Catfish are a diverse group of ray-finned fish of the order Siluriformes (historically Nematognathi). 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 (such as maxillae modified into the barbels' bases) 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. Some species, such as plecos of the genus Pterygoplichthys, are prolific invasives due to the aquarium hobby.
Most catfish are bottom feeders. In general, they are negatively buoyant, which means that they 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. Some families, though, notably the Loricariidae and Astroblepidae, have a suckermouth that allows them to fasten themselves to objects in fast-moving water.
Catfish do not have scales; their bodies are often naked. In some species, their 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. In loricarioids and in the Asian genus Sisor, the armor is primarily made up of one or more rows of free dermal plates. Similar plates are found in large specimens of Lithodoras. These plates may be supported by vertebral processes, as in scoloplacids and in Sisor, but the processes never fuse to the plates or form any external armor. By contrast, in the subfamily Doumeinae (family Amphiliidae) and in hoplomyzontines (Aspredinidae), the armor is formed solely by expanded vertebral processes that form plates. Finally, the lateral armor of doradids, Sisor, and hoplomyzontines consists of hypertrophied lateral line ossicles with dorsal and ventral lamina.
Juvenile catfish, like other 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. For many species, pigmentation pattern is also similar in juveniles and adults. Thus, juvenile catfish generally resemble and develop smoothly into their adult form without distinct juvenile specializations. Exceptions to this are the ariid catfish, where the young retain yolk sacs late into juvenile stages, and many pimelodids, which may have elongated barbels and fin filaments or coloration patterns.
The maxilla is a tooth-bearing bone in vertebrates, and modified in neopterygian fish to facilitate the protrusion of the mouth and enable suction feeding. Catfish, despite being a group of neopterygians, reduced the maxilla into a support for the maxillary barbels; this means that they are unable to protrude their mouths as other fish such as carp. Catfish barbels typically occur in pairs, and up to four pairs of barbels may be present in some species; these being the nasal, maxillary (on each side of mouth), and two pairs of "chin" barbels termed the internal and external mandibular barbel, though the various families often have fewer pairs, some species may have branched or duplicated barbel pairs, and a number of families only have extremely reduced maxillary barbels. The palatine-maxillary system is responsible for moving the maxillary barbels; it is a system of ligaments and muscles centred on these two skeletal elements. If severed, the barbels grow back over time, but the maxillary barbels cannot regenerate if their basal element (the maxilla) is lost.
Many larger catfish have chemoreceptors across their entire bodies (especially the barbels), which means they "taste" anything they touch, and "smell" any chemicals in the water. "In catfish, gustation plays a primary role in the orientation and location of food". Because barbels and chemoreception are more important in detecting food, their eyes are generally small, and many species lost them entirely as they adapted to underground environments, becoming cavefish. 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 and sound production.
All catfish other than members of the Malapteruridae (electric catfish), possess a strong, hollow, bony, 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, enabling them to inflict severe wounds. In numerous catfish species, these fin rays can be used to deliver a stinging protein if the fish is irritated; as many as half of all catfish species may be venomous in this fashion, making the Siluriformes overwhelmingly the vertebrate order with the largest number of venomous species. This venom is produced by glandular cells in the epidermal tissue covering the spines. In members of the family Plotosidae and of the genus Heteropneustes, this protein is so potent it may hospitalize humans who receive a sting; in Plotosus lineatus, the stings can be lethal. The dorsal- and pectoral-fin spines are two of the most conspicuous features of siluriforms, and differ from those in other fish groups. Despite the widespread use of the spines for taxonomic and phylogenetic studies the fields have struggled to effectively use the information due to a lack of consistency in the nomenclature, with a general standard for the descriptive anatomy of catfish spines proposed in 2022 to try to resolve this problem.
In many catfish, the "humeral process" is a bony process extending backward from the pectoral girdle immediately above the base of the pectoral fin. It lies beneath the skin, where its outline may be determined by dissecting the skin or probing with a needle.
The retinae of catfish are composed of single cones and large rods. Many catfish have a tapetum lucidum, which may help enhance photon capture and increase low-light sensitivity. Double cones, though present in most teleosts, are absent from catfish.
Sexual dimorphism is reported in about half of all families of catfish. The modification of the anal fin into an intromittent organ (in internal fertilizers) as well as accessory structures of the reproductive apparatus (in both internal and external fertilizers) have been described in species belonging to 11 different families.
The anatomical organization of the testis in catfish is variable among the families of catfish, but the majority of them present fringed testis: Ictaluridae, Claridae, Auchenipteridae, Doradidae, Pimelodidae, and Pseudopimelodidae. In the testes of some species of Siluriformes, organs and structures such as a spermatogenic cranial region and a secretory caudal region are observed, in addition to the presence of seminal vesicles in the caudal region. The total number of fringes and their length are different in the caudal and cranial portions between species. Fringes of the caudal region may present tubules, in which the lumen is filled by secretion and spermatozoa. Spermatocysts are formed from cytoplasmic extensions of Sertoli cells; the release of spermatozoa is allowed by breaking of the cyst walls.