The earliest known fossil siluriforms date from the late Cretaceous period and are in freshwater and marine deposits. Catfish live, or have lived, on all continents including Antarctica where there are fossil records from the Eocene era. Because of this diversity, they are of particular interest to ecologists and evolutionary biologists (Lundberg & Friel, 2003). The taxonomy of this order is undergoing changes, and there is quite a divergence of opinion among ichthyologists concerning several classifications. As of 2007, there are 36 extant families with some 3,023 species (Ferraris, et al. 2007). The makes the order the second or third most diverse of the vertebrates: 1 in 4 freshwater fish, 1 in 10 fish, and 1 in 20 vertebrates is a catfish.
Catfish vary considerably in size, from some of the smallest vertebrates at 20mm (less than 1 inch) to some of the largest freshwater fish at close to 5 metres (over 16 feet). Aquarists commonly keep species from several different families, though by far the most occur in the Callichthyidae [Corydoras, etc.] and Loricariidae [plecostomus, whiptails, Farlowella, Sturisoma, etc.].
The common name "catfish" arose from the barbels resembling a cat's whiskers. While there are some other fish (for instance many of the cyprinids) that have them, all catfish have barbels on the head: normally up to four pairs - one nasal, one maxillary, and two on the chin or lower jaw (mandible). The nasal and chin barbels may be absent in some species. Eyes are usually small, as the barbels are sensory organs used to locate food, a particular advantage that allows the catfish to live in very dark or murky waters. Most species possess an adipose fin. Many have the ability to use aerial respiration (extract oxygen from the air). Some can "crawl" over land to find a pool of water - a very useful adaptation during the dry season when pools frequently dry up. And except for species in the Malapteruridae (electric catfishes) family, the rays of the dorsal and pectoral fins are preceded by one (frequently two on the dorsal) spine-like rays referred to as "spines" that can be locked into position; these are very sharp and can inflict a serious wound, and many have a stinging protein venom that may irritate, cause temporary paralysis, or in a few species can even kill a human.
Catfish have no scales; their bodies are either naked or covered by bony plates called scutes. Most catfish have a cylindrical body with a flatted ventral surface allowing for benthic (bottom) feeding; and for this they are negatively buoyant - a heavy bony head and reduced gas bladder means they will sink rather than float. The head is often flattened to allow for digging in the substrate. The mouth may be inferior (pointing downward) or subterminal and frequently expandable to suck in food; there are no incisor (cutting) teeth.
Most species use external fertilization but species in 11 different families have been discovered to use internal fertilization. The majority of catfish provide parental care to the eggs, and a few species also to the fry. Many commonly kept by aquarists such as the Callichthyidae do neither.
Requirements respecting water parameters, water flow (this is critical to many species), and aquascaping are varied for the many species and will be discussed in the profile for each species. All catfish are highly sensitive to chemicals and medications, partly due to their lack of scales but also to their physiology, and will usually be the first fish in the aquarium to show signs of stress from treatments. It is important to keep the water as free of these as possible.
Ferraris, Carl J. Jr., M. Miya, Y. Azuma and M. Nishida (2007), "Checklist of catfishes, recent and fossil (Osteichthyes: Siluriformes), and catalogue of siluriform primary types," Zootaxa 1418, pp. 1-628.
Lundberg, John G. and John P. Friel (2003), "Siluriformes," Tree of Life project. http://tolweb.org/Siluriformes/15065