Catfish

Catfish belong to the order Siluriformes (once called Nematognathi) and are a varied group of ray-finned fish. Their name comes from the barbels that stick out like a cat’s whiskers, though not every species has obvious ones. What all members of this order share is a lack of scales—they either have smooth skin or bodies covered in bony armor. These fish are defined by skull features, such as the maxillae that form the base of the barbels, and by their swimbladder. In size and lifestyle, they range from the three biggest living species—the Mekong giant catfish in Southeast Asia, the wels catfish in Eurasia, and the piraíba in South America—to bottom feeders that scavenge or eat detritus, all the way down to the tiny parasitic candiru. In the Southern United States, local slang gives them names like "mud cat," "polliwogs," or "chuckleheads," though these terms vary by region and aren’t standardized; for instance, "chucklehead" might mean a bullhead catfish in one area but a blue catfish elsewhere. Commercially, catfish matter a lot: larger species are farmed or caught for food, like the Pangasius (a shark catfish) and Clarias (such as the walking catfish), while smaller ones, including many Corydoras, are popular in aquariums. Some, like plecos from the genus Pterygoplichthys, have become invasive because of the aquarium trade.

Most catfish feed near the bottom. They tend to be negatively buoyant, so they sink rather than float, thanks to a reduced gas bladder and a heavy, bony head. Their body shapes vary, but many have a cylindrical form with a flattened underside for benthic feeding. A flattened head helps them dig through the substrate and may also act as a hydrofoil. Some have mouths that can open wide and lack incisor-like teeth; they usually feed by sucking or gulping instead of biting and cutting. However, families like the Loricariidae and Astroblepidae have a suckermouth that lets them attach to objects in fast currents. Without scales, their bodies are often bare. In some species, the mucus-covered skin allows cutaneous respiration—breathing through the skin. Others have bony plates called scutes; various forms of armor appear across the order. In loricarioids and the Asian genus Sisor, the armor consists of one or more rows of free dermal plates. Similar plates show up in large specimens of Lithodoras. These plates may be supported by vertebral processes in scoloplacids and Sisor, but the processes never fuse to the plates or create external armor. In the subfamily Doumeinae (family Amphiliidae) and in hoplomyzontines (Aspredinidae), the armor is made entirely from expanded vertebral processes that form plates. Meanwhile, the lateral armor of doradids, Sisor, and hoplomyzontines comes from hypertrophied lateral line ossicles with dorsal and ventral lamina.

Juvenile catfish, like other young fish, have relatively large heads, eyes, and posterior median fins compared to adults. These juveniles can often be placed in their families, especially those with highly derived fin or body shapes; sometimes even the genus is identifiable. As far as is known for most catfish, features typical of species—such as mouth and fin positions, fin shapes, and barbel lengths—differ little between juveniles and adults. Pigmentation patterns are also similar in many species. So juveniles generally resemble adults and develop smoothly into them without distinct specializations. Exceptions include ariid catfish, whose young keep yolk sacs late into the juvenile stage, and many pimelodids, which may have elongated barbels, fin filaments, or different coloration.

The maxilla is normally a tooth-bearing bone in vertebrates, modified in neopterygian fish to help protrude the mouth for suction feeding. Catfish, though neopterygians, reduced the maxilla into a support for the maxillary barbels, so they cannot protrude their mouths like carp can. Barbels usually come in pairs, and some species have up to four pairs: nasal, maxillary (one on each side of the mouth), and two pairs of chin barbels called the internal and external mandibular barbels. Different families often have fewer pairs, some species have branched or duplicated barbels, and a number of families have only very reduced maxillary barbels. The palatine-maxillary system, a network of ligaments and muscles around these two skeletal elements, moves the maxillary barbels. If cut, the barbels grow back over time, but they cannot regenerate if the maxilla—their basal element—is lost. Many larger catfish have chemoreceptors all over their bodies, especially on the barbels, so they taste anything they touch and smell chemicals in the water. Gustation plays a primary role in how they find and orient toward food. Because barbels and chemoreception are more important for detecting food, their eyes are usually small, and many species have lost them entirely after adapting to underground environments, becoming cavefish. Like other ostariophysans, they have a Weberian apparatus. Their well-developed Weberian apparatus and reduced gas bladder improve hearing and sound production.

All catfish except members of the Malapteruridae (electric catfish) possess a strong, hollow, bony, leading fin spine.