylodictis olivaris, ever caught was in Independence, Kansas, weighing 123 lb 9 oz (56.0 kg). In July 2009, a catfish weighing 193 pounds was caught in the River Ebro, Spain, by a 11-year old British schoolgirl. However, these records pale in comparison to a giant Mekong catfish caught in northern Thailand in May 1, 2005 and reported to the press almost 2 months later that weighed 293 kilograms (650 lb). This is the largest giant Mekong catfish caught since Thai officials started keeping records in 1981. The giant Mekong catfish are not well studied since they live in developing countries and it is quite possible that they can grow even larger.